Magnetic Nanocatalysis. Volume 1: Synthetic Applications [1] 9783110735185

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Rajender S. Varma, Bubun Banerjee (Eds.) Magnetic Nanocatalysis

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Magnetic Nanocatalysis Synthetic Applications Edited by Rajender S. Varma and Bubun Banerjee

Editors Dr. Rajender S. Varma Regional Centre of Advanced Technologies and Materials Palacky University Šlechtitelů 27 783 71 OLOMOUC Czech Republic [email protected] Dr. Bubun Banerjee Department of Chemistry Akal University Talwandi Sabo, Bathinda Punjab 151302 India [email protected]

ISBN 978-3-11-073518-5 e-ISBN (PDF) 978-3-11-073035-7 e-ISBN (EPUB) 978-3-11-073047-0 Library of Congress Control Number: 2022930076 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2022 Walter de Gruyter GmbH, Berlin/Boston Cover image: Wittayayut/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Dedicated to Sant Baba Iqbal Singh Ji, Founder Chancellor, Akal University

Shiromani Panth Rattan, Vidya Maartand, Padam Shri Sant Baba Iqbal Singh Ji, fondly known as ‘Baba Ji’, was born on May 1, 1926 in village Bharial Lahri, Tehsil Pathankot, District Gurdaspur (now Pathankot). Inspired by Sant Teja Singh Ji, the devout disciple of Sant Attar Singh Ji, he made a resolve to dedicate his life towards serving humanity and thus he started his journey of relentless service towards humanity following the footsteps of his Guru, Sant Attar Singh Ji Maharaj. He did B.Sc. in Agriculture and retired as the ‘Director of Agriculture’ from Himachal Pradesh Government in 1987. Realizing the poor condition, he felt to do something for the betterment of the society. With his own money he started a school called Akal Academy at Baru Sahib, Sirmaur, Himachal Pradesh, India (a prelude to Eternal University) with just five students in 1986. He tried to educate underprivileged children and treated thousands of patients at his charitable institution under The Kalgidhar Trust at Baru Sahib, Sirmaur, Himachal Pradesh, India. Baba Ji worked relentlessly in only one direction – imparting values-based education in rural India so that every rural child can have access to low-cost values-based education which is literacy embedded with moral, ethical and spiritual education. To carry forward this noble mission, he worked tirelessly and one by one, he established 129 Akal Academies in Punjab, Himachal Pradesh, Haryana, Uttar Pradesh and Rajasthan, wherein more than 70,000 students are being imparted the blend of modern scientific education. These students have not only excelled in academics but also imbibed spiritual ethos. Apart from making their life ideal and successful, they have also transformed the lives of their families and relatives who were previously addicted to consumption of various drugs. Far away from the urban milieu, these schools focus on values-based education to children from marginalized sections of society. As a social worker Baba Iqbal Singh ji didn’t restricted himself just to the education sector, he was involved in every facet of community life i.e. Schools, Hospital, Colleges, Women Care Center, De-addiction Centers. Baba Ji established Akal University in 2015 which is running successfully at Talwandi Sabo in Bathinda, Punjab. Baba ji sincerely believed that real contribution to society constitutes in working relentlessly for the poorest, oppressed and most backward sections of the society. https://doi.org/10.1515/9783110730357-202

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Dedicated to Sant Baba Iqbal Singh Ji, Founder Chancellor, Akal University

For his philanthropic and selfless services for social welfare, he was conferred various awards and honors: Dedicating his entire life to the service of humanity and social welfare, Baba Ji merged with the Divine Light on 29th January, 2022 at 2.30 p.m at the age of 96.

Foreword

Sustainability is the key criteria for the development of modern science and is an unavoidable aspect in chemistry, among all other disciplines. Catalysis occupies the center stage of sustainable development in chemistry, and especially organic synthesis where it has been recognized as an indispensable tool for green and sustainable chemical developments to combat the environmental challenges. By lowering the activation energy, catalysts help to form the product through an energetically favorable pathway. Various homogeneous catalysts display significant catalytic activity but are difficult to recover from the reaction medium after completion of the reaction. Thus, many organic methodologists slowly shifted towards the development of heterogeneous catalysts. The past two decades have witnessed tremendous surge in the development of nanoscience and nanotechnology; a large number of diverse methods have been advanced for the assembly of nanoparticles (NPs) with varying size, shape, and composition. Comparative to bulk materials, high surface-to-volume ratio makes nanomaterials a viable alternative to conventional catalysts. Incessant efforts have been made to prepare newer heterogeneous nanocatalysts or immobilize the homogeneous catalysts onto the solid support nanoscale surfaces to render them heterogeneous for easy recovery. Furthermore, metal contamination in the product is considered a major disadvantage for the large-scale industrial applicability of the metal-based heterogeneous nanocatalysts because of possible leaching. The deployment of magnetic nanocatalysts, however, enables their quantitative recovery using a simple external magnet. Thus, the recovered nano catalysts can be reused for further reactions without any significant loss in its catalytic activities. Under this purview, I personally believe that this book entitled ‘Magnetic Nanocatalysis: Synthetic Applications’ edited by Dr. Rajender S. Varma and Dr. Bubun Banerjee is going to be a valuable resource for the researchers working in the fascinating field of nanocatalysis. The first chapter describes the catalytic applications of various magnetic nanoparticles for the formation of diverse carbon-heteroatom bonds and thereby synthesis of structurally diverse biologically promising heterocyclic scaffolds. Chapter 2 deals with the uses of magnetic nanoparticles for the disulfide synthesis while the 3rd chapter demonstrates the utility of the hybrid magnetic nanocatalysts in assorted organic transformations. Chapter 4 focuses on the reduction of https://doi.org/10.1515/9783110730357-203

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Foreword

nitroarenes by utilizing magnetic nanoparticles and magnetic copper ferrite (CuFe2O4) nanoparticles-catalyzed diverse organic transformations are discussed in Chapter 5. Chapter 6 provides the an elaborative literature related to the appliance of magnetic nanocatalysts in C-C bond forming reactions through various coupling reactions and representative oxidation reactions. Chapter 7 specifically describes the importance of magnetic nanocatalysis in Suzuki cross-coupling reactions whereas magnetic nickel nanoparticles catalyzed synthesis of various biologically promising heterocyclic scaffolds are deliberated in Chapter 8. Synthesis and catalytic applications of various sulfonic acid functionalized magnetic nanoparticles are summarized in Chapter 9. Chapter 10 specifically updates about the scope of silica coated magnetic nanomaterials in organic synthesis. Prof. Chintamani Nagesa Ramachandra Rao (C.N.R. Rao) Bharat Ratna, D.Sc., Ph.D., FRS, Hon. FRSC Linus Pauling Research Professor & Honorary President Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India

About Professor C.N.R. Rao C.N.R. Rao (born on 30 June 1934, Bangalore, India) received the M.Sc. degree from Banaras, Ph.D. from Purdue, D.Sc. from Mysore universities. He is Honorary President and Linus Pauling Research Professor at the Jawaharlal Nehru Centre for Advanced Scientific Research. He is also an Honorary Professor at the Indian Institute of Science. His main research interests are in solid state and materials chemistry. He is an author of over 1770 research papers and 53 books. He has received honoris causa doctorate degrees from 82 universities including Purdue, Bordeaux, Banaras, Calcutta, Delhi, IITs (Bombay, Kharagpur, Kanpur, New Delhi, Guwahati), IISERs (Bhopal, Kolkata, Mohali, Pune), The Assam Royal Global University,Northwestern, Notre Dame, Novosibirsk, Oxford, Stellenbosch, Temple, Université Joseph Fourier, Grenoble, Uppsala, Wales, Wroclaw, Caen, Liverpool, St. Andrews, Canberra, Taiwan and Desikottama from VisvaBharati. Prof. Rao is a member of several of the science academies in the world, including the Royal Society, London, the National Academy of Sciences, U.S.A., the Russian, French and Japan Academies as well as the American Philosophical Society. He is a Member of the Pontifical Academy of Sciences and Foreign Fellow of Academia Europaea, the Royal Society of Canada and the Chinese Academy of Sciences. He is a distinguished visiting professor of the University of California. Among the various medals, honours and awards received by him, mention may be made of the Marlow Medal of the Faraday Society (1967), Bhatnagar Prize (1968), Padma Shri (1974), Royal Society of Chemistry (London) Medal (1981), Padma Vibhushan (1985), Honorary Fellowship of the Royal Society of Chemistry, London (1989), Blackett Lectureship of the Royal Society (1991), Einstein Gold Medal of UNESCO (1996), Linnett Professorship of the University of Cambridge (1998), Centenary Medal of the Royal Society of Chemistry (2000), Hughes Medal of the Royal Society for original discovery in physical sciences (2000), Karnataka Ratna (2001), the Order of Scientific Merit (Grand-Cross) from the President of Brazil (2002) and the Somiya Award of the International Union of Materials Research (2004). He is the first recipient of the India Science Award by the Government of India and received the Dan David Prize for science in the future dimension for his research in Materials Science in 2005. He was named Chemical Pioneer by the American Institute of Chemists (2005), Chevalier de la Légion d’Honneur by the President of the French Republic (2005) and received the Honorary Fellowship of the Institute of Physics, London (2006) and of St. Catherine’s College, Oxford (2007). He received the Nikkei Asia Prize for Science, Technology and Innovation in 2008 andwas awarded the Royal Medal by the Royal Society (2009) and the AugustWilhelm-von-Hoffmann Medal by the German Chemical Society (2010). He received the Ernesto Illy Trieste Science Prizefor materials research in 2011 and was Albert Einstein Professor of the Chinese Academy of Sciences in 2012.The President of India conferred the title Bharat Ratna in 2014. The Emperor of Japan bestowed the Order of the Rising Sun, Gold and Silver Star in 2015. He was conferred the highest award for materials research, the von Hippel award by the Materials Research Society, in 2017. He is the first Asian to receive this award. The Centre for Advanced Materials of Ras Al Khaima has conferred the First Sheikh Saud International Prize for Materials Research (2019), Karnataka Science and Technology Academy Lifetime Achievement Award in STEAM (Science, Technology, Engineering, Agriculture & Medicine); Eni Award 2020 Edition of the Energy Frontiers Prize. Prof. Rao was Chairman, Scientific Advisory Council to the Prime Minister during 2004–2014 and also 1985–89. He was President of The Academy of Sciences for the Developing World (TWAS). He is Founder-President of both the Chemical Research Society of India and of the

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Materials Research Society of India. Prof. Rao was President of the Indian National Science Academy (1985–86), the Indian Academy of Sciences (1989–91) and the International Union of Pure and Applied Chemistry (1985–97). He was the Director of the Indian Institute of Science (1984–94), and Chairman, Scientific Advisory Committee to the Union Cabinet (1997–98) and Albert Einstein Research Professor (1995–99).

Contents Dedicated to Sant Baba Iqbal Singh Ji, Founder Chancellor, Akal University V Foreword

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Brindaban C. Ranu, Laksmikanta Adak, Nirmalya Mukherjee, Tubai Ghosh Chapter 1 Magnetic metal nanoparticle-catalyzed carbon-heteroatom bond formation and synthesis of related heterocycles 1 Bablee Mandal, Basudeb Basu Chapter 2 Magnetic nanocatalysts in disulfide synthesis

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Nahid Ahmadi, Ali Ramazani Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

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Mohammadreza Shokouhimehr, Ho Won Jang Chapter 4 Magnetic nanostructured catalysts for reduction of nitroaromatics Abhijeet Singh, Trisha Ghatak, Shalini Agarwal, Ramendra Pratap, Mahendra Nath Chapter 5 Applications of CuFe2O4 magnetic nanoparticles in organic synthesis

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Sanjay Paul, Asish Ranjan Das Chapter 6 Magnetic nanoparticle catalysis: a potential platform for the fabrication of C–C bond generation and oxidation reaction to achieve important structural motifs 191 Nirjhar Saha, Asim Kumar, Antarlina Maulik, Asit K. Chakraborti Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

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Yadavalli Venkata Durga Nageswar, Jayathirtha Rao Vaidya Chapter 8 Ni nanoparticles-mediated synthesis of various heterocycles

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Yogesh B. Wagh, Yogesh A. Tayade, Dipak S. Dalal Chapter 9 Sulfonic acid functionalized magnetic nanocatalysts in organic synthesis

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Manavi Yadav, Sriparna Dutta, Anju Srivastava, Rakesh K. Sharma Chapter 10 Silica-coated magnetic nanocatalysts as efficient green catalysts for organic synthesis 379 Manmeet Kaur, Anu Priya, Arvind Singh, Aditi Sharma, Gurpreet Kaur, Bubun Banerjee Chapter 11 Multicomponent synthesis of biologically promising pyrans and pyran annulated heterocycles using magnetically recoverable nanocatalysts Author list Index

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Brindaban C. Ranu*, Laksmikanta Adak, Nirmalya Mukherjee, Tubai Ghosh

Chapter 1 Magnetic metal nanoparticle-catalyzed carbon-heteroatom bond formation and synthesis of related heterocycles 1.1 Introduction The past decade saw an exponential growth in the area of nanoscience and nanotechnology. One of the most interesting features of nanotechnology is its useful applications in various fields. The discovery and easy accessibility of nanoparticles (NPs) of different shapes, sizes, and compositions have prompted investigations on their applications in catalysis. The “nano” in nanocatalysis refers to the size of the particles in the nanoscale range. As nanoparticles have a high surface-to-volume ratio, compared to bulk materials, they offer a viable alternative to conventional catalysts [1, 2]. Recent reports have showed their remarkable performance as catalysts, in terms of reactivity, selectivity, and product yields [3–8]. Nanocatalysts offer numerous advantages over convention catalyst systems, but isolation and recovery of these small nanocatalysts from the reaction mixture is tedious. The development of an efficient, and industrially and environmentally acceptable catalyst constitutes one of the important goals towards sustainability and economic growth. To achieve this goal, the use of magnetic nanoparticles has received much interest as they are easily separable and reusable. Thus, magnetic nanoparticles have appeared as a practical alternative. Their magnetic and insoluble properties offer easy and efficient separation of the

Acknowledgments: B. C. Ranu gratefully acknowledges the support of the Indian National Science Academy, New Delhi for offering him the position of INSA Honorary Scientist. L. Adak thanks SERB, DST, Government of India (Project: SRG/2020/001350) and the WBDST-BT for their support via government order [Memo No: 1854(Sanc.)/ST/P/S&T/15G-7/2019]. T. Ghosh thanks the UGC-DSKPDF (UGC Award Letter No. & Date: F.4-2/2006 (BSR)/CH/19-20/0088; 24.01.2020) for his postdoctoral fellowship. *Corresponding author: Brindaban C. Ranu, School of Chemical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India, e-mail: [email protected] Laksmikanta Adak, Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Botanic Garden, Howrah 711103, India, e-mail: [email protected] Nirmalya Mukherjee, School of Chemical Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India Tubai Ghosh, Department of Chemistry, Jadavpur University, Kolkata 700032, India https://doi.org/10.1515/9783110730357-001

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catalyst from the reaction mixture – by the application of an external magnet without the requirement of filtration, centrifugation, or other complex workup. The carbon-heteroatom bond formation constitutes the backbone of the synthesis of heterocycles. Thus, the carbon-heteroatom bond formation at a high efficiency is of great interest. The formation of the transition metal catalyzed carbon-heteroatom bond by cross-coupling is an efficient tool and has received wide application in the synthesis of pharmaceutically active heterocyclic compounds, drugs, and natural products [9–15]. In general, as the use of metal nanoparticles provides more advantages over parent metals in catalysis, more investigations are now directed towards the application of magnetic nanoparticles for various fundamental reactions [16–22]. This chapter will highlight the applications of magnetic metal nanoparticles in the carbon-heteroatom bond formation and the related synthesis of heterocycles.

1.2 Carbon-nitrogen bond formation and the synthesis of related heterocycles Carbon-nitrogen (C–N) bond formation is one of the useful processes in organic synthesis, as the consequent amines are broadly used as intermediates or precursors in the preparation of fine chemicals, pharmaceuticals, agrochemicals, and natural products. The developments in the formation of carbon-nitrogen bonds and in the synthesis of related heterocycles by magnetic metal nanoparticles as catalysts have been described below.

1.2.1 C–N bond formation via cross-coupling reactions Martínez et al. [23] reported the reaction of aromatic amines with substituted benzylic alcohols to produce the respective benzyl imines, in the presence of magnetite (Figure 1.1). The reaction involved 20 mol% of magnetite (Fe3O4) and two equivalents of t-BuOK (potassium t-butoxide) as a base in 1,4-dioxane at 90 °C for a period of 7 days. Diversely substituted anilines (1) reacted with various benzyl alcohols (2) under standardized reaction conditions and the corresponding N-substituted benzylated derivatives (3) were obtained at excellent yields. Benzylic alcohols were used as electrophiles here. The best result was observed when less nucleophilic 3-chloroaniline was used, and the use of more nucleophilic aniline derivatives led to lower yields of the corresponding products. No reaction was observed with aliphatic amine or aliphatic alcohols, which clearly indicates the preference of the catalyst. Using a simple external magnet, the catalyst was recovered and reused eight times without any loss of significant catalytic activity.

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Figure 1.1: Fe3O4-catalyzed N-monoalkylation of aromatic amines with benzylic alcohols.

The single-pot reductive amination of carbonyl compounds have been demonstrated in the synthesis of secondary or tertiary amines (6) using environmentally benign magnetically separable sulfonic acid, supported by hydroxyapatite-encapsulated-γ-Fe2O3 [γ-Fe2O3@HAP-SO3H] as the catalyst at room temperatures (Figure 1.2) [24]. Various aromatic aldehydes (4), including furan-2-carboxaldehyde, thiophene-2carboxaldehyde and nicotinaldehyde, reacted with both electron-deficient and electron-rich anilines (5) to produce the alkylated amines (6) at considerably good yields. The protocol is also successful in the reductive amination of aliphatic aldehydes, including α,β-unsaturated one and ketones.

Figure 1.2: Direct reductive amination of carbonyl compounds using NaBH4-[γ-Fe2O3@HAP-SO3H] heterogeneous catalyst.

Copper ferrite nanoparticle-mediated N-arylation of N-heterocycles with aryl chlorides, under refluxed conditions, was developed by Panda and co-workers (Figure 1.3) [25]. Several nitrogen heterocycles (7), such as indole, pyrazole, imidazole, benzotriazole, pyridine, and carbazole, coupled with halobenzenes (8) in optimized reaction conditions and corresponding products (9) were obtained at excellent yields. CuFe2O4 was recovered and was reused up to 3 times. Atomic absorption spectroscopy (AAS)

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indicated a 60.6 ppm leaching of Fe and Cu in three successive cycles; the leaching lies within a permissible range of toxicity for humans.

Figure 1.3: CuFe2O4 magnetic nanoparticle catalyzed N-arylation of various N-heterocycles.

Karvembu and co-workers [26] developed a convenient and efficient lepidocrocitesupported copper oxide catalyst (Fe–CuO) for the N-arylation of imidazole with several substituted aryl halides (Figure 1.4). Different nitrogen heterocycles (7) reacted with aryl halides (10) in the presence of Fe–CuO as the active catalyst, K2CO3 as the base in DMAc (N,N-dimethylacetamide) solvent at 120 °C for 24 h for the synthesis of N-arylated heterocycles (11). The Fe–CuO was characterized by XRD (X-ray diffraction), XPS (X-ray photoelectron spectroscopy), and TEM (transmission electron microscope) images data.

Figure 1.4: Fe-CuO nanoparticles for the C-N bond formation.

In 2017, Yadav and his group also reported the N-arylation of N-heterocycles (e.g., indole (13) and imidazole (15) with aryl halide (12) using magnetically recoverable spinel CuFe2O4 magnetic nanoparticles (Figure 1.5) [27]. The reaction requires the use of K2CO3 as a base in the case of imidazole, but indole reacts only in the presence of K3PO4. The reaction provides the corresponding products (14 and 16) at considerably good yields using CuFe2O4 MNPs as the catalyst under ligand-free conditions, and the

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catalyst was reused up to six times without much activity loss. These spinel CuFe2O4 magnetic nanoparticles were further characterized by several techniques, such as XPS, SEM (scanning electron microscope), TEM, XRD, EDXA (energy dispersive X-ray analysis), TGA (thermo gravimetric analyzer), FT-IR (Fourier transform infrared), and N2 adsorption and desorption techniques. A possible mechanism is shown in Figure 1.6.

Figure 1.5: Copper-ferrite magnetic nanoparticles for N-arylation of imidazole and indole.

Figure 1.6: Possible reaction mechanism.

The application of nano-Fe catalyst for N- and C-alkylations of anilines (18) and ketones (21) by alcohols (17 and 20) via hydrogen auto-transfer was demonstrated by Nallagangula et al.(Figure 1.7a) [28]. It was found that the reaction yield was not sensitive to the electronic character of the substituent present on the aryl moiety of the aniline. Diversely substituted amines (18) reacted with different alcohols (17) under optimized reaction conditions and the corresponding products (19) were obtained at good yields. However, methanol failed to produce the coupling product. This heterogeneous catalyst was further used for the Friedlander synthesis of quinolines (22),

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starting from 2-amino benzyl alcohols (20) and methyl ketones (21) (Figure 1.7b). Deuterium labeling experiments suggest that the reaction goes via the hydrogen auto-transfer pathway.

Figure 1.7: Nano-Fe2O3 direct N- and C-alkylations of anilines and ketones using alcohols.

In a recent report, Shadjou and his group [29] reported CuI/Fe3O4NPs@IL-DFNS (dendritic fibrous nanosilica, functionalized by ionic liquid modified Fe3O4NPs and CuI), supported on wrinkled fibrous nanosilica (WFNS) and functionalized by bis-imidazole ionic liquid, as an efficient and reusable heterogeneous catalyst for the one-pot synthesis of N-sulfonylamidines (26) by the reaction of phenyl acetylene (23)-substituted sulfonyl azide (25) and different amines (24) under solvent-free conditions at room temperatures (Figure 1.8). The green magnetic nanocatalyst, CuI/Fe3O 4 NPs@IL-DFNS, was further characterized by FE-SEM (field emission scanning electron microscopy), TEM, FT-IR, FAAS (flame atomic absorption spectroscopy), EDX (energy-dispersive X-ray), and XRD, VSM (vibrating sample magnetometer), and BET-BJH (Brunner-Emmett-Teller and Barrett-JoynerHalenda) analysis.

Figure 1.8: CuI/Fe3O4NPs@IL-DFNS catalyzed three component reactions to synthesize N-sulfonylamidines.

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1.2.2 Synthesis of five-membered nitrogen-containing heterocycles Nanocrystalline CuFe2O4 catalyzed the efficient and green preparation of chromeno [4,3-b]pyrrol-4(1H)-one derivatives (30) in water. This was reported by Das and coworkers (Figure 1.9) [30]. The CuFe2O4 magnetic nanoparticles were obtained by a citric acid complex method and characterized by TEM, HRTEM (high-resolution transmission electron microscopy) images, XRD, EDX, and FT-IR techniques. A series of chromeno[4,3-b]pyrrol-4(1H)-ones (30) was synthesized under standardized reaction conditions at good yields. The catalyst was separated and reused up to six times, which makes this protocol attractive, cost-effective, and sustainable.

Figure 1.9: Synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives.

Rakhtshah et al. [31] reported the use of dioxomolybdenum complex, supported on functionalized Fe3O4 magnetite nanoparticles, containing a Schiff-base ligand for the synthesis of pyrazole derivatives (34) (Figure 1.10). This protocol proceeds by the one-pot three-component reactions of phenyl hydrazine (31), malononitrile (32), and aromatic aldehydes (33) in a short reaction period. Moghaddam et al. [32] reported the use of nickel ferrite (NiFe2O4) as a viable heterogeneous catalyst to synthesize the poly-substituted pyrroles (38) (Figure 1.11). The chemical stability and the synergistic effect of the two cations and the magnetically recoverable nature of NiFe2O4 were utilized for the multicomponent reaction of amine (35), aldehyde (33), 1,3-dicarbonyl (37), and nitromethane (36) at 100 °C. This procedure of poly-substituted pyrrole synthesis provided high yields in a comparatively short reaction period (3–4 h). The catalyst NiFe2O4 was separated by an external magnet and was reused nine times.

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Figure 1.10: Preparation of 5-amino-pyrazole-4-carbonitrile derivatives.

Figure 1.11: NiFe2O4-catalyzed synthesis of poly-substituted pyrroles.

Suman et al. [33] reported an efficient method for the preparation of 2-pyrazole3-amino-imidazo-[1,2-a]pyridines derivatives (42) by the use of sulfonic acid-functionalized silica-coated CuFe2O4 magnetic (CuFe2O4@SiO2-SO3H) mixed oxide nanoparticles in the presence of ethanol as a solvent at excellent yields. (Figure 1.12). The paramagnetic property of the catalyst helps its separation by an external magnet, which makes the recovery of the catalyst simpler.

Figure 1.12: CuFe2O4-catalyzed synthesis of 2-pyrazole-3-amino-imidazo[1,2-a]pyridines.

El-Remaily and Abu-Dief [34] described the preparation and characterization of the non-toxic magnetic CuFe2O4 nanoparticles and its application for an access to 1,2,4,5-tetrasubstituted imidazoles (45) (Figure 1.13). The catalyst can be easily

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recovered by means of an external magnet and can be reused six times. The easy workup, reusability of the catalyst, and simple isolation are the most significant advantages of this procedure.

Figure 1.13: CuFe2O4 nanoparticles-catalyzed synthesis of imidazoles.

Maleki et al. [35] described the preparation and characterization of urea-functionalized silica-based magnetite hybrid core-shell Fe3O4 nanoparticles and employed them for the synthesis of imidazole derivatives (49) by a one-pot three-component reaction of benzil (43) or benzoin (46) with various aldehydes (47) and ammonium acetate (48) with good to excellent yields under milder reaction conditions (Figure 1.14). The catalyst was separated and reused at least five times without any notable change in its catalytic efficiency.

Figure 1.14: Three-component synthesis of substituted imidazoles.

A similar synthesis of imidazole was also reported by Marzouk et al. [36]. They reported the preparation of magnetic nanoparticles ZnFe2O4 hydrothermally and its application for the preparation of pharmaceutically active multi-substituted imidazoles (52) by the reaction of benzil (43) with a variety of aromatic aldehydes (51), aliphatic amines (50), and ammonium acetate in the absence of any solvent (Figure 1.15). A library of 1,2,4,5-tetrasubstituted imidazoles (52) was obtained under optimized reaction conditions. The attractive advantages of this procedure are improved yield, fast

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reaction, and simple operation. The ZnFe2O4 magnetic nanoparticles were characterized by several experimental techniques, including PXRD (powder X-ray diffraction), FT-IR, SEM and TEM images, and TGA analysis. The newly synthesized imidazole derivatives were screened for their anti-inflammatory activity.

Figure 1.15: ZnFe2O4 magnetic nanoparticles for the synthesis of tetra-substituted imidazoles.

A single-pot three component synthesis of 1,2,3-triazoles using magnetic NiFe2O4, supported by a glutamate-copper catalyst, was developed by Zhang and co-workers (Figure 1.16) [37]. This catalyst demonstrated remarkable efficiency for the preparation of 1,4-disubstituted-1,2,3-triazoles (58, 59 and 60) by a single-pot click reaction of terminal alkynes (56), sodium azide (57), and epoxides (53), or benzyl chlorides (54), or aryl boronic acids (55) in a green reaction medium, water. The catalyst was

Figure 1.16: NiFe2O4–glutamate–Cu catalyzed preparation of 1,2,3-triazoles at room temperature in water.

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easily separated by an external magnet and was used for subsequent reactions several times. The other benefits of this procedure include high yields, excellent substrate scope, operation at room temperature, etc. 1,4-dihydroxyanthraquinone–Cu(II), supported by a super paramagnetic Fe3O4@SiO2 catalyst, was employed to synthesize 1,2,3-triazole derivatives (63) by the threecomponent condensation of the aryl boronic acid (61), alkyne (62), and sodium azide (57) (NaN3) in a water/acetonitrile (1:1) solvent system at room temperature by Zahmatkesh et al. (Figure 1.17) [38]. Diversely substituted boronic acids (61) reacted with both aromatic and aliphatic alkynes and sodium azide to provide the corresponding 1,2,3-triazole derivatives (63) at excellent yields. The recovered catalyst was used again for six runs without much activity loss.

Figure 1.17: Synthesis of 1-aryl-1,2,3-triazoles using Fe3O4@SiO2–DAQ–Cu(II) as catalyst at room temperature.

In 2016, Wang and his group introduced a cost-effective and powerful catalyst system CuFe2O4 to synthesize N-2-aryl-substituted 1,2,3-triazoles (66) (Figure 1.18) [39]. A variety of chalcones (64) reacted with sodium azide under standardized reaction conditions and the respective triazole derivatives (66) were formed at excellent yields. The chalcones bearing electron-withdrawing groups produced a higher yield of the products than those having electron-donating groups.

1.2.3 Synthesis of six-membered rings containing one nitrogen atom Bamoniri and Fouladgar [40] reported an environment friendly heterogeneous solid acid, prepared by the immobilization of SnCl4 on the surface of Fe3O4@SiO2 (silica coated magnetite), and demonstrated its use to prepare 1,4-dihydropyridine derivatives (69) by the coupling reaction of an aldehyde (67) and the 1,3-dicabonyl compound (68) in the presence of ammonium acetate (48), under sonication (Figure 1.19).

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Figure 1.18: Synthesis of N-2-aryl-substituted 1,2,3-triazoles catalyzed by CuFe2O4.

The magnetically recoverable catalyst was further characterized by X-ray diffraction (XRD), emission-scanning electron microscopy (FE-SEM), Fourier transform-infrared spectroscopy (FT-IR), field energy dispersive X-ray (EDX) analysis, vibrating sample magnetometer (VSM), transmission electron microscopy (TEM), and N2 adsorption–desorption (BET) study.

Figure 1.19: Ultrasound-mediated synthesis of 1,4-dihydropyridines using magnetically recoverable Fe3O4@SiO2-SnCl4 catalyst.

The synthesis of Hantzsch 1,4-dihydropyridines (69) in water using heterogeneous bimetallic ZnFe2O4 was demonstrated by Naik and his group (Figure 1.20) [41]. The reaction occurs through a multicomponent interaction of aldehydes (67), 1,3-dicarbonyl compound (68), and ammonium acetate (48). The important advantages of this procedure are the use of water, shorter reaction time, high yields, and an easy work-up.

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Figure 1.20: Preparation of 1,4-dihydropyridine using ZnFe2O4 catalyst at room temperature.

Zhang et al. [42] reported the preparation of magnetically recoverable graphene oxide-supported molybdenum (Fe3O4/GO-Mo) nanoparticles for the single-pot threecomponent reaction of malononitrile (32), isatins (70) and anilinolactones (71) under microwave heating for the preparation of spirooxindole dihydropyridines (72) at excellent yields (Figure 1.21). No reaction occurs with 4-(pyridin-2-ylamino)furan-2(5H)one. The catalyst was reused eight times with a marginal decrease of its catalytic activity.

Figure 1.21: Fe3O4-catalyzed synthesis of spirooxindole dihydropyridine derivatives.

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1.2.4 Synthesis of six-membered rings containing two nitrogen atoms A novel and green copper(II)/L-histidine-coated magnetic nanoparticles[Cu(II)/LHis@Fe3O4] catalyst was used to synthesize 2,3-dihydroquinazolin-4(1H)-ones (74), a class of pharmaceutically important molecules (Figure 1.22) [43]. The Cu(II)/LHis@Fe3O4 catalyst was recovered and used for six runs. The composition and structure of the heterogeneous catalyst were established by EDS, TGA/DTG (derivative thermogravimetry), VSM, FT-IR, XRD, and SEM.

Figure 1.22: Cu(II)/L-His@Fe3O4-catalyzed synthesis of 2,3-dihydroquinazolin-4(1H)-one derivatives.

In 2012, an efficient Fe3O4 magnetic nanoparticle-supported Cu(I) catalyzed synthesis of quinazolinones and bicyclic pyrimidinones was reported (Figure 1.23) [44]. The reaction occurs among amidine hydrochlorides (76), substituted 2-halobenzoic acids (75), and 2-bromocycloalk-1-enecarboxylic acids using 10 mol% of magnetically recoverable Fe3O4-supported Cu(I) catalyst. Among the different 2-halo benzoic acids, 2-iodobenzoic acid provided the best reactivity. Various substituted 2-halo benzoic acid derivatives reacted with different amidine hydrochlorides (76), and quinazolinone derivatives (77) were obtained at moderate to excellent yields. Benzimidamide and cyclopropanecarboximidamide demonstrated higher reactivities than acetimidamide.

1.3 Carbon-oxygen bond formation and the synthesis of related heterocycles In polymer and pharmaceutical industries, diaryl ether constitutes an important structural motif [45–47] as many of the natural products having a diaryl ether

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Figure 1.23: Fe3O4 nanoparticle-supported Cu(I) catalyzed synthesis of quinazolinones and bicyclic pyrimidinones.

bridge, such as antibiotic vancomycin [48] and anti-HIV chloropeptins [49], show significant biological activities. Thus, intensive efforts have gone in for their efficient and convenient synthesis. Traditionally, diaryl ethers are prepared by the Ullmann C–O cross-coupling reaction of alcohols with aryl halides. However, the elevated reaction temperature, use of stoichiometric amount of copper salts, and relatively low yields limit their large-scale applications. In recent years, efforts are directed to utilize the transition metals for the O-arylation reactions. Simultaneously, magnetic metal nanoparticles have been effectively used in the design of green heterogeneous catalysts. The efficacy of a magnetically separable catalyst for the C–O bond formation was first demonstrated in 2011 by Sun and co-workers [50], reporting the application of copper ferrite nanoparticles to catalyze the reaction of aryl halides and phenols. The catalytic activity of CuFe2O4nanoparticles, with an average diameter of 5–10 nm, has been studied for diaryl ether formation by the reaction of 4-methyliodobenzene with phenol, under varied conditions. From the optimization of the reaction conditions, it was found that the best result was obtained with caesium carbonate as the base and acetyl acetonate as an assisting ligand in DMF at 135 °C (Figure 1.24). From substrate screening, the aryl halides containing an electron-withdrawing substituent and the phenols having electron-donating groups were most effective to accelerate the reaction. In the presence of a powerful electron-withdrawing group in the para position of the aryl iodide or bromide, the reaction proceeded even without a catalyst. The catalyst is not much effective in the case of aryl bromides bearing an electron-donating substituent. The coupling of chlorobenzene and phenol did not initiate under the same reaction conditions even after 24 h. The catalyst was used for a further six times with a marginal decrease in its efficiency. To extend the scope of this methodology, Pallapothula and his group demonstrated the efficient CuFe2O4catalyzed diaryl ether (80) formation via the C–O cross-coupling of phenols (79) and

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aryl halides (78), without any ligand, in the presence of a base (KOH) at 120 °C using DMSO (dimethyl sulfoxide) as a solvent [51]. Better reactivity was observed with CuFe2O4 nanoparticles, compared to other metal oxide nanoparticles – Sb2O3, Y2O3, Bi2O3, Co3O4, and SnO2.

Figure 1.24: CuFe2O4-catalyzed diaryl ether formation.

Later, Sharma and Gawande [52] described a similar procedure for the aryl-oxygen bond formation by maghemite (γ-Fe2O3)-supported copper nanoparticles in DMF at 130 °C (Figure 1.25). In the presence of maghemite (γ-Fe2O3)-supported copper nanoparticles, a series of phenols (79) and aryl iodides (78) underwent the reaction to produce the cross coupled products (80) at high yields. Very recently, Khorsandi et al [53] showed that the Cu/ascorbic acid@MNPs worked as an efficient catalyst for the coupling of aryl halides and phenols.

Figure 1.25: Maghemite-Cu NPs-catalyzed O-arylation of phenol with aryl halides.

Until 2013, the reaction was limited only to aryl substrates. Xu and Yang [54] first showed that, together with aryl substrates, the heteroaryl units, such as, 2-bromopyridine, 2-chloropyridine and 3-bromopyridine, could be employed as substrates

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for the C–O coupling reaction using CuFe2O4 as a catalyst in the presence of a ligand, 2,2,6,6-tetramethylheptane-3,5-dione, and NMP as solvent at 135 °C (Figure 1.26). Aiming to expand the substrate scope, the same group described that the coupling of unactivated alkyl alcohols and aryl halides was performed using the same CuFe2O4 nanoparticles as catalyst in the presence of 1,10-phenanthroline and Cs2CO3 without any other solvent (alcohols used in excess as solvent) [55]. Interestingly, in this case, after the first run, the agglomeration of the magnetic nanoparticles significantly decreased the catalytic efficiency for the next cycles. However, it was observed that the grinding of the recovered CuFe2O4 catalyst, before use, can dramatically increase the catalytic activity in the next run.

Figure 1.26: CuFe2O4 NPs-catalyzed C-O cross-coupling of phenols and aryl halides.

To avoid such an agglomeration, Likhar and co-workers [56] introduced a Fe3O4@mesoporous polyaniline (mPANI) core–shell nanocomposite, which acts as an effective catalyst for the cross-coupling of phenols (79) with aryl halides (78) (Figure 1.27). It was suggested that the presence of mesoporous polyaniline (mPANI) shell avoids the agglomeration and oxidation of the Fe3O4 magnetic nanoparticles. In addition, its porosity increases the direct contact of the reactant to the core Fe3O4 nanoparticles. Its catalytic activity was established for the coupling of 4-chloroanisole and phenol in the presence of K2CO3 in DMF. A wide range of substituted aryl chlorides and phenols underwent the reaction to provide fairly high yields of the corresponding products at 110 °C within 10–12 h. This catalyst system also works efficiently for the cross-coupling of cyclohexanol and benzyl alcohol with aryl chlorides. The catalyst was used further for five times with uniform efficiency after recovery of the same with the help of an external magnet. Recently, Heydari et al. [57] introduced a new route to prepare carbon nanotube (CNT)-supported α-Fe2O3@CuO nanocomposite. The catalytic effectivity of the prepared CNT-supported α-Fe2O3@CuO has been studied in the Ullman-type coupling in the presence of acetyl acetone as a ligand to form the diaryl ether (80) (Figure 1.28).

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Figure 1.27: Fe3O4@mPANI-catalyzed diaryl ether formation.

Several (hetero)aryl iodides and bromides produced the corresponding coupled products at fairly good yields. The catalyst was separated from the reaction mixture by a simple magnet and was used further up to six times without any loss in efficiency.

Figure 1.28: Carbon nanotube (CNT)-supported α-Fe2O3@CuO catalyzed diaryl ether formation.

Zolfigol and co-workers [58] developed palladium(0) with a phosphine ligand, supported on silica coated magnetite particles [Fe3O4@SiO2@PPh2@Pd] nanocatalyst, which exhibited excellent activity and reusability in the aqueous phase O-arylation of phenols. The report described the procedure to prepare silica-coated magnetic particles (Fe3O4@SiO2) by the interaction of Fe3O4 with tetraethyl orthosilicate (TEOS) in an aqueous solution of ammonia and the phosphorylation of hydroxyl groups on the surface of the support (SiO2) with ClPPh2, and the subsequent integration of Pd by the reaction with PdCl2. This catalyst was found effective for the coupling of phenols and aryl halides in water. The optimization of reaction conditions showed that an aqueous NaOH medium is effective for accessing the quantitative transformation of bromobenzenes to the diaryl ethers (Figure 1.29). Thus, the

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scope of the reaction has been extended to different aryl halides (78) and naphthols (81). In general, good yields were achieved within the period of 1.5–15 h. The catalyst was recovered easily by an external magnet and was used further for six runs with a slight decrease in efficiency.

Figure 1.29: O-Arylation of phenols and naphthols using Fe3O4@SiO2@PPh2@Pd catalyst.

Prompted by this result, Nagarkar’s group [59] described the application of a solid super-paramagnetic Pd-ZnFe2O4 (palladium supported on zinc ferrite) catalyst for the coupling of phenols (79) and nitro-substituted aryl halides (83) (Figure 1.30). The synthesis of nitro-substituted diaryl ethers (84) is of much importance as these molecules are useful precursors to amine compounds and several bioactive compounds. It was found that the electron-rich phenols offer better results, in comparison to the electron-poor phenols, in terms of product yields.

Figure 1.30: Pd-ZnFe2O4-catalyzed nitro-substituted diaryl ether formation.

In 2014, Zhang and co-workers [60] introduced Fe3O4-encapsulated CuO nanoparticles as an effective catalyst for access to diaryl ethers (80) by the coupling of aryl

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halides (78) and phenols (79) (Figure 1.31). In the presence of Fe3O4-encapsulated CuO nanoparticles, a variety of phenols (79) underwent reaction with diversely substituted aryl halides (78) to provide the corresponding diaryl ethers (80) in good yields. Similar to other magnetic nanoparticle catalysts, it can also be recovered from the reaction mixture by a simple magnet and can be reused up to three consecutive runs without losing its catalytic activity. More recently, Zhou and Peng [61] reported a similar work involving the aryl-oxygen bond formation by using a new copper complex, supported on the surface-modified Fe3O4/SiO2 nanoparticles (Fe3O4@SiO2-Glycerole-Cu(II)). While in another approach, Khalili and co-workers [62] described a ligand-free procedure, where O-arylation of aryl halides has been accomplished in an open atmosphere using the impregnated copper ferrite on the mesoporous graphitic carbon nitride (CuFe2O4/g-C3N4) as catalyst.

Figure 1.31: The CuO-Fe3O4 catalyst for the C–O coupling reaction.

In addition to diaryl ether formation, the other type of C–O bond formation was also achieved by using the magnetic metal nanoparticles. Park and Song et al [63] reported the preparation of the cobalt coated on silica (Co@SiO2) yolk-shell nanocatalysts by a simple reduction of CoO@SiO2 core-shell nanoparticles. The thermal reduction generates pores in the outer silica shell, which increases the approachability of the substrate molecules to the active cobalt center. The catalytic activity of the prepared Co@SiO2 yolk-shell nanocatalysts has been investigated for the carbonylative crosscoupling of phenol (87) with iodobenzene (85) (Figure 1.32). Only 1 mol% Co@SiO2 yolk-shell nanocatalysts is required for the successful carbonylative cross-coupling of phenol (87) with iodobenzene (85) in toluene at 180 °C, in the presence of carbon monoxide (2.0 MPa) and Et3N, to obtain the corresponding ester (88) in a quantitative yield. No leaching of cobalt into the reaction medium was found after four runs and, thus, the efficacy of the catalyst remains unchanged.

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Figure 1.32: Carbonylative cross-coupling of phenol with iodobenzene using Co@SiO2 yolk-shell nanoparticles catalyst.

Allylic ethers are important synthetic intermediates for a wide range of organic reactions and are also being used as popular protecting groups for alcohols [64]. Varma and co-workers [65] have found that Fe3O4-dopamine-PdII showed excellent catalytic activity and recyclability in the O-allylation of phenols (89) by the reaction of allylic acetates (90) at 100 °C in water using NaHCO3 in an open air atmosphere (Figure 1.33). In continuation, the same group reported the magnetic silica-supported palladium (Fe3O4@SiO2-Pd) as a simple catalyst for the O-allylation of phenols [66].

Figure 1.33: Fe3O4-Dopamine-Pd-catalyzed O-allylation of phenols by allylic acetates.

Using a similar concept, Prasad et al [67] reported the preparation of Fe3O4-dopamine-Pd0 by the reaction of dopamine functionalized Fe3O4-nanoparticles with an aqueous solution of Na2PdCl4 using hydrazine monohydrate as a reducing agent. The supported Pd0 catalyst shows excellent catalytic activity in the alkoxycarbonylation of aryl iodides (92) in an atmospheric pressure of CO in the presence of K3PO4 (Figure 1.34). The catalyst was separated and used further for several times. However, the catalytic efficacy for the less reactive coupling partner such as aryl bromides or chlorides was not reported. Wang and his group [68] reported the application of magnetic metal nanoparticles for the synthesis of benzoxazoles (96) from substituted N-(2-halophenyl)benzamides (95), catalyzed by magnetic copper ferrite nanoparticles (Figure 1.35). The catalyst screening showed that best results are achieved in DMSO at 120 °C using K2CO3. The scope of the reaction for the diversely substituted N-(2-halophenyl)benzamides has been investigated in standardized reaction conditions. It was observed that the compounds having electron-donating benzoyl substituents showed higher

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Figure 1.34: Fe3O4-Dopamine-Pd(0)-catalyzed alkoxycarbonylation of aryl iodides.

reactivity, in comparison to those having electron-withdrawing substituents. The catalyst was recovered using an external magnet and was used further up to seven times with a consistent catalytic efficiency.

Figure 1.35: CuFe2O4-catalyzed synthesis of benzoxazoles from substituted N-(2-halophenyl) benzamides.

Recently, Farahi and Tanuraghaj [69] introduced a new approach for the preparation of sodium carbonate-tagged silica-coated nano-Fe3O4 (Fe3O4@SiO2@(CH2)3OCO2Na) heterogeneous basic catalyst. The activity of nano-Fe3O4@SiO2@(CH2)3OCO2Na catalyst has been investigated for the access to a series of pyranocoumarins (99) by the reaction of dialkyl acetylene dicarboxylates (98) and 5,7-dihydroxy coumarin derivatives (97) at 100 °C in the absence of any solvent (Figure 1.36). A variety of 5,7-dihydroxy coumarins (97), diethyl acetylene dicarboxylates, and dimethyl acetylene dicarboxylates have been employed to give the corresponding desired pyranocoumarins (99) at considerably good yields using the nano-Fe3O4@SiO2@(CH2)3OCO2Na catalyst. The catalyst was used further up to five runs without an appreciable decrease in its catalytic efficiency.

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Figure 1.36: Synthesis of pyranocoumarins catalyzed by nano-Fe3O4@SiO2@(CH2)3OCO2Na.

More recently, Maleki and group [70] demonstrated the use of magnetic Fe3O4@polyvinyl alcohol nanoparticles as a recyclable catalyst for the preparation of a series of pyrano[2,3-d]pyrimidine derivatives (101) at room temperature in water (Figure 1.37). The catalyst was separated and used further up to six consecutive runs without any significant loss in catalytic efficiency.

Figure 1.37: Fe3O4@polyvinyl alcohol magnetic nanoparticles-catalyzed synthesis of pyrano[2,3-d] pyrimidine derivatives.

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1.4 Carbon-chalcogen bond formation and the synthesis of related heterocycles The formation of carbon-chalcogen bond is less investigated, in comparison to other carbon-heteroatom bonds. The organo-chalcogenides constitute the core structural units in various molecules of pharmaceutical, biological, and material interest [71–73], and they are also used as useful intermediates in several syntheses [74, 75]. The developments toward the formation of carbon-chalcogen bonds and the synthesis of related heterocycles using magnetic metal nanoparticles as catalysts are discussed below. Jun and Lee [76] developed a modest and efficient protocol for the C–S crosscoupling of various aryl halides (102) with thiols (103) by NHC-nickel catalyst, immobilized on magnetite/silica-nanoparticles (MNP-Si-NHC-Ni) (Figure 1.38). The catalyst separation is much simpler with the help of an external magnet, and the recovered catalyst was reused up to three times with an almost consistent activity.

Figure 1.38: C–S cross-coupling reaction of aryl halides with thiols.

Nageswar and his group [77] established an efficient procedure for the thio-arylation of aryl/alkyl halides with aromatic/aliphatic thiols using a heterogeneous CuFe2O4nanoparticle catalyst in the absence of any ligand (Figure 1.39). This clean, mild, and inexpensive protocol offers an expedient route for the synthesis of a range of substituted organic sulfides (104), giving excellent product yields with good chemoselectivity and functional group tolerance. This is the first report on the use of magnetically separable and recyclable nano-CuFe2O4-catalyst for the C–S bond formation by the reaction of aryl/alkyl iodides (92) and aryl/alkyl thiols (103) to provide the aryl and alkyl sulfides. The catalyst is air-stable, inexpensive, and magnetically separable, and recycled up to four cycles without any appreciable loss of activity. The proposed

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mechanism for the CuFe2O4nanoparticle-catalyzed coupling reaction proceeds via an oxidative addition and reductive elimination (Figure 1.40).

Figure 1.39: Cross-coupling of aryl iodides with thiols in the presence of nano-copper ferrite.

Figure 1.40: Possible mechanism for C–S cross-coupling.

Kovács and Novák [78] reported the preparation and application of a simple copperon-iron catalyst in the coupling of aryl halides (92) and thiols (103) (Figure 1.41). Regarding the reaction pathway, it was suggested that a quick oxidation of the thiol took place at the start of the reaction and, subsequently, the direct formation of the sulfides was accomplished in the presence of iron by the reductive cleavage of the disulfide intermediate. This protocol offers an efficient, cost-effective, and green

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procedure for the synthesis of a range of organic sulfides (104) without an inert atmosphere. Furthermore, the copper-on-iron catalyst was easily recovered from the reaction mixture by an external magnet and the catalyst was recycled.

Figure 1.41: Cu/Fe-catalyzed coupling of aryl iodides with thiols.

Panda and Mohapatra [79] described a convenient approach for the preparation of monodispersed, super paramagnetic copper ferrite (CuFe2O4) nanoparticles with a large surface area. The developed material was characterized by several techniques, such as TEM, XRD, N2 adsorption/desorption measurements, etc. The catalytic activity of the synthesized CuFe2O4 nanoparticles was tested for the cross-coupling of thiols (105) with various aryl halides (102) containing different functional groups (Figure 1.42). Aryl iodides and bromides provided biaryl sulfides (106) in considerably good yields (62–98%). The scope of this procedure was extended to one-pot synthesis of pharmaceutically significant tricyclic dibenzothiazepines (110 and 111) (Figure 1.43). The magnetic character of CuFe2O4 nanoparticles provides the advantage of quick, easy, and quantitative separation of the heterogeneous catalyst from the reaction mixture. Minimum leaching of Cu and Fe, in successive cycles, makes the catalyst cost-effective and environmentally benign.

Figure 1.42: C-S cross-coupling of aryl halides with thiols in the presence of CuFe2O4 catalyst.

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Figure 1.43: Synthesis of dibenzothiazepines.

A sustainable, green, and efficient procedure for the C–S coupling of aryl halides (102) with thiophenols (112) was established using a magnetically removable and recyclable heterogeneous Cu catalyst via a single-pot multi-component reaction under microwave irradiation in isopropanol (Figure 1.44) [80]. This unique C–S coupling reaction in isopropanol progressed successfully with the use of a magnetically recoverable heterogeneous catalyst with minimum copper loading (0.82%). High stability of the catalyst, easy recovery using an external magnet, and efficient recycling make the procedure economical and sustainable.

Figure 1.44: Cross-coupling of aryl halides with thiols.

Likhar and his group [81] developed an efficient and magnetically separable porous magnetic Fe3O4 catalyst, stabilized with mesoporous polyaniline (PANI), for the thioarylation of a variety of aryl, heterocyclic and alkyl halides (102) with thiophenol (114) to produce unsymmetrical diaryl sulfides (115) in fairly good yields (Figure 1.45). The use of the catalyst was further extended to the thio-arylation of several aryl iodides with thiourea to provide symmetrical diaryl sulfides, selectively. The most interesting advantages of this procedure are the simple preparation of the catalyst from

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easily available reagents and the reaction in water. The catalyst was separated with an external magnet and used further in the next successive thio-arylation reaction.

Figure 1.45: mPANI/pFe3O4-catalyzed C-S bond formation in water.

Sreedhar et al. [82] described an efficient and environmentally benign catalyst system having magnetically separable copper ferrite (CuFe2O4) nanoparticles for the crosscoupling of aryl sulfinic acid salts (117) with diverse alkyl or aryl halides (116) and boronic acids (119) to produce the respective aryl sulfones (118 and 120) (Figures 2.46 and 2.47). The protocol shows a broad functional group tolerance. Magnetically separable catalyst is an additional advantage of this protocol. The reusability of the catalyst system makes the reaction potentially and economically viable for commercial applications. Moreover, this protocol can be extended further for the synthesis of vinyl sulfones.

Figure 1.46: Synthesis of aryl sulfones from various organohalides.

The magnetically separable non-toxic and nano-CuFe2O4 catalyzed synthesis of symmetrical aryl/heteroaryl sulfides (123) by the reaction of thiourea (122) with a variety of aryl halides (121) has been reported (Figure 1.48) [83]. High yields of

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Figure 1.47: Preparation of aryl sulfones from various boronic acids.

products were obtained in the absence of any ligand and without the use of any costly catalyst, like palladium.

Figure 1.48: The reaction of aryl halides and thiourea in the presence of CuFe2O4 nanoparticles.

Gholinejad et al. [84] reported the application of copper ferrite nanoparticles (CuFe2O4) for the single-pot synthesis of aryl alkyl thioethers (126) by the reaction of thiourea (122), aryl halides (124), and alkyl bromides (125) in a mixture of water and polyethylene glycol (Figure 1.49). The catalyst system was also useful for the one-pot synthesis of symmetrical diaryl trithiocarbonates (128) through the reaction of aryl iodides (92), sodium sulfide, and carbon disulfide under heterogeneous reaction conditions (Figure 1.50). The magnetic CuFe2O4 nanoparticles were synthesized using copper (II) chloride and iron (III) chloride, and characterized by TEM, XRD, FT-IR, and AAS (atomic absorption spectroscopy) analysis. The catalyst was recovered from the reaction mixture using a simple magnet and used further for five successive runs in the reaction of iodobenzene, benzyl bromide and thiourea without any significant loss of activity. Several mono-dispersed magnetic nanoligands (MNLs) (CoFe2O4–NH2 (MNL A), Fe3O4@Si(CH2)3NH2 (MNL B), Fe3O4@Si(CH2)3NHC(O) (CH2)2PEI (MNL C) and Fe3O4@Si(CH2)3NHC(O)PEI (MNL D) have been prepared by the simple solvo-thermal method (Figure 1.51).The catalytic activity of the prepared MNLs was investigated for the cross-coupling reaction of aromatic iodides and heterocyclic thiols [85]. The reactions were performed using MNL (10 mol% N), CuI (5 mol%), and K2CO3 (1.3 eq.) in DMF at 120 °C. A wide range of heterocyclic sulfides (106) were obtained

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Figure 1.49: Copper ferrite nanoparticles-catalyzed thioetherification reaction.

Figure 1.50: Synthesis of symmetrical diaryl trithiocarbonates catalyzed by CuFe2O4.

in high yields (up to 98%) by the cross-coupling reaction of heterocyclic thiols (105) with several substituted aryl iodides (92) in the presence of the catalyst, MNL B (Figure 1.52). The magnetic nature of these ligands led to their rapid and easy separation from the reaction mixture; MNL B was found to have remarkable catalytic efficiency even after five cycles. Palladium, supported on silica-coated magnetic nanoparticles [Fe3O4@SiO2@C22Pd(II)], has been prepared and characterized by X-ray analysis, vibrating sample magnetometer, X-ray photoelectron spectroscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, elemental analysis, energy dispersive inductively coupled plasma, and differential thermal analysis [86]. This heterogeneous catalyst was employed as an efficient catalyst for the Suzuki coupling of aryl halides with aryl boronic acids, and the cross-coupling reaction of aryl halides with thiols for the formation of aryl-sulfur bonds. The catalyst was easily recovered using an external magnet and used further for five cycles without any substantial decrease in its catalytic efficiency. Gholinejad and Firouzabadi [87] reported a user-friendly procedure for the production of aryl-alkyl sulfides (130) in high yields by the reaction of aryl halides (121) with elemental sulfur and electron-poor alkenes (129) using iron powder as a reducing agent and copper-ferrite nanoparticles as catalyst (Figure 1.53). The important features of this protocol are: a) easy recovery of the catalyst using an external magnet and reuse for further runs with similar catalytic efficiency, b) the applicability in

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Figure 1.51: Synthesis of MNLs A, B, C, D [aninopropyl-triethoxysilane (APTES), polyethylene imine (PEI), 3-Isocyanatopropyltriethoxysilane (ICPTS)].

Figure 1.52: Cross-coupling of heterocyclic thiols and aryl iodides in presence of MNL B.

large-scale operation, c) the use of readily available and cheap sulfur powder as the source of sulfur atom, and d) the operation of reactions under non-odorous conditions in PEG (200) as an eco-friendly media. These advantages make the procedure more attractive commercially. However, aryl chlorides that are much cheaper than bromides and iodides do not undergo the reaction. A magnetically separable, efficient, and green maghemite-Cu-nano catalyst has been prepared from low-cost materials and was applied for C–S bond-formation reactions (Figure 1.54) [88]. The synthesized maghemite-Cu nanocatalyst was characterized by several spectroscopic techniques, such as XRD, atomic absorption spectroscopy, TEM, field-emission gun SEM with energy-dispersive spectroscopy, X-ray photoelectron spectroscopy, FTIR spectroscopy, and Mössbauer spectroscopy. Moreover, the

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Figure 1.53: Thia-Michael reaction of aryl halides, electron-deficient alkenes and elemental sulfur in the presence of copper ferrite nanoparticles.

cross-coupling reactions proceeded well with high yields. This may be due to the nanoparticle size of the maghemite solid support material that led to the immobilization of a high amount of complex and increased the dispersal of the catalytically effective sites in the reaction medium and, thus, enhanced their availability to the substrate. Outstanding catalytic activity, easy separation of the catalyst, and reusability without an appreciable loss in efficiency make the protocol effective to deal with environmental and industrial concerns.

Figure 1.54: C-S cross-coupling of thiophenol and substituted iodobenzenes in presence of Maghemite-Cu.

CuFe2O4 magnetic nanoparticles were employed in the cross-coupling of phenyl boronic acid (131) and aryl/heteroaryl/benzyl halides (116) in the presence of S8 as a sulfur source in a PEG solvent, leading to a single-pot synthesis of aryl/heteroaryl/ benzyl sulfides (133) (Figure 1.55) [89]. These catalysts were properly characterized by standard techniques. Similarly, the synthesis of symmetrical sulfides from aryl/ heteroaryl halides using S8, KF, and Cs2CO3 at 120 °C in PEG was demonstrated. A mechanism was suggested for this reaction (Figure 1.56). The magnetic nanocatalyst CuFe2O4 was separated efficiently by an appropriate external magnet and used further for several times (6 runs) with little loss in efficacy.

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Figure 1.55: Synthesis of unsymmetrical sulfides using CuFe2O4 MNPs catalyst.

Figure 1.56: Suggested mechanism for the synthesis of phenyl aryl sulfides from phenyl boronic acid and aryl halides using S8 and CuFe2O4 as a catalyst.

A single-pot synthesis of symmetrical diaryl/alkyl sulfides in high yields within a short period, from the reaction between aryl/alkyl halides and S8, was accomplished using nickel(II)-histidine, supported on silica-coated magnetic nanoparticle [Fe3O4@SiO2@His@Ni(II)] as a reusable catalyst [90]. This procedure offers the advantages of high efficiency, clean reaction, and avoidance of the use of a toxic catalyst. Most significantly, simple magnetic separation of the catalyst makes the isolation of product much easy. NiFe2O4 magnetic nanoparticles (MNPs) were prepared, characterized, and used as an inexpensive, air-stable and magnetically separable nanocatalyst for the preparation of structurally varied sulfides [91]. Simple procedures were established for the synthesis of unsymmetric diaryl sulfides (136 and 138) via one-pot reactions of aryl halides (134) or nitroarenes (139) as starting materials with triphenyl tin chloride/S8 or aryl boronic acid/S8 as thiolating agents, using a base (K2CO3 or NaOH) and NiFe2O4MNPs as a catalyst in water or polyethylene glycol as solvent (Figure 1.57). The ligand-free reaction and the use of magnetically recyclable nanocatalyst and green solvents made these methods more eco-friendly and practical than the existing protocols for the synthesis of sulfides.

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Figure 1.57: Preparation of diaryl sulfides via nickel ferrite‐catalyzed C─S bond formation.

An efficient and non-hazardous catalyst was prepared by the anchored palladium complex onto the surface of Fe3O4 nanoparticles from inexpensive materials by a simple procedure [92]. The prepared nanostructure was characterized by several spectroscopic techniques. Besides, the catalytic efficiency of this nanostructured material has been examined for the synthesis of sulfide (123) and the oxidation reactions (Figure 1.58). The fast reaction, excellent yields, simple magnetic separation of catalyst, and environment-friendliness are useful benefits of this protocol.

Figure 1.58: Fe3O4-AMPD-Pd-catalyzed synthesis of sulfides and sulfoxides (AMPD = 2-amino-2methyl-1,3-propanediol).

Atashkar and his group [93] described a green procedure for the one-pot syntheses of unsymmetrical and symmetrical diaryl sulfides via C–O bond activation using NiFe2O4 magnetic nanoparticles as a reusable heterogeneous catalyst. The synthesis of unsymmetrical sulfides (113) was achieved by the cross-coupling of phenolic esters (142), such as acetates, triflates and tosylates, with aryl boronic acid (143)/S8 or triphenyl tin chloride/S8 as thiolating agents in the presence of a base and in poly (ethylene glycol) (Figure 1.59). Additionally, the synthesis of symmetrical diaryl sulfides (113) from phenolic esters (142) using S8 as the sulfur source and NiFe2O4 as

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catalyst in dimethyl formamide at 120 °C was demonstrated. A suggested mechanism for the C–S bond formation by the reaction of aryl boronic acid and phenolic esters in the presence of S8 and NiFe2O4 is shown in Figure 1.60.

Figure 1.59: C-S bond formation of phenolic ester and aryl boronic acids using S8 and NiFe2O4 MNPs.

Figure 1.60: Suggested mechanism for C-S bond formation by the reaction of aryl boronic acid and phenolic esters in the presence of S8 and NiFe2O4.

A copper complex supported on surface-modified Fe 3 O 4 /SiO 2 nanoparticles (Fe3O4@SiO2–Glycerole–Cu(II)) was successfully fabricated and characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA), N2 adsorption and desorption (BET), energy dispersive inductively coupled plasma optical emission spectroscopy (ICP-OES), and Vibrating Sample Magnetometer (VSM) techniques. This magnetic nanomaterial was employed as an efficient catalyst for the C–S and C–O cross-coupling of aryl halides with thiourea and phenol [94]. The catalyst was separated by an external magnet and reused for six cycles without an appreciable loss in catalytic efficiency.

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A magnetically supported ionic liquid on Fe3O4@SiO2 nanoparticles (MNPs@SiO2-IL) has been prepared and applied as a reusable catalyst for the single-pot synthesis of 1,3-thiazolidin-4-ones (147) in good--high yield in the absence of any solvent (Figure 1.61) [95]. The MNPs@SiO2-IL catalyst was characterized by different spectroscopic techniques. Also, the catalyst was simply recovered by an external magnet and used further for ten times without an appreciable loss in its catalytic efficiency.

Figure 1.61: Synthesis of 1,3-thiazolidin-4-one derivatives catalyzed by MNPs@SiO2-IL.

A convenient three-component synthesis of 1,3-thiazolidin-4-ones (150) was developed by the single-pot condensation of aromatic amine (149), aldehydes (148) and thioglycolic acid with nano-CoFe2O4@SiO2/PrNH2 (cobalt ferrite nanoparticle supported on amine functionalized silica) as a robust heterogeneous catalyst (Figure 1.62) [96]. The primary advantages of this procedure are high yield of products and the use of magnetically recoverable CoFe2O4@SiO2/PrNH2 nanoparticles as an eco-friendly catalyst.

Figure 1.62: Synthesis of 1,3-thiazolidin-4-ones catalyzed by CoFe2O4@SiO2/PrNH2 nanoparticles.

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Nageswar’s group [97] demonstrated a convenient -ferrite (CuFe2O4) nanoparticles-catalyzed cross-coupling reaction of aryl halides (92) with diphenyl diselenide (151) to produce diaryl selenides (152) in the absence of any ligand (Figure 1.63). This protocol provides major improvements with respect to operational simplicity and general applicability to the synthesis of both aryl and alkyl selenides in high yields of products, using an efficient, cost-effective and recyclable catalyst. This is the first report of using a magnetically separable non-toxic CuFe2O4 nanoparticles for a cross-coupling reaction to furnish aryl and alkyl selenides.

Figure 1.63: Cross-coupling of aryl iodides and diphenyl diselenide using nano-CuFe2O4-catalyst.

An efficient and simple method for the preparation of unsymmetrical diaryl selenides (154) was reported by CuFe2O4 nanoparticle-catalyzed reaction of phenyl selenyl bromide (153) and various organoboranes (135) in polyethylene glycol (PEG400) medium using Cs2CO3 as a base (Figure 1.64) [98]. Using this procedure, a range of unsymmetrical selenides was obtained at high yields. The copper ferrite (CuFe2O4) nanoparticles were recovered using an external magnet and used further for four cycles.

Figure 1.64: CuFe2O4 nanoparticle-catalyzed synthesis of unsymmetrical diaryl selenides.

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Magnetite (Fe3O4) nanoparticles, supported on charcoal, graphene, or SBA-15, have been prepared by simple solid-state grinding and subsequent thermal treatment [99]. The Fe3O4 nanoparticles supported on activated charcoal showed remarkable catalytic efficiency and provided high yields of alkynyl selenides (156) by the cross-coupling of diphenyl diselenide (151) with alkynes (155) through C_H and Se_Se bond activation under environment-friendly conditions (Figure 1.65).

Figure 1.65: Synthesis of alkynyl selenides via C-H activation.

Silva Júnior and his group [100] developed a convenient procedure for the synthesis of selenated naphthaquinones and anthraquinones. The carbon nanotube-supported copper-ferrite was used as a catalyst for the synthesis of selenium-containing quinoidal derivatives (159) in the presence of AgSeR-salts (Figure 1.66). In addition, the compounds exhibited activity against T. cruzi. These new molecules may open up new possibilities for the design of compounds that could be potential against the parasitecausing Chagas disease. Our group [101] reported an efficient and convenient protocol for the preparation of organo-chalcogenides (162) (selenides and tellurides) by the reaction of organoboronic acid/boronic ester/trifluoroborate (160) and diphenyl diselenide/ ditelluride (161) in the presence of CuFe2O4 nanoparticles in PEG-400 with a small amount of DMSO as an additive (Figure 1.67).This procedure is very simple and is applicable to various boronic acids, such as heteroaryl, alkyl, alkenyl, and alkynyl ones. In general, the reactions provided high yields. The products with different functionalities, such as Cl, F, OMe, OCF3, CHO, COMe, COOEt, CN, NO2, SMe, and NHCOMe, were obtained successfully by this procedure. The catalyst was separable by a magnet and used further for eight times without any considerable loss in catalytic efficiency. After the reaction, the solvent was evaporated, followed by column chromatography, to provide products with high purities. A probable reaction pathway is outlined (Figure 1.68). In cycle I, the CuFe2O4 nanoparticle underwent

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Figure 1.66: Preparation of selenated naphthaquinones and anthraquinones.

oxidative addition with diphenyl ditelluride/diselenide to produce an intermediate A and it went through a transmetallation with phenyl boronic acid to afford the intermediate B, which under reductive elimination led to the product. On the other hand, in cycle II, the boronic acid reacted with CuFe2O4 to form the intermediate C, which then reacted with another half of Ph2Te2 to provide B that finally provided the product via a reductive elimination step. Thus, one-half of ditelluride/diselenide units of (PhTe)2/(PhSe)2 was consumed in the process, making it atom-economic.

Figure 1.67: Condensation of boronic acids and diaryl diselenides/ditellurides.

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Figure 1.68: Possible mechanism.

The construction of carbon-selenium and carbon-tellurium bonds is of synthetic and pharmaceutical importance. Graphene oxide-based nano-Fe3O4(nano-Fe3O4@GO) was employed as a recyclable catalyst for the useful synthesis of diselenides and ditellurides (165) by the coupling of aryl iodides (163) with Se(0) or Te(0) (164) (Figure 1.69) [102]. The magnetic heterogeneous catalyst was easily recovered from the reaction mixture and used further for several runs without major loss in catalytic efficiency.

Figure 1.69: Preparation of diselenides and ditellurides by a single-pot coupling reaction.

Braga and his group [103] developed a mild and efficient methodology for the access to alkynyl chalcogenides (168) from terminal acetylenes (166) and diorganyl dichalcogenides (167) using Fe3O4 nanoparticles as catalyst (Figure 1.70). This protocol furnished the products in fairly good yields. In addition, the catalyst was easily separated by a simple magnet and used further for successive runs without an appreciable decrease in catalytic efficiency. A probable mechanism is given (Figure 1.71). It was postulated that in the first step, the [alkenyliron] cluster D was formed from the catalyst and the terminal alkyne. Instantly, this cluster reacted with diorganyl dichalcogenide, leading to E, which on reductive elimination, provided the corresponding alkynyl chalcogenides and species F. Finally, the catalyst was regenerated in the presence of a base and the catalytic cycle is completed.

Chapter 1 Magnetic metal nanoparticle-catalyzed carbon-heteroatom bond formation

Figure 1.70: Magnetite (Fe3O4) nanoparticles-catalyzed synthesis of alkynyl chalcogenides.

Figure 1.71: Plausible reaction pathway.

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1.5 Conclusion This chapter has focused on the preparation of magnetic metal nanoparticles and their application for carbon-heteroatom bonds, such as C–N, C–O, C–S, C–Se, and C–Te, forming reactions and synthesis of the related heterocyclic compounds. The advantages of using magnetic nanoparticles as a catalyst over other homogeneous and heterogeneous ones have been highlighted. The major advantages include easy recovery of the catalyst from the reaction mixture using an external magnet without any requirement for centrifugation, filtration, or other tedious workup processes and recyclability of the catalyst for several runs without an appreciable loss in efficiency. In addition, a large surface area and a superb distribution of magnetic nanoparticles in different solvents increase their catalytic efficiency. The growing concern of environment and economy prompted interest in the development of procedures involving green and cost-effective catalysts. Thus, applications of magnetic nanoparticles empowered with several green features are progressing fast to achieve this objective. We believe, this chapter will create more interest in academia and industry to explore various facets of magnetic nanoparticles and open a new route to the synthesis of pharmaceutically important molecules.

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benzoxazoles via Ullmann-type coupling under ligand-free conditions. Synlett 2014, 25, 0729–0735. Tanuraghaj HM, Farahi M. Preparation, characterization and catalytic application of nanoFe3O4@SiO2@(CH2)3OCO2Na as a novel basic magnetic nanocatalyst for the synthesis of new pyranocoumarin derivatives. RSC Adv 2018, 8, 27818–27824. Maleki A, Niksefat M, Rahimi J, Taheri-Ledari R. Multicomponent synthesis of pyrano[2,3-d] pyrimidine derivatives via a direct one-pot strategy executed by novel designed copperated Fe3O4@polyvinyl alcohol magnetic nanoparticles. Mater Today Chem 2019, 13, 110–120. Mugesh G, Du Mont WW, Sies H. Chemistry of biologically important synthetic organoselenium compounds. Chem Rev 2001, 101, 2125–2180. Stuhr-Hansen N, Bescers EHA, Engman L, Jansen RA. Organoselenium-substituted poly(pphenylenevinylene). Heteroatom Chem 2005, 16, 656–662. Okamoto Y. The Chemistry of Organic Selenium and Tellurium Compounds. Patai S, Rappoport Z, eds. Wiley, Chichester, U.K., 1986, Vol. 1, 331. Paulmier C. Selenium Reagents and Intermediates in Organic Synthesis, Organic Chemistry Series 4. Baldwin JE, ed. Pergamon Press Ltd, Oxford, 1986. Petragnani N. Tellurium in Organic Synthesis. Katritzky AR, Meth-Cohn O, Rees CW, eds. Academic Press, San Diego, 1994. Yoon H-J, Choi JW, Kang H, Kang T, Lee S-M, Jun B-H, Lee Y-S. Recyclable NHC-Ni complex immobilized on magnetite/silica nanoparticles for C-S cross-coupling of aryl halides with thiols. Synlett 2010, 2518–2522. Swapna K, Murthy SN, Jyothi MT, Nageswar YVD. Nano-CuFe2O4 as a magnetically separable and reusable catalyst for the synthesis of diaryl/aryl alkyl sulfides via cross-coupling process under ligand-free conditions. Org Biomol Chem 2011, 9, 5989–5996. Kovács S, Novák Z. Oxidoreductive coupling of thiols with aryl halides catalyzed by copper on iron. Org Biomol Chem 2011, 9, 711–716. Panda N, Jena AK, Mohapatra S. Heterogeneous magnetic catalyst for S-arylation reactions. Appl Catal A: General 2012, 433–434, 258–264. Baig RBN, Varma RS. A highly active and magnetically retrievable nanoferrite-DOPA-copper catalyst for the coupling of thiophenols with aryl halides. Chem Commun 2012, 48, 2582–2584. Damodara D, Arundhathia R, Likhar PR. High surface and magnetically recoverable mPANI/ pFe3O4 nanocomposites for C–S bond formation in water. Catal Sci Technol 2013, 3, 797–802. Srinivas BTV, Rawat VS, Konda K, Sreedhar B. Magnetically separable copper ferrite nanoparticles-catalyzed synthesis of diaryl, alkyl/aryl sulfones from aryl sulfinic acid salts and organohalides/boronic acids. Adv Synth Catal 2014, 356, 805–817. Hajipour AR, Karimzadeh M, Azizi G. Highly efficient and magnetically separable nanoCuFe2O4 catalyzed S-arylation of thiourea by aryl/heteroaryl halides. Chin Chem Lett 2014, 25, 1382–1386. Gholinejad M, Karimi B, Mansouri F. Synthesis and characterization of magnetic copper ferrite nanoparticles and their catalytic performance in one-pot odorless carbon-sulfur bond formation reactions. J Mol Catal A: Chem 2014, 386, 20–27. Liu Y, Zhou L, Hui X, Dong Z, Zhu H, Shao Y, Li Y. Fabrication of magnetic amino-functionalized nanoparticles for S-arylation of heterocyclic thiols. RSC Adv 2014, 4, 48980–48985. Movassagh B, Takallou A, Mobaraki A. Magnetic nanoparticle-supported Pd(II)-cryptand 22 complex: An efficient and reusable heterogeneous precatalyst in the Suzuki–Miyaura coupling and the formation of aryl–sulfur bonds. J Mol Catal A Chem 2015, 401, 55–65. Gholinejad M, Firouzabadi H. One-pot odorless thia-Michael reaction by copper ferrite nanoparticle-catalyzed reaction of elemental sulfur, aryl halides and electron-deficient alkenes. New J Chem 2015, 39, 5953–5959.

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[88] Sharma RK, Gaur R, Yadav M, Rathi AK, Pechousek J, Petr M, Zboril R, Gawande MB. Maghemite-copper nanocomposites: Applications for ligand-free cross-coupling (C−O, C−S, and C−N) reactions. ChemCatChem 2015, 7, 3495–3502. [89] Amiri K, Rostami A, Rostami A. CuFe2O4 magnetic nanoparticle catalyzed odorless synthesis of sulfides using phenyl boronic acid and aryl halides in the presence of S8. New J Chem 2016, 40, 7522–7528. [90] Azadi G, Taherinia Z, Naghipour A, Ghorbani-Choghamarani A. Synthesis of sulfides via reaction of aryl/alkyl halides with S8 as a sulfur-transfer reagent catalyzed by Fe3O4magnetic-nanoparticles-supported L-Histidine-Ni(II). J Sulfur Chem 2017, 38, 303–313. [91] Farzin S, Rahimi A, Amiri K, Rostami A, Rostami A. Synthesis of diaryl sulfides via nickel ferritecatalysed C–S bond formation in green media. Appl Organometal Chem 2018, 32, e4409. [92] Tamoradi T, Moeini N, Ghadermazi M, Ghorbani-Choghamarani A. Fe3O4-AMPD-Pd: A novel and efficient magnetic nanocatalyst for synthesis of sulfides and oxidation reactions. Polyhedron 2018, 153, 104–109. [93] Atashkar B, Rostami A, Rostami A, Zolfigol MA. NiFe2O4 as a magnetically recoverable nanocatalyst for odourless C–S bond formation via the cleavage of C–O bond in the presence of S8 under mild and green conditions. Appl Organometal Chem 2019, 33, e4691–6. [94] Ashraf MA, Liu Z, Peng W-X, Zhou L. Glycerol Cu(II) complex supported on Fe3O4 magnetic nanoparticles: A new and highly efficient reusable catalyst for the formation of aryl-sulfur and aryl-oxygen bonds. Catal Lett 2020, 150, 1128–1141. [95] Azgomi N, Mokhtary M. Nano-Fe3O4@SiO2 supported ionic liquid as an efficient catalyst for the synthesis of 1,3-thiazolidin-4-ones under solvent-free conditions. J Mol Catal A Chem 2015, 398, 58–64. [96] Safaei-Ghomi J, Navvab M, Shahbazi-Alavi H. CoFe2O4@SiO2/PrNH2 nanoparticles as highly efficient and magnetically recoverable catalyst for the synthesis of 1,3-thiazolidin-4-ones. J Sulfur Chem 2016, 37, 1–12. [97] Swapna K, Murthy SN, Nageswar YVD. Magnetically separable and reusable copper ferrite nanoparticles for cross-coupling of aryl halides with diphenyl diselenide. Eur J Org Chem 2011, 1940–1946. [98] Reddy KHV, Satish G, Ramesh K, Karnakar K, Nageswar YVD. Magnetically separable CuFe2O4 nanoparticle catalyzed C–Se cross-coupling in reusable PEG medium. Chem Lett 2012, 41, 585–587. [99] Mohan B, Park JC, Park KH. Mechanochemical synthesis of active magnetite nanoparticles supported on charcoal for facile synthesis of alkynyl selenides by C−H activation. ChemCatChem 2016, 8, 2345–2350. [100] Jardim GAM, Bozzi IAO, Oliveira WXC, Mesquita-Rodrigues C, Menna-Barreto RFS, Kumar RA, Gravel E, Doris E, Braga AL, Da Silva Júniora EN. Copper complexes and carbon nanotube– copper ferrite-catalyzed benzenoid A- ring selenation of quinones: An efficient method for the synthesis of trypanocidal agents. New J Chem 2019, 43, 13751–13763. [101] Kundu D, Mukherjee N, Ranu BC. A general and green procedure for the synthesis of organochalcogenides by CuFe2O4 nanoparticle catalysed coupling of organoboronic acids and dichalcogenides in PEG-400. RSC Adv 2013, 3, 117–125. [102] Kassaee MZ, Motamedi E, Movassagh B, Poursadeghi S. Iron-catalyzed formation of C–Se and C–Te bonds through cross-coupling of aryl halides with Se(0) and Te(0)/nanoFe3O4@GO. Synthesis 2013, 45, 2337–2342. [103] Godoi M, Liz DG, Ricardo EW, Rocha MST, Azeredo JB, Braga AL. Magnetite (Fe3O4) nanoparticles: An efficient and recoverable catalyst for the synthesis of alkynyl chalcogenides (selenides and tellurides) from terminal acetylenes and diorganyl dichalcogenides. Tetrahedron 2014, 70, 3349–3354.

Bablee Mandal, Basudeb Basu*

Chapter 2 Magnetic nanocatalysts in disulfide synthesis 2.1 Introduction S–S bond containing organic compounds, commonly known as disulfides (or disulfanes), revealed their importance over the years to chemists as well as biologists. Their versatile applications in biochemistry, polymer industry, fine chemical industry, in fields like peptidomimetics, self-assembled monolayers, etc. have encouraged researchers of both chemical and biological backgrounds to explore and establish novel techniques of disulfide synthesis [1–3]. Hence, the organyl disulfides (bearing S–S bonds) are extremely important in diverse fields, and a few of them are mentioned hereunder, to highlight their more specific importance and function.

2.1.1 Disulfides in biomolecules Proteins and various other bioactive natural molecules contain large disulfide-linked aggregates [4]. Disulfides play a crucial role in thiol-containing protein folding – an imperative phenomenon that gives the three dimensional structure to a protein molecule and makes it biologically functional [5, 6]. The oxidation of thiol groups of two cysteine units results in the covalent disulfide linkage in the protein. The disulfide bonds are reactive and easily reversible by thiol–disulfide exchange reactions. In vitro formation of disulfide bond occurs spontaneously in the presence of any appropriate electron acceptor (e.g., oxygen) but the in vivo process has to be catalyzed [7]. Although disulfide bond formation in proteins is mainly catalyzed by disulfide isomerase a1 (PDIA1) [8], ensuing studies have identified a variety of other enzymes which act as efficient catalysts [9–12]. Although some little information have been gathered about of how enzymes form, break, and shuffle disulfide bonds, there are, nevertheless, many unanswered questions, providing wide scope for intense exploration. The disulfide bonds cause oxidative refolding of a protein, which uniquely determines the protein sequencing and, hence, its quaternary structure. As demonstrated

*Corresponding author: Basudeb Basu, Department of Chemistry, Cotton University, Guwahati 781001, India, e-mail: [email protected], https://orcid.org/0000-0002-7993-2964 Bablee Mandal, Department of Chemistry, Surya Sen Mahavidyalaya, Siliguri 734004, India, https://orcid.org/0000-0002-3409-8611 https://doi.org/10.1515/9783110730357-002

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by Anfinsen, reducing the disulfide bonds in the natural condition of RNase A generated unfolded protein. It folds reversibly, forming the four native S–S bonds from eight cysteine residues, under oxidizing conditions [13–15]. The fact that these four native S–S bonds are always formed out of 105 possibilities seems quite intriguing; so the knowledge of the correlation between the disulfide bond formation and protein folding is highly crucial from the chemical as well as biophysical point of view. Currently, various research groups are actively engaged in a variety of experimental as well as computational studies aimed at gathering information about this highly complicated correlation [16–18]. Another significant biochemical application involves the crucial role of disulfide linkages (intra as well as inter) in the study of drug export by multidrug transporters, which is important to address the rapidly increasing antibiotic resistance [19–22]. Studies proposing an amalgamation of known antibiotics and disulfide-bond-forming inhibitors seem to be the solution to augment the efficiency of antibiotics and restrain this antibiotic resistance [23–25]. An alternative solution may be unlocking the potential of an appropriate biocatalyst (enzyme with poor stability outside its native lipidbilayer context) by tuning it to be water soluble [26].

2.1.2 Disulfides in industries Apart from their significant role in protein folding, disulfides have applications in the myriad industrial sectors, as well. Self-assembled monolayers of organic disulfides have been used as environmentally benign lightweight cathode in rechargeable lithium batteries. Here, redox processes involving thiol–disulfide exchange assist the lithium ion transfer [27]. In the rubber industry, disulfides are not only used as vulcanizing agents and oils of rubber and elastomers [28, 29] by imparting tensile strength, but also find applications in the self-healing of rubbers or elastomers through exchange or metathesis reaction. This process finds huge application in reclaiming waste rubber, thereby reducing the environmental burden [30]. Their importance in the agro-chemical industry stems from the fungicidal properties of the disulfides. Dimethyl disulfide is found to have a huge control on the growth of a wide variety of plant pathogens, with minimum toxic effect on the plant [31]. Hence, they find enormous application as crop protection chemicals. A few oxidized species of disulfides, such as thiosulfinate or thiosulfonate, play a significant role in the body’s defense against cyanide intoxication [32]. The sulfur donor activates the enzyme rhodanese by transferring sulfur, and this enzyme converts cyanide to thiocyanate. Disulfide sulfur donors, such as bis(4-methoxyphenyl) disulfide or diallyl disulfide (present in garlic), are effective in vivo therapeutic agents against cyanide intoxication. Thus, disulfides are valuable to the pharmaceutical industry [33].

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51

2.1.3 Disulfides in organic synthesis In the sphere of organic synthesis, disulfides are a well-known class of key and versatile intermediate and reagent. They are extensively used for the generation of sulfenyl, sulfinyl compounds, and other sulfur-containing heterocyclic compounds [34, 35]. They have applications in cross dehydrogenative coupling (CDC) [36], thioannulation reactions [37], in glycobiology [38] and, most of all, in peptide synthesis – enhancing their activity and selectivity – thereby applying them as scaffolds in drug design [39–41]. The disulfide formation also has an important role in protection– deprotection technique. The thiol groups can be efficiently protected as disulfides [42] and can again be easily recovered from these disulfides in two ways – (a) by reduction and (b) by selective cleavage of S–S bonds, using efficient and specific agents such as nitriles, alcohols, or hydrazines [43]. It also finds immense application as a protecting group for cystein side chain [44]. Thus, not only from an industrial or biological perspective, but also from the viewpoint of organic synthesis, the design and development of effective, inexpensive, and eco-friendly catalytic systems/synthetic methodologies for the disulfide synthesis is a challenge eagerly taken up by research chemists of today.

2.1.4 Nanotechnology & nanocatalysis in chemistry Nanotechnology is the technology of applying science and engineering syntheses conducted in nanoscale (1 to 100 nm). Though first conceived by the eminent physicist, Richard Feynman, in 1959 [45], it was not until 1974 that the term “nanotechnology” was coined by Norio Taniguchi. Nanotechnology emerged as a full-fledged branch in 1981, with the discovery of scanning tunneling microscope. The technology involves the ability to manipulate and control individual atoms and molecules. The advantages of the technology stems from certain advanced properties of nanomaterials as compared to large scale materials, e.g., lighter weight, greater strength, better control of light spectrum, and increased chemical reactivity. Thus, though discovered by a physicist, the technology, today, has spread to all other branches of science like chemistry, biology, material science, and engineering. Although the modern age of science has given its name and elucidation, this technology has its roots in the 4th Century AD, when the Romans used to decorate glasses and cups with nanosized materials. The Lycurgus cup or the stained glass windows of the medieval church are very famous examples, where gold and silver nanoparticles were embedded in glass to create a beautiful coloring effect. This forgotten technology was reborn in the modern age, with the development of technologies, such as atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning tunneling microscope (STM).

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In the field of chemistry, “nanocatalysis” is a very hot topic of research that involves the generation and application of various kinds of nanomaterials as catalysts [46]. Since the catalyst or its support is of nanoscale, the surface area is considerably increased [47], allowing a better and more efficient interaction with the reactants, thereby increasing the catalytic activity and selectivity. Their stability and recyclability are further added advantages over conventional catalysis. Moreover, the activity and selectivity of the catalyst can be manipulated by tailoring their physical and chemical properties like shape and size and/or composition and morphology [48]. Tuning the properties of metals is easier at the nanoscale than in macroscopic levels. These properties depend on the procedure employed for the preparation of the catalysts. The various synthetic procedures include thermal decomposition, chemical precipitation, hydrothermal methods, chemical vapor synthesis, antisolvent precipitation, photochemical methods, sonochemical methods, and microwave irradiation [47, 48]. In fact, the catalytic efficiency of metallic nanocatalysts is, to a great extent, governed by the particle size and support used for the metal dispersion.

2.1.5 Magnetic nanocatalysis In conventional catalysis, the most tedious, time-consuming, and expensive task is catalyst separation and recovery. Heterogeneous catalysis solves this problem to some extent, as being in a different phase allows its easy recovery by filtration [49, 50]. But the selectivity and efficiency of these catalysts are lower than their homogeneous counterparts. Hence, a catalyst able to combine the advantageous features of both homogeneous and heterogeneous catalysis was, indeed, a tremendous development in the field of organic synthesis. The greatest advantage of a magnetic nanocatalyst is its insolubility and paramagnetic nature, which allows it to be easily separated from the reaction mixture, simply by applying an external magnet, thereby avoiding any filtration or workup processes [51–53]. Moreover, they retain their activity even after several recycles, thus matching up to the advantages of heterogeneous catalysis. Again, magnetic nanoparticles (MNPs) are associated with several impressive features like great surface area to bulk ratios, thermal stability, high activity, minimum toxicity, their ability for being superficially modified easily, and ready dispersion [54–56]. Thus, magnetic nanocatalysis being an appealing and splendid approach, possesses the advantages of both homogenous and heterogeneous catalysis. Among the different magnetic supports explored, magnetite (Fe3O4) and maghemite (γ-Fe2O3) nanoparticles are the most widely investigated core nanomagnetic support [57–59], as they offer the most versatile range of technological applications. When these ferrite particles are reduced to less than 128 nm in size, they become superparamagnetic, avoid self agglomeration, and exhibit their magnetic behavior only in the presence of an external magnetic field. The catalytic system is immobilized on the nanoparticles

Chapter 2 Magnetic nanocatalysts in disulfide synthesis

53

through encapsulation using surfactants, silica, silicones, diamines, etc., which also serves to increase the stability of the MNPs in solution. X-ray diffraction (XRD) techniques, Fourier transform-IR spectroscopy, Transmission electron microscopy (TEM), Field emission-SEM analysis, Vibrating sample magnetometric analysis, inductively coupled plasma-optical emission spectrometer, and dispersive spectroscopic analysis are some of the techniques usually applied for the characterization of the MNPs [60, 61]. The techniques generally employed for the preparation of MNPs include co-precipitation, microemulsion, thermal decomposition, flame spray synthesis, and hydrothermal synthesis [62].

2.2 MNPs (Magnetic Nanoparticles) in disulfide synthesis 2.2.1 Magnetite MNP core The versatility, low toxicity, low cost, and ready abundance makes copper the most popular choice, compared to other transition metals in modern catalytic research. Thus, the catalytic activity of various copper complexes heterogenized as nanoparticles on numerous solid supports such as alumina, silica, zeolite, mesoporous materials, etc., has been massively investigated in the past couple of decades [63–65]. Although the techniques have their own advantages, a major drawback is the side reaction due to the catalytic activity of the support itself, thereby decreasing the yield of the desired product. Continued research in this area has now focused on Fe3O4 nanoparticles as the support due to its great surface–area ratio, minimum toxicity, cost effectiveness, huge dispersion and reactivity, immense chemical stability, and above all, simplicity of its preparation procedure and easy magnetic separation, after the completion of the reaction [47, 66]. One representative example of the technique employs Cu(II) immobilized on Fe3O4–diethylenetriamine (DETA) as the catalytic system [67]. Oxidation of thiols to disulfides was brought about using a mixture of thiol (1 mmol), hydrogen peroxide (0.5 ml), and Cu(NO3)2/Fe3O4–DETA (10 mg) in ethanol (2 ml), at ambient temperature. Cu(NO3)2/Fe3O4–DETA was prepared using Cu(NO3)2⋅3H2O (0.25 g) and Fe3O4– DETA (0.5 g) in absolute ethanol (30 ml) under reflux for 24 h (Figure 2.1). Another example of application of copper catalysis, also investigated by the same group, is found in the specific oxidation of an array of aliphatic, aromatic, and heteroaromatic thiols in the presence of 33% hydrogen peroxide (0.5 mL), as an eco-friendly oxidant, in ethanol, as a green solvent, at ambient temperature [68]. The catalytic prospective of a new Schiff base complex of CuII supported on superparamagnetic Fe3O4 (Fe3O4/salen of CuII) with a usual size of 10–20 nm was studied in the preparation of disulfides (Figure 2.2). The salen complex of CuII was immobilized on the magnetic

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RSH

Cu(NO3)2/Fe3O4-DETA H2O2, EtOH, rt

R = Aryl,Br-aryl, heteroaryl, alkyl

FeCl3.6H2O + FeCl2.4H2O H2N

N H

NH4OH, N2 rt

toluene, reflux, 42 h

S

R

O Fe3O4 O Si MNPs O

CPTMS EtOH/H2O 40oC, 8h

NH2 Fe3O4 MNPs

S

89-98%

Fe3O4 MNPs

30 min

R

O O Si O

H N

Cl

NH2

N H

EtOH, 80oC CuNO3.3H2O O Fe3O4 O Si MNPs O

H N

NH O3N Cu NH 2 O3N Cu(NO3)2/Fe3O4-DETA

Figure 2.1: Oxidative coupling of thiols using Cu(II) immobilized on Fe3O4–diethylenetriamine (DETA).

RSH

Fe3O4 / salen of CuII H2O2, EtOH, rt

R = aryl, heteroaryl, Br-aryl, CH2CO2H, (CH2)2OH

R

S

S

FeCl3.6H2O + FeCl2.4H2O

R

57-98%

N2, 80oC

Distilled water, NH4OH Br

OH OH HO HO OH Fe3O4 HO OH HO HO OH HO OH

Br (a) (EtO)3Si

NH2

CHO

EtOH, rt, 8h

OH

O N Cu O N

Si(OEt)3 Si(OEt)3 +

(b) Cu(NO3)2.9H2O, EtOH, reflux

EtOH, reflux

Br

15h, N2

Br O Si O O

OH Fe3O4

O N Cu O N

Br

OH

O O Si O

OH

Fe3O4 / salen of CuII

Figure 2.2: Disulfide synthesis using Schiff base complex of CuII supported on superparamagnetic Fe3O4.

OH OH OH

Chapter 2 Magnetic nanocatalysts in disulfide synthesis

55

support following the procedure as outlined in Figure 2.2. The catalyst showed no loss of activity for 10–12 successive recycles, thereby showing excellent reusability and recoverability. Depending on the substrate, the reaction time ranges from 15 min to 2 h. In the same year, the group developed yet another copper-catalyzed efficient methodology for the conversion of thiols to dilsulfides, using hydrogen peroxide as the oxidant and a Fe3O4–Schiff base of CuII as a novel heterogeneous nanocatalyst in ethanol, at ambient temperature [69]. Except the oxazolyl (52% yield), all other thiols yielded the disulfide in 92–99% within 5 min to 2 h, and a good catalytic activity was observed for 10 consecutive runs (Figure 2.3).

O OEt Si O N OH Cu N OH

Fe3O4 O Si O OEt RSH

R

H2O2, EtOH, rt

S

R

S

92-99%

S SH

Me

SH

CH2SH

Br

SH

N SH

O SH

CO2H HO2C CH2SH

SH

N

N SH

NH2 SH

HOH2C CH2SH

Figure 2.3: Cu(II)–Schiff base complex-functionalized magnetic Fe3O4 nanoparticles.

Encouraged by the success of Cu, the above group investigated the Schiff-base complexes of a series of metals like Ni, Co, Cr, Cd, and Zn dispersed on magnetite MNPs (M-salen-MNPs) with a typical size of about 15 nm in the specific oxidation of various aliphatic and aromatic thiols at ambient temperature to their corresponding disulfides, using H2O2 as the green oxidant (Figure 2.4). The experiments clearly demonstrated the success of all the nanocatalysts in these oxidative couplings, generating the products in superb yields (up to 99%) in less than an hour [70]. The nanocatalyst, after recovery with an externally applied magnetic field, was successfully recycled in all cases, up to 10 consecutive runs. It has already been discussed, that the activity of magnetic nanoparticles can be enhanced by encapsulation; and coating with a layer of silica on the surface allows easy functionalization. Silica encapsulation is also popular due to its easy availability, low cost, thermal stability, mechanical robustness, high binding strength, biocompatibility, and chemical inertness. They have found application in numerous organic reactions, and oxidation of mercaptans to disulfides is, definitely, one of them [71].

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O OEt Si O NM O N O

Fe3O4

M = Ni, Cd, Cr, Co, Zn

O Si O OEt RSH

H2O2, EtOH, rt

R

S

S

R

upto 99%

R = Tolyl, benzyl, 4-BrC6H4, 4-MeC6H4, 2-NH2C6H4, napthyl, thiazolyl,OH(CH2)2, CO2HCH2

Figure 2.4: Ni, Co, Cr, Cd, and Zn complexes dispersed on magnetic nanoparticles in oxidative coupling of thiols.

Cobalt nanocatalysts are one of the metal nanocatalysts that show high catalytic activity in oxidation reactions. An effective process for the synthesis of cobalt phthalocyanine- functionalized magnetic silica beads (50–100 nm average particle size) catalyzed by supporting tetrasulfonated Co(II) phthalocyanine (CoPcS) on the silica-coated MNPs (with amino functionalization) (Fe3O4@SiO2, SMNP) through a sulfonamide linkage was discovered by Singh and coworkers [72], with the aim of providing an economical, clean, and effective process for converting the mercaptans present in petroleum products into less detrimental disulfides (Figure 2.5). The MNPs were prepared by co-precipitating FeCl2 and FeCl3 in aqueous HCl at ambient temperature followed by silica coating using tetraethoxysilane (TEOS) and ammonia solution. Apart from the aforementioned advantages, the silica coating prevents the undesirable oxidation of the nanomagnetite surface, thereby improving its stability. The myriad applications of manganese nanocatalysts in oxidation reaction as well as in MRI, biofibers, textiles, etc. [73] has led to a growing interest in the development and application of Mn-MNPs. An MnIII complex supported on silica-functionalized magnetite nanoparticles (Fe3O4@SiO2–NH2@Mn(III)) with an approximate size of 14–50 nm was demonstrated as an efficient and reusable catalyst for the specific oxidative coupling of aromatic thiols (Figure 2.6), applying urea hydrogen peroxide (UHP) as the oxidant in a solvent mixture of CH3OH/CH2Cl2 (1: 1), at ambient temperature [74]. Dopamine sulfamic acid MNPs (DSA@MNP) are one of the first reported solid acid catalysts for the oxidation of sulfur compounds. The MNPs prepared from chlorosulfuric acid and dopamine (with an approximate diameter of 24 nm) proved to be easy to prepare and inexpensive. Dopamine sulfamic acid-functionalized magnetite nanoparticles successfully catalyzed oxidative coupling of thiols, using H2O2 as a green oxidizing in ethyl alcohol at room temperature (Figure 2.7) [75]. The catalyst exhibited recyclability up to four runs, without any appreciable loss of catalytic activity.

Chapter 2 Magnetic nanocatalysts in disulfide synthesis

CoPcS@ASMNP RSH

R

H2O, O2 70oC

R = C6H5, C6H4-OCH3, C6H4-Cl, tolyl, ethyl and other linear alkyl chains

S

S

57

R

upto 96%

NH2 SO2R Si O OO

H2N

H2N

O O Si O O Si O Fe3O4 O O OO O O O Si Si

N N N Co N N N N N

O H N S O

SO2R

RO2S

NH2 CoPcS@ASMNP

Figure 2.5: Magnetic silica beads functionalized with cobalt phthalocyanine oxidizing mercaptans to disulfides.

RSH

Fe3O4 @SiO2-NH2 @Mn(III)/UHP CH3OH:CH2Cl2 (1:1), rt, 2h

R

S

S

R

81-90%

R = Ph, 4-BrC6H4, 4-MeC6H4 benzyl, naphthyl O N Fe3O4 @SiO2

O H2N

Mn O N O

Fe3O4 @SiO2-NH2 @Mn(III)

Figure 2.6: Mn(III) complex immobilized on magnetite nanoparticles for specific oxidation of thiols.

A novel silver based MNP, Ag-AcPy@ASMNP, (with an average size of 20–24 nm) has been developed as an effective catalyst for the oxidative coupling of thiols, in water, at room temperature, in 30 min under aerobic conditions, thereby providing a sustainable methodology for the preparation of disulfides (Figure 2.8) [60]. The catalyst has recyclability up to seven runs, with negligible leaching. The antibacterial, antiviral, and fungicidal properties of Ag-nanoparticles render this method an extremely important one, suitable for large scale disulfide synthesis in the pharmaceutical industry. Ni has been immobilized on magnetite coated with adenine-generating MNPs (of approximate size 10–30 nm), which has been successfully used for the oxidation

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Bablee Mandal, Basudeb Basu

Figure 2.7: Application of DSA@MNPs as heterogeneous and reusable nanocatalysts for oxidative coupling of thiols.

AgAcPy@ASMNPs (15 mg) RSH

H2, 30 min, rt, air

R = Ph, 4-MeC6H4, 4-BrC6H4, 4-OMeC6H4, 4-NH2C6H4, alkyl, naphthyl

(i)Tetra-ethyl orthosilicate (ii) 3-aminopropyltriethoxysilane Fe3O4 (iii) N (iv) AgNO3

R

S

S

R

95-100%

Fe3O4 N

O

AgNO3 N

AgAcPy@ASMNPs

Figure 2.8: Oxidative coupling of thiols using silver-based magnetic catalyst.

Figure 2.9: Ni(II)-Adenine complex-immobilized magnetite nanoparticles as recyclable nanocatalyst for oxidation of thiols.

of thiols to disulfides, with hydrogen peroxide as the oxidant, under EtOH at room temperature, within 30–90 min [76]. A mechanism as proposed by the authors has been outlined in Figure 2.9

Chapter 2 Magnetic nanocatalysts in disulfide synthesis

59

Zn was another metal successfully immobilized on the same MNP by the same group, under similar reaction conditions. Zinc (II)-Adenine complex functionalized on Fe3O4 nanoparticles (diameter ranging from 9 to 19 nm) was found to catalyze the oxidation of thiols (aromatic, heteroaromatic, and aliphatic) in the presence of hydrogen peroxide, in ethyl acetate at room temperature [77]. The yield ranged 86–99%, within 40–130 min (Figure 2.10).

Figure 2.10: Oxidation of thiols using Fe3O4–adenine–Zn.

The use of bromine in organic synthesis is very hazardous and necessitates extreme caution during handling. An MNP-based heterogeneous bromine source has negated this drawback and has been successfully employed as a fascinating catalyst for the conversion of thiols to disulfides [55]. Tribromide ion immobilized superficially on silica-coated magnetite nanoparticles with DETA/Benzyl functionalization retained its catalytic activity for five successive runs (Figure 2.11). Fe3O4 MNPs-DETA/Benzyl Br3 RSH

R

H2O2, EtOH, rt, 15-55 min R = Aryl, alkyl heteroaryl

O Fe3O4 O Si MNPs O

Ph Br3

N

Br3 Ph

N

Br3 Ph Fe3O4 MNPs-DETA/Benzyl Br3 Ph

R

S

89-98%

Ph

Ph

S

N

Ph Ph

Figure 2.11: Oxidative coupling of thiols using Fe3O4 MNPs-DETA/Benzyl-Br3.

Apart from thiols, thiolacetates have also been explored as a substrate for disulfide synthesis, using lipase-immobilized MNP, based on chitosan coated magnetite [78] {Magnetite-chitosan-poly[N-benzyl-2-(methacryloxy)-N,N-dimethylethanaminium

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Bablee Mandal, Basudeb Basu

bromide]} nanoparticles (Fe3O4-CS-PQ), the average particle size being 13–17 nm. This biocatalytic synthesis can prove to be an extremely remarkable alternative to the existing processes for the preparation of a variety of disulfide compounds (Figure 2.12). The reaction carried out under anaerobic condition exhibits a broad range of substrate specificity [79]. The hydrolysis of long chain thiol acetates is sluggish compared to the short chain ones, based on which, the authors propose that the hydrolysis of organic thioesters by the enzyme generates thiol radical, resulting in immediate generation of disulfides under anaerobic conditions [80]. O R

S

lipase-(Fe3O4-CS-PQ)

R1

R, R1 = Alkyl, aryl, heteroaryl

R

Cyclohexane, N2, 35oC

S

S

R1

33-55%

Figure 2.12: Lipase-immobilized magnetic nanoparticle-catalyzed synthesis of disulphide compounds.

Magnetite nanoparticles have also successfully catalyzed the conversion of aryl halides to disulfide, using K2S as the sulfur transfer agent at 120 °C in DMF under N2atmosphere (Figure 2.13) [81]. Though the method exhibited a wide range of functional group tolerance, an electron withdrawing group accelerates the rate of formation of the corresponding diaryl disulfide. Aryl iodides and bromides Fe3O4 NP 1,10-phenanthroline Ar

ArX + K2S

DMF, KOH Ar = Ph, 4-OMeC6H4, 120oC, N2 2-MeC6H4, 4-NO2C6H4, 3-CF3C6H4,

S

S

Ar

X = I, Br; yield =70-98% X = Cl; yield = 58-97%

Fe3O4 NP Ln ArX

Fe3O4 NP

ArSK

Ln Fe3O4 NP

Ar

Ar

Fe3O4 NP SK

X Ln

Ln K2S

Figure 2.13: Synthesis of symmetrical disulfides from aryl halides using magnetic iron oxide nanoparticles/K2S.

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were proved to be better substrates than chlorides. The catalyst gave a recoverability record of five successive cycles, with no appreciable loss of activity. The authors proposed a mechanism in which 1,10-phenanthroline coordinates with the Fe-center, followed by oxidative addition to ArX, generating a complex that undergoes subsequent nucleophilic substitution with K2S and reductive elimination to generate the thiolate, which is then converted to disulfide, under the reaction conditions (Figure 2.13).

2.2.2 Maghemite MNP core The maghemite (γ-Fe2O3) nanoparticles are superparamagnetic materials that have extensive use in the realm of catalytic reactions, sensors/detectors, drug delivery, magnetic resonance imaging (MRI), and biology [82]. Moreover, the γ-Fe2O3 nanocomposites immobilized with metals like Sn, Ni, or Ag, or functionalized with silica, graphene oxide, and starch have applications not only in organic catalysis but also in optical devices and elimination of heavy metal ions from aqueous solution, in a selective manner [83]. An interesting example of immobilization of acidic reagent MNPs and their application has been studied by Sabet and coworkers in the chemoselective transformation of a variety of aromatic thiols, using recyclable immobilized iron oxide nanoparticles (γ-Fe2O3–SO3H) (average size 14 nm) catalyst, H2O2, as the oxidizing agent in methanol at ambient temperature (Figure 2.14) [84]. The progress of the reaction as observed by potentiometric titration usually yielded the product in 1 min (99% yield) for aromatic thiols, but aliphatic thiols gave no reaction even after 6 h. The catalyst could be reused for six consecutive runs, without any appreciable loss of activity. A plausible mechanism explaining the role of iron has been outlined in the Figure 2.14.

Figure 2.14: Conversion of aromatic thiols to disulfides using maghemite-supported sulfonic acid catalyst.

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An oxo-vanadium salen-type complex has quite recently been successfully immobilized on magnetic γ-Fe2O3 nanoparticles and utilized as an effective and recyclable catalyst for the oxidation of thiols to disulfides, with urea H2O2 (UHP) as the oxidizing reagent (Figure 2.15). Vanadium’s ability to make a quick transition from + IV and + V oxidation states makes it compatible for catalytic oxidations [85]. The catalyst was recycled for five successive runs with minimum leaching. A mechanism proposing the role of the catalyst and oxidant has been delineated by the authors, in Figure 2.15.

RSH

γ-Fe2O3@[VO(salenac-OH)]

R

EtOH, UHP, rt

R S 85-95%

O O Si O

R = Ph, 4-BrC6H4, 4-MeC6H4, 2-NH2C6H4, benzyl, oxazolyl thiazolyl, naphthyl

R

S

+ H2O

S

NH2CONH2

γ-Fe2O3@[VO(salenac-OH)]

O

H S

O

Me

Me

γ-Fe2O3@[VO(salenac-OH)]

R

RSH

NO N V O O

S

NH2CONH2

H

H

H2O2

O O

γ-Fe2O3@[VO(salenac-OH)]

γ-Fe2O3@[VO(salenac-OH)] NH2CONH2

O γ-Fe2O3@[VO(salenac-OH)]

OH

NH2CONH2 RSH

Figure 2.15: Maghemite nanoparticle-supported vanadium-catalyzed oxidative coupling of thiols.

2.2.3 Spinel ferrite MNPs Apart from magnetite, a great surge of interest has recently been witnessed in the exploration of spinel ferrites (MFe2O4; M = Ni, Zn, Mn, or Co) due to their applicability as magnetic catalyst immobilizing systems, in drug delivery, high-density storage, etc. [86–88]. Of these, the inverse spinel structure of nickel ferrite (NiFe2O4) gives it a lot of significance as a soft magnetic material. High thermal stability, great magnetic anisotropy, and good saturation magnetization are some of its characteristics that render it so important. The catalytic prospective of nickel ferrite NPs (14–20 nm) was explored in the oxidation of thiols, in the presence of H2O2 as the oxidizing agent, at ambient temperature, in acetonitrile solvent (Figure 2.16) [89].

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It has been established that, a rapid electron exchange between M2+ and M3+ ions occurs in inverse and mixed spinel structures [90]. It is a well-known fact that Fe2+ ions generate HO radical and OH– ion during its oxidation with H2O2 [91, 92]. On this basis, a probable mechanism has been outlined for the above reaction, in Figure 2.16.

H+

RSH

NiFe2O4, H2O2 CH3CN, rt

R

S

S 88-96%

R = 4-BrC6H4, 4-OMeC6H4, 2-NH2C6H4, benzyl, furyl, thiazolyl, alkyl

H2O2

.

H +

R

R

S

2OH-

Ni2+ + Fe3+

Ni3+ + Fe2+ S

.

R

2OH RSH

.

RS

RSH H2O

Figure 2.16: Nickel ferrite nanoparticles – hydrogen peroxide-catalyzed oxidative coupling of thiols.

2.2.4 Magnetic clay Clay, in both natural and ion-exchange forms, is a well-known catalyst for various organic transformations [93, 94]. Of the various types explored, Montmorillonite has exceptional characteristics such as swelling capacity, cation exchange ability, etc. and, hence, is capable of accepting a variety of foreign species in its interlayers. In addition, K10-Montmorillonite is noncorrosive, nontoxic, inexpensive, reusable, and eco-friendly. Magnetically recoverable cation-exchanged Al3+ Montmorillonite (Al-MMT) was synthesized using co-precipitation method, using Al-MMT (5.0 g), FeCl2.4H2O (9.94 g, 50 mmol), and FeCl3.6H2O (0.27 g, 100 mmol), under ultrasound. Al-MMT was successfully applied as a magnetically reusable catalyst for the oxidative dimerization of thiols in THF at 50 °C, within 60–120 min [95]. Aluminum ions existing between MMT layers facilitate electron transfer, thereby activating the thiol functionality to dimerize (Figure 2.17).

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Al-MMT / THF / 50oC RSH R = Ph, 4-MeC6H4, 4-BrC6H4, 3-ClC6H4, alkyl, naphthyl, furyl

Interlayer Cations

R S R S 65-96%

1.Cation Exchange Na

Na

Na

Na

Montmorillonite K-10 (MMT)

Al3

2.Magnetization

Al3

Al3

Al3

Magnetic Al3+-montmorillonite (Al MMT)

Figure 2.17: Aluminum-MMT-catalyzed oxidative coupling of thiols.

2.3 Conclusions The disulfide (-S-S-) bond formation constitutes an important reaction, and disulfanes find wide applications in diverse fields like organic synthesis, biochemistry, industrial chemistry, making of self-assembled monolayers (SAMs), and biology. Apart from various homogeneous and heterogeneous catalysts employed for making the disulfide bonds from thiols and other S-containing compounds, the magnetic nanocatalysts based on magnetic cores, such as magnetite, maghemite, magnetic clay, spinel ferrites, etc., are found to have exhibited excellent catalytic performances in the -S-Sbond formation. Notable advantages of using these magnetic nanocatalysts are high catalytic performance, excellent selectivity, easy separation, and recyclability. The most extensively explored magnetic core, however, appears to be the magnetite, till now. The field of research includes versatile uses of different magnetic materials that are easy to prepare, with high degree of catalytic efficacies. Catalytic community and material scientists are still busy exploring the arena for yet more precise, economic, and eco-friendly approaches that are likely in the years to come. Conflict of interests: Authors declare that there is no conflict of interest.

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[81] Soleiman-Beigi M, Mohammadi K, Mohammadi F. Magnetic iron oxide nanoparticles/K2S: A simple and scale-up method for the direct synthesis of symmetrical disulfides from aryl halides. J Iran Chem Soc 2018, 15, 1545–1550. [82] Kumar AP, Sudhakara K, Kumar BP, et al. Synthesis of γ-Fe2O3 nanoparticles and catalytic activity of azide-alkyne cycloaddition reactions. Asian J Nanosci Mater 2018, 1, 172–182. [83] Subramaniyan A, Veeraganesh V. Preparation and characterization of SnO–Fe2O3 nanocomposites. Asian J Nanosci Mater 2019, 1, 92–98. [84] Sabet A, Kolvari E, Koukabi N, Fakhraee A, Ramezanpour M, Bahmannia G. Oxidative coupling of aromatic thiols to corresponding disulfides using magnetic particle-supported sulfonic acid catalyst and hydrogen peroxide under mild conditions. J Sulfur Chem 2015, 36, 300–307. [85] Nikoorazm M, Ghorbani-Choghamarani A, Khanmoradi M. Immobilization of a vanadium complex onto functionalized nanoporous MCM-41 and its application as a catalyst for the solvent-free chemoselective oxidation of sulfide to sulfoxide. Appl Organomet Chem 2016, 30, 236–241. [86] Senapati KK, Roy S, Borgohain C, Phukan P. Palladium nanoparticle supported on cobalt ferrite: An efficient magnetically separable catalyst for ligand free Suzuki coupling. J Mol Catal A: Chem 2012, 352, 128–134. [87] Karimi B, Farhangi E. A highly recyclable magnetic core-shell nanoparticle-supported TEMPO catalyst for efficient metal- and halogen-free aerobic oxidation of alcohols in water. Chem–Eur J 2011, 17, 6056–6060. [88] Song Q, Zang ZJ. Shape control and associated magnetic properties of spinel cobalt ferrite nanocrystals. J Am Chem Soc 2004, 126, 6164–6168. [89] Kulkarni AM, Desai UV, Pandit KS, Kulkarnia MA, Wadgaonka PP. Nickel ferrite nanoparticles–hydrogen peroxide: A green catalyst-oxidant combination in chemoselective oxidation of thiols to disulfides and sulfides to sulfoxides. RSC Adv 2014, 4, 36702–36707. [90] Reithwisch DG, Dumesic JA. The effects of metal-oxygen bond strength on properties of oxides: II. Water-gas shift over bulk oxides. Appl Catal 1986, 21, 97–109. [91] Dumitrescu AM, Samoila PM, Nica V, Doroftei F, Iordan AR, Palamaru MN. Study of the chelating/fuel agents influence on NiFe2O4 samples with potential catalytic properties. Powder Technol 2013, 243, 9–17. [92] Ogata Y, Sawaki Y. Oxidations with metal compounds and peroxides. In Organic Synthesis by Oxidation with Metal Compounds. Mijs WJ, Jonge de CRHI, eds. Plenum Press, New York, 1996, 839–876. [93] Nagendrappa G, Chowreddy RR. Organic reactions using clay and clay-supported catalysts: A survey of recent literature. Catal Surv Asia 2021, 25, 231–278. [94] Kumar BS, Dhakshinamoorthy A, Pitchumani K. K10 montmorillonite clays as environmentally benign catalysts for organic reactions. Catal Sci Technol 2014, 4, 2378–2396. [95] Masnabadi N, Ghasemi MH, Beyki MH, Sadeghinia M. Oxidative dimerization of thiols to disulfide using recyclable magnetic nanoparticles. Res Chem Intermed 2017, 43, 1609–1618.

Nahid Ahmadi, Ali Ramazani*

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis 3.1 Introduction In recent years, the reaction catalytic conditions in the synthesis of organic reactions in industrial and experimental processes have been further investigated. The conditions include high yield [3, 7, 8], low temperature [5], eco-friendly solvent or free solvent [9, 10], enantioselectivity [8, 11], and reaction time [3, 11] as well as mechanical and thermal stability, easy separation, and availability of the catalyst. Therefore, researchers have tried to improve the items with the design of a new nanocatalyst. It is common to have two kinds of catalysts – heterogeneous [7–15] and homogeneous [6], which have different features (phase, temperature, selectivity, diffusivity, heat transfer, recycling, active site, modification, separation, and reaction mechanism). These catalysts include organic, inorganic, or mixed, which can create a multifunctional system with new physical and chemical properties in the original components due to formation of a covalent bond, a complex between metal (or metal compounds) and organic material (clusters, nanotube, mesoporous, etc.), and/or polymerization. These compounds are called hybrid catalysts. That is to say, the hybrid nanocatalyst is different from the composite nanocatalyst, although the structure, particle size, and chemical bond of both are the same. In fact, composites exhibit the properties of individual components in operation. Nowadays, catalyst separation of reaction product using magnetic nanoparticles is common in order to address the concern of environment pollution. As per reports, employing iron oxides nanoparticles is more common than other magnetic particles (Ni and Co) because of high magnetic permeability, cost, availability, large specific area, small size, good stability, and easy separation [21–26]. However, you can find hybrid nanocatalysts consist only of Ni [54] or Co [16, 28–32], and/or mix of each of them with iron oxides [28, 31, 32]. Some reports even introduced magnetic nanocatalysts iron oxide combined with nonmagnetic metals like Mn [7], Zn [10], and Cu [8, 9]. The catalysts are designed to function as “core-shell” [10, 11]; the core can be an inorganic material such as metal, metal oxide, and the shell can also be metal oxide and or organic particle. Generally, the core is encapsulated in polymers [10, 12–19], dendrimers, carbon compounds [20–22], ionic liquid [23–27], and metal *Corresponding author: Ali Ramazani, Department of Chemistry, Faculty of Science, University of Zanjan, Zanjan 45371-38791, Iran, e-mail: [email protected]; [email protected] Nahid Ahmadi, School of Mahdiyeh Shahed, Education of Zanjan, Zanjan 45186-17981, Iran https://doi.org/10.1515/9783110730357-003

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oxides in the core-shell systems. The first shell in most catalysts is SiO2 [23, 45, 49], which is coated by organic compounds. It is clear that the surface of magnetic nanoparticles (MNPs) is modified by organic compounds, which increases active sites on the catalyst. The sites may be created by acid or base agents through -COOH and -SO3H or -NH2 and –OH in the related structures. Altogether, they form an organic– inorganic hybrid magnetic nanocatalyst, which is designed to synthesize various methods, such as sol-gel, ultrasonic, electrochemistry, microwave, precipitation, and thermal-based conditions. Here, we attempt to describe the synthesis and functionalization of organic–inorganic hybrid magnetic nanocatalysts. To continue, we will discuss characterization techniques, applications, and reuse of nanocatalysts.

3.2 Synthesis of hybrid magnetite nanocatalysts The fabrication of hybrid magnetite nanocatalysts has been developed in chemistry due to their application. Significantly, iron particles (ferric and ferrous) as the magnetic agent are in the structure of magnetite nanocatalysts. However, there are some in which Nickle [28] and Cobalt [29] have been employed as magnetic agents. The iron that has been used in nanocatalysts is iron oxide MNPs (Fe3O4) that was further prepared by thermal decomposition (Figure 3.1) [4, 9, 10, 30] or co-precipitation method (Figure 3.2) [5, 8, 31–38]. Thermal method About 350 ml of diethylene glycol and ethylene and 15 g of FeCl3·6H2O (1) were added to the reaction container. Thereafter, about 2 g of each of NaAc and polyvinyl pyrrolidone (PVP) with long sodium citrate (Na3Cit) were poured into the mixing reaction. The mixture was under ultrasonic irradiation for 1 h. After that, the solution was stamped and left in the autoclave. The temperature of the autoclave was raised to 200–220 °C, and after about 12 h, the temperature was reduced to 25 °C. The black nanoparticles (2) were gathered by an external magnet. Washing was done with double distilled water and ethanol. The final products were dried at 50 °C for 12 h (Figure 3.1) [4].

1) NaAc, FeCl3. 6H2O

2)PVP,

Fe3O4

3)Na3Cit 1

MNP 2

Figure 3.1: Preparation of Fe3O4 nanoparticle via thermal method.

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

FeCl3.6H2O +

73

1) N2, 2)NH4OH,

Fe3O4

3)80 °C, 30 min

FeCl2.4H2O

MNP 2

3 Figure 3.2: Preparation of Fe3O4 nanoparticle via co-precipitation method.

3.2.1 Surface modifications of the coated silica on magnetic nanocatalyst Catalysts can be loaded onto MNPs support, either by modification of MNPs shells or by co-precipitation during the MNPs synthesis. Therefore, there are a number of magnetic nanocatalysts that iron oxides leave in the center of the catalyst; then, they are coated by silica on their surfaces. Often, for the shell, a coating of iron oxides with silica is used. The sol-gel process makes core-shell structures (Fe3O4@SiO2 (4)). It is known that silica cover prevents aggregation of MNPs and can stabilize functional ligands. Synthesis of Fe3O4@SiO2: The prepared Fe3O4 particles (2 g) were dispersed in a mixture of ethanol (150 ml) and deionized water (about 50 ml), and NH4OH (25 wt. %, 5 ml) was added and stirred at 40 °C for 15 min. In the next step, 5 ml of the diluted tetraethyl orthosilicate (TEOS) in ethanol (20 ml) was poured dropwise and was also stirred for 12 h at room temperature. After collecting the prepared Fe3O4@SiO2nanoparticles by magnetic separation, they were eluted with ethanol and double distilled water, and dried under vacuum at 60 °C (Figure 3.3) [1–3]

1) H2O, EtOH

Fe3O4

2) NH4OH, 40 °C 3) TEOS

2

Fe3O4@SiO2 4

Figure 3.3: Preparation of Fe3O4 nanoparticle coated with silica on their surfaces.

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Some examples with identical substitutes of silica [33, 35, 39], are furnished, depending on the kind of catalyst and its application. Thus, there are two kinds of catalysts, acidic and basic. The acidic catalyst (Fe3O4@SiO2@Ph-SO3H) (5) is synthesized as described in Figure 3.4. After, the synthesis of magnetic nanoparticles, a mixture of 1, 4-bis(triethoxysilyl)benzene (BTEB), and tetramethoxysilane (TMOS) were added to the dispersed solution. The mixture is followed using in a setting of phenyl moieties with chlorosulfonic acid [35]. As regards the result, adding BTEB in the presence of chlorosulfonic acid leads to unstable nanocatalyst during the reaction, so it is better to use the 50:50 ratio of BTEB and TMOS. In addition, the sulfonic acid groups are immobilized on the modified surface of MNPs via chemical bonding. The study of catalyst activity illustrated that the optimized conditions for the condensation of benzaldehydes in a three-component reaction containing 0.2 mol of the catalyst, water as a solvent, and at 25 °C for 25 min can obtain excellent yield.

Figure 3.4: Preparation of Fe3O4@Ph-SO3H nanocatalyst.

In 2018, in a similar work, Maleki et al. [39] used o-phenylendiamine instead (OPDA) of BTEB in previous work. O-phenylendiamine was immobilized on the Fe3O4@SiO2 through the formation of a covalent bond between amine and carbon. In the next step, adding chlorosulfonic acid dropwise to Fe3O4@SiO2@propyltriethoxysilane@ o-phenylendiamine (OPSF) (7) (Fe3O4@SiO2@propyltri ethoxysilane@o-phenylenediamine-SO3H-HCl) (8) is formed as an organometallic nanocatalyst (Figure 3.5). The effect of the catalyst is considered in the synthesis of quinoline derivatives in the threecomponent reaction under ultrasonic irradiation, different solvents, and temperatures. According to the result, yield > 95% was achieved for such reactions under conditions of free-solvent, room temperature, and ultrasonic bath. The acid agent generally is -SO3H or -COOH in the acid catalysts, as seen in the previous work, and will be observed in the next works. In this context, Aghahosseini and coworkers [40] used L-proline for having an acid agent. They firstly afforded O-acryloyl-trans-4-hydroxy-L-proline hydrochloride in the absence of any kind of chromatography or protecting processes of reaction between the acidic acylation and trans-4-hydroxy-L-proline in the presence of TFA, diethyl ether, and at room temperature for 6 h. Then, the obtained compound was added (Fe3O4@SiO2), s amine-functionalized, and led to the formation of OAc-HPro@Fe3O4 through the

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75

Figure 3.5: Preparation of Fe3O4@SiO2@propyltri ethoxysilane@o-phenylendiamine-SO3H-HCl nanocatalyst.

Michael addition. Finally, the prepared catalyst was employed to synthesize imidazole derivatives to investigate the high catalytic activity. Mohammadi and coworkers [41] successfully synthesized the nanocatalyst (12). MNPs were coated with ortho isopropoxystyrene ligands by covalent bonds. The modified magnetic nanoparticles (Fe3O4@SiO2) with guanidine have worked with the polyacrylic acid (11), which is grafted on the Fe3O4@SiO2-guanidine (10) by a noncovalent bond (Figure 3.6). From the obtained structure, it is clear the carboxylic group has an electrostatic bond with two amine groups. The number of catalysts is evaluated to affect the yield, as an increasing amount of catalyst increases yield, but the increase is followed to 70 mg. A value higher than 50 mg could not enhance the yield.

Figure 3.6: Preparation of Fe3O4@SiO2-poly acrylic acid.

The Fe@Si-Gu-Prs(14) was synthesized by the brief route outlined in Figure 3.7. Fe3O4 coated-silica was modified with 3-chloroopropyl-trimethoxysilane and guanidine hydrochloride to functionalized silica Fe3O4 nanoparticles (Fe@Si-Gu)(13). Ultimately, the modified silica MNPs were immobilized by noncovalent interaction of the Preyssler heteropolyacid, resulting in the formation of the nanomagnetic organic–inorganic hybrid catalyst (Fe@Si-Gu-Prs) [43]. One of the most successful acid catalysts is prepared from magnetic nanoparticles and Preyssler heteropolyacid.

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1) CPTS

2) 4

NH

H2N

3) NaHCO3

NH2

O O Si O

4) Reflux, 28h

NH2

N

NH2

13

9

1) H14[NaP5W30O110]

2) MtOH, 50 0C, 12 h

NH3 H12[NaP5W30O110]2-

O O Si O

N

NH3

14 Figure 3.7: Preparation of Fe@Si-Gu-Prs.

Rostamizadeh et al. [26] introduced the novel magnetic nanocatalyst [(γ-Fe2O3)@MCM-41] in which the mesochannels of mesoporous material have been bound to l-prolinium nitrate ionic liquid. These catalysts act as a Brønsted acid and also as an oxidant. In addition, the catalyst showed excellent activity in the direct synthesis of quinazolin-4(3H)-one derivative from isatoic anhydride, primary amines, and an aldehyde or an aryl halide, at 100 °C, with a high yield of the products within short reaction times. They synthesized quinazoline-4(3H)-one derivative with prepared nanocatalyst, in the absence of any other extra oxidant for the first time. Also, there are magnetic nanocatalysts that are combined with organic compounds – organic moiety contains urea or Choline Cl-urea,Fe3O4@SiO2-urea(15)and Fe3O4@IPS-DES(17), respectively. For preparing such catalyst, Maleki et al. [2] used Fe3O4@SiO2-C l(6) and developed a simple strategy to change the surface of the catalyst (Figure 3.8). In a similar work, Tavakol and coworkers [42] also used Fe3O4@SiO2Cl and introduced a new magnetic nanocatalyst with Choline Cl–urea deep eutectic solvents (DES).1 The image of TEM displayed a particle size of about 20–100 nm for the prepared catalyst, and also the FT-IR result confirmed setting iodopropyltrimethoxysilane (IPS) and DES on the surface of Fe3O4@SiO2. In addition, compared to the two works indicated in the first work, urea directly has reacted with organosilicon and formed a covalent bond. However, in the second work, quaternary ammonium salts

1 Deep eutectic solvents: A eutectic mixture of Lewis or Brønsted acids and bases, which can possess a spread of anionic or cationic particles, is fashioned into a category of systems that are called Deep Eutectic Solvents (DES). They are classified as sorts of ionic solvents with special features, which are obtained by mixing one or more compounds with a melting point much less than either of the original elements. A mixture of choline chloride and urea within the quantitative relation of 1:2 mole makes one of the important deep eutectic phenomena. The ensuing mixture has a melting point of 12 °C (choline chloride, 302 °C, and urea, 133 °C have melting points that are higher than that of the obtained mixture) that makes it liquid at room temperature.

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

77

(choline chloride) were attached to urea with a hydrogen bond. In fact, as observed in Figure 3.9, initially, iodopropyltrimethoxysilane loaded on the surface of magnetic nanoparticle via a covalent bond; after that, the hydroxyl groups of DES attack carbon involving iodide. Since iodide is a better leaver, it leaves the compound. Finally, the covalent bond formed between EDS and propyltrimethoxysilane. On the other hand, the catalyst is employed in the three-component reactions just like in the work carried out by Elhamifar group. Based on the result, both catalysts with acid properties show the same catalytic activity in similar conditions (room temperature, solvent (water), and amount of catalyst (0.1 mmol)), except reaction time.

Figure 3.8: Preparation of Fe3O4@SiO2-urea nanoparticles.

O O Si O

O O Si O

NaI, 80 °C, Acetone

Cl

I

16

6

Cl

ChCl. Urea 18h, 90 0C

O O Si O

O

O

N H2N

NH2

17

Figure 3.9: Preparation of Fe3O4@IPS-DES.

There have been numerous studies of the modified surface of Fe3O4@SiO2 nanoparticles. These syntheses are of interest not only because they are efficient and flexible, but also because they are recoverable catalysts. Hence, two groups of chemists have functionalized magnetic iron ferrite (γ − Fe2O3) using 1-(3,5-bis(trifluoromethyl)phenyl)3-(3-(trimethoxysilyl)propyl)thiourea. The first group developed the catalyst as observed in Figure 3.10. The catalyst was prepared in two steps – in the first step, organosilane (20) is prepared, and in the second step, N-(3,5-bis trifluoromethylphenyl) isothiocyanate reacted with trimethylsilyloxypropyl amine via addition reaction and produced 1-(3,5-bis(trifluoromethyl)phenyl)-3-(3-(trimethoxysilyl) propyl)thiourea. Then, the synthesized thiourea is added to magnetic hydroxyl apatite (19). In this step, the hydroxyl groups of hydroxyapatite are substituted with the methanol

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groups on organosilane. Finally, Ag nanoparticles embedded on the surface of [γ-Fe2O3@Hap-Si-(S)] [43]. The second team [44], just followed the step producing [γ-Fe2O3@Hap-Si-(S)] using the same method. Although they are used in the different syntheses (oxidation of primary amines and selective reduction of nitro compounds and synthesis of β-ketonitriles, respectively) they have excellent activity. Both of them were employed and extracted by an external magnet in 10 successful runs.

Figure 3.10: Preparation of [γ-Fe2O3@HAp-Si-(S)-Ag].

The hybrid magnetic systems can introduce many new catalysts supported by silica and iron oxide due to the presence of various organic compounds. Herein, a new hybrid magnetic with cyclodextrin will be introduced. Cyclodextrins (CDs) are natural products that are extracted from starch by the enzyme cyclodextrinase; their structure contains an interior cavity thatis made of cyclic oligosaccharides. For this reason, CDs are more attractive. Therefore, they are appropriate for use in drug systems, chemical industrials, and catalysis. The catalyst of Fe3O4@SiO2-PGMACD (28) was synthesized by Kang and coworkers, according to the common methods. In this study, the sol-gel process was utilized for coating of MNPs with TEOS. Then, Fe3O4@SiO2is functionalized with amine and Br groups. The next step, polymerization of the poly(glycidyl methacrylate) (PGMA) (25) was performed through atom transfer radical on the surfaces of MNPs (24) and Fe3O4@SiO2-PGMA (26) (Figure 3.11) [45] obtained. On the other hand, the surface of CDs was modified by ethylene diamine, and the prepared media (27) were attached to the compound (26) by the ring-opening reaction of epoxy groups with amino groups via a covalent bond, and Fe3O4@SiO2-PGMACD(28) was synthesized eventually. The resulting production of benzaldehyde illustrated high activity of the catalyst (28) compared to Fe3O4@SiO2-PGMA and easy recovery to CD. In this case, the authors predicted benzyl alcohol (starting material) reacted polymers on the surface of Fe3O4@SiO2-PGMA and created hydrogen bond; in the result the yield reduced < 40%. Additionally, they reported that the hybrid catalyst (28) acts as the

79

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

substrate-selective in the oxidation of alcohols, because 1-octanol was oxidized by Fe3O4@SiO2-PGMACD.

23 O MeO MeO

Si

NH2

Br

O O Si MeO

MeO Toluene, stirring, 24h, 40°C

NH2

Br

Toluene, stirring, 4h

22

4 O O O O Si MeO

Br

O

O

25

O

CuBr, 50°C, stirring, 4h

*

24

O

n

26 H N

NH2

27

O O

DMF, 60°C, stirring,24h

*

OH

N H

H N

n

28 Fe3O4@SiO2-PGMACD Figure 3.11: Preparation of β-CD-modified MNPs (Fe3O4@SiO2-PGMACD).

In recent works, in order to apply heterogeneous nanocatalysts in organic synthesis, a magnetic dichromate hybrid with triphenylphosphine surface-modified SPIONs (Super Paramagnetic Iron Oxide Nanoparticles), Fe3O4@SiO2@PPh3@Cr2O72‐ as a recoverable and efficient nanocatalyst was synthesized and used for the synthesis of polyhydroquinolines by Maleki et al. Co‐precipitation method was applied for the synthesis of SPIONs, and a modified Stober method was used for preparing of Fe3O4@SiO2. After silica-coating, SPIONs were functionalized with triphenylphosphine. Finally, the dichromate anion was immobilized on the phosphonium groups to give the functionalized MNPs [46]. They also synthesized a similar catalyst with a difference, in which bromochromate was substituted for the dichromate anion on triphenylphosphine-functionalized Fe3O4@SiO2nanoparticles. So, a relatively strong chemical interaction occurs between the bromochromate anion and the phosphonium groups, such as early catalysts (Figure 3.12) [1].

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O O Si O

PPh3, Toluene

O O Si O

Cl 6

[CrO3Br]-

PPh3Cl 29

O O Si O

PPh3[CrO3Br] 30

Figure 3.12: Preparation of Fe3O4@SiO2@PPh3@ [CrO3Br].

Rossi et al. [47] and Yi et al. [48] synthesized MNPs individually to get a complex with Pd nanoparticles. The surface of Fe3O4@SiO2 NPs has been functionalized by organosilane, which is able to form a chelate with Pd particles (Figure 3.13). These functional groups influenced by the catalytic features of metal NPs are not well clarified yet, but in the fact, changes in selectivity, activity, metal loading, and morphology of NPs prepared saturation salts are reported in the literature. As Yi’s team investigated catalytic activity with all of X were functionalized for hydrogenation of nitrobenzene. In this case, the highest conversion speed was for γ − Fe2O3@SiO2-NH2-Pd. Rossi et al. also prepared MNPs that were functionalized by ethylene diamine groups. They show that there are about 1.7% Pd particles submitted on the surface of the ligand-modified γ-Fe2O3@SiO2 [49].

Figure 3.13: Magnetically recoverable Pd NPs immobilized on the surface of silica-coated iron oxide NPs functionalized with organosilanes.

An interesting strategy was developed by applying heterogeneous catalysts using a mesoporous silica (mSiO2) shell with perpendicularly aligned channels. The synthesis of a novel hybrid Fe3O4@mSiO2@Cu4 (33) nanocatalyst was followed to load an active Cu4 unit onto Fe3O4@mSiO2(32), in an oxidative C-H functionalization (Figure 3.14) [11]. As the result of the VSM display, the prepared catalyst will be quickly separated from reaction products with an external magnet and agglomerated easily. The activity of the catalyst was measured in the oxidation of cycloalkanes and cycloalcohols. As the result, the yield was lower than 20% for all of them. As a compound of constant research to synthesize organic–inorganic hybrid catalytic systems for different organic conversations, Sharma’s team has employed

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Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

Figure 3.14: Preparation of the Fe3O4@mSiO2@Cu4 nanocatalyst.

copper-based magnetically nanocatalytic systems. They also found that the copper compounds are an impressive catalyst for N-alkylation of amines. That is why they used (Cu-AcTp@Am-SiFe3O4)(35) for the preparation of diverse industrially significant amines. In order to generate such catalyst, Fe3O4 was encapsulated by 3-aminopropyltriethoxysilane (APTES),in the next step, the obtained nanoparticles were functionalized with acetylthiophene (AcTp), and metallized with copper acetate to achieve(Cu-AcTp@Am-Si-Fe3O4)(35) (Figure 3.15) [50]. N-alkylation of amines was performed in the presence of the synthesized catalyst; The highest yield was 98% while Cu@Am-Si-Fe3O4 was (59%), however both of them were prepared in the same conditions. O S (OH)Si(CH ) ) NH

O O Si O

NH2 EtOH, Reflux

O O Si O

22

4

Stirring

OAc Cu

N

S

34

SH Cu(OAc)

N

N Cu

AcO

SH

35 CuAcTp@Am-Si-Fe O

Figure 3.15: Preparation of the CuAcTp@Am-Si-Fe3O4 core-shell nanocatalyst.

Dutta and coworkers prepared a similar catalyst, with a small difference. In this case, the ligand-grafting process was carried out by Am-SiO2@Fe3O4 (22) nanoparticles with the ligand 2,2′-bipyridyl ketone (BPy)(36) and was covalently immobilized on the shell

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of the functionalized nanoparticles [34]. As shown in Figure 3.16, Cu(OAc)2 completely got chelated nitrogen of 2,2′-bipyridyl in magnetic nanocatalyst. Hence, the most common reaction occurs between copper and a chelating element like N, O, S, etc., as you can find in the next examples.

Figure 3.16: Preparation of Cu-BPy@Am-SiO2@Fe3O4 Core-Shell Nanocatalyst.

Sharma et al. [51] employed 2-acetylbenzofuran (ABF) (39) as a ligand, which was the attached amino-functional group on the surface of Fe3O4@SiO2magnetic nanoparticles with covalent bond via Schiff base reaction to achieve ABF-graftedAmino-Fe3O4@SiO2(ABF@Amino-Fe3O4@SiO2) (40). In the long run, the obtained nanoparticles were metalized with copper (II) acetate to produce the copper-based magnetic nanocatalyst, (Cu-ABF@Amino-Fe3O4@SiO2) (41) (Figure 3.17). The result of FT-IR, the peaks appearing in 1,623, 1,655, and 1,647 cm−1 confirmed functionalization of amino group, to form imine group (C = N), and to set copper particles on the catalyst. The catalysis activity of (41) was also considered, using cycloaddition reactions. Therefore, parameters contain organic bases, solvents, and the number of catalysts was optimized. It is necessary to explain that cycloaddition reactions could not be performed without either base compound or catalyst. Organic bases act as CO2 activators, which help to develop the reaction. In spite of the assistance of these bases including 1, 8Diazabicyclo (5.4.0) undec-7-ene) (DBU), PPh3 (Triphenylphosphine), [4-(dimethylamino) pyridine], (Triethylamine) and (1,5,7-triazabicyclo [4.4.0] dec-5-ene) that could activate CO2 at atmospheric pressure, DBU is the best organic base for the foregoing synthesis. In the presence of DBU, (41) (50 mg) and solvent-free obtained the yield > 90%. The researchers are still attempting to search for and develop a good deal of new magnetite nanocatalysts. Hence, they have utilized other metals with or without silica for supporting iron oxides. In this way, Mohammadizadeh and coworkers [38] introduced a new nanomagnetic catalyst [Fe3O4@TiO2-Pr-2AB@Cu](46) as a hybrid nanostructure that gets copper II acetate to form a chelate with amino-amido

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

83

Figure 3.17: Preparation of Cu-ABF@Amino- Fe3O4@SiO2 nanocatalysts.

groups. The result of the chelation is that the donor atom (N) becomes effectively more electronegative, and Cu ions act as Lewis acid. On the other hand, it is noted in this work that magnetic nanoparticles coated TiO2 instead of SiO2 (Figure 3.18). O

44

O

EtOH, MeCN, r.t

2

N H

O O Si O

APTES

TEOT

NH2

EtOH

43

Fe3O4@ TiO2 42

OAc AcO

H2N

O O Si O

HN

Cu(OAc) EtOH, r.t

O 45

O

Cu H2N

O O Si O

HN O 46

Figure 3.18: Preparation of hybrid nanomaterial (nano-Fe3O4@TiO2-Pr-2AB@Cu).

Cyclodextrins (CDs) are cyclic oligosaccharides that include six (α‐CD), seven (β‐CD) or eight (γ‐CD) D‐glucopyranose units. They have the shape of a tapered cylinder with hydrophobic inner and hydrophilic exterior holes, which are more attractive because of their low cost, availability, and unique supramolecular properties. Nowadays, CDs are applied for the preparation of various homogeneous–heterogeneous catalysts. According to the prospect, new magnetic hybrid nanoparticles were prepared, as depicted in Figure 3.19. The Stober method was used for coating the surface of MnFe2O4 (47) with tetraethylorthosilicate (TEOS). The obtained MnFe2O4@SiO2 nanoparticles were functionalized by (3‐aminopropyl) triethoxysilane (APTS). Then, MnFe2O4@SiO2‐Pr‐NH2 (49) was reacted with the prepared Ts‐β‐CD@Cu(OAc)2 complex

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Nahid Ahmadi, Ali Ramazani

to produce the final hybrid MNPs@β‐CD@Cu(OAc)2 (50) nanomaterial. Based on characterization techniques, the prepared structures are identical, and the covalent binding of copper in β‐CD cavity is confirmed [8].

Figure 3.19: Preparation of MnFe2O4@SiO2@β‐CD@Cu (OAc) 2 nanoparticles.

Aghahosseini and coworkers [5] synthesized an artificial metalloenzyme as a hybrid magnetic nanocatalyst, using a complex with Au nanoparticles. In this way, they firstly prepared Au (III) polypyridyl coordination complex [Au (1, 7-Phen) Cl3] (51) via a simple process. First, 1, 7-phenanthroline ligand reacts with HAuCl4.3H2O in the presence of acetonitrile and KOH. Next, Fe3O4@SiO2-Cl was added to the prepared complex, and finally, Fe3O4@SiO2-[Au(1,7-Phen) Cl3] (52) was obtained, as observed in Figure 3.20. Accordingly, as the complex crystallizes in the monoclinic crystal system, Au (III) cation coordinated with one N atom from one 1, 7-phenanthroline ligand and three chloride anions in a square-planar configuration. On the other hand, there is a weak interaction between C–C and intermolecular C–H . . . Cl hydrogen bond in this structure, which can be effective in the stabilization and the foundation of the three-dimensional supramolecular complex.

Figure 3.20: Preparation of Au@MNP (Fe3O4@SiO2-[Au(1,7-Phen)Cl3]) as an artificial metalloenzyme.

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

85

For several years, compounds containing triethoxysilane moiety have been applied as a linker to attach the organic compounds to the mineral supports. In fact, silica-coating prevents MNPs from aggregating. In an attractive work, for the first time, Karimi and coworkers [52] used this reagent because of both the cationic part and linker for the syntheses of γ-Fe2O3@SiO2@[bis-APTES]Cl2-NPs by stabilizing of triethoxysilane on the γ-Fe2O3@SiO2-NPs surface. They introduced ionic liquid with a bis-dicationic acidic and an aliphatic spacer on γ-Fe2O3@SiO2-NPs for the impressive promotion of the synthesis of pyrimidine derivatives. The polyhedral oligomeric silsesquioxane (POSS) has a crystal structure with a particle size of about 0.45 nm with eight branches located in a cubic environment and can be linked by varying the organic pendant arms. Additionally, these nanoparticles are interesting because of their easy synthesis, chemical modification, and cageshaped structures. Safaei et al. [53] have functionalized Fe3O4 using glutathione and dopamine followed by complexation with various metals, and demonstrated that their application in organic synthesis supported inorganic–organic hybrids based on POSS will have a high surface and internal organic content, which may be useful as catalyst (Figure 3.21). Organic parts play a key role in the structure of nanocatalysts and impacts their properties. So, the good organic selection is important for immobilizing the MNPs. Ionic liquids are a good example of industrial chemicals that consist of an organic cation, such as alkyl imidazolium, alkyl pyridinium, alkyl phosphonium, or alkyl ammonium, and an organic or inorganic anion, such as tosylate, tetrafluoroborate, bis (trifluoromethylsulfonyl)imide, or hexafluorophosphate. A covalent bond, polymerizations, encapsulations, or sol-gel condensations are generally methods that these compounds can follow in the immobilization process. Besides, the coating of 1-butyl3-methylimidazolium hexafluorophosphate (BMIm-PF6) within silica was recently developed in Omar group. The BMIm-PF6@SiO2 microreactors were asserted to be a useful host for catalytic palladium NPs. Ionic liquids are known to provide a precious medium for catalysis mediated by metal nanoparticles, as they can dramatically increase their selectivity, activity, and stability. This magnetic nanocatalyst is synthesized by sol-gel process and the organosilane-based ionic liquid is copolymerized with tetraethoxysilane on MNPs. Then, palladium NPs were captured and stabilized by nitrogen of bridges of ionic liquid (Figure 3.22) [54]. (MNPs@SiO2-IL)(61) [24] was prepared by the same previous work following the procedure shown in Figure 3.23. This work has not utilized palladium nanoparticles for more chelation. Sajjadifar and coworkers, in a similar work, prepared the same catalyst and employed it for the synthesis of pyrano[2,3-d]pyrimidine derivatives [27]. Also, Fe3O4@SiO2-DABCO was synthesized by the sol-gel process. Organosilane, including diazoniabicyclo[2.2.2] octane dichloride groups as a bridge double-charged, [(MeO)3Si(CH2)3N+(CH2CH2)3N+(CH2)3 Si(OME)3]Cl2 (62), was easily synthesized and employed as a precursor reagent to achieve core-shell compound, using Fe3O4 as the core and the mentioned organosilane as the shell. That is why the surface of the

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Figure 3.21: Preparation of Fe3O4@APTPOSS.

synthesized MNPs was successfully designed by organic–inorganic hybrid silica with the ionic liquid framework and the alkoxysilanes groups consisting of ammonia of organosilane precursor and tetraethylorthosilicate. The organic parts of catalysts are often immobilized on the MNPs because of the availability of reactants to the active sites of the nanocomplex. The steps of Fe3O4@SiO2-DABCO(63) preparation are exhibited in Figure 3.24 [33]. For the engineering of advanced catalytic systems, it is required to use suitable functionalizing agents and linker, depending on the surface of MNPs. Such functionalized MNPs not only exhibit thermal stability and superior chemical but also provide the field for presenting additional functionalities, such as metal complexes and organic ligands. Reports propose that the surface modification of MNPs can be performed by two likely routes: noncovalent adsorption of surfactants, polymers, and bifunctional molecules have been attracted as a nanocovalent bond, and hydroxyl groups on the surface of nanoparticles and the make-fast agents create a covalent bond for the formation of a sort of constant ligand. Many ligands have been used successfully for functionalizing Fe3O4@SiO2@L-proline (LPSF) (65) (Figure 3.25)

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Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

Cl

O TEOS, Bridge IL

N

Si

O

O O O

Si

N

O 2

MNP@IL-SiO2 57

O

N

Si

O O Na2PdCl4

O Si O O

N

PdCl4 2-

ion exchange

O

O O

Si

O

N

Si

N

O O

58

O O O

N

Si

O Si O O

N

NaBH4

Pd O

O O O

N

Si

Si

N

O O

MNP@IL-SiO2-Pd(nano) 59

Figure 3.22: Preparation of MNP@IL-SiO2-Pd (nano).

Cl

Cl

EtO +

EtO EtO

4

N

Si 60

O N

NaH, THF

O O

N

Si 61

MNPs@SiO2-IL

Figure 3.23: Preparation of 1-methyl-3-(3-trimethoxysilylpropyl)-1H-imidazol-3-ium chloride-supported nano-Fe3O4@SiO2.

N

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Nahid Ahmadi, Ali Ramazani

MeO Si MeO MeO

N

OMe + OMe Si OMe

N

Sol-Gel, 80°C

2Cl-

TEOS, EtOH

62

Fe3O4@SiO2-DABCO 63 O Si O O

N

O O Si O

N 2Cl64

Figure 3.24: Preparation of Fe3O4@SiO2-DABCO.

[55], sporopollenin-1-(2-hydroxyethyl) piperazine (Sp-HEP) (66) (Figure 3.26) [56], N-[3(triethoxysilyl) propyl]isonicotinamide (TPI)(71) (Figure 3.27) [57], 4-aminoquinaldine (73) (Figure 3.28) [58] and frequently employed in catalytic applications.

APTES

O

Toulene

O O

Si

NH2 22

4

O H N

O O

NH

N O

EtOH

O O O

Si

N H

O

65 Figure 3.25: Preparation of (Fe3O4@SiO2@L-proline) LPSF magnetic nanocatalyst.

Zhou et al. synthesized Fe3O4@SiO2-HPG-NCs (77) and found various metal ions, for example, Pt2+, Au3+, and Pd2+can divide into the hyper branched polyglycerol (HPG); due to their high density concluded from hyper branched polyglycerol on the surface of magnetic hybrids, they can form a strong complex with carboxylic groups. In this case, HPG, including numerous carboxylic groups, are in its structure. Following the HPG, metal ion complexes were reduced by NaBH4, where zerovalent nanocrystals

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Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

+

Sporopollenin

OMe OMe Si OMe

O

Dry, Reflux, 72h

Si

O

C

C

O

N

N 68

67

66 NH

HN N

Stirring,15 h

N OH

O O Si O

C 69

O

Ma

gn e ti

NH

za

tio n

NH N O O Si O

C

70

O

NH

Figure 3.26: Preparation of 1-(2-hydroxyethyl)piperazine-functionalized magnetic sporopollenin (MNPs@Sp-HEP).

Figure 3.27: Preparation of Fe3O4@HPA-TPI nanocatalyst.

Figure 3.28: Preparation of novel basic hybrid nanomaterial (Fe3O4@SiO2-AQ).

were generated and sterically snared. Lastly, the aforesaid magnetic nanocatalysts of Fe3O4@SiO2-HPG-Pt-Au-Pd were obtained (Figure 3.29). The studying of such catalysts illustrated the concentration of metal ions in solution, before and after the adsorption process, and determined that the contents of Pt, Au, and Pd on the surface of MNPs are approximately 0.296, 0.243, and 0.268 mmol/g,

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respectively. These contents are much more than that of (Fe3O4@SiO2-NH2) reported for Pd, 0.075 mmol/g and for Pt, 0.053 mmol/g, and even more than the PAMAM dendrimer, which grafted Fe3O4@SiO2 (for Pd, 0.246 mmol/g). These values are attributed to the atypical branches of HP and the functional support effect of dendrimers. Comparing the structures of the prepared nanocatalyst display ions of Pt and Pd get to form a complex with two carboxylic groups, but Au ions need three groups of that [59].They found that Fe3O4@SiO2-HPG-Au is selective for the aerobic oxidation of alcohols treated in an atmosphere of air at room temperature, and Fe3O4@SiO2-HPG-Pd is selective for Heck coupling reactions. Ramazani and coworkers synthesized [email protected] (81) (Figure 3.30). Various noble metal nanocatalysts, for example, Cu, Co, and Pd, directly flourished on the surface of Fe3O4@SiO2 with high dispersion and loading capacity. They demonstrated that their MNP-supported catalyst was highly active in the oxidation of alcohols to their corresponding carbonyl compounds, using oxone at room temperature in water [60]. Tarasi et al. [19] developed a methodology for the immobilization of Mn on core-shell magnetic nanoparticles. The prepared catalyst was active in the oxidation of cyclohexene, ethyl benzene, and toluene in the presence of H2O2. Therefore, the works of Ramazani and Tarasi groups show this class of hybrid magnetic nanocatalysts has high efficiency in oxidation reaction, without depending on the kind of metal. Copper ferrite-coated chitosan has been introduced as a magnetic organic–inorganic hybrid nanocatalyst for the synthesis of organic compounds in multicomponent systems by Maleki et al. The catalytic activity was tested in the synthesis of α-aminonitrile derivatives by using aromatic aldehydes, aniline, and trimethylsilyl cyanides in ethanol at room temperature; the high yields confirmed the selectivity and efficiency of the catalyst for such syntheses [61]. Boehmite NPs is an aluminum oxide hydroxide (γ-AlO(OH)), which contains aluminum ions, was furnished with ions of oxide and hydroxide as octahedral double layers in the shape of an orthorhombic structure. It could react with metal ions and organic ligands because of hydroxyl groups arranged on its surface. In addition, the special features such as nontoxicity, availability, the thermal and mechanical stability of boehmite NPs make it a great support in heterogeneous catalysis. With regard to such properties of boehmite, Mohammadizadeh et al. designed to synthesize MNPsbased boehmite with excellent magnetic properties and many tiny particle sizes. To continue the design process, MNPs were coated with boehmite and functionalized with (3-choloropropyl) triethoxysilane. In the next step, triethylene tetramine reacts with Fe3O4@Boehmite as a ligand containing amine groups that are suitable for chelating. The obtained Fe3O4@Boehmite-NH2 (86),in the presence of an ethanolic solution of CoCl2.6H2O, reacts to give Fe3O4@Boehmite-NH2-CoII NPs(87). It can also be appropriate for catalyzing the Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions. In Figure 3.31, the preparation strategy of the Co(II) complexing on aminated Fe3O4@Boehmite nanoparticles [62] is shown.

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Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

OH

OH

O

HO

O

HO

O

OH

O OH HO CH3OK, 95°C

OH

O

HO

O O

O

Fe3O4@SiO2

OH

O

75

OH

4

Fe3O4@SiO2@HPG

COOH

COOH O

O

O HOOC

HOOC

O

O

COOH

O

O

O

HOOC HOOC

O HOOC

O

O

COOH O COOH

76 Fe3O4@SiO2@HPG-COOH

O O C C HO

OH

HO

O C

O

O

HO

O

OH O

HO

C

O

C

Metal ions 2) NaBH4

O

C

O

C

OH

O

O

O

O OH

O

O HO

=Metal nanocrystal

C O O

77

Fe3O4@SiO2@HPG-NCs

Figure 3.29: Preparation of Fe3O4@SiO2@HPG-NCs.

C

C

O OH

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Nahid Ahmadi, Ali Ramazani

H2N

NH O O

O Si O

+

H N

+ *

N N

100°C, 24h,

O

Reflux

HPEI

EPO

2

*

79

NH

78

NH2

MCl2 M = Cu,Co,Pd 81 [email protected]

Fe3O4@HPEI 80

H2N

NH O O Si O

O N H

H N

N

*

N n

OH NH NH2

Figure 3.30: Preparation of Fe3O4@HPEI, and Pd, Co, and Cu get chelation with the nitrogen ligands in Fe3O4@HPEI to [email protected].

Faisal and coworkers have also functionalized magnetic Fe3O4@boehmite using the reaction with 3-(chloropropyl)triethoxysilane, in the presence of toluene and refluxed to examine nickel-catalyzed Suzuki reaction using dichloro(dimethoxyethane) nickel, potassium bis(trimethylsilyl)amide, and prolinol in isopropanol, to furnish terpyridine-Pr-Fe3O4@boehmite nanoparticles (89) (Figure 3.32). They have also studied Suzuki reaction in other conditions, but the reaction did not perform for alkyl chloride of n-PrCl-Fe3O4@boehmite nanoparticles, which have inactive nature [63].

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

93

Figure 3.31: Preparation of Fe3O4@Boehmite-NH2-CoII NPs.

Maleki group was introduced to the synthesis of Fe3O4 NPs-coated vinylsilane as shown in Figure 3.33. Under normal conditions of hybrid magnetic nanocatalyst preparing, trimethoxy (vinyl) silane reacted with Fe3O4@SiO2 NPs to fabricate a vinyl layer on the MNPs. In this method, after dissolving the mixture of FeCl3 and FeCl2 salts in the deionized water, the pH of the mixture rose to about 12 by adding

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OH

HO OH HO

OH

OH

HO

OH OH

HO

OH

1)FeCl3.6H2O , FeSO4.7H2O

HO OH OH OH

HO 82

2) NaOH, Ar

OH

HO

83 Fe3O4@Boehmite

HO Boehmite NPs O

Toluene, Reflux, 30h

O

Si

O

OEt

Cl 84

EtO Si

Cl

Fe3O4@Boehmite -nPrCl

OEt

N

NiCl2,KHMDS, i-propanol 12% prolinol, 60°C, 30h

O O

N

Si

O HO

B

OH

N 89

Fe3O4@Boehmite -Pr -Terpyridine

N N

N 88

Figure 3.32: Preparation of terpyridine-Pr-Fe3O4@boehmite nanoparticles.

ammonia, and MNPs are generated. For encapsulation of MNPs with silica, tetraethyl orthosilicate (TEOS) solution was applied. After that, trimethoxy (vinyl) silane was immobilized on the surface of Fe3O4@SiO2 NPs via a covalent bond. A vinyl layer was formed on the MNPs coated silica as a second shell. The catalyst was used in organic syntheses as heterogeneous dienophile sites [46]. Arpanahi and coworkers developed a new catalyst by combining iron and phosphorus ligands, Fe-diethylenediamine Penta (methylene phosphonic acid) (DTPMP). The obtained catalyst has an average particle size of about 14–20 nm. Fe-DTPMP

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

Trimethoxy vinyl silane

95

O O

4

Si O 90

Figure 3.33: Preparation of Fe3O4@SiO2@vinyl NPs.

catalyst was tested to synthesize derivatives of three-component imidazole, which obtained a high yield of compounds in the short reaction time [13]. Babaei and coworkers also synthesized HAp-encapsulated-α-Fe2O3-based Cu (II) (96) (Figure 3.34). They subjected the prepared catalyst to generating multicomponent cyclo addition reaction of alkynes. Compared to other copper catalysts, the catalyst (96) was efficient in the same conditions, with a short reaction time and room temperature. The catalyst separation of the reaction mixture and the product isolation without the need for time-consuming purification procedures is possible due to the magnetic temper of the catalyst [31].

Figure 3.34: Preparation of α-Fe2O3@Hap-Cu (II).

In continuation of the research works, amplification of new methodologies for the preparation of magnetic nanoparticles was done. Thus, the Fe3O4nanoparticles were synthesized by a precipitation method just like the previous works, using ferric and ferrous ions. Then, a layer of hydroxyapatite was coated on the surface of the Fe3O4 nanoparticles; and the calcinations of the mixture was performed at 450 °C. α-Fe2O3@HAp (93) was reacted with 3-aminopropyltrimethoxysilane to synthesize an

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Nahid Ahmadi, Ali Ramazani

organic–inorganic hybrid. After that, the prepared hybrid got a reaction with 2-hydroxybenzaldehyde and obtained a bidentate ligand, which was ready to get a complex with metals. Surface-bound ligands got copper (ΙΙ) to form a complex using CuCl2 (Figure 3.35) [64].

Figure 3.35: Preparation of ([γ-Fe2O3@HAp Si (CH2)3 AMP-Cu]).

It is reported in the methodology discussion concerning organic linkers or functionalizing groups that, generally, linker and functionalization occur only on the surface of magnetic nanoparticles that have special structure features. But there is some methodology that just changes metal to a complex without handling MNPs, organic linkers, and functionalizing groups. On the other hand, the nano-γ-Fe2O3 has a high activity for a high space of the particles, which can be suitable for substrate activation, because of the coordinated sites of unsaturated iron and vacancies. Accordingly, Barazandehdoust and coworkers decided to design a nanocatalyst by supporting Pd nanoparticles on dopamine-modified nanoferrites. HAp-encapsulated-γ-Fe2O3based Pd (II) (101) as an organic–inorganic hybrid nanocatalyst was synthesized (Figure 3.36) and employed in the Heck cross-coupling reaction, in the presence of air and moisture [65]. Magnetic hydroxyapatite-encapsulated supporting(2-aminomethyl) phenol and then, ([γ-Fe2O3@HAp Si (CH2)3 AMP]) (100) were prepared, as shown in Figure 3.37. The one-pot synthesis of carbonitrile derivatives requires using tiny amounts of nontoxic and environment-friendly [γ-Fe2O3@HAp Si (CH2)3 AMP] as synthesized catalyst. Khoobi and coworkers designed and synthesized all synthesis steps, as reported in the literature. Their group used the prepared catalyst without metal complexion and also indicated the hybrid nanocatalyst attached to carbonyl group of aldehydes and aromatic organic compounds via amino agent and leads to the formation of imine. Then, the ring forming occurs, while the catalyst has taken part in

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

97

Figure 3.36: Preparation of ([γ-Fe2O3@HAp Si (CH2)3 AMP-Pd]).

the reaction via covalent bond; however, it presents with no structural change and loss of activity in reaction. Hence, it would be deduced that this green and impressible catalyst, with a common experimental and work-up system, which needs to use the small volumes of perilous organic solvents, makes it a useful replacement to the previous methodologies for the growth of three-component reactions [66].

Figure 3.37: Preparation of ([ɣ-Fe2O3@HAp Si (CH2)3 AMP]).

3.2.2 Iron and cobalt magnetic nanoparticles Magnetic nanoparticles are frequently employed as cores in the synthesis of heterogeneous catalysts. Iron oxides are a common and popular class of magnetic nanoparticles, with the general formula of MFe2O4, where M can be Fe or a noble metal (M: Mn, Co, Fe, and Zn) in well-known bimetal ferrite oxides. Compared to Fe3O4 MNPs, they have adjustable particle size. Amongst these ferrites, cobalt ferrite exhibits interesting properties, such as high magnetic capability, and superior chemical, mechanical, and thermal stability [32, 67, 68]. Cobalt ferrites are found to have potential applications in various fields; a support for catalyst immobilization is one of them, because of

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Nahid Ahmadi, Ali Ramazani

easily modified surfaces [22]. Hence, the synthesis of CoFe2O4is developed using a solvothermal synthetic strategy. Yadav et al. designed to synthesize a hybrid catalyst of CoFe2O4@Mn-BDC MOF(104). In Figure 3.38 the steps and compounds in the preparation of the catalyst have been demonstrated. However, benzene dicarboxylic acid is immobilized on the surface of MNPs (CoFe2O4)(102) and, Mn ions, in the presence of carboxylic acid groups, lead to form a complex with a carboxylate group. The onepot synthesis of “click” coupling some compounds including terminal alkynes, aryl, or alkyl halides, have been observed to provide excellent yield in the presence of the prepared catalyst [29].

Figure 3.38: Preparation of CoFe2O4@Mn-BDC Hybrid nanocatalyst.

Cobalt ferrite can be utilized as magnetic support for its moderate saturation magnetization, low cost, and chemical and mechanical stability. As observed in Figure 3.39, for preparing such catalysts, firstly, the cobalt ferrite nanoparticles were encapsulated in silica by Stober method. Then, CoFe2O4@SiO2 nanoparticles get to functionalize with (3-aminopropyl) teriethoxysilane (APTES) in toluene and CoFe2O4@SiO2-PrNH2 (106) was obtained after refluxing for 24 h. In continuation, CoFe2O4@-SiO2-PrNH2, along with formaldehyde, and urea was confronted in ultrasonic irradiation and refluxed for 24 h too, to produce MNPs@SiO2@PUF (107). Eventually, adding Zn(OAc)2.2H2O to the mixture leads to the trapping of atoms of nitrogen and was immobilized. The final precipitate was washed and dried to achieve the nanocatalyst (MNPs@SiO2@PUF@Zn) (108) [32]. Regarding the result of characterizations, peaks of FT-IR confirmed grafting in the catalyst structure and determined the diameter of particle size 30–60 nm, and 20–40 nm with SEM and TEM, respectively, and deformation in the temperatures of more than 650 °C. The activity of the catalyst was also subjected to the preparation of spirooxindolopyrans in optimized conditions, which display the high catalytic activity and efficiency for such synthesis. CoFe2O4@[Cu2(Si-N¼SA)2(bipy)2(PTA)] (111) has an intricate structure for the formation of a bridge by the molecule of phosphotungstic acid (PTA) between a couple of catalysts of CoFe2O4@ [Cu2(Si-N¼SA)2(bipy)2]. It seemed to coordinate Cu particles that supported cobalt ferrite nanoparticles. The catalysts designed by Kooti’s team and the steps involved are shown in Figure 3.40. Cobalt ferrite silica-coated with

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

99

Figure 3.39: Preparation of MNPs@SiO2@PUF@Zn as a novel hybrid material.

salicylaldehyde (SA) gives CoFe2O4@Si-N¼SA. PTA, 2, 2′‐bipyridine (bipy), and copper (II) sulfate were augmented to the last compound to achieve the CoFe2O4@ [Cu2(SiN¼SA)2(bipy)2(PTA)] hybrid. The presence of Cu ions leads to the formation of chelating with nitrogen atoms of an available compound and the creation of a complex, and PTA became a bridge between two groups of these compounds. On the other hand, Cu ions were attracted by oxygen ions on the catalyst. The result of FT-IR indicated increasing of bond length in W–O and P–O and decrease in the force constant, which confirmed the coordination of PTA with copper metal. The catalytic study proved to be two active acidic sites concurrently on the catalyst, Cu as a Lewis acid, and PTA as a Brønsted acid, and also the synthesis of acetaminophen gave an excellent yield. Such properties can make it an eco-friendly (for solvent conditions) and efficient catalyst [67]. Chitosan is extracted from natural sources. It is a jack-of-all-trades and renewable biopolymer that has prominent features, for instance, biocompatibility, plenty, high thermal consistency, and low cost. Significantly, it has free hydroxyl and amine functional groups, which can furnish the coordination of transition metals on the polymer. Moreover, chitosan hydrogel beads can be easily achieved via the same functional groups to ameliorate the metal binding capacity of chitosan. Some literature has reported the application of the modified magnetic catalysts with chitosan inorganic synthesis. Therefore, the surface of CoFe2O4 can be modified by chitosan, followed by the preparation of CoFe2O4@chitosan hybrid as stabilizers. Thereafter, different metallic nanoparticles can be put out of action on the designed support, and their catalytic properties in diverse organic reactions assayed. Baran and Nasrollahzadeh designed a new nanocatalyst of cobalt ferrites with chitosan-complexed Pd nanoparticles (Figure 3.41). They dissolved chitosan in acetic

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Nahid Ahmadi, Ali Ramazani

Figure 3.40: Preparation of CoFe2O4@[Cu2 (Si-N¼SA)2(bipy)2(PTA)].

acid, and adding CoFe2O4 to the mixture produced CoFe2O4@chitosan hydrogel after stirring for 3 h. Hydrogel is cross-linked with glutaraldehyde in methanol at 70 °C. In the next step, the produced beads were stirred for 6 h at the same temperature in the presence of PdCl2, and the yield precipitate was collected, lastly. The findings of the analysis illustrated that Pd NPs got coordinating CoFe2O4@chitosan, and their particle size is determined to be about 20 nm by TEM. The FT-IR of both CoFe2O4@chitosan and Pd@CoFe2O4@chitosan were similar. Hence, the authors claim that the slight shift observed at the FT-IR of Pd@CoFe2O4@chitosan is for interaction of Pd with CoFe2O4@chitosan. They evaluated the catalytic features of Pd@CoFe2O4@chitosan (112) hybrid nanocatalyst in the aryl halide cyanation and 2-NA reduction. Various benzonitriles were successfully produced with high yields, in the presence of Pd@CoFe2O4@chitosan hybrid nanocatalyst. Moreover, Pd@CoFe2O4@chitosan nanoparticles converted 2-NA o-phenylenediamine within only 65 s. Therefore, it shows that the catalyst could be decreased in the reaction time [22]. Maleki and coworkers also used Cu (NO3)2 and Fe (NO3)3 to synthesize CuFe2O4@chitosan magnetic nanoparticles, a new catalyst replacing Cu. They tried to extend a simple strategy to improve a catalyst that was employed in the selective synthesis of α-aminonitrile derivatives [61]. Yadollahi and coworkers prepared magnetic metal-organic frameworks (MOF) (Figure 3.42), CoFe2O4@TMU‐17‐NH2(115), using a simple embedding approach. In this method, the solution contains CoFe2O4 (102),(113),and (114) after stirring to leave in the autoclave at 90 °C for 3 days. The time was suitable for growing crystals. Synthesis of pyrimidine derivatives was chosen for studying catalytic activity (115), (102), and TMU-17-NH2 under the same conditions. The result indicated the high activity for (115) to two other cases [68].

101

Chapter 3 Hybrid magnetic nanocatalysts for organic synthesis

OH

OH

Pd Chitosan

HO HO

cross-linked

NH2

Glutaraldehyde

CoFe2O4

Pd

O

O

O HO

O NH2 n

Pd

PdCl2, EtOH, Reflux

112

102

Pd@CoFe2O4@chitosan

Figure 3.41: Preparation of Pd@CoFe2O4@chitosan.

114 +

N N N N

CoFe2O4 102

H 2N HOOC

COOH

Zn(NO3)2 DMF, 90 °C

CoFe2O4@TMU -17-NH2 115

113

Figure 3.42: Preparation of Co2Fe3O4@TMU‐17‐NH2.

Yao and coworkers employed graphene oxide for modifying of the surface of CoFe2O4. They presented a facile method for preparing CoFe2O4-reduced graphene oxide (rGO). In this way, GO sonicated for 2 h to obtain carboxylate ions. Then, the solution of Fe(NO3)3.9H2O and Co(NO3)3.6H2O was poured into the mixture and stirred. Finally, for reducing GO in CoFe2O4-GO, the solution of hydrazine hydrate was used. The findings show that CoFe2O4 reacts attractively as an electrostatic with GO. CoFe2O4-rGO hybrids exhibited better catalytic activity than pure CoFe2O4. The reduction of Phenols on CoFe2O4-rGO was studied by the first-order kinetics and the activation energy was 15.8 kJ/mol [69]. In the other works, Yao group replaced Mn with Co. They reported a facile method for preparing MnFe2O4 and MnFe2O4-reduced graphene oxide (rGO), and their catalytic properties were studied on generating sulfate radicals from peroxy monosulfate for degradation of organic dyes [7]. Some researchers have shown ring-opening metathesis (ROM) polymerization on the surface of silica [14–16, 70], some on polystyrene resins [70, 71], some for trapping Nb-tagged N-hydroxy succinimide used ROM polymerization [72], and some described the load increasing by such a process. According to this result, Schatz and coworkers designed to synthesize a nanomagnetic catalyst using such a process. Therefore, they used Cu(I) for catalyzing cycloaddition reaction azide (117) with alkyne (118). Grafting (119) and (120) was performed covalently on cobalt nanoparticles by ROM polymerization (Figure 3.43) [17].They employed Pd(OAc)2 to create a complex between (122)(in the PPh3) and Pd. The prepared catalyst was applied in the reactions of Suzuki-Miyaura cross-coupling that performed between phenylhalids and phenylboric

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acid. In regard to the result, the catalyst acts as selective for phenylhalids with I and Br, but for Cl, the yield was < 50%.

Figure 3.43: Preparation of azide-tagged Co@C-NPs 122.

3.2.3 Nickel and iron magnetic nanoparticles As observed in Figure 3.44, MNPs@TiO2–ILPip (128) was synthesized, based on the reported procedure. At first,NiFe2O4@TiO2‐PrCl was dispersed in dry toluene. After that 2, 2, 6, 6‐tetramethyl‐4‐amino piperidine was added to the mixture and was

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refluxed for 24 h. Next, an amount of NiFe2O4@TiO2-4‐APip was dispersed in dried toluene and methyl iodide was added to the suspension. Finally, the MNPs@TiO2–ILPip was synthesized and collected by an external magnet, rinsed with dry toluene, and dried at 80 °C [25]. The FT-IR spectrum confirmed the functional group attached to suit the structure of MNPs, and the particle size was determined at an average of about 34 nm by SEM. Carbonitrile derivatives were synthesized in the presence of (127), with a high yield, compared to other catalysts.

Figure 3.44: Preparation of hybrid MNPs@TiO2-ILPip.

3.2.4 Gold nanoparticles Metal-organic frameworks (MOFs) are a class of organic–inorganic hybrid particles that contain clusters of metal or ions coordinated with an organic bridge. Even if MOFbased catalysts illustrate high catalytic activity, the work-up of the bare MOFs is hard. For this reason, using magnetic nanoparticles in their structures or combined with them is an intelligent move. Interestingly, catalytic applications of Fe-based MOFs (MIL-100(Fe)) have increased recently. Ma and coworkers prepared MOF-based nanocatalysts (Fe3O4@Au@MIL-100 (Fe)) (131) (Figure 3.45). Solvothermal, Au seed (129) growth, and low-temperature cycling self-assembly were three processes that were performed in the synthesis of nanocatalyst. The magnetic MOF-based nanocatalysts are employed for the oxidation of the 3, 3, 5, 5-tetramethylbenzidine substrate as new peroxidase mimics. Tiny gold colloidal nanoparticles as seeds were rising to the surface of the Fe3O4 nanoparticles. The magnetic nanocatalysts of Fe3O4@Au@MIL-100(Fe) were fabricated by the cyclic plunging of the thioglycolic acid-functionalized Fe3O4@Au (130) nanoparticles in the solutions of FeCl3 and trimesic acid (BTC), which were dissolved in ethanol at 70 °C, separately [73]. The catalyst was investigated in peroxidase-like reactions. Fang et al. designed a unique simple synthetic for the Fe3O4@Au-polydopamine (PDA) microspheres with the core-shell nanostructure of sandwich-like structure. The hybrid layer with PDA shell-protected Au nanocrystals was coated in situ on the Fe3O4 core via the reaction between HAuCl4 and dopamine, by a redox polymerization. They

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also used this method for preparing β-FeOOH@Au-PDA, SiO2@Au-PDA, and CNT@AuPDA nanocatalysts with different morphologies and functionalities. Finding of SEM and TEM indicated that the particle size is about 170–240 nm for Fe3O4@Au-PDA, and the shell thickness was determined at an average of about 10–20 nm. With regard to the results, conducting the reduction of p-nitrophenol was considered for the catalytic performance of all the nanocatalysts. Nanocatalysts containing Au display high recycling activity because of protection by PDA shells [10].

Figure 3.45: Preparation of magnetic MOF-based nanocatalysts.

3.2.5 Iron nanoparticles Fe3O4, synthesized in the presence of Irish moss (IM), which led to the synthesis of Fe3O4@IM(135) was designed to fabricate by Hemmati’s team (Figure 3.46). IM contains either protein or carbohydrate, of which carbohydrates are composed of L and k-carrageenan. In general, carrageenans are polysaccharides that can combine with MNPs. Hence, the design of such catalysts is a way to develop sustainable chemistry. In this synthesis, IM (134) was added to a mixture of iron oxides and stirred at 80 °C. During the synthesis, IM reacts with Fe3O4 nanoparticles through both hydroxyl and sulfate groups that are in carbohydrate. SEM shows the morphology of nanocatalyst and has a homogenous structure and IM anchored on the MNPs. For preparing imidazopyrimidine derivatives, Fe3O4@IM were used, which were found to be highly efficient when EtOH is a solvent; in other words, the reaction could not be performed in solvent-free [74].

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134 IM FeCl2 + FeCl3

1) r.t. 2) NH3, 80°C, 30min

135 Fe3O4@IM Figure 3.46: Preparation of Fe3O4@IM catalyst.

The synthesis process of magnetic konjac glucomannan-graphene oxide hybrid mooring palladium is illustrated in Figure 3.47. The Fe3O4@GO-KGM-Pd catalysts have great reusability to Fe3O4@Pd, because of the strong hydrogen bonding between KGM and GO, and the hierarchical porous structure. With regard to these properties, Jia and coworkers described an easy approach to synthesize and modify a magnetic konjac glucomannan-graphene oxide hybrid (KGM-GO-Fe3O4) with Pd NPs. After dissolving KGM in water, MNPs and graphene oxide (GO) were poured. Then, the mixture was stirred for one hour, and pH of 12 was reached by NaOH. The resultant KGM-GO-Fe3O4 was dispersed into PdCl2 (136) and was stirred for 24 h. The solution was stirred constantly for another 24 h, after adding NaBH4. The catalytic activity was evaluated for the hydrolysis of ammonium borane. They further investigated the influence of the dosages of Fe3O4@GO-KGM-Pd (137) and ammonium borane on the reaction rate of ammonium borane hydrolysis, under the present conditions. The corresponding activation energy (Ea) is also computed according to the Arrhenius equation, which studied the different reaction temperatures effect on the reaction speed. Compared to other catalysts of palladium, the prepared catalyst had the lowest activation energy [75]. HCl, NaOH,

PdCl2 136

Fe3O4@GO-KGM-Pd Fe3O4@GO-KGM

137

Figure 3.47: Preparation of Fe3O4@GO-KGM-Pd.

GO can improve peroxymonosulfate (POMs) catalytic activity. That is why researchers try to design and develop the nanomagnetic catalyst, loading of GO on that – because of key properties, for example magnetic separation, the large surface area, and ease of functionalizing with various chemical groups – to raise their dependence toward object combinations heterogeneous catalysts. Accordingly, Yao and coworkers designed and synthesized MnFe2O4-rGO hybrids. At first, GO was prepared

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with the Hummer method and added into the mixture of MNPs containing Mn and Fe. Finally, the solution of hydrazine was poured into the mixture and stirred at 80 °C for 4 h. The findings showed that growing MnFe2O4 among sheets of GO resulted in heaping of GO layers during the reaction. The catalysts are tested to reduce the oxidation of various polluted organics in water by activation of peroxymonosulfate. It was found that MnFe2O4-rGO displayed high activity in activating POMS to produce sulfate radicals for degradation of organic dyes (Methyl violet, Methyl orange, Methylene blue, Orange II, and Rhodamine B). MnFe2O4-rGO has lower activation energy (25.7 kJ/mol), which justifies the higher chemical activity than that of MnFe2O4 (31.7 kJ/mol), suggesting that graphene plays an important role in the increasing degradation of dyes [7]. Melchianno et al. synthesized Fe@CNTs-Pd@ MO2 (142) nanohybrid, as indicated in Figure 3.48. They integrated iron-carbon nanotubes (Fe@CNTs)(138) with an inorganic sample made of palladium nanoparticles set within TiO2 or CeO2nanocrystals. They chose two different metal oxides for a specific catalyst scope: TiO2 as photoactive material in the photocatalytic hydrogen improvement and CeO2 as cracking patronage in the water-gas shift reaction (WGSR). The synthesis methods of both were similar; they were just different in the used metal compound – in TiO2 Ti (OBu)4 was used, and in CeO2 Ce(ODe)4(ODe = decyloxide) was employed. The magnetic Fe@CNTs core can restrain a general and facile sifting process to achieve the most active CNT-based catalyst package, as illustrated by the measuring of the catalytic activity in two main processes: H2 photocatalytic progress and WGSR. Additionally, the development of the usage of these nanohybrids in other chemical transformations may point to direct participation of the endohedral Fe phase, which can simplify charge transfer pathways [76]. O 139

N

O HOOC COOH

NH2 , 90°C

138 COOH

140

141 142 hydrolysis

Figure 3.48: Preparation of Fe@CNTs-Pd@MO2.

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Moini and coworkers reported the synthesis of Fe3O4@Tryptophan-Cu, Fe3O4@Tryptophan-Co, and Fe3O4@Tryptophan-Fe (144), as observed in Figure 3.49. After preparing magnetic nanoparticles, tryptophan was poured into the previous solution as a ligand. In this step, tryptophan reacted through oxygens of carboxyl groups, with Fe3O4 as a covalent bond. Then, the precursor material was linked to the metal (Cu, Co, and Fe) with a coordinating bond via amine groups on the tryptophan part. These sustainable magnetic nanocatalysts result in the oxidation of a wide area of sulphides and thiols with relatively high efficiency and selectivity, under mild conditions. Such catalysts displayed their excellent efficiency in increasing product yield, decreasing reaction time, and making the reaction mild [77].

Figure 3.49: Preparation of Fe3O4@Tryptophan-M (Cu, Co, and Fe) nanocatalysts.

Carbon dots (C-dots) have key important features, such as below 10 nm size of particles, excellent conductivity, and rapid electron transfer, so they have been employed in many fields of organic syntheses. Guo and coworkers were designed to synthesize core-shell C-dots@MFe2O4 (M = Mn, Zn, and Cu) hybrid materials (Figure 3.50). In their design, the hydrothermal method is predicted for the fabrication of C-dots under conditions at 200 °C for 3 h. After preparing C-dots and MNPs, both of them were poured into the reaction container and were ultrasonicated for one hour. The catalytic activity was evaluated in the reduction of p-nitrophenols for every three catalysts, which have good activity. However, C-dots@CuFe2O4 exhibit high activity that can be attributed to its specific features and the synergistic effect of CuFe2O4 and C-dots [78]. It is known that Cu plays a key role in the catalyst, and authors claim that both Fe3+ and Cu2+ presented in the octahedral structure to handle a transferring electron between Cu+-Cu2+ and Fe2+-Fe3+; this factor justified increasing of catalytic activity. In that case, electro transfer about Mn2+and Zn2+ is difficult. In continuation, Rathore’s group attempted to synthesize Fe3O4@LD-Cu(147). They tried to react Fe3O4 with L-3, 4-dihydroxyphenylalanine (LD). LD through hydroxyl groups reacts with Fe3O4 via a covalent bond in the aromatic part. During the reaction, covalent bonds are also formed via oxygen of carboxyl groups and coordinating bond via amine group on the aliphatic part of LD with Cu ions (Figure 3.51). The important purpose of preparation of such catalysts is to introduce a new, available, and effective catalyst whose organic molecules grafted MNPs and got to form a complex with copper particles to expand the use of these types of nanocatalysts for

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P-AP

P-NP

NaBH4

e e-

e-

e-

e-

-

e-

e-

C-dot I

II

III

C-dot

C-dot IV

V

Cu2+ Fe3+ Fe2+ Cu+

Figure 3.50: Schematic of the catalytic reduction of p-NP on C-dots@CuFe2O4 surface: (I) Adsorption of BH4 − on the C-dots@CuFe2O4 surface, formation of electrons. (II) Accepting those electros by Cu2+ and Fe3+ to convert Cu + and Fe2+. (III) Adsorption of p-NP on the C-dots@CuFe2O4 surface. (IV) p-NP received electrons of Cu+ and Fe2+ as donor electrons and turn into Cu2+ and Fe3+. (V) p-NP accept electrons to produce p-AP.

oxidation reactions. It has been pointed that this kind of catalytic system is reported for the aerobic and H2O2 oxidation of alcohols in literature. In the beginning, they employed benzyl alcohol as a model system, to facilitate the analysis and to speed up the screening speed. In consequence, the optimized conditions were utilized for the synthesis of different aromatic aldehydes [79]. A copper complex of the Fe3O4-LDOPA conjugates afforded magnetically recoverable nanocatalyst for oxidation of benzylic alcohols. The catalyst displayed a high turnover number (TON) and great selectivity [80].

Figure 3.51: Preparation of Fe3O4@LD-Cu.

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Nasir Baig and coworkers [79] synthesized MNPs functionalized with an organic compound complexed copper similar to the previous one. Herein, glutathione (148) was used instead of LD, and sulfur attached to the magnetic nanoparticle through covalent bond; OH and NH of glutathione also linked to Cu with a coordinate bond (Figure 3.52).

Figure 3.52: Preparation of (Fe3O4@-FGT-Cu (0)) catalyst.

Rahimi and coworkers synthesized Fe3O4@PVA-Ag NPs(153).They were prepared by an in situ co-deposition method in which PVA(151) strands (72,000) dissolved in the water at 80 °C. In the next step, Fe3O4@PVA (152) magnetic NPs were ultrasonicated, silver nitrate was poured, and the media were stirred at ambient temperature. Finally, dark brown particles in the yield were washed and collected with an external magnet. Ag nanoparticles were loaded onto the surface of Fe3O4@PVA, which are accountable for the catalytic activity of this nanocatalyst via their electronic attraction with heteroatoms (Figure 3.53). Moreover, they have a biological effect on the presented nanocatalyst for their antibacterial property [81].

Figure 3.53: Preparation of the Fe3O4@PVA-10%Ag nanocatalyst.

Hydrogenation reaction is essentially carried out in the presence of a metal catalyst, usually Pd, Sn, which takes a long time to complete the reaction. Today, efforts have been enhanced for generating and developing colloidal catalysts containing metal. Hence, coating the surface of MNPs directly to Palladium NPs or on silica-coated MNPs in the beginning has been beneficial. Then, designs has been developed to modify the surface of magnetic NPs, which involves the chemical modification of

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the metal atoms. Examples of surface modifiers are many; herein, there are two cases: mercapto propyl acid (MPA) (154) and dopamine molecules (156). As observed in Figure 3.54, these ligands were directly linked to MNPs through a covalent bond. They are stabilizers for the Pd NPs that could be recovered via magnetic separation. The use of bare magnetic NPs is possible, but the silica-coating of the MNPs improves the chemical stability or protects the MNPs and enables surface silanol groups to functionalize with alkoxyorganosilanes [49]. HO H 2N

O

NH2 156

HO

O

154

O HO

Toluene, 80°C

Reflux

SH

O

SH

O

157

155

2

Figure 3.54: Amino- and thiol-functionalized magnetite NPs using MPA and dopamine molecules.

To continue, the Fe3O4@CMC-Ru (III) (160) nanocatalyst was synthesized in three steps by a facile, inexpensive, and convenient self-assembly method (Figure 3.55). In the beginning, a solution of FeCl2 and CMC-Na was stirred at room temperature to form the CMC-Fe (II). Next, NaOH solution was added into the solution of CMC-Fe (II) to afford Fe3O4@CMC-Nahybrid. Finally, the yield of Fe3O4@CMC-Na (159) was plunged in RuCl3 solution and stirred at room temperature to obtain Fe3O4@CMC-Ru (III) coordinated nanocatalyst through ion-exchange reaction [82]. The images of analysis confirmed grafting of CMC on the MNPs and the presence of Ru as a complex with oxide ions.

OCH2CO2Na

OH HO O

O HO

OCH2CO2Na

OH Fe2+

O

HO O

OH

O HO

NaOH, air

OCH2CO2Na

Ru

CO 2 H2 OC

OCH2CO2Na

158

159

OH Ru3+

HO O

O HO

O O OH

OCH2CO2

O OH

Ru 160

Figure 3.55: Preparation of Fe3O4@CMC-Ru (III)organic–inorganic hybrid catalyst.

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Fe3O4@R. Tinctorum2-Ag (162 )was synthesized by Veisi and coworkers through the two-step modification of Fe3O4MNPs with the extraction of R. Tinctorum. Eventually, Ag+ ions were adsorbed on the modified surface of MNPs, reduced, and stabilized by the extracted natural chemical (Figure 3.56). Hence, this method helps reduce the dimension of the MNPs from 15–20 to 2–5 nm. This may be one of the advantages of the method. Moreover, R. tinctorum is bimolecular, which gets MNPs to stand out agglomeration. Additionally, there are the phenolic compounds in the R. tinctorum root including hydroxyl and ketone groups that can link to metals; herein, Ag leads to the formation of chelating. [email protected] NPs were described to be suitable for the catalyst of three-component reactions of amines, aldehydes, and alkynes [83]. OH

OH R. Tinctorum

O O

2

161

AgNO

OH

O O

Ag

O 162

O

Fe O @R. Tinctorum-Ag

Figure 3.56: Preparation of Fe3O4@R. Tinctorum-Ag NPs nanocatalyst.

The Fe3O4@L-lysine-Pd (0) magnetic nanoparticles, as a novel L-lysine-Pd Complex supported on Fe3O4, were prepared and explained as a systematic nanocatalyst for the cross-coupling reactions of C–C by Ashrafi groups. For preparing Fe3O4@L-lysine MNPs, L-lysine has been grafted on Fe3O4 MNPs through the covalent bond of -COOH groups with OH groups on the surface of Fe3O4 MNPs. Finally, the catalyst was synthesized by the reaction of Fe3O4@L-lysine MNPs with Pd(OAc)2 (Figure 3.57). L-lysine functionalized on magnetic nanoparticles and chelated with Pd. The TEM images illustrated that the diameter of the hybrid nanocatalyst is an average of about 7.8 nm. The catalyst was employed in the cycloaddition synthesis of 5-substituted 1H-tetrazoles [21, 84]. The C–C cross-coupling was successful with an efficiency of > 90% for all compounds that were synthesized in the presence of the obtained nanocatalyst. In the other work, Amirnejad and coworkers synthesized alginate-functionalized MNPs coated with arginine Fe3O4@Alg@CPTMS@Arg, using the layer-by-layer techniques. As in the first, MNPs and sodium alginate were stirred under an N2 atmosphere for two hours. In the next step, 3-chloropropyltrimethoxysilane (CPTMS) was added to the solution and got ultrasonic irradiation for 20 min. After pouring l-arginine into the

2 R. tinctorum: Rubia. tinctorum is a herbaceous plant species that belongs to the bedstraw family Rubiaceae. Its roots contain an acid ruberthyrin which was treated to change sugar, alizarin, and purpurin according to conditions. The polyphenolic compounds does not color all of them, for example, purpurin is not colored, but it will be red when it is dissolved in alkaline. Alizarin is a red compound, and it is known to rose madder as a red dye.

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Figure 3.57: Preparation of Fe3O4@L-lysine-Pd (0) magnetic nanoparticles (MNPs).

reaction container, it was refluxed to obtain Fe3O4@Alg@CPTMS@Arg. The catalytic study of Fe3O4@Alg@CPTMS@Arg as a heterogeneous hybrid inorganic–organic nanocatalyst was applied for the pyrazole derivatives synthesis [85]. Pyrrole-2-carboxylic acid functionalized on the surface of Fe3O4 nanoparticles using the covalent bonding via –COOH group as a novel organic–inorganic hybrid heterogeneous catalyst, was fabricated by Poormirzaei et al. (Figure 3.58). The result of characterization showed that average particle size is about 14 nm. Its high catalytic activity and short reaction time were properties to be discovered in the synthesis of indazoles through the nucleophilic substitution of hydrogen; besides, the prepared catalyst has antibacterial properties too [86]. O 165 N H

OH

NH4OH, N2, 80°C, 6h

O

H N 166

O

2 Fe3O4@Pyrrole -2 -carboxylic acid

Figure 3.58: Preparation of PCA-Fe3O4 nanoparticles.

Mohammadi and coworkers [87] introduced Hercynite3@L-Methionine-Pd (169) as an organic–inorganic hybrid magnetic nanocatalyst to the chemical society. For synthesizing this type of nanocatalyst, it is necessary to prepare hercynite MNPs (167), which were simply prepared by the co-precipitation process. But there are some ferric ions (Fe3+) in their structure. To resolve this problem, hercynite nanoparticles were synthesized under an N2 atmosphere to stop the oxidation of iron ions. Next, L-Methionine was attached to the surface of hercynite MNPs via covalent bonding. Then, the LMethionine-Pd complex was generated by the reaction of Hercynite@L-Methionine (168) with Palladium (II) acetate (Figure 3.59). The design and synthesis of MNPs of Fe3O4@O2PO2-NH2-Schiff base-Pd (174) were carried out by Aghayee and coworkers [88] for the first time. In this work, they

3 Hercynite: is a crystal mineral with the formula FeAl2O4.

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Figure 3.59: Preparation of Hercynite@L-Methionine-Pd.

used the Schiff base as a ligand consisting of duplex chelating and Pd metal as a complex. It is known that 2-aminoethyl dihydrogen phosphate plays two of many active roles – spacer and coating agent – in this process. The synthesis performs in the condition of alcohol solvents and room temperature at the early steps, then for the formation of complex, using toluene and 70 °C under N2 atmosphere. Finally, Fe3O4@O2PO2-NH2-Schiff base-Pd was prepared (Figure 3.60). Meanwhile, the integration of Schiff base and two groups of 2-aminoethyl dihydrogen phosphate via the formation of eimin has created a strong bridge and a suitable trap for capturing metal particles, herein, palladium. Therefore, the present study can serve as a guide for the methodologic design of different magnetic nanoparticle complexes, both metal particles and MNPs, with or without silica and metal oxide coating. The obtained catalyst is used for Sonogashira and Mizoroki–Heck C–C coupling reactions.

Figure 3.60: Preparation of Fe3O4@O2PO2-NH2-Schiff base-Pd.

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Shoueir and teammates introduced 3,5-dinitrosalicylic acid@Chitosan@MnFe2O4 (DNSA@CS@MnFe2O4) nanocatalyst. They synthesized DNSA-modified CS(DNSA@CS), which had a particle size of about 50 nm and a soft coral shape. For the first time, they increased the specific surface area and grafted with MnFe2O4 to produce DNSA@CS@MnFe2O4 nanocatalyst. FT-IR shows a strong electrostatic attraction force between MnFe2O4 and DNSA@CS matrix. Mooring of MnFe2O4 appended thermal stability to not more than 4% because of the spinal phase nature [18]. The catalytic activity of about 99% was obtained in decolorization, in about 30 min.

3.3 Characterization techniques There are various techniques for the characterization of physical and chemical properties of the prepared magnetite nanocatalysts, such as Fourier Transform Infrared Spectroscopy (FT-IR), Energy-dispersive X-ray spectroscopy (EDX), Vibrating sample magnetometer (VSM), Scanning electron microscopy(SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XRP), Thermo-gravimetric Analysis (TGA), Atomic absorption spectrometry (AAS), atomic force microscopy (AFM), and Inductively coupled plasma spectroscopyatomic emission spectroscopy (ICP-AES) analysis. Researchers have used some of them, which are available and necessary for characterization of their synthesized nanocatalysts,. For example, you will find FT-IR, TEM or SEM, TGA, and XRD are techniques are used in most literature. In hence, we will follow our discussion about these themes.

3.3.1 FT-IR spectroscopy FT-IR is an important instrument for functional group analysis and is employed to identify nanomaterials and to confirm the surface modification of the magnetic nanocatalysts. Study of FT-IR in iron nanoparticles indicated the characterized peaks at 450–650 cm−1, which have been attributed to the Fe–O bond [43, 51, 89–91]. They are found in all magnetite nanocatalysts, for example, at 597 cm−1 in Fe3O4@CTSN-Cunanoparticle [90], 632 cm−1 and 579 cm−1 in magnetic dextrin nanobiomaterial [8, 92], and 580 cm−1–620 cm−1 for Fe3O4@SiO2-based organic–inorganic hybrid copper(II) [33, 46, 50]. The vibrational frequencies in 584–631 cm−1 are related to the Fe–O bonds in nano-Fe3O4@TiO2-Pr-2AB@Cu [8, 38], FT-IR spectrum of CuFe2O4@chitosan, 599 cm−1 band represents the Fe-O bond of CuFe2O4 [61], and the absorption peaks at 589 cm−1 has been imputed to the characteristic stretching vibration of Fe–O bonds in Fe3O4@CMC-Ru(III) [34, 82].

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In the case of the CoFe2O4 spectrum, we found the presence of vibration typical of Fe-O and Co-O revealed at around 609 and 872 cm−1 [29]. Moreover, the hydroxyl (OH) groups on CoFe2O4 are confirmed by the presence of a broad band at near 3,420 cm−1. In addition, the FT-IR spectra of CoFe2O4 and the final catalyst were compared, and the adornment of Mn-BDC metal organic framework with inverse spinel CoFe2O4 was confirmed. The final catalysts spectra revealed peaks at 1,560, 1,390, and 748 cm−1, which are ascribed to the metal organic framework, while it is known that the peak near 617 cm−1 is due to the Fe-O vibrations of the CoFe2O4 [29]. Moreover, the sharp peak at 564 cm−1 was attributed to Fe–O of Fe3O4@Sp-HEP [56]. Herein, we considered a sample of the FT-IR spectrum of a few magnetic compounds containing Fe3O4, Fe3O4@TPI, and Fe3O4@TPI-HPA nanoparticles, which are in the structure of the majority of magnetic nanocatalysts in the wave number area of 4,000–400 cm−1, as demonstrated in Figure 3.61. The characteristic Fe–O absorption near 590 cm−1 appeared in the FT-IR spectrum of magnetic Fe3O4 nanoparticles (Figure 3.61) [32, 33, 35, 39, 46, 57, 58, 78]. The FT-IR spectrum of Fe3O4@SiO2 nanoparticles shows peaks at about > 3,200, 1,087–1,150, 950, 631–781 in-plane bending modes and 461 cm−1 in phase rocking mode of the Si-O-Si group, and affirms the evolution of SiO2 shell [31, 32, 35, 39, 57, 58, 78]. The range 3,200–3,700 cm−1 indicates the broad peaks, which are related to the stretching and vibration modes of Si-OH and at the range of 1,650 cm−1 are obvious; the weak peak in the spectrum is because of the twisting vibration mode of H-O-H adsorbed by the silica skin. The IR spectrum of Fe3O4@TPI-HPA nanoparticles contained broadband at about 1,020–1,075 cm−1, which proved strong interaction between pyridino-Fe3O4and the heteropolyacid. In addition, consideration of the presence of peaks in the fingerprint region (800–1,100 cm−1) of the Keggin structure manifested loading of H5PW10V2O40 on the surface of Fe3O4@TPI. Presumably, the result shows there is a slight shift in the frequencies after disabling, may be because of the interaction between heteropolyacid and the hydroxyl functional groups on the support material. Hence, the data prove that the functional groups were successfully linked, in situ [57].

3.3.2 EDX Energy-dispersive X-ray spectroscopy was named with different abbreviations containing EDS, EDX, EDXS, or XEDS. It is an analytical method that is utilized or the elemental analysis or chemical characterization of a specimen. It depends on an interaction of some principles of X-ray excitation and a sample. Its characterization capabilities are due, in large parts, to the fundamental rules that each element has an individual atomic structure authorizing an inimitable set of peaks on its electromagnetic emission spectrum (which is the basic rule of spectroscopy). Therefore, it is employed for consideration of atomics percent, which is in nanocatalyst structure; for instance, 49%, 35%, and 15%,the atomic percentages, belong to the oxygen, iron, and carbon

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Fe3O4-TPI-HPA

1150

Fe3O4-TPI 900 1150

Fe3O4 1650 950

450

2900 3400

590

4000 3600 3200 2800 2400 2000 1800 1600 1400 1200 1000 800 600 400 1/cm Figure 3.61: The FT-IR spectrum of Fe3O4, Fe3O4@TPI, and Fe3O4@TPI-HPA [57].

elements in magnetic dextrin nano biomaterial structure, respectively [92]. With regard to the results, the energy dispersive X-ray spectroscopy (EDS) affirm the presence of the C [8, 32, 35, 39], N [8, 32, 34, 39], S [35, 39], Cl [39], Ti, Si [8, 32, 34, 35, 39, 56, 58], O [8, 32, 34, 35, 39, 58, 93], Cu [8, 31, 34], Mg [94], Zn [32], Co [32], Au [93], Mn [8], and Fe [8, 32, 34, 35, 39, 56, 58, 93, 94] elements in the respective nanocatalyst structure. The intensity rate of the Ti peak to the Fe ascertains that the Fe3O4 NPs are coated by TiO2 and support the core-shell structure of the NPs. Also, this data shows that to get metals for the complexation on the surface of MNPs, a catalyst such as Cu ions on the surface of nano-Fe3O4@TiO2-Pr-2AB@Cu is useful [38]. Since all prepared magnetic nanocatalysts do not have the analysis of EDX, it chooses one of them that will be investigated in detail. EDX with elemental mapping was used for the compositional analysis of the prepared nanocatalysts. Elemental mapping images of the hybrid [email protected] NPs material (Figure 3.62) displays that it is composed of six elements, namely, Fe, O, S, N, C, and Ag. According to the results achieved from elemental analysis, the dispersing of silver nanoparticles in the hybrid was discovered to be on the extracted shell, and it has completely coated on the surface of MNPs. Moreover, the S-peaks indicate the alizarin molecules,

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which are extracts of the R. tinctorum, confirmed effective modification of the Fe3O4 NPs, provided the Ag signal confirms functionalization of Ag NPs and preparation of the designed nanocatalyst successfully [83]. OKα 5000

FeKα FeLα

4000

3000

NKα 2000

CKα 1000

SKα

AgLβ AgLα FeKβ

SKβ keV

0 0

5

10

Figure 3.62: EDX of the [email protected] NPs.

3.3.3 VSM A beneficial scientific instrument for measuring magnetic properties is a vibrating sample magnetometer (VSM). Hence, researchers used it for measuring the magnetic properties of prepared magnetite nanocatalysts. Prepared nanoparticles tested for VSM analysis in the range of −10,000 to 10,000 Oe, at ambient temperature were considered for the magnetic field [8, 29, 31–33, 67]. Generally, the saturation magnetization amounts can be observed for Fe3O4,30–60 emu/g [1, 2, 19, 33–35, 38, 41, 53, 56, 60, 64, 74, 77, 78, 83, 92], and for CoFe2O4, 57 emu/g [29], for Fe3O4@Ph-SO3H,40 emu/g [35, 39], ZnS‐ZnFe2O4 and ZnFe2O4, 52.9 and 66.9 [95],for Fe3O4@APTPOSS, 41.87 emu/g [53], for Cu-ABF@ASMNPs, 26 emu/g [51], 41.94 and 33.35 emu/g for Fe3O4@SiO2 [1, 34, 46], and PAA–modifiedFe3O4@SiO2 [41], 24.7 emu/g forFe3O4@SiO2-urea [2], 23.2 emu/g for α-Fe2O3@Hap@Cu [31], 3.71 emu/g for α-Fe2O3@Hap@Cu [64], 29 emu/g for Fe3O4@SiO2@PPh3@Cr2O72‐ [46], 31 emu/g for Fe3O4@SiO2@PPh3 [1], 60 emu/g for Magnetic dextrin [92], 15.64 emu/g for MNPs-Sp-HEP [56], 8.3 emu/g for CF@[Cu2(Si―N¼SA)2(bipy)2

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(PTA)] [67], 7.24 and 4.37 emu/g for[-γ-Fe2O3@HAp] and γ-Fe2O3@HAp Si (CH2)3 AMP)] [66], 31.1 and 18.4 emu/g for nano-Fe3O4@ D-NH2 and Fe3O4@D-NH2-HPA [37], 35.733 emu/g for OPSF- SO3H-HCl [39], 33.3 emu/g for Fe3O4@IM [74], 30.5 emu/g for C-dots@MnFe 2O4, 26.9 emu/g for C-dots@ZnFe2O4, and 18.2 emu/g for C-dots@CuFe2O4, 49 emu/g for Fe3O4@SiO2/DABCO [33], 2.5 emu/g for MnFe2O4, 44.2 emu/g for ZnFe2O4,30.6 emu/g for CuBPy@Am-SiO2@Fe3O4 [34], and 39.2 emu/g for CuFe2O4 [78], 67.9 and 71.8 emu/g for the square and circular β-Fe3O4 [96], 22 emu/g for [email protected]%Pd [36], and for nano-Fe3O4@ D-NH2 and Fe3O4@D-NH2-HPA, 75.2, 31.1, and 18.4 emu/g [37, 68]. According to the reports, the magnetic properties of iron oxides reduced because of increasing the coating on the surface of iron NPs. This was predicted due to the coating contribution from a nonmagnetic functionalized material [29, 56]. Initially, with increasing magnetic field, the magnetization increases rapidly, and saturation magnetization is completed. It was further found that the saturation magnetization and the remnant magnetization can be mildly affected by the size of the Fe3O4 nanoparticle. As Chen and coworkers evaluated this topic, they showed Fe3O4 nanoparticles with a particle size of 100–700 nm have a remnant magnetization of about 3.26–4.28 emu/g [4]. The saturation magnetization of nano-Fe3O4@TiO2-Pr-2AB@Cu is also altered from 30 emu/g(Fe3O4 MNPs) to 22 emu/g [38], which suggests that the magnetic properties of the nanoparticles are covered by nonmagnetic TiO2 layers and organic groups. Moreover, the obtained magnetic curves, before and after functionalization of the MNPs, do not exhibit hysteresis for the great paramagnetic properties. It is necessary to investigate the VSM of one of the synthesized nanocatalysts; so the magnetic properties of the Fe3O4, Fe3O4@Tryptophan-Co,Fe3O4@Tryptophan-Cu nanoparticles, and the measuring of Fe3O4@Tryptophan-Fenanoparticles were performed at ambient temperatures observed in Figure 3.63. VSM measurements for Fe3O4 NPs-coated (for example, for Fe3O4@Tryptophan-Fe is about 30 emu/g) is less than uncoated (62.82 emu/g) in the same field. Based on these results, the magnetization of Fe3O4 decreased with the coating of nonmagnetic substrates (organic compounds and metal complex) on the surface of iron oxide. With regard to the magnetization curve of the particles that cross the origin graph, and any coercive force and remaining magnetism that are not in the VSM curve, the results showed a complete reversal of the magnetization process that affirms the superparamagnetic behavior of the core MNPs [77].

3.3.4 SEM A scanning electron microscope (SEM) is a kind of electron microscope, that makes images of a sample by scanning the surface with a concentrated beam of electrons. Hence, images were used for determining morphology and particle size. Regarding SEM properties, the mean particle size of the magnetic dextrin was determined to

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80 a b c

60 40 Magnetization (emu/g)

d 20 0 –10000

–5000

0

5000

10000

–20 –40 –60 –80 H(Oe)

Figure 3.63: VSM curve of Fe3O4 (a), Fe3O4@Tryptophan-Co (b) Fe3O4@Tryptophan-Cu nanoparticles (c), and Fe3O4@Tryptophan-Fe nanoparticles (d) [77].

be 84 nm [92], and at nano-Fe3O4@TiO2-Pr-2AB@Cu is 32 nm [38]; Fe3O4@CMC-Ru (III)hybrid catalyst measured approximately 28 and 16 nm [82] and affirms the spherical and regular image of hybrid nanocatalyst. Owing to the small size of the particles, the ratio of the surface to the volume of these particles is high; therefore, more contiguity with the reactants has taken place, and it has played its catalytic role well. Therefore, there is a mean particle size of 10–100 nm for synthesized nanocatalysts, depending on the kind of prepared method [31–33, 35, 39, 43, 46, 56, 58, 78, 94]. According to the above analysis, it can be found that dispersing and stabilizing of the structure were well for all samples [30]. However, some nanocatalysts have an average particle size higher than 100 nm [10]. Figure 3.64 depicts the SEM images of Fe3O4 and Fe3O4@SiO2-DABCO microspheres. The Fe3O4and Fe3O4@SiO2-DABCO have particle sizes of about 48 and 74 nm, respectively, which show MNPs successfully have been coated by organosilane [33].

3.3.5 TEM Transmitting a beam of electrons for creating a microscopy image is executed by a technique called Transmission electron microscopy (TEM). TEM and SEM are two of the highly powerful techniques that are used to determine both morphological features and the size of designing nanoparticles. TEM analysis of MNPs@SiO2@PUF@Zn showed the size of the obtained particles is 20–40 nm [32]. Based on these results, the result of SEM for this catalyst was confirmed. The TEM images of Fe3O4@CMCRu(III) catalysts [82], CoFe2O4@Mn-BDC [29], Fe3O4 nanospheres [4], MP@RF [30],

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Figure 3.64: The SEM images of (a) Fe3O4 and (b) Fe3O4@SiO2-DABCO microspheres [33].

SiO 2@Fe3O 4 [34], Fe3O4@Au-PDA [10], CuFe2O 4 MNPs [78], Fe3O4@D-NH2-HPA [37], and Fe3O4@SiO2-AQ [58] confirm the result of SEM. TEM images of a magnetic sample of iron oxide nanoparticles and MNPs coated with silica were studied. TEM images were found to achieve correct vision into the morphological specifications of the nanoparticles. It was found that Fe3O4 nanoparticles have an approximately globular morphology, with a mean dimension of 8– 12 nm (Figure 3.65a). The selected area electron diffraction pattern entered on a single particle display the presence of white cloudy refraction rings determinable to the (220), (311), (400), (422), (511), and (440) grates of the cubic inverse spinel structure of magnetite (Figure 3.65b). The outward appearance of these discrete refraction rings also corroborated the polycrystalline matter of the nanoparticles. The HRTEM4 image of the pristine magnetite nanocatalysts was interesting because of the detail of the structure (Figure 3.65c) – it reflected the image of two-dimensional lattice edges that could be demonstrated to the (220) grate of net magnetite, with a value of interfacial separation about 0.25 nm. The representative TEM image of Fe3O4@SiO2 asserted the morphology of the core-shell structure very well, as manifested in Figure 3.65d. The black inner core has been encapsulated within the light gray silica shell with the thickness of about 10 nm as can be observed distinctly. Next, the TEM image of the final catalyst (Cu-BPy@Am-Fe3O4@SiO2) was also obtained, which exhibited that the metal complex is uniformly imprinted over the core-shell surface, and the particles display a globular morphology similar to that of the pure Fe3O4 nanoparticles [34].

4 HRTEM: high-resolution transmission electron microscopy.

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Figure 3.65: (a) TEM image of Fe3O4 nanoparticles, (b) SAED pattern of Fe3O4, (c) HRTEM image of the single nanocrystal entity with lattice fringe, and (d) TEM image of Fe3O4@SiO2 [34].

3.3.6 XRD The atomic and molecular structure of a crystal is determined by an experimental science instrument, which is called X-ray crystallography (XRC).The crystalline structure is because of a beam of incident X-rays that refracts into many specific directions. The angles and intensities of these refracted beams are measured to produce a three-dimensional image of the congestion of electrons within the crystal. Using this electron density, the average location of the atoms in the crystal, chemical bonds, crystallographic disorder, and different other information can be estimated. Based on this description, it has been illustrated for magnetite dextrin at 2θ = 17.53°, 30.1°, 35.52°, 37.87°, 57.40° and 69.68° [92] that XRD of Fe3O4@TiO2-Pr-2AB@Cu nanoparticles display several diffraction peaks at 2 h ¼ 0.54,35.89,3.9,54.28, 57.55, 63.3, and 74.33 that ascribed to the miller planes 220, 311, 400, 422, 511, 440, and 533, respectively [38]. These data are in agreement with the standard templates. The results prove that the spinel structure of the cubic crystals has been preserved in the functionalization process of the Fe3O4 nanoparticles. The XRD pattern of some synthesized MNPs-based iron and pure iron oxide such as Fe3O4, Fe3O4@CMC,andFe3O4@CMC@Ru(III) hybrid [82], CoFe2O4, Mn-BDC, CoFe2O4@Mn-BDC [29], MgO-MgFe2O4 [4], [Ag@mHAp–Si–(S)] [43], CoFe2O4, MNPs@SiO2@PUF@Zn [32], MP@RF-Pd@hSiO2 [30], Fe3O4 NPs [46], MnFe2O4, MnFe2O4@SiO2-

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NH-β-CD, and MNPs@β–CD@Cu(OAc)2 [8], Fe3O4@Au/PDA [10], C-dots@MFe2O4 [78], Fe3O4@D-NH2-HPA [37], and Fe3O4@SiO2-AQ [58] show diffraction peaks at 2θ = 30.4°, 35.8°, 43.3°, 53.9°,57.3°, 63°, and 74.5°, which can be allocated to the (220),(311), (400), (422), (511), (440) and (533) planes of Fe3O4, respectively. This data displays that the Fe3O4 particles in the nanocatalyst were pure in a cubic spinel structure; these are in good agreement with the standard patterns. The WAXRD5 spectrum of Fe3O4@SiO2 indicated the presence of a broad hump near 2θ = 20 − 23°, which could be ascribed to the silica layer circumambient of the core material; the prominent XRD peaks, which could be ascribed to(111), (200), (220), (311), and (222) of Au, were obviously remarked in diffraction spectra of Fe– Au nanoparticles; then, the appearance of this new peak confirmed that the magnetite nanoparticles were encapsulated successfully. In addition, the crystallite size of the nanoparticles was evaluated by picking the peak of the highest intensity in the WAXRD spectrum of Fe3O4and using the Debye − Scherer formula (D = 0.9λ/β cosθ, where D is the mean crystalline size, λ is the X-ray wavelength (0.154 nm), β defines the full width in radians subtended by the half maximum intensity width of the (311) powder peak, and θ corresponds to the Bragg angle of the (311) peak in degrees) in the characterization of all hybrid magnetic nanocatalysts [34, 93]. To continue obtaining more information, XRD of CoFe2O4, CoFe2O4@chitosan, and CoFe2O4@chitosan-Pd hybrid nanocatalyst have been studied, the results of which can be seen in Figure 3.66. The beholding diffractions are in good accord with the structure of CoFe2O4 and additionally, the diffraction peaks near18.25°, 30.14°, 35.55°, 43.11°, 53.75°, 57.06° and 62.68°, were imputable to the (111), (200), (220), (311), (400), (422), (511), and (440) planes of CoFe2O4, respectively, and were easily recognized from the XRD pattern, which matches well with the structure ofCoFe2O4. In the case of the synthesis of CoFe2O4@chitosan microbeads, a new wide peak became visible at 20.06°, because of chitosan. As a result, five new diffraction peaks were clearly revealed at 40.15°, 46.66°, 68.12°, 82.06°, and 86.50°, which belonged to (111), (200), (220), (311), and (222) crystalline planes of palladium as a face-centered cubic lattice, respectively, confirming the embedded of Pd nanoparticles on CoFe2O4@chitosan nanobeads [22].

3.3.7 XRP X-ray photoelectron spectroscopy (XPS or XRP) is a surface-sensitive quantitative spectroscopic technique according to the photoelectric effect, which can recognize if the elements are materials or are coating its surface..

5 WAXRD: Wide-Angle X-Ray Diffraction.

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(111)

(200) (c) (311) (222)

Intensity (deg)

(220)

(b) (311) (220) (111)

15

(400)

30

(511) (422)

45

60

(440) (a)

75

90

2-theta (deg) Figure 3.66: The XRD pattern of(a)CoFe2O4, (b) CoFe2O4@chitosan, and (c) CoFe2O4@chitosanPdhybrid nanocatalyst [22].

It is powerful instrument that illustrates what elements are present or what bonds they are created to. The technique can be utilized in line profiling of the elemental composition athwart the surface, or in-depth profiling, when coupled with ionbeam etching. For more study, the XPS investigation of the prepared magnetic nanocatalyst, MnFe2O4-rGO, is chosen. As observed in Figure 3.67b for Mn 2p, peaks relating to Mn 2p3/2 and Mn 2p1/2 are clearly seen at 642.2 eV and 653.2 eV, which is clear evidence for Mn2+ chemical connects on the sample surface. All Fe 2p spectra (Figure 3.67c) indicated at binding energies of 711.5 and 725.1 eV are two main peaks and are marked as Fe 2p3/2 and Fe 2p1/2, respectively. Moreover, two follower peaks were revealed to prove the presence of Fe3+ cations. The Mn and Fe reaction with PMS and dyes decreased the area of peaks, and the related values slightly shifted towards lower values of binding energy. It suggested that Mn and Fe particles on the prepared catalyst surface are in a mixed-valence. Also, Mn (II) and Fe (III) transformed to Mn (III) and Fe (II), since the deconvolution of Mn (2p) and Fe (2p) lead to the formation of Mn (III) and Fe (II) species about 41% and 42%, respectively. Therefore, the activation of PMS

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by MnFe2O4 NPs proved to occur in redox Mn (II)/Mn (III), and Fe (III)/Fe (II) species in MnFe2O4 NPs, for high catalytic activity. The C1s peak remarked at 284.8 eV relates to the elemental carbon in graphene (Figure 3.67d), which is in accordance with the XRD and Raman results. The O1s patterns of XPS illustrate two peaks (Figure 3.67e). The O1s peak near 529.9–530.3 eV and 531 ~ 532 eV corresponds to the O1s peak of MnFe2O4, and the chemisorbed oxygen groups (such as -OH and -COOH) bonded with C atoms in the graphene layers and in the O-MnFe2O4 interfacial bonding structure. The most active oxygen species make the chemisorbed oxygen on the MnFe2O4-rGO catalyst surface. These species play a key role in the oxidation reaction. The PMS activation via oxidation was suitable for the fabrication of graphene oxides. Magnetic separation displays no clear dissolution of MnFe2O4 and MnFe2O4-rGO hybrid, meaning that their structure is completely stable. Hence, MnFe2O4 and MnFe2O4-rGO hybrid are appropriate for water treatment [7]. The methodology in the MFe2O4-rGO synthesis was reported one more time by Yao group [69] with changed metal. They synthesized CoFe2O4-rGO and proved that MFe2O4 is combined and chemisorbed with rGO layers via different analyses. Some, not all, of the chemical states of surface elements have been determined in various materials by XPS. As has been explained previously for the magnetic particle sample, Fe3O4O1s (531 eV) and Fe (712 eV) peaks are observed in XPS spectra [30, 78, 93, 97]. But for other elements, based on the detail of organic or inorganic compounds, XPS can be different. We cannot discuss that here, so if further information is required, reference to the original sources, which were pointed earlier, is suggested.

3.3.8 AAS Determination of the quantity of chemical elements utilizing the optical radiation absorption by free atoms in the gaseous phase is called Atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES). This spectroscopy worked according to light which is absorbed by free metallic ions. Hence, using AAS is determined, 0.499 mmol/g of manganese has been loaded in CoFe2O4@Mn-BDC [29].

3.3.9 ICP-AES The technique employed for the detection of chemical elements is called inductively coupled plasma atomic emission spectroscopy (ICP-AES) or inductively coupled plasma optical emission spectrometry (ICP-OES). In this method, the excited atoms and ions are generated by applying the inductively coupled plasma, the particles that emitted electromagnetic radiation at wavelengths identifying a specific element. Factually, it is a kind of technique to indicate how many grams, percent, or mole of an atom

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(a) After reaction

After reaction (b)

Survey

C 1s

Mn 2p

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O 1s Fe 2p Mn 2p

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Before reaction

1000

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400

200

0

660

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740

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282

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Binding Energy (eV)

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Before reaction

525

530

535

540

545

Binding Energy (eV)

Figure 3.67: XPS spectrum of MnFe2O4@rGO before and after the reaction in the (a) survey scan; (b) Mn2p region; (c) Fe2p; (d), C1s; and (e) O1s energy regions [7].

is loaded in the synthesized particles. In continuation, we will discuss a few prepared magnetic nanocatalysts. The prepared nanocatalyst evaluation shows that one gram of nanocatalyst combined with 0.72 g (72% w/w) of Fe3O4 and 0.28 g (28% w/w) of dextrin polysaccharide

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[92]. The exact amount of copper in Fe3O4@TiO2-Pr-2AB@Cu was 1.47 mol/g [38], the amount of Ru(III) in the samples of Fe3O4@CMC-Ru(III) was 0.238 mmol/g [82], each gram of the functionalized magnetic HAp is about 0.425 mmol of Ag nanoparticles [43], the concentration of the Pd nanoparticles inMP@RF-Pd@hSiO2 in the solution is 3.8 µg/ml in [30], the exact amount of Cu in MNPs@β-CD@Cu(OAc)2 and Cu-BPy@Am-SiO2@Fe3O4 were 1.73 × 10−3 mol/g [8] and 0.1542 mmol/g, respectively [34].

3.3.10 TGA Measuring the synthesized sample’s mass done by a technique of thermal analysis over time as the temperature changes is called thermo gravimetric analysis or thermal gravimetric analysis (TGA).The analysis of TGA provides information about physical and chemical parameters related to adsorption and decomposition. Thermometric studies for a few hybrid magnetic nanocatalysts were carried out in the range of room temperature to 900 °C, under N2 atmosphere. Firstly, a low amount of weight, about 1–4%, reduced at a temperature less than 200 °C. It happened because the molecules of solvent (mainly water) and surface hydroxyl groups had physically adsorbed to remove. In the next step, at the temperature range between 200 °C and 500 °C is when the organic part in the nanocatalysts was decomposed. Therefore, the lost weight loss at that temperature range is related to the organic attaches to the magnetic catalyst [8, 10, 31, 32, 35, 37, 38, 43, 58, 61, 82]. For example, the residual mass achieved for magnetic dextrin until 500 °C was about 80%, whilst for net dextrin it was less than 5% [92]. The thermal analysis of Fe3O4@TiO2-Pr-2AB@Cu [19], MNPs@SiO2@PUF@Zn [53], and all organic-inorganic hybrid magnetic nanoparticles indicate organic groups were linked on the surface of MNPs and also shows stability at high temperatures. TGA was performed in the temperature range of 25–800 °C under air flow (Figure 3.68), to investigate the thermal stability of MNPs@ SiO2-NH2 and Fe3O4@HPro-OAc and to locate the loading of coated organic groups on the surface of MNPs. In the range of 0–200 °C, it was slightly sloping in the area of thermal decomposition of the catalyst. The area of 0–100 °C was ascribed to the loss of moisture (the removal of surface-adsorbed water). At the range of temperature between 100 and 200 °C, weight loss was because of the loss of structural water within amorphous SiO2. A sharp decay rate was seen at temperatures ranging from 220–425 °C, which can be related to the decomposition of organic (L-proline) coating. The iron oxide and silica combined at high temperatures. The TGA curve of MNPs@ SiO2-NH2 displayed a cleavage profile similar to that of Fe3O4@HPro-OAc, except for the disappearance of the decomposition step associated with the L-proline moiety (Figure 3.68a). TGA analysis showed that the L-proline add-up of Fe3O4@HPro-OAc was ca. 0.007 mmol/g, and the total weight loss of Fe3O4@HPro-OAc was estimated to be 9.5% [40].

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100

2.5 % a

Weight loss (%)

98 96

9.5 % 94 92

b

90 88 0

100

200

300

400

500

600

700

800

Temperature (ºC) Figure 3.68: TGA data of MNPs@ SiO2-NH2 (a) and Fe3O4@HPro-OAc (b).

3.3.11 BET Brunauer–Emmett–Teller (BET) analysis is used for investigating the surface area, pore-volume, and pore diameter of nanoparticles. As for the reported results, utilizing magnetic material in spite of the synthesis method had caused all the nanoparticles to have pores with large diameters and an area with high surface. Yielding media of the precipitation-hydrothermal and syntheses with a multiplex way, including precipitation and ultrasonic, has the highest and relatively high surface area, respectively, and the maximum pore diameter and volume in comparison to other samples because of the interconnected nanosheets, as seen in the FESEM images. Therefore, it can predict all the compounds containing mesoporous morphology; the comparison of the results indicates that the combustion prepared specimen has a much more mesoporous structure than the other specimens, as forestalled. The combustion sample also has a mean pore diameter lower than the other samples because the surface and pores are covered with an active phase layer [94]. Herein, the BET results of a few nanocatalysts have been explained for better understanding of the topic. For example, the specific surface area and total pore volume for the nanoparticles of Fe3O4@TiO2-Pr-2AB@Cu were 15.02 m2/g and 0.115 cm3/g, respectively [38], for MNPs@SiO2@PUF@Zn was 237.21 m2/g, 0.6173 cm3/g, and the average pore size was 25.71 nm [32], for MP@Void-Pd@mSiO2 are estimated to be as high as 536.6 m2/g and 0.37 m3/g [30], for OPSF-SO3H-HCl were evaluated at 34.88 m2/g with a pore volume of 0.12 cm3/g and pore size 12.52 nm [39], for [Ag@mHAp-Si-(S)] it was 198 m2.g/1 to 112 m2/g [43]. One of the applied properties of heterogeneous catalysts is recoverability, which is important both from the environmental and industrial perspectives. Hence, several

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experiments were designed for proving these applied features of the synthesized organic–inorganic hybrid catalysts. Accordingly, the synthesized magnetic nanocatalysts were recovered and reused in the synthesis of organic reactions, some of them recycled several times without a change in their chemical and physical properties. For instance, Magnetite dextrin [92], CuFe2O4@chitosan [61],CoFe2O4@Mn-BDC [29], MNPs@SiO2@-PUF@Zn [32], MnFe2O4@SiO2-NH-β-CD@Cu(OAc)2 [8], Fe3O4@Au@PDA [10], C-dots@CuFe2O4 [78], Fe3O4@SiO2-AQ [58], MNPs-Sp-HEP [56], MP@Void-Pd@mSiO2, MP@Void-Au@mSiO2,MP@Void-Pt@mSiO2 [30], Fe3O4@SiO2-DABCO [33], and IM@Fe3O4 [74] reused after five-six consecutive runs, the nano-Fe3O4@TiO2-Pr-2AB@Cu [38], MgO@MgFe2O4(C) mesoporous [94], Cu-BPy@Am-SiO2@Fe3O4 [34], α-Fe2O3@Hap@Cu [31], Ag@mHAp-Si-(S) [43], Fe3O4@CMC-Ru(III) [82], Fe3O4@Ph-SO3H [35], [email protected]%Pd [36], and OPSF-SO3H-HCl [39] were recycled seven-ten times, without any considerable decrease in its activity. The FT-IR spectra and the XRD analysis, and in some samples, the SEM and VSM images of the recycled catalyst were proved, after several sequential reuses in the reactions – the catalyst properties, and activity had not changed in comparison to the fresh catalyst. In addition, the obtained products in the presence of the prepared magnetic nanocatalysts were synthesized in high yields, without using harmful solvents (or solvent-free) and in short reaction times. Also, all the catalysts can be readily separated and recycled from the reaction system by a magnet [1–4, 6–19, 21–39, 41–46, 49–136].

3.4 Functionalization and applications One of the remarkable features of MNPs is their easy separation of reaction product via an external magnet that not only deletes the filtration and centrifugation method but also reduces energy consumption, catalyst loss, and saves time in obtaining catalyst recycle. This is what caused it to be applied widely in the synthesis of various organic compounds. The range of applications enhanced a lot with MNPs-coated silica, since the supporting of organosilicon prevents agglomeration of MNPs. This combined MNPs and silicones increases the properties of MNPs such as thermal and mechanical stability, and the presence of silanol groups on the surface leads to the addition of a wide variety of functionalities. Actually, SiO2NPs have functioned as hardly flexible supports for disabling metal complexes, heteropolyacids, and so forth. Hence, paving the surface of the metal with silica is a method for the preparation of inorganic–organic hybrid nanocatalysts. Literally, the functional groups linked to the silica and metal or inorganic species have intermolecular interaction with each other, which is very important in making many reactive catalytic centers. Accordingly, several classes of organic reactions employing MNP-based organic–inorganic catalysts have been reported, in recent years. Two of the most important features of magnetic nanoparticles

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are their size and shape, which give them other unique properties. Using parameters that contain time, temperature, concentration and the use of covering agents to retouch their magnetic and surface properties were tried, to control the magnetic nanoparticle’s size and morphology. As for reports on papers, organic–inorganic hybrid magnetic nanocatalysts are synthesized based on the knowledge, available chemical, and required organic compounds. In this case, such nanocatalysts are applied in various reactions. For example, magnetic dextrin nanocatalyst in synthesis of polyhydroquinoline derivatives [92], Fe3O4@TiO2-Pr-2AB@Cu for synthesis of substituted 1,4-dihydropyridines, amido-alkylnaphtholes and pyrano[3,2-c]chromen-5(4H)-one derivatives [38], CuFe2O4@chitosan for α-aminonitrile derivatives via the reaction of different aromatic aldehydes with aniline and trimethylsilyl cyanides [61], Fe3O4@CMC@ Ru(III) for the synthesis of 1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrilesand polyhydroquinoline derivatives [82], CoFe2O4@MnBDC in the multicomponent click protocol to access 1,4-disubstituted 1,2,3-triazoles [29], and urea-functionalized Fe3O4@SiO2 magnetic nanocatalyst for the synthesis of the one-pot multicomponent of substituted imidazole derivatives [2], MnFe2O4@SiO2@β–CD@Cu (OAc)2 for the synthesis of spiropyrans and high yields [8], Fe3O4@IL-SiO2-Pd in three various types of C–C bond formation reactions: Heck, Suzuki, and carbonylation reactions (Figure 3.69) [54], Fe3O4@Sp@HEP for removal of noxious Pb(II) and As(III) metal ions from aqueous media through a batchwise method [56], Fe3O4@ SiO2-IL for the one-pot synthesis of thiazolidin under solvent-free conditions and in high yield [24], [α- Fe3O4@ Hap-SiO (PO4)-Pr-N-Prydin-Cu] [31] and-γ-Fe2O3@HAp Si (CH2)3 AMP] [66] for Three-component 1,3-dipolar azide–alkyne and pyran derivatives cycloaddition reaction, Fe3O4@SiO2AQ for the green synthesis of substituted 2-amino-4H-chromenes through the one-pot condensation reaction of an aldehyde, malononitrile and α-naphthol/ β-naphthol/ phenol under solvent-free condition [58], [Ag@mHAp-Si-(S)] for oxidation of primary amines and reduction of aromatic nitro compounds (Figure 3.70) [43], Cu-BPy@AmSiO2@Fe3O4 for tandem oxidative cyclization reactions [34], Fe3O4@SiO2-DABCO for the synthesis of pyran derivatives through multicomponent reaction [33], CoFe2O4@SiO2@PUF@Zn(OAc)2 for the synthesis of spirooxindolopyran and spirooxindoloxanthene derivatives in aqueous medium [32], Fe3O4@GO-KGM-Pd for the hydrolysis of ammonia borane [75], Fe3O4@SiO2-PGMACD for substrate-selective oxidation of alcohols system [45], CuFe2O4@chitosan for synthesis of α-aminonitriles (Figure 3.71) [61], CoFe2O4@[Cu2(Si-N¼SA)2(bipy)2(PTA)] for ultrafast synthesis of acetaminophen [67], Fe3O4@mSiO2@Cu4 for the mild oxidation of cycloalkanes and an alcohol [11], Fe3O4@SiO2@PPh3@[CrO3Br] for a one-pot four-component synthesis of 1,4-DHP derivatives through the Hantzsch reaction of dimedone, β-ketoesters, and aldehydes [1], Fe3O4@SiO2@propyltri ethoxysilane@o-phenylendiamine-SO3H-HCl for the synthesis of 7-aryl-8H-benzo[h]indeno[1,2-b]quinoline-8-ones derivatives [39], Fe3O4@SiO2@propyltriethoxysilane@L-proline for the one-pot synthesis of 2,4,6triarylpyridines through three-component reaction of acetophenone, aryl aldehydes

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and ammonium acetate [55], and Fe3O4@IPS/DES [42] and Fe3O4@APTPOSS [53] in three-component reaction for syntheses of pyran derivatives were used.

Figure 3.69: Hech, Suzuki, and Carbonylation reactions catalyzed by Fe3O4@IL-SiO2-Pd(59).

21 [Ag@mHAp–Si–(S)]

RNH2 181

RNHOH Et2O, UHP, r.t.

182

Figure 3.70: Oxidation of primary amines by the [Ag@mHAp-Si-(S)](21).

Ph R PhNH2 + TMSCN 183

184

N CuFe2O4@chitosan

+ O 185

HN

EtOH, r.t. R

186

Figure 3.71: Synthesis of α-aminonitrile derivatives by CuFe2O4@chitosan.

Also, [email protected] [email protected](II), as a novel silicon-containing polymeric organic–inorganic hybrid nanomaterial for the oxidation of cyclohexene, ethylbenzene, and toluene in the presence of H2O2 [19], the CuAcTp@Am-Si-Fe3O4 for aerobic N-alkylation of amines with different alcohols [50], Fe3O4@LD–Cu for the aerobic and H2O2 oxidation of benzyl alcohol [91], [email protected] [email protected] for the oxidation of primary and secondary alcohols, thio urea-functionalized MNPs@Hap for the synthesis of beta ketonitrile [44] were applied.

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3.5 Conclusions The important factor that has led to the use of MNPs in the synthesizing of all nanocatalysts is the easy separation of the reaction mixture by an external magnet. Other advantages of organic–inorganic magnetic nanocatalysts apply to other catalysts, immobilizing organic compounds on the surface of MNPs with or without silica coating through a covalent bond or chemisorb. On the other hand, hybrid magnetic nanocatalysts are known to be reactive and recoverable nanocatalysts. However, the unavoidable constraint related to the intrinsic mutability of magnetic nanoparticles over a longer frame of time has to be addressed, in order to increase the reusability of the supported catalysts and, thus, keep cost at a minimum for the catalyst precursors. It is necessary to decrease or clear the leaching of active sites during the reaction. Additionally, the maximum surface area of supported magnetic nanoparticle sis used and their immobilizing is stable and active. Also, organic–inorganic hybrid magnetic nanocatalysts are being prepared to be further used in multicomponent reactions.

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Mohammadreza Shokouhimehr*, Ho Won Jang

Chapter 4 Magnetic nanostructured catalysts for reduction of nitroaromatics 4.1 Introduction Reduction of organic compounds is an indispensable transformation in industrial chemistry [1–3]. For example, the reduction of nitroarenes to their arylamine counterparts is a common route to produce important intermediates that are utilized for the production of agrochemicals, cosmetics, dyes, pesticides, pharmaceuticals, polymers, etc. [4–7]. In general, these transformations are expedited by diverse catalysts [8–10]. Therefore, the appropriate composition and structure of the catalysts can significantly affect the process efficiency and sustainability, and reusability of the applied catalysts [11]. In this regard, magnetic nanocomposite catalysts have superior advantages compared to their non-magnetic peers as they can be easily separated from the reaction media after completion of the reactions using an external magnet [12–14]. Consequently, researchers have incorporated magnetic nanoparticles to variable nanostructured materials possessing active metal nanocatalysts for the accomplishment of durable and reusable magnetic nanocomposite catalysts [15]. In general, these catalysts presented high stability and recyclability compared to their non-magnetic counterparts, providing green and environmentally benign protocols [16, 17]. Herein, selected representative examples expatiating the rationale and the concepts behind the magnetic nanocomposite catalysts applied for the reduction of nitroaromatics are presented via the composition and structure of the introduced catalysts.

4.2 Magnetic nanostructured catalysts for the reduction of nitroaromatics Patra and colleagues prepared a magnetic silver (Ag) nanocomposite catalyst by decorating Ag nanoparticles (NPs) on the surface of Fe2O3 NPs for the reduction of nitroarenes. First, Fe2O3 NPs were hydrothermally synthesized by admixing sodium

*Corresponding author: Mohammadreza Shokouhimehr, Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea, e-mail: [email protected] Ho Won Jang, Department of Materials Science and Engineering, Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Republic of Korea https://doi.org/10.1515/9783110730357-004

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hydroxide and sodium salicylate, followed by the injection of aqueous iron (III) nitrate. Then, Ag NPs were decorated on the Fe2O3 NPs [18]. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images of the prepared magnetic nanocomposite catalyst show that the Ag NPs were well adorned on the bitruncated octahedron-shaped iron oxide NPs (Figure 4.1).

Figure 4.1: (a and b) TEM, and (c and d) HRTEM images of magnetic Ag nanocomposite catalyst. Reproduced with permission from reference [18].

The catalytic activity of the magnetic Ag nanocomposite catalyst was investigated in the reduction of different substituted nitroaromatics (Figure 4.2a). The kinetics study of the reaction was explored in the reduction of 4-nitrophenol (0.1 mmol/L, 2 mL) with sodium borohydrate (10 mmol/L, 200 μL). The process needed ~10 min for the complete conversion as monitored by ultraviolet-visible (UV-vis) spectroscopy (Figure 4.2b). The reusability of the magnetic Ag nanocomposite catalyst was verified for the reduction of 4-nitrophenol as a model reaction for 10 cycles, presenting high catalytic activity (Figure 4.2c). Varma and colleagues developed a sustainable procedure for the reduction of nitroaromatic compounds using a magnetically reusable magnetite-supporting gold (Au) nanocatalysts [19]. They first prepared maghemite NPs by co-precipitation of FeSO4 and FeCl3. Then, the gold precursor was impregnated magnetically by separable maghemite NPs, followed by a chemical reduction (Figure 4.3), to produce magnetic Fe-Au nanocatalyst.

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Figure 4.2: (a) Reduction of nitroaromatics, catalyzed by the magnetic Ag nanocomposite catalyst. (b) Kinetics study for the reduction of 4-nitrophenol. (c) Reusability of the magnetic Ag nanocomposite catalyst in the reduction of 4-nitrophenol in water. Reproduced with permission from reference [18].

Figure 4.3: Synthesis of magnetic Fe-Au nanocatalyst.

The catalytic performance of the prepared catalyst was investigated in the reduction of nitro compounds using HCOONH4 as reductant, exhibiting excellent activity and selectivity for various functional-substituted substrates (Table 4.1). Importantly, the magnetic nanocatalysts were easily separated using an external magnet and reused for five cycles with negligible loss of activity. Magnetic heteroatoms nanocatalysts are generally composed of two different NPs, merging the magnetic NPs properties and the advantage of catalytic metal NPs [20]. Jang and colleagues synthesized magnetic Rh-Fe3O4 heterodimer nanocatalysts and utilized them for the reduction of nitroaromatics [21]. The magnetic heterodimer catalyst was synthesized by a heating one-pot procedure. Fe(acac)3 and Rh(acac)3 were admixed with oleylamine and oleic acid and heated at 300 °C. Figure 4.4 shows the HRTEM images of the magnetic Rh-Fe3O4 heterodimer nanocatalysts, clarifying that the prepared catalysts are comprised of Rh NPs (2–3 nm) and Fe3O4 NPs (~16 nm). The magnetic Rh-Fe3O4 heterodimer nanocatalysts presented high catalytic activity for the reduction of nitro compounds with different functional groups using hydrazine reductant in 1 h. In addition, the catalyst showed stable catalytic activity for eight cycles in the reduction of nitrobenzene.

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Table 4.1: Reduction of nitroaromatic compounds by magnetic Fe-Au nanocatalyst.a

Entry

Nitroaromatic

Time (h)

Product

Isolated yield (%)





































a

Reaction conditions: nitroaromatic compound (1 mmol), HCOONH4 (8 mmol), magnetic Fe-Au nanocatalyst (80 mg), EtOH (3 mL), 70 °C.

Although nano-sized magnetic catalysts generally have active sites exposure, their high surface energy leads to aggregation and instability in the reaction media, decreasing their high catalytic activity. Furthermore, their scale-up production is expensive and sophisticated. To resolve these drawbacks, metal catalyst NPs and magnetic NPs have been supported on variable assorted nanostructured materials, particularly porous supports, to integrate their catalytic and reusability properties in hybrid nanocrystals [22]. When the porous materials are utilized to support both nanocatalysts and magnetic NPs, the overall surface area of the heterogeneous catalyst can be significantly increased. Shokouhimehr et al. introduced a scalable procedure to prepare a magnetically recyclable porous carbon nanocomposite catalyst by incorporating nitrogen-doped porous carbon, magnetic NPs, and ~3 nm platinum (Pt) nanocatalysts [23]. Figure 4.5 shows the synthetic procedure for the preparation of the magnetically recyclable carbon nanocomposite catalysts. The designed catalysts presented a relatively high surface area, containing abundant and accessible Pt

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Figure 4.4: HRTEM images of magnetic Rh-Fe3O4 heterodimer nanocatalysts. Reproduced with permission from reference [21].

nanocatalysts NPs. Consequently, the prepared catalyst presented high catalytic activity for the reduction of nitroaromatic compounds. It could also be reused in the reduction of nitrobenzene using hydrazine as the reductant, which successfully recycled for five consecutive cycles without a significant loss of activity.

Figure 4.5: Synthetic procedure for the preparation of magnetic Pt nanocomposite catalyst. Reproduced with permission from reference [23].

In another important study, Shokouhimehr and colleagues prepared an interesting magnetically recyclable hollow nanocomposite catalyst with high surface areas at a large scale for the reduction of nitroaromatic compounds [24]. Hollow catalysts are generally prepared by sophisticated hard-template and etching methods in small scales. In addition, they have unstable structures in the reaction media, which leads to their aggregation and scaffold collapse, causing catalytic activity decay [25]. However, the magnetically recyclable hollow nanocomposite catalyst was prepared by a simple method, incorporating magnetic properties and self-assembled hollow nanostructure.

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This catalyst could be prepared at a large scale of ~2.3 g in only one-batch synthesis. The nanocomposite catalysts provided excellent catalytic activities for the reduction of nitroaromatics. The synthetic procedure is shown in Figure 4.6. The porous carbon coating layer preserved the hollow catalyst from structural collapse during the catalytic reactions, while it offered accessible paths to the nitroaromatic compounds, enhancing the diffusion of the substrates and products. This catalyst could also be reused for five cycles in the reduction of nitrobenzene without a major loss in activity.

Figure 4.6: Synthetic procedure for the preparation of magnetic carbon hollow nanocomposite catalyst-Rh NPs. Reproduced with permission from reference [24].

Polymeric materials are also very appropriate scaffolds to support various nanocatalysts as their layers can protect the nanocatalysts and magnetic NPs from aggregation, corrosion, and dissolution in the reaction media [26]. Ayad and co-workers prepared Ag NPs, adorned on a polyaniline-chitosan-magnetite (Ag@PANI-CS-Fe3O4) nanocomposite catalyst (Figure 4.7) for the reduction of 4-nitrophenol in aqueous sodium borohydride media, which affirmed rapid reduction of 4-nitropehonl to 4-aminophenol in ~10 min and the magnetic recycling remained reactive with an efficacy of ~95% after a four cycle [27]. Varma and colleagues synthesized a magnetically recyclable nanocomposite decorated with Pd NPs, which could be utilized for the reduction of nitroaromatics in aqueous sodium borohydride solution [28]. Polymerization of pyrrole monomers was achieved in the presence of iron metal NPs and Pd precursor, forming Pd NPs on the polypyrrole scaffold, without using any reductant (Figure 4.8). Field emission scanning electron microscopy (FESEM) and TEM images of the magnetic polymer catalyst confirmed Pd NPs (∼2 nm) adorned on the polypyrrole (Figure 4.9). The magnetic catalyst could be separated and reused by a magnet for seven cycles of reduction of nitrobenzene (99–95%). Similarly, Rahimi and colleagues prepared an organic-inorganic hybrid magnetic nanocomposite catalyst made of polyvinyl alcohol (PVA), iron oxide NPs, and Ag NPs (Fe3O4/PVA/Ag) [29]. The magnetic Fe3O4/PVA/Ag nanocomposite catalyst was synthesized by dropping a solution containing FeCl2 and FeCl3 into a PVA solution by an in situ method, as shown in Figure 4.10. The prepared catalyst provided excellent activity for the reduction of a series of functionalized nitroaromatic compounds in

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Figure 4.7: Preparation procedure of Ag@PANI-CS-Fe3O4 nanocomposite catalyst.

Pd precursor in EtOH

Pd NPs

Fe NPs

Pyrrole in EtOH Polymer

Fe NPs in EtOH

Figure 4.8: Synthetic procedure for the preparation of magnetic polypyrrole nanocomposite Pd nanocatalysts. Reproduced with permission from reference [28].

a short reaction time (5–16 min). In addition, the nanocomposite catalyst could be reused for five cycles in the reduction of nitrobenzene to aniline, losing its activity significantly, perhaps due to the catalytic loss during the separation. Although the magnetic catalysts generally provide facile separation and recycling, their complete and fast retrieval, in particular, for the small-sized magnetic nanocatalyts is very challenging and typically leads to rapid transformation efficiency loss [30]. Consequently, it is highly demanding to accomplish a strong and steady magnetic separation to address the reusability issues of the magnetic nanostructured catalysts. To resolve the aforementioned problems, Zeng et al. prepared gold (Au) NPs on polydopamine (PDA)-encapsulated magnetic microspheres (Fe3O4@PDA-Au) nanocomposite catalyst for the reduction of nitro benzene [31]. Core Fe3O4 NPs (~400 nm) were coated with PDA with a thickness of ~45 nm on which abundant and accessible small-

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Figure 4.9: (a) TEM image and (b) HRTEM image of iron NPs. (c) FESEM image of iron NPs. (d) TEM image and (e) HRTEM image of the magnetic polypyrrole nanocomposite Pd nanocatalysts. (f) FESEM image of the magnetic polypyrrole nanocomposite Pd nanocatalysts. Reproduced with permission from reference [28].

Figure 4.10: Synthetic pathway of the Fe3O4/PVA/Ag nanocomposite catalyst. Reproduced with permission from reference [29].

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sized Au NPs were deposited simultaneously, as PDA functioned as a reductant and stabilizer so that no additional reducing agent was needed (Figure 4.11). The Fe3O4@PDA-Au nanocomposite catalyst provided a high catalytic performance in the reduction of nitroaniline, with a conversion of 99% in 7 min using NaBH4. Importantly, the catalyst was easily separated using an external magnet and reused with excellent reusability (8 cycles) with high conversion (>98%) for nitrobenzene reduction (Figure 4.12).

Figure 4.11: Synthetic procedure for the preparation of Fe3O4@PDA-Au nanocomposite catalyst. Reproduced with permission from reference [31].

Recently, the application of metal organic frameworks (MOFs) has attracted substantial attention in diverse research areas, including heterogeneous catalysis [32]. MOFs have generally high surface areas and accessible pores, presenting excellent performance in the catalytic transformations [33]. Furthermore, their combination with magnetic NPs can provide facile recovery by a magnetic field and present excellent reusability. Chen and co-workers introduced a layer-by-layer assembly procedure for the preparation of noble metal (M = Au, Pd, Pt) nanocatalysts encapsulated in magnetic MOFs (Fe3O4@M/MIL-100) for the efficient reduction of nitroaromatics [34]. Figure 4.13 shows the synthetic procedure for the preparation of the Fe3O4@M/MIL100 catalyst. The catalytic performance of the three prepared magnetic MOFs catalysts (Au, Pd, Pt) was examined in the reduction of p-nitrophenol with NaBH4. Pd

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100 90

Conversion (%)

80 70 60 50 40 30 20 10 0

1

2

3

4

5

6

7

8

Cycle No. Figure 4.12: Recycling of the Fe3O4@PDA-Au nanocomposite catalyst in the reduction of nitrobenzene. Reproduced with permission from reference [31].

and Pt-incorporated magnetic MOFs catalysts presented higher catalytic activity and reusability than the Au catalyst. The Pt magnetic MOFs catalyst could efficiently reduce various functionalized nitroaromatics to extend the substrate breadth of the reduction transformation and be reused for 10 cycles without a significant loss of activity.

Figure 4.13: Synthetic procedure for the preparation of Fe3O4@M/MIL-100 catalyst. H3btc: 1,3,5-benzenetricarboxylic acid; MAA: mercaptoacetic acid. Reproduced with permission from reference [34].

The combination of mesoporous silica with magnetic NPs is an important methodology in the preparation of heterogeneous catalysts because the mesoporous silica

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provides high surface area, excellent stability, and tunable pore size for the supported metal nanocatalysts, enhancing their catalytic performance and durability [34]. Yao and co-workers synthesized core-shell FexOy/Pd@mesoporous magnetic NPs in an autoclave, followed by calcination at 600 °C (Figure 4.14) [35]. The catalytic efficiency of the prepared core-shell catalyst was ascertained for the reduction of nitroaomatics, which was achieved in ~40 min. Consecutive recycling of 4-nitrophenol hydrogenation was accomplished with excellent conversion for each cycle (~100%).

Figure 4.14: Synthetic procedure for the preparation of FexOy/Pd@mesoporous magnetic catalyst. TEOS: tetraethyl orthosilicate; CTAB: cetrimonium bromide.

Dong et al. synthesized porous silica sphere with a dandelion shape, decorated with Ni@Pd NPs, for the reduction of 4-nitrophenol (Figure 4.15). The porous silica spheres (with size of 200–300 nm) effectively prevented the agglomeration of Ni@Pd nanocatalysts, presenting highly accessible nanocatalysts for the reduction of nitroaromatics [36]. The magnetically recyclable porous silica sphere catalyst could be reused in the reduction of 1-nitrophenol for six consecutive runs with negligible loss of catalytic activity, demonstrating its promising applicability for various catalytic transformations and industrial usages. The application of biomaterials and benign processes for the green synthesis of magnetic nanostructured catalysts has found their edge over the traditional physicochemical procedures due to their environmental and ecological compatibility. Li et al. decorated Au NPs on pectin as a protective shell and as a bioreductant – a

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Functionalization

SH

Toluene

SH

Mesoporous silica

SH Ni@Pd NPs

Functionalized mesoporous silica

SH

Ni@Pd NPs supported on mesoporous silica

Figure 4.15: Synthetic procedure for the preparation of magnetic porous silica Ni@Pd nanocatalyst. Reproduced with permission from reference [36].

common fiber found in most plants – by incorporating Fe3O4 NPs; preparing a magnetic nanocomposite catalyst (Fe3O4/Pectin/Au) for the reduction of nitroaromatic compounds (Figure 4.16) [37]. The Fe3O4/Pectin/Au magnetic nanocomposite catalyst could be easily separated and reused for eleven cycles without a significant loss of activity.

Figure 4.16: Synthetic procedure for the preparation of Fe3O4/Pectin/Au magnetic nanocomposite catalyst.

Recently, enzyme catalysts have attracted substantial consideration because of their superb specificity and excellent efficiency. However, their practical application is limited due to their high expense and difficulties in recovery and reusability. To overcome these deficiencies, Tang and colleagues synthesized a magnetically reusable biocatalyst by the immobilization of nitroreductase, which was purified from Enterobacter cloacae onto the iron oxide NPs [38]. Figure 4.17 shows the synthetic procedure for the synthesis of the magnetically retrievable biocatalyst. The prepared magnetic enzyme catalyst could easily reduce substituted nitrobenzene moieties at room temperature, with yields up to 60.9%. Importantly, it could be recycled seven times, maintaining its high activity at ~85.0% of the last cycle; this study could resolve the reusability issues of enzyme catalysts encountered in the practical applications of biocatalysts and demonstrated a green method for the synthesis of substituted anilines. Figure 4.18 presents examples of reduction nitroaormatics, achieved by the magnetically reusable nitroreductase, by utilizing the designed magnetic heterogeneous nanostructure. The separation and recycling of the enzyme catalyst could be productively achieved.

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Figure 4.17: Synthetic procedure for the synthesis of magnetic biocatalyst.

Figure 4.18: Magnetically reusable biocatalyst in the reduction of substituted nitroaromatics. Reproduced with permission from reference [38].

4.3 Conclusions Magnetic nanostructured catalysts have been broadly applied in organic transformations, including in the reduction of nitroaromatics. The reduction of nitroaromatics is factually known to be a pivotal pathway in the preparation of polymers, agrochemicals, pharmaceuticals, dyes, pesticides, etc. The reduction of nitroaromatics has been widely exploited for decades, providing expeditious transformation of nitro groups to amino moieties. The application of inexpensive and reusable heterogeneous catalysts is indispensable for the completion of these transformations, in particular, at the industrial scale. Consequently, diverse nanostructured materials have been explored to support precious nanocatalysts for the development of

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economical, durable, and green catalytic reduction processes. Among them, the application of magnetic nanostructured catalysts has unveiled tremendous opportunities for the reduction of nitroaromatics, resolving the severe challenges and difficulties in catalysts’ separation and recyclability. For industrial applications, however, important deficiencies should be resolved, e.g., catalytic activity decay, catalysts’ leaching, attainment of region- and chemo-selectivity, etc. In addition, the utilization of biowaste and naturally abundant magnetic materials and green procedures need to be developed for the practical applications of the reduction of nirtoaromatics in the future.

References [1]

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[32] Karimi H, Heidari MA, Banna Motejadded Emrooz H, Shokouhimehr M. Carbonization temperature effects on adsorption performance of metal-organic framework derived nanoporous carbon for removal of methylene blue from wastewater; experimental and spectrometry study. Diam Relat Mater 2020, 108, 107999. [33] Tajik S, Beitollahi H, Garkani Nejad F, Sheikhshoaei I, Nugraha AS, Jang HW, Yamauchi Y, Shokouhimehr M. Performance of metal–organic frameworks in the electrochemical sensing of environmental pollutants. J Mater Chem A 2021, 9, 8195–8220. [34] Zhang H, Qi S, Niu X, Hu J, Ren C, Chen H, Chen X. Metallic nanoparticles immobilized in magnetic metal–organic frameworks: Preparation and application as highly active, magneticallyisolable and reusable catalysts. Catal Sci Technol 2014, 4, 3013. [35] Yao T, Cui T, Fang X, Fang C, Wu J. Preparation of yolk−shell FexOy/Pd@mesoporous SiO2 composites with high stability and their application in catalytic reduction of 4-nitrophenol. Nanoscale 2013, 5, 5896. [36] Dong Z, Le X, Dong C, Zhang W, Li X, Ma J. Ni@Pd core−shell nanoparticles modified fibrous silica nanospheres as highly efficient and recoverable catalyst for reduction of 4-nitrophenol and hydro dechlorination of 4-chlorophenol. Appl Catal B Environ 2015, 162, 372. [37] Li Y, Li N, Ma G, Zangeneh MM. In situ decorated Au NPs on pectin-modified Fe3O4 NPs as a novel magnetic nanocomposite (Fe3O4/Pectin/Au) for catalytic reduction of nitroarenes and investigation of its anti-human lung cancer activities. Int J Biol Macromol 2020, 163, 2162–2171. [38] Zhang Q, Yu L, Li F, Tang B. Reduction of nitroarenes by magnetically recoverable nitroreductase immobilized on Fe3O4 nanoparticles. Sci Rep 2020, 10, 2810.

Abhijeet Singh, Trisha Ghatak, Shalini Agarwal, Ramendra Pratap, Mahendra Nath*

Chapter 5 Applications of CuFe2O4 magnetic nanoparticles in organic synthesis 5.1 Introduction Magnetic nanoparticles-based catalytic systems are widely utilized to accelerate diverse organic transformations in organic synthesis. They have been extensively employed in various areas, including in catalysis, biomedicine, tissue specific targeting, magnetically tunable colloidal photonic crystals, micro-fluidics, magnetic resonance imaging, data storage, environmental remediation, optical filters, defect sensors, and cation sensors [1]. Further, these nanocatalysts often play a pivotal role in enhancing the product yields with better enantioselectivity under mild conditions during asymmetric synthesis. In fact, these heterogeneous catalytic systems provide a high surface area, which significantly enhances the rate of the reaction, to afford products at high yields. After completion of the reaction, these catalysts can be easily recovered from the reaction medium by using external magnets and efficiently recycled multiple times without a loss in their efficiency. These properties make them highly desirable candidates in the field of catalysis, particularly in the realm of green chemistry. Generally, magnetic nanoparticles are composed of a number of single magnetic nanoparticles, known as magnetic nanobeads, with a diameter of 50–200 nanometers [2]. They consist of a magnetic material, such as iron, nickel, cobalt, chromium, manganese, and gadolinium, and are highly economical, offering efficient synthetic solutions due to their reusable properties. Among these, ferrites or ferrimagnetic oxides contain ferric oxide along with another metal oxide. Ferrites are of three types, namely spinel, garnet, and hexagonal ferrites. They have been studied very extensively due to their low prices, easy to fabricate characteristics, and abundant uses in technological and industrial applications. The application of spinel ferrites usually depends upon the chemical composition, sintering temperature, preparation methods, and the trend of occupying

Acknowledgments: Mahendra Nath and Ramendra Pratap are thankful to the IoE, University of Delhi, India for their financial support. Abhijeet Singh, Trisha Ghatak, and Shalini Agarwal are grateful to the CSIR and the UGC, New Delhi, India for providing Junior and Senior Research Fellowships, respectively. *Corresponding author: Mahendra Nath, Department of Chemistry, Faculty of Science, University of Delhi, Delhi 110 007, India, e-mail: [email protected] Abhijeet Singh, Trisha Ghatak, Shalini Agarwal, Ramendra Pratap, Department of Chemistry, Faculty of Science, University of Delhi, Delhi 110 007, India https://doi.org/10.1515/9783110730357-005

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tetrahedral or octahedral sites. In ferromagnetic ceramic compounds, ferrites have spinel-type structures. The magnetic properties of ferrites occur due to the structure and distribution arrangement of the ions in the sublattice. Most of the ferrites have a spinel structure, with formula AFe2O4, where A is a divalent ion, such as Cu2+, Mg2+, Co2+, Ni2+, or Mn2+. Spinel ferrites are homogeneous in nature and both metallic cations, A2+ and Fe3+, occupy two different crystallographic sites – tetrahedral and octahedral [3]. Ferrites are basically categorized into normal and inverse spinel. In normal spinel, the divalent cation A and the trivalent cation Fe3+ occupy tetrahedral and octahedral sites, respectively. However, in inverse spinel, one-half of the trivalent Fe ions occupy the tetrahedral site and other half resides with the divalent metal ions in the octahedral site, as presented in Figure 5.1.

Figure 5.1: Categories of ferrites and their ionic distribution.

5.2 General methods of preparation and the structural characteristics of CuFe2O4 MNPs Nanoscale copper ferrite particles have shown a variety of applications in catalyzing organic reactions due to their unique electrical, optical, and magnetic properties. These properties are highly dependent on the size and shape of the nanoparticles [4]. Ideally, CuFe2O4 has an inverse spinel structure, having eight Cu2+ ions in the octahedral sites and sixteen Fe3+ ions equally distributed in the tetrahedral and octahedral sites. It is reported in literature that copper ferrite has two crystallographic spinel structures. The first is a cubic phase (c-CuFe2O4), which is observed at high temperature, and the second is the tetragonal phase (t-CuFe2O4), which is observed at low temperature [5]. That is why it is crucial to control the particle size during the synthetic process. According to literature reports, CuFe2O4 nanoparticles can be synthesized by a variety of ways as discussed below.

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5.2.1 Co-precipitation method This method involves a simple mixing of Cu2+ and Fe3+ salts under alkaline conditions, followed by heat treatment. Wang and co-workers synthesized CuFe2O4 nanoparticles by stirring a mixture of Fe(NO3)3.9H2O (0.02 mol) and CuCl2 (0.01 mol) in anhydrous ethanol for 30 min, followed by a slow addition of 3 M NaOH solution. Thus, the precipitate obtained was washed thoroughly with ethanol and water. Then, it was calcinated at 700 °C for an hour. X-ray diffraction (XRD) studies of the calcinated precipitate of CuFe2O4 nanoparticles showed a polycrystalline structure with a cubic jacob site. A particle size of around 20 to 40 nm was obtained, as confirmed by the transmission electron microscopy (TEM). Further, these nanoparticles have shown a ferromagnetic behavior at room temperature, as detected by the magnetometer [4].

5.2.2 Sonochemical method In general, homogeneous precipitation with a controlled particle size of the nanoparticles was observed in a significantly reduced time by using ultrasonic waves. Therefore, Zhang et al. utilized a sonochemical method to synthesize CuFe2O4 nanoparticles by mixing the aqueous solutions of CuCl2.2H2O (0.25 M) and Fe(NO3)3.9H2O (0.5 M). The desired nanoparticles were precipitated on slow addition of 0.5 M aqueous solutions of NaCO3 and NaOH as precipitation agents for 6.5 h under ultrasonic conditions. The resulting precipitate was washed with ethanol, followed by water, and dried at 850 °C for 2 h to afford CuFe2O4 nanoparticles. XRD studies revealed that the prepared CuFe2O4 nanoparticles have a crystallite size of 60 nm as calculated by the Scherrer’s formula, which was further confirmed by scanning electron microscopy (SEM) [6].

5.2.3 Ceramic method In this method, a stoichiometric mixture of CuO and α-Fe2O3 was ground in an agate mortar at 1,223 K for 20 h in three cycles to ensure completion of the reaction. The resulting material was cooled at a rate of 2 K/min during the third cycle. Then, a milling process was applied to reduce the particle size of the synthesized CuFe2O4 MNPs from 200 nm to 6 nm. The CuFe2O4 nanoparticles having a smaller size (6 nm) displayed super paramagnetic relaxation effects at ambient temperature, and their magnetization was not even saturated after applying a magnetic field of 9 Tesla [5].

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5.2.4 Sol-gel method In a sol-gel approach, a solution of Cu(NO3)2 (0.005 mol) and Fe(NO3)3 (0.01 mol) was poured into a 0.3 M solution of citric acid at 80 °C. The temperature of the reaction medium was maintained at 80 °C until a viscous and transparent gel was formed. Further, the resulting gel was dried in an oven at 130 °C for 3 h, and then annealed at 850 °C for 3 h at a heating rate of 10 °C/min. The XRD data of CuFe2O4 MNPs obtained by the sol-gel method showed a tetragonal structure. In addition, the TEM analysis of the fabricated material revealed an almost uniform particle size of 80 nm [7].

5.2.5 Green synthesis Wendari and co-workers reported a green synthesis of CuFe2O4 nanoparticles by using Morus alba L. leaf extract as a basic capping agent. The desired nanoparticles were synthesized by mixing Cu(NO3)2 (0.015 M) and Fe(NO3)3 (0.03 M) with Morus alba L. leaf extract at 60 °C for 2 h. Then, the reaction mixture was heated at 120 °C for 10 h to afford a viscous gel, which was further calcinated at 700 °C for 5 h. Interestingly, the synthesized nanoparticles demonstrated a cubic structure, as suggested by the XRD data. However, the SEM and TEM data analyses revealed spherical-shaped nanoparticles with an average particle size of 32 nm. The magnetic properties of these CuFe2O4 nanoparticles were analyzed by a vibrating sample magnetometer (VSM). Based on the data, these nanoparticles displayed a ferromagnetic behavior [8].

5.2.6 Microwave-hydrothermal method Thongtem et al. synthesized the CuFe2O4 nanoparticles using microwave irradiations. According to the reported procedure, a solution of Cu(NO3)2.6H2O (1 eq.) and Fe(NO3)3.9H2O (2 eq.) was prepared in distilled water and 3 M KOH solution was added. The reaction mixture was transferred into a fluoropolymer lining vessel, which was then heated to 150 °C for 60 min using a CEM Mars-5 microwave oven. After cooling, the precipitate was collected, washed with water, followed by ethanol, and dried at 80 °C for 24 h. The XRD data indicated a cubic structure of the synthesized CuFe2O4 nanoparticles, with a particle size of 2 to 10 nm, as calculated by the Scherrer’s formula. The TEM analysis of the sample further supported the particle size of the catalyst, obtained from the XRD data. The synthesized CuFe2O4 nanoparticles exhibited a ferromagnetic behavior with strong magnetic anisotropy [9].

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5.2.7 Microemulsion method Copper ferrite nanoparticles have also been prepared using the microemulsion technique. The first microemulsion was prepared at an ambient temperature by adding 5 mL solution each of K2Cu(CN)4 (0.05 M) and K3[Fe(CN)6] (0.1 M) into 80 mL octanol (Octanol acts as an organic phase and a co-surfactant). Furthermore, 10 mL of an aqueous solution of cetyl trimethylammonium bromide (CTAB) was added and the mixture was stirred at 1,200 rpm. The second microemulsion was prepared similarly, as described above, by mixing HCHO and CH3COONa. Then, both microemulsions were mixed and left for 3 h at room temperature. Finally, the resulting precipitate was separated by centrifugation, and washed thoroughly with absolute EtOH and hot water several times. The material was dried at 120 °C in an air oven and calcinated at 700 °C for 2 h to obtain nano-CuFe2O4 [10]. Furthermore, the surface of the copper ferrite nanoparticles is often modified by mobilizing surfactants, silica, silicones, or phosphoric acid derivatives to increase their stability in the solution. Such modified particles proved to be more efficient catalysts than the normal CuFe2O4 magnetic nanoparticles in catalyzing various organic reactions. In particular, ferrite nanoparticle clusters with a narrow size distribution consisting of super paramagnetic oxides coated with silica shell have some advantages, such as higher chemical and colloidal stability, over simple metallic nanoparticles. In addition, they also retain their super paramagnetic properties and the silica surface enables straightforward covalent functionalization [11].

5.3 Copper ferrite magnetic nanoparticles-catalyzed organic reactions Thorough literature survey revealed that copper ferrite magnetic nanoparticles (CuFe 2O 4 MNPs) have shown excellent ability to catalyze a variety of organic transformations, including acetylation of the functional groups, C-C bond coupling reactions, and one-pot multi-component strategies for the synthesis of diverse biologically relevant heterocycles and oxidation reactions. A detailed discussion about the ability of the CuFe2O4 magnetic nanoparticles in accelerating various organic reactions is presented in the following sections.

5.3.1 Acetylation reactions Although acetylation is a generic reaction and many effective procedures are available in the literature, surprisingly the use of magnetic nanoparticles (MNPs) often affords good yields of the products, with better chemo-selectivity. In one of the

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successful attempts, CuFe2O4 MNPs efficiently synthesized from the tea leaves extract containing polyphenols, flavonoids, and catechins have been employed for the acetylation of various primary amines, phenols, and alcohols, using acetic anhydride as an acetylating agent (Figure 5.2). All reactions proceeded well under neat conditions and afforded high yields of acetylated products (3) at ambient temperature [12]. Surprisingly, better chemo-selectivity was achieved for acetylating the alcohols under the same reaction conditions. For example, when a mixture of 4-nitrobenzyl alcohol and 4-nitrophenol was treated with Ac2O in the presence of a catalytic amount of CuFe2O4 MNPs, the 4-nitrobenzyl alcohol was exclusively acetylated to form 4nitrobenzylacetate in almost quantitative yield.

Figure 5.2: CuFe2O4-catalyzed acetylation of primary amines and alcohols.

Furthermore, Mousavi and co-workers reported the acetylation of aromatic amines (4) and one-pot reductive acetylation of nitroarenes (6) in the presence of CuFe2O4 nanoparticles in water (Figure 5.3) [13]. The use of NaBH4 as a reducing agent, along with Ac2O, yielded the N-arylacetamide (5) very efficiently under reflux conditions.

Figure 5.3: CuFe2O4 MNPs-promoted conversion of arylamines and nitroarenes to acetanilides.

5.3.2 Coupling reactions Cross-coupling reactions are very well documented in literature, which mainly requires the use of transition metals-based catalytic systems. Often, these catalysts are difficult to recycle and, sometimes, provide lower yields of the coupling products.

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However, the introduction of CuFe2O4 MNPs in organic synthesis demonstrated many advantages, including improved yields of the products, easy recovery and reusability of the catalyst after the first cycle of the reaction.

5.3.2.1 Sonogashira coupling reaction Sonogashira reaction is a palladium-catalyzed carbon–carbon cross-coupling reaction between aryl or vinyl halides and terminal alkynes. To this end, Gholinejad and co-workers [14] developed a methodology to assemble an effective catalytic system (Pd-CuFe2O4@SiO2) by incorporating CuFe2O4 MNPs and Pd nanoparticles on the surface of silica microparticles. The energy dispersive X-ray (EDX) analysis and TEM images confirmed the presence of Pd and CuFe2O4 nanoparticles on the surface of SiO2. Further, Sonogashira coupling reactions of aryl halides (7) and terminal alkynes (8) were carried out using 0.3 mol% magnetic Pd-CuFe2O4@SiO2 as a catalyst under basic conditions at 50 °C (Figure 5.4). Particularly, the use of 1,4-diazabicyclo[2.2.2]octane (DABCO) as a base and N,N-dimethylacetamide (DMA) as a solvent provided the best results, with products yields up to 97%. After completion of the reaction, the catalyst was recovered using a magnetic rod, and it could be reused up to three times without any loss in catalytic activity.

Figure 5.4: Pd-CuFe2O4@SiO2-catalyzed Sonogashira coupling to form internal alkynes.

5.3.2.2 Suzuki coupling reaction Suzuki coupling reaction is one of the most useful strategies for the construction of new carbon–carbon bonds in the products by reacting organoboranes (11) or boronic acids (13) with organic halides (10) using palladium catalysts under basic conditions. Instead, Ranu et al. [15] reported the use of CuFe2O4 nanoparticles to catalyze the Suzuki-type coupling reactions. The coupling of alkynyl bromides (10) and ester of alkynyl boranes (11) was carried out in dimethyl carbonate (DMC) by employing CuFe2O4 nanoparticles as a catalyst to form unsymmetrical 1,3-diynes (12) in moderate-to-good yields under basic conditions (Figure 5.5(i)). This synthetic methodology was further applied to synthesize a series of conjugated 1,3-enynes (14) in good yields from the reaction of alkynyl bromides (10) and alkenyl boronic acid (13) (Figure 5.5(ii)). After the

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completion of the reaction, CuFe2O4 nanoparticles were recovered from the reaction mixture by using an external magnet and were recycled up to ten times.

Figure 5.5: CuFe2O4 MNPs-catalyzed Suzuki coupling reaction.

In addition, self-coupling between the two moles of p-tolylboronic acid (15), in the presence of CuFe2O4 nanoparticles in methanol at room temperature, provides 4,4ʹ-dimethyl-1,1ʹ-biphenyl (16), as reported by Tapsale et al. [16] (Figure 5.6). The CuFe2O4 MNPs prepared using the sol-gel auto-combustion method were examined for reusability during the synthesis of biphenyl derivative (16). It was observed that the catalyst can be recycled up to 3–4 cycles at room temperature without any significant loss in its catalytic activity and afforded the coupling product at an excellent yield.

Figure 5.6: Synthesis of 4,4ʹ-dimethyl-1,1ʹ-biphenyl from 4-methylphenylboronic acid.

5.3.2.3 Carbon–heteroatom coupling reactions 5.3.2.3.1 C–N bond formation Yadav et al. [17] reported a C–N cross coupling using CuFe2O4 magnetic nanoparticles. According the reported methodology, indole and imidazole were coupled separately with aryl iodide/bromide (17) in the presence of a catalytic amount of CuFe2O4 MNPs under basic conditions to afford arylated indoles (18) and imidazoles (19) in good yields (Figure 5.7). It is interesting to mention that N-methylpyrrolidone (NMP) and K3PO4 were found to be the best solvent and base, respectively, when indole was

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taken as a substrate. However, DMF as a solvent and K2CO3 as a base gave good results in the case of imidazole (Figure 5.7).

Figure 5.7: CuFe2O4-catalyzed N-arylation of indole and imidazole scaffolds.

The plausible mechanism for a representative CuFe2O4 MNPs-catalyzed N-arylation of indole is depicted in Figure 5.8. This includes a well-known step of oxidative addition of aryl halide with the catalyst, followed by a nucleophilic attack of nitrogen containing heterocycle, to form a key intermediate, which on reductive elimination and desorption affords the product and also regenerates the catalyst for the next cycle.

Figure 5.8: Plausible mechanism for N-arylation of indole.

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Another ligand-free approach for the synthesis of N-arylated heterocycles has been reported by Panda et al. [18], where the catalytic activity of CuFe2O4 nanoparticles was investigated for the N-arylation of a wide range of nitrogen-containing heterocycles, such as pyrrole (20a), pyrazole (20b), imidazole (20c), indole (20d), benzotriazole (20e), pyrrolidin-2-one (20f), pyridin-2-one (20g), and carbazole (20h), using bromobenzene (21). Under optimized conditions, DMF was found to be the best solvent for the reaction of nitrogen-containing heterocycles (20a–h) and bromobenzene (21), in the presence CuFe2O4 as a catalyst and tBuOK as a base at 155 °C in a nitrogen atmosphere and provided N-arylated heterocycles (22a–h) in good-to-excellent yields within 24 h (Figure 5.9). Copper ferrite magnetic nanoparticles were also used by Hosseinzadeh and co-workers [19] in the synthesis of N,N,N-trisubstituted guanidine derivatives (25). The reactions of anilines (23) with dicyclohexylcarboxamide (DCC; 24) in a solvent-free condition at 80 °C were efficiently catalyzed by CuFe2O4 nanoparticles to afford the desired products in good yields (Figure 5.10).

Figure 5.9: CuFe2O4 MNPs-catalyzed N-arylation of nitrogen containing heterocycles.

5.3.2.3.2 C–O bond formation Buchwald and Bolm [20] developed a protocol for the formation of new C–O bonds through the cross-coupling of aryl iodides with alcohols using a catalytic amount of Cu2O and ferric chloride in the presence of diamine ligand in toluene at 135 °C. The

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Figure 5.10: Synthesis of N,N,N-trisubstituted guanidines from anilines using CuFe2O4MNPs.

main drawback of this procedure was the non-recyclability of the catalyst. In contrast, CuFe2O4 MNPs can be easily recovered from the reaction mixture and reused multiple times without the loss of its catalytic efficiency. To this end, Zhang et al. [21] synthesized a series of diaryl ethers (28) in high yields by stirring a mixture of phenols (26) and aryl halides (27) in the presence of Cs2CO3, along with 10 mol% of CuFe2O4 MNPs in DMF containing acetyl acetone as a ligand at 135 °C in an argon atmosphere (Figure 5.11). After the completion of the reaction, the catalyst was recovered using an external magnet, washed with distilled water, followed by ethyl acetate, and dried under reduced pressure. The recovered catalyst was reused directly for six times in the next cycles of reactions. Similarly, Pallapothula and coworkers [22], synthesized different diaryl ethers (28) from the reaction of phenols (26) and aryl halides (27) using CuFe2O4 as a catalyst in DMSO, containing KOH as a base at 120 °C, in moderate-to-excellent yields in 20 h (Figure 5.11).

Figure 5.11: CuFe2O4 MNPs-catalyzed synthesis of diaryl ethers.

5.3.2.3.3 C–S bond formation Ranu and co-workers [23] successfully catalyzed a C–S bond formation reaction by utilizing CuFe2O4 nanoparticles as a catalyst. The aryl or heteroarylthiols (29) and styrenyl or aryl halides (30 or 31) were coupled using 10 mol% copper ferrite nanoparticles in

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water to produce products (32 and 33) in excellent yields (Figure 5.12). This coupling reaction occurs in the presence of tetrabutyl ammonium bromide (TBAB) as a phase transfer catalyst and K3PO4 as a base to support the nanocatalyst. This synthetic procedure was found to be efficient for the synthesis of unsymmetrical diaryl- or diheteroarylsulfides in excellent yields. After the reaction, CuFe2O4 nanoparticles were successfully recovered and recycled. The SEM and TEM analyses showed that the morphology of nanoparticles was retained even after the fourth catalytic cycle.

Figure 5.12: C–S bond formation reaction, catalyzed by CuFe2O4 nanoparticles in water.

Furthermore, Hajipour et al. [24] have also reported the synthesis of diaryl sulfides (36) using CuFe2O4 nanoparticles as a catalyst and potassium carbonate as a base in dimethylformamide (DMF) at 120 °C. Under optimized reaction conditions, aryl halides (34) were reacted with thiourea (35) to produce the desired products in moderate-to-good yields (Figure 5.13). Generally, C–S coupling reactions often require expensive palladium catalysts, but the reported CuFe2O4 MNPs-catalyzed protocol is found to be an economical alternative for the synthesis of diaryl sulfides. Moreover, CuFe2O4 MNPs have shown reusability up to six cycles without losing their catalytic efficiency.

Figure 5.13: Synthesis of diaryl sulfides using CuFe2O4 nanoparticles as a catalyst.

5.3.2.3.4 C–Se bond formation Nageswar and co-workers [25] efficiently employed CuFe2O4 nanoparticles for the synthesis of unsymmetrical diaryl selenides (39). The reported methodology described the reaction of phenylselenyl halides (37) with arylboronic acids (38) using

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CuFe2O4 nanoparticles as a catalyst in recyclable PEG-400 medium, containing cesium carbonate at 80 °C, to afford the desired products (39) in good-to-excellent yields (Figure 5.14). Furthermore, CuFe2O4 nanoparticles were efficiently recovered from the reaction mixture and recycled at least four times without any notable loss in catalytic activity.

Figure 5.14: Synthesis of diaryl selenides using CuFe2O4 nanoparticles as a catalyst.

Similarly, the same group utilized CuFe2O4 MNPs for catalyzing another C–Se bond formation reaction [26]. The cross-coupling reactions between aryl iodides (40) and diphenylselenide (41) were efficiently catalyzed by 5 mol% CuFe2O4 MNPs in DMSO, containing KOH as a base at 120 °C, in a nitrogen atmosphere and provided diaryl selenides (42) in moderate-to-excellent yields (Figure 5.15).

Figure 5.15: Nano-CuFe2O4-catalyzed cross-coupling reaction of aryl iodides with diphenyldiselenide.

5.3.2.4 A3-coupling reaction A traditional A3-coupling strategy for the formation of propargylamine derivatives involves a dehydrative condensation between aldehydes and amines in the presence of a metal catalyst to form an iminium cation, which finally undergoes addition reaction with nucleophilic alkynes. In addition, these A3-coupling reactions can also be catalyzed using CuFe2O4 nanoparticles, as reported by Kantam and coworkers [27]. A wide range of biologically active propargyl amines (46) have been synthesized via a one-pot three-component coupling reaction of aldehydes (43), amines (44), and terminal alkynes (45) in toluene at 80 °C using CuFe2O4 nanoparticles as a magnetically reusable catalyst (Figure 5.16). The reported methodology

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Figure 5.16: CuFe2O4 MNPs-catalyzed A3-coupling strategy to form propargylamine derivatives.

does not require any co-catalyst or/and additive to facilitate the reaction and magnetically separable nanocatalyst can be efficiently recycled up to three cycles.

5.3.3 Multicomponent synthesis of heterocycles Multi-component reactions (MCRs) involve two or more reacting substrates and are carried out in a one-pot operation. These protocols are often considered as environmentally benign processes because of their ability to suppress unwanted side reactions and generate less chemical waste. Although MCRs afford products in high yields in comparison to the multi-step synthesis, it was observed that the yields of the desired products can be further incremented using certain MNPs as catalysts. Among these, CuFe2O4 MNPs-catalyzed one-pot strategies for the synthesis of a variety of heterocycles are discussed in the following subsections.

5.3.3.1 Synthesis of imidazoles El-Remaily and co-workers successfully synthesized a new series of 1,2,4,5tetrasubstituted imidazoles (50) by engaging copper ferrite magnetic nanoparticles as a catalyst. The four-component synthesis of 1,2,4,5-tetrasubstituted imidazoles (50) in excellent yields was carried out by the reaction of benzil (47), aromatic aldehydes (48), aryl amines, and ammonium acetate (49) using CuFe2O4 nanoparticles as a catalyst in aqueous ethanol under reflux conditions (Figure 5.17). In contrast, the same reaction, using CuFe2O4 powder, provided 1,2,4,5-tetrasubstituted imidazoles in moderate yields [28]. In another attempt, Sansasi and co-workers [29] synthesized 1,2,4,5-tetrasubstituted and 2,4,5-trisubstituted imidazoles (50 and 51) in excellent yields by using CuFe2O4 nanoparticles as a catalyst in ethanol under ultrasonic conditions, as presented in Figure 5.17. The catalyst was successfully recovered and recycled up to 5 times. However, only moderate yields (up to 45%) of the desired imidazoles were obtained when experiments were performed in the absence of sonication.

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Figure 5.17: Synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles.

5.3.3.2 Synthesis of 1,2,3-triazoles Saha and co-workers [30] utilized CuFe2O4 MNPs as a catalyst in the Huisgen 1,3dipolar cycloaddition reaction to construct various 1,2,3-triazole heterocycles in good yields. The CuFe2O4 MNPs prepared using the ultrasonication method were successfully employed to synthesize arylazides (55) by stirring organic bromides (52)/boronic acids (53)/epoxides (54) with sodium azide in water at 80 °C. Finally, these arylazides (55) underwent 1,3-dipolar cycloaddition with alkynes to form the corresponding 1,2,3-triazoles (56) in good yields (Figure 5.18). The magnetically recoverable CuFe2O4 MNPs were recycled up to five catalytic cycles without any significant loss in their efficiency.

Figure 5.18: CuFe2O4 MNPs-promoted synthesis of 1,2,3-triazoles in water.

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5.3.3.3 Synthesis of tetrazoles Shreedhar and co-workers efficiently catalyzed [2 + 3] cycloaddition reaction between aryl nitriles (57) and sodium azide (58) using reusable copper ferrite nanoparticles as a nanocatalyst in DMF at 120 °C to generate tetrazoles (60) in 70–93% yields (Figure 5.19). The benzonitriles (57), having electron-donating and electron-withdrawing groups, afforded tetrazoles in good yields, whereas the reaction did not proceed using aliphatic nitriles as the starting material [31]. Furthermore, Pamulaparthy and co-workers extended this methodology to construct tetrazoles (60) by using aldoximes (59) as the starting substrates. In optimized conditions, aldoxime derivatives (59) were reacted with sodium azide (58) using CuFe2O4 nanoparticles as a catalyst in DMF at 120 °C for 12 h to generate the corresponding tetrazoles (60) in good yields (Figure 5.19) [32].

Figure 5.19: CuFe2O4 MNPs-catalyzed synthesis of tetrazoles using benzonitriles and aldoximes as substrates.

5.3.3.4 Synthesis of 2-iminothiazolidin-4-ones Das et al. [33] reported a CuFe2O4 nanoparticles-catalyzed regio-controlled cascade reaction, involving 1,4-addition, followed by electrophilic intramolecular cyclization steps, to afford 2-iminothiazolidin-4-one derivatives (64) in excellent yields. A typical procedure involves the reaction of phenyl isothiocyanate (61), primary amines (62), and dialkylacetylenedicarboxylates (63) in ethanol at room temperature by using 10 mol% CuFe2O4 nanoparticles to produce the title products in appreciable yields within 30 min (Figure 5.20). It is interesting to note that this synthetic protocol tolerates various acid and base sensitive aliphatic and heteroaromatic amines.

5.3.3.5 Synthesis of 1,4-dihydropyridines Vishwanath et al. [34] reported the use of nano copper ferrite as a reusable catalyst in the synthesis of 1,4-dihydropyridines (67). According to the procedure, aromatic aldehydes (65) were reacted with ethyl acetoacetate (66) in the presence of ammonium

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Figure 5.20: Copper ferrite nanoparticles catalyzed synthesis of 2-iminothiazolidin-4-ones.

acetate, using 10 mol% copper ferrite nanoparticles in ethanol at room temperature (Figure 5.21). After completion of the reaction, the catalyst was filtered and recycled for 3–4 times. XRD studies of the recovered catalyst showed that nanoparticles retained their morphology after first catalytic cycle and can be reused in another reaction.

Figure 5.21: Synthesis of 1,4-dihydropyridine using CuFe2O4 nanoparticles as catalyst.

5.3.3.6 Synthesis of indoles Nguyen and co-workers [35] prepared various functionalized indole derivatives (70) in 23 to 55% yields through a CuFe2O4 MNPs-catalyzed coupling reaction between 2bromo/iodoanilines (68) and 1,3-dicarbonyl compounds (69) in DMSO under basic inert conditions at 80 °C for 16 h (Figure 5.22).

Figure 5.22: Synthesis of indoles from the reaction of 2-bromo/iodoanilines and 1,3-diketones/β-ketoesters.

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According to the proposed mechanism, 2-iodoaniline reacts with acetylacetone to form imine intermediate (I), which on oxidative addition in the presence of CuFe2O4 MNPs, followed by base-catalyzed intramolecular cyclization, affords benzene-fused metallacycle (II). On reductive elimination, the six-membered copper adduct (II) finally provides 3-acetyl-2-methylindole (Figure 5.23).

Figure 5.23: Plausible mechanism for a CuFe2O4 MNPs-catalyzed synthesis of indole derivative.

5.3.3.7 Synthesis of imidazo[1,2-a]pyridines Srivastava et al. [36] reported a one-pot synthesis of 2-pyrazole-3-aminoimidazo[1,2-a]pyridines (74) using CuFe2O4@SiO2–SO3H nanoparticles. 2-Aminopyridines (71) reacted with isocyanides (72) and functionalized pyrazoles (73) in ethanol, containing a catalytic amount of CuFe2O4@SiO2–SO3H, at 78 °C for 10 min to afford the title products at excellent yields (Figure 5.24). For these transformations, CuFe2O4 nanoparticles were synthesized by co-precipitation, after mixing the solutions of Cu (CH3COO)2.H2O and FeCl3.6H2O in the presence of an alkali. Then, these CuFe2O4 nanoparticles were coated with a SiO2 shell using a sol–gel process, in which welldispersed CuFe2O4 nanoparticles in ethanol were reacted with TEOS (tetraethyl orthosilicate) in the presence of ammonia to obtain the core–shell, CuFe2O4@SiO2. On treatment with chlorosulfonic acid, the silica coated CuFe2O4 nanoparticles resulted in sulfonic acid-functionalized silica coated CuFe2O4 nanoparticles [36]. The reaction is believed to have proceeded through a protonation of the aldehydic group of pyrazoles by the CuFe2O4@SiO2–SO3H nanoparticles, followed by a dehydrative condensation with 2-aminopyridine, to form an imine intermediate. The resulting imine undergoes CuFe2O4@SiO2–SO3H-catalyzed [4 + 1] cycloaddition to give a cyclic adduct, which on aromatization, affords the desired imidazo[1,2-a]pyridines (Figure 5.25).

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Figure 5.24: CuFe2O4@SiO2–SO3H-catalyzed synthesis of imidazo[1,2-a]pyridine derivatives.

Figure 5.25: Plausible mechanism for the synthesis of imidazo[1,2-a]pyridine derivatives.

5.3.3.8 Synthesis of benzoxazoles Nagarkar and co-workers [37] reported a new one-pot redox cascade condensation reaction between benzylamines (75) and 2-nitrophenols (76) using copper ferrite nanoparticles as a catalyst in N-methylpyrrolidone (NMP) at 130 °C for 16 h to provide the benzoxazole derivatives (77) at good yields (Figure 5.26). CuFe2O4 MNPs provide a surface to benzylamines for autoxidation, to form benzaldehydes with a loss of ammonia. The resulting ammonia acts as a hydrogen source for the reduction of 2nitrophenols to produce the corresponding 2-aminophenols, which on condensation with benzaldehydes in situ to afford 2-arylbenzoxazoles.

5.3.3.9 Synthesis of uracil-fused pyrroles Das et al. [38] described a one-pot three-component methodology for the synthesis of uracil-fused pyrrole scaffolds (79). The synthetic procedure available in literature revealed a reaction of aldehydes (48), 6-aminouracil derivatives (78), and nitromethane in the presence of copper ferrite nanoparticles as a heterogeneous catalyst under reflux conditions for 4 h to give the desired products at good yields (Figure 5.27).

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Figure 5.26: CuFe2O4-catalyzed redox condensation of benzylamines and 2-nitrophenols to form benzoxazoles.

Figure 5.27: CuFe2O4 nanoparticles-catalyzed synthesis of uracil-fused pyrroles.

The easy recovery and recyclability of the catalyst makes this protocol green and sustainable.

5.3.3.10 Synthesis of dihydropyrano[2,3-c]pyrazoles The copper ferrite-catalyzed multi-component synthesis of dihydropyrano[2,3-c]pyrazoles (84) was reported by Das and co-workers [39]. A wide range of biologically relevant dihydropyrano[2,3-c]pyrazole derivatives (84) were constructed by reacting hydrazines (80), ethyl acetoacetate (81), dialkylacetylenedicarboxylates (82), and alkylnitriles (83) using CuFe2O4 nanoparticles as a catalyst in water at 60 °C (Figure 5.28). In mild reaction conditions, the reaction proceeded at a high efficiency to produce the products at good yields. The magnetically recovered catalyst can be reused for six catalytic cycles.

Figure 5.28: Synthesis of dihydropyrano[2,3-c]pyrazoles in water.

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5.3.3.11 Synthesis of quinolines and quinazolines Baghbanian and co-workers [40] have attempted the synthesis of quinolines (87) and quinazoline (89) derivatives using reusable copper ferrite nanoparticles as a catalyst in an aqueous medium. The CuFe2O4 nanoparticles were prepared by a thermal decomposition method and characterized by XRD, SEM, and TEM techniques. It was further used as a catalyst in the synthesis of quinolines (87) and quinazolines (89). The 2-aminoacetophenone/benzophenone derivatives (85) were reacted with 1,3dicarbonyl compounds (86) using CuFe2O4 nanoparticles as a catalyst in water at 80 °C to generate diverse quinoline scaffolds (87) at good yields (Figure 5.29). For the synthesis of quinazolines (89), 2-aminobenzophenones (88) were reacted with a wide range of aryl or heterocyclic aldehydes (48) and ammonium acetate (49) in the presence of a catalytic amount of CuFe2O4 nanoparticles in water at 80 °C (Figure 5.30).

Figure 5.29: CuFe2O4-catalyzed synthesis of quinolines in water.

Figure 5.30: CuFe2O4-catalyzed synthesis of quinazolines in water.

5.3.3.12 Synthesis of 4H-chromenes The use of CuFe2O4 nanoparticles has also been reported for the synthesis of 4Hchromene-3-carbonitriles (92) by Rajput et al. [41]. The CuFe2O4 nanoparticles obtained under ultra-sonic irradiation are found to be small in size with a high surface area and provided better product yields due to their high catalytic activity. According to the reported methodology, the synthesis of 4H-chromene-3-carbonitrile derivatives (92) was carried out via a one-pot three-component reaction of aldehydes (65) with malononitrile (90) and cyclohexane-1,3-diones (91) in the presence of 7 mol% CuFe2O4 as a catalyst at 30 °C in ethanol (Figure 5.31). After the reaction,

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Figure 5.31: CuFe2O4-catalyzed synthesis of 4H-chromene-3-carbonitrile scaffolds.

the catalyst was efficiently recovered and recycled for 5 runs without a loss of catalytic activity, and produced the desired products at good yields.

5.3.3.13 Synthesis of 4-methylcoumarins Traditionally, 4-methylcoumarins were synthesized through the acid-catalyzed Pechmann condensation between ethyl acetoacetate and phenols. To develop an environmentally benign strategy, Baghbanian and co-workers [42] modified the reaction conditions and synthesized 4-methylcoumarins (94) at 80–98% yields by employing CuFe2O4 nanoparticles as a catalyst in an aqueous medium at room temperature (Figure 5.32). In this study, CuFe2O4 MNPs were prepared by a thermal decomposition method and the resulting 5 mol% of CuFe2O4 MNPs were found to be sufficient to catalyze the formation of 4-methylcoumarin derivatives (94) at good yields. Under optimized conditions, the reaction of ethyl acetoacetate (81) and the substituted phenol derivatives (93) using reusable magnetic copper ferrite nanoparticles as a catalyst in water at room temperature afforded the desired coumarin products (94) within 15 to 34 min.

Figure 5.32: CuFe2O4 MNPs-catalyzed aqueous phase synthesis of 4-methylcoumarins.

5.3.3.14 Synthesis of spirohexahydropyrimidines Dandia et al. [43] employed recyclable and magnetically recoverable CuFe2O4 nanoparticles as a heterogeneous catalyst for the synthesis of highly substituted spirohexahydropyrimidines (98) in mild reaction conditions for the first time. In this attempt,

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a three-component condensation of cyclohexanones (95), formaldehyde (96), and aniline derivatives (97) was carried out in the presence of 10 mol% CuFe2O4 in ethanol at ambient temperature to afford the products at moderate-to-good yields (Figure 5.33).

Figure 5.33: CuFe2O4-promoted synthesis of spirohexahydropyrimidines.

The mechanism for the formation of spirohexahydropyrimidine derivatives (98) is presented in Figure 5.34. The reaction is proceeded by the formation of an imine intermediate by a dehydrative nucleophilic condensation of aromatic amine and formaldehyde. On reaction with enolate anion (generated in-situ), imine intermediate gives β-aminocarbonyl intermediate. This intermediate reacts further in the same manner to form the substituted propane-1,3-diamine, which on condensation with another mole of formaldehyde furnishes the desired spirohexahydropyrimidines (98) (Figure 5.34). It is important to mention that the oxide (O2–) of metal

Figure 5.34: Proposed mechanism for the synthesis of spirohexahydropyrimidines.

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oxide acts as a Lewis base and Fe3+as a Lewis acid. Once the Fe3+ coordinates with the carbonyl oxygen, it increases the electrophilicity of carbonyl carbon, thereby making it possible to carry out the reaction at room temperature in short reaction times. Moreover, the reusability of the catalyst and the use of ethanol as a solvent make the procedure eco-friendly.

5.3.3.15 Synthesis of 4H-benzo[g]chromene-5,10-dione Cahyana et al. [44] reported the use of CuFe2O4 MNPs in a one-pot synthesis of 2amino-5,10-dioxo-4-styryl-5,10-dihydro-4H-benzo[g]chromene-3-carbonitrile (101). During the reaction, a mixture of malononitrile (90), 2-hydroxy-1,4-napthoquinone (99) and cinnamaldehyde (100) was heated at 60 °C in 50% aqueous ethanol, containing CuFe2O4 MNPs as a catalyst, to provide benzo[g]chromene-3-carbonitrile (101) at 90% yield (Figure 5.35). The use of CuFe2O4 MNPs has been proven beneficial in the preparation of the compound (101), as only a poor yield (31%) of the desired compound was obtained in the absence of the catalyst.

Figure 5.35: Synthesis of 2-amino-5,10-dioxo-4-styryl-5,10-dihydro-4H-benzo[g]chromene-3carbonitrile using Cu-Fe MNPs as catalyst.

5.3.3.16 Synthesis of 1,8-dioxo-octahydroxanthenes Mokhtary and co-workers [45] used chlorosulfonic acid to modify the surface of CuFe2O4 MNPs and prepared the CuFe2O4@SO3H nanoparticles as a heterogeneous catalyst. This modified catalyst has shown its ability to accelerate the rate of onepot cyclo-dehydration reaction of dimedone (102) and arylaldehydes (103) in ethanol at room temperature to provide 1,8-dioxooctahydroxanthenes (104) at excellent yields (Figure 5.36). Similarly, Davoodnia’s group [46] synthesized the SiO2-coated CuFe2O4 magnetic nanoparticles, which also contains the highly acidic SO3H groups. The SO3H-functionalized CuFe2O4 MNPs with the SiO2 shell coating was efficiently utilized for the synthesis of 1,8-dioxooctahydroxanthenes (104) at good yields under solvent-free heating conditions (Figure 5.36).

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Figure 5.36: Synthesis of 1,8-dioxo-octahydroxanthenes.

5.3.3.17 Synthesis of naphthoxazinones Saffari and co-workers [47] successfully synthesized copper ferrite nanoparticles via a microwave-assisted co-precipitation method. These nanoparticles were further employed as a heterogeneous catalyst for the one-pot three-component synthesis of naphthoxazinones (107) by reacting aryl aldehydes (48) with β-naphthol (105) and urea (106) in PEG-400 in basic conditions (Figure 5.37). The reaction mixture was stirred at room temperature for 30 min to obtain the desired naphthoxazinones (107) at excellent yields. After the complete consumption of the starting materials, the catalyst was separated by filtration and it was further reused for four cycles with negligible loss in catalytic activity.

Figure 5.37: CuFe2O4-accelerated synthesis of naphthoxazinones.

5.3.3.18 Synthesis of chromeno[4,3-b]chromenes Mokhtary and co-workers [45] synthesized a nano-CuFe2O4@SO3H as a heterogeneous catalyst to accelerate the reaction of benzaldehydes with cyclic 1,3-dicarbonyl compounds and 4-hydroxycoumarin (108) or 3-hydroxycoumarin (109) in ethanol at 70 °C to provide polycyclic chromeno[4,3-b]chromene heterocycles (110 and 111) at good-to-excellent yields (Figure 5.38).

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Figure 5.38: Nano-CuFe2O4@SO3H-catalyzed synthesis of chromeno[4,3-b]chromenes.

5.3.3.19 Synthesis of spirooxindoles Khodabakhshi et al. [10] reported a three-component synthesis of spirooxindoles (113–116) by using copper ferrite magnetic nanoparticles as the catalyst. The reaction was carried out by refluxing a mixture of malononitrile (90), isatin derivatives (112), 4-hydroxycoumarin or barbituric acid or thiobarbituric acid or cyclohexane-1,3-dione in EtOH:H 2 O (1:1) using a catalytic amount of CuFe 2 O 4 magnetic nanoparticles to afford the corresponding spirooxindoles products at good yields (Figure 5.39). Similarly, Baazgir et al. [48] have reported an aqueous phase green strategy, in which a three-component reaction of 3-hydroxy-1H-phenalen-1-one (117) or 2-methylpyrimidine-4,6-diol (118) or 4-hydroxy-6-methyl-2H-pyran-2-one (119), substituted acetonitriles and isatin derivatives. The reactions were carried out in refluxing water using copper ferrite nanoparticles as a catalyst to give spirooxindoles (120) at 81–96% yields. However, only ~23% of the product was obtained when the reaction was performed in the absence of a catalyst. During the workup, the catalyst was easily recovered by using a magnet and washed with chloroform. It was dried in vacuum and recycled four times for the same reactions (Figure 5.40).

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Figure 5.39: Copper ferrite nanoparticles-catalyzed synthesis of spirooxindoles (113–116).

Figure 5.40: CuFe2O4-promoted green synthesis of spirooxindoles (120).

5.3.3.20 Synthesis of benzodiazepines Maleki et al. [49] reported an efficient green one-pot three-component synthesis of benzodiazepines (122) by using copper ferrite nanoparticles. As per the reported method, a mixture of aromatic aldehydes (48), dimedone (102) and o-phenylenediamine (121) was ball-milled at 28 Hz for 25–46 min in the presence of CuFe2O4 nanoparticles as a heterogeneous catalyst (Figure 5.41). After the completion of the reaction, ethanol was added to dissolve the products. The heterogeneous catalyst was removed by a magnet

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Figure 5.41: CuFe2O4 MNPs-catalyzed synthesis of benzodiazepines using ball-milling process.

and the desired products were obtained at good yields after the evaporation of the solvent in vacuum. The crude products were purified by recrystallization.

5.3.4 Oxidation reactions CuFe2O4 has been successfully employed for the oxidation of several substrates and is also useful in enhancing the yields of oxidized products. For example, the oxidation of substituted toluenes (123) to the corresponding benzoic acid derivatives (124) was carried out in solvent-free conditions at 60 °C in the presence of 3 mol% CuFe2O4 and hydrogen peroxide as an oxidant (Figure 5.42). The use of magnetic nanoparticles as a catalyst gives a better product yield with a catalyst reusability of up to five

Figure 5.42: Oxidation reactions of toluenes, cumene, and ethylbenzene in the presence of CuFe2O4 MNPs.

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times. Interestingly, when cumene (125) and ethylbenzene (127) were subjected to oxidation in the presence of this catalyst, 2-phenylpropan-2-ol (126) and acetophenone (128) were obtained as the main product (Figure 5.42) [50]. It is worth mentioning that the CuFe2O4 MNPs used as a catalyst in the above reactions were synthesized by using the extract of azadirachta indica. It is believed that the polyphenols, catechins, flavonoids, and their metabolites present in the leaves extract act as powerful reducing agents to reduce the metals. During the synthesis of the catalyst, the extract was filtered and added to a solution containing a mixture of Fe3O(OAc)6.(H2O) and copper acetate. Then, 0.1 N NaOH solution was added to the reaction mixture and stirred for an hour at room temperature. The resulting solution was centrifuged and the precipitate was washed with ethanol, followed by annellation at 800 °C for four hours, which provided CuFe2O4 MNPs. Ramazani et al. [51] synthesized copper ferrite nanoparticles by a sol-gel method using Tragacanth gum, a naturally occurring acidic polysaccharide. The resulting material was used for the oxidation of primary and secondary alcohols (129) to their corresponding aldehydes and ketones (130). The oxidation reaction was carried out in acetonitrile (ACN) in the presence of oxone as an oxidant at 40 °C in the presence of 7 mol% of nano-catalyst (Figure 5.43).

Figure 5.43: CuFe2O4-accelerated oxidation of secondary alcohols to ketones.

In another attempt, benzyl alcohols (131) were oxidized to benzaldehydes (132) in an aqueous medium using TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) as an oxidant, along with 10 mol% CuFe2O4 at reflux conditions. This reaction provides up to 95% yields of the desired products (Figure 5.44) [52].

Figure 5.44: Oxidation of benzyl alcohols to benzaldehydes using TEMPO and CuFe2O4.

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5.4 Conclusions In summary, this chapter gives a brief overview on the synthesis and structural characteristics of CuFe2O4 MNPs. In addition, CuFe2O4 MNPs-catalyzed diverse organic transformations have also been discussed that efficiently generate reaction products in good-to-excellent yields. The synthetic strategies described herein demonstrated many advantages, such as mild reaction conditions, high yields of the products, short reaction times and reusability of the catalyst, over the traditional methods. We believe that this article motivates synthetic chemists worldwide to develop efficient ecofriendly methodologies for diversity-oriented synthesis of diverse pharmacologically important molecules using CuFe2O4 MNPs as a heterogeneous catalyst.

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[29] Sanasi PD, Majji RK, Bandaru S, Bassa S, Pinninti S, Vasamsetty S, Korupolu RB. Nano copper ferrite catalyzed sonochemical, one-pot three and four component synthesis of poly substituted imidazoles. Mod Res Catal 2016, 5, 31–44. [30] Mondal B, Kundu M, Mandal SP, Saha R, Roy UK, Roychowdhury A, Das D. Sonochemically synthesized spin-canted CuFe2O4 nanoparticles for heterogeneous green catalytic click chemistry. ACS Omega 2019, 9, 13845–13852. [31] Shreedhar B, Kumar AS, Yada D. CuFe2O4 nanoparticles: A magnetically recoverable and reusable catalyst for the synthesis of 5-substituted 1H-tetrazoles. Tetrahedron Lett 2011, 52, 3565–3569. [32] Akula RK, Adimulam CS, Gangaram S, Kengiri R, Banda N, Pamulaparthy SR. CuFe2O4 nanoparticle mediated method for the synthesis of 5-substituted 1H-tetrazoles from (E)-aldoximes. Lett Org Chem 2014, 11, 440–445. [33] Pal G, Paul S, Das AR. A facile and efficient synthesis of functionalized 4-oxo-2-(phenylimino) thiazolidin-5-ylideneacetate derivatives via a CuFe2O4 magnetic nanoparticles catalyzed regio-selective pathway. New J Chem 2014, 38, 2787–2791. [34] Vishwanath IVK, Murthy YLN. One-pot, three-component synthesis of 1,4-dihydropyridines by using nano crystalline copper ferrite. Chem Sci Trans 2013, 1, 227–233. [35] Vu CM, Le KB, Vo UN, Van VDT, Nguyen AT, Phan NTS, Le NTH, Nguyen TT. Recyclable CuFe2O4 for the synthesis of 2,3-disubstituted indoles. Tetrahedron Lett 2020, 61, 152629. [36] Swami S, Agarwala A, Shrivastava R. Sulfonic acid functionalized silica-coated CuFe2O4 core–shell nanoparticles: An efficient and magnetically separable heterogeneous catalyst for the synthesis of 2-pyrazole-3-amino-imidazofused polyheterocycles. New J Chem 2016, 40, 9788–9794. [37] Sarode SA, Bhojane JM, Nagarkar JM. An efficient magnetic copper ferrite nanoparticle: For one pot synthesis of 2-substituted benzoxazole via redox reactions. Tetrahedron Lett 2015, 56, 206–210. [38] Paul S, Pal G, Das AR. Three-component synthesis of a polysubstituted pyrrole core containing heterocyclic scaffolds over magnetically separable nanocrystalline copper ferrite. RSC Adv 2013, 3, 8637–8644. [39] Pradhan K, Paul S, Das AR. Magnetically retrievable nano crystalline CuFe2O4 catalyzed multicomponent reaction: A facile and efficient synthesis of functionalized dihydropyrano[2,3-c] pyrazole, pyrano[3,2-c]coumarin and 4H-chromene derivatives in aqueous media. Catal Sci Technol 2014, 4, 822–831. [40] Baghbanian SM, Farhang M. CuFe2O4 nanoparticles: A magnetically recoverable and reusable catalyst for the synthesis of quinoline and quinazoline derivatives in aqueous media. RSC Adv 2014, 4, 11624–11633. [41] Rajput JK, Arora P, Kaur G, Kaur M. CuFe2O4 magnetic heterogeneous nanocatalyst: Low power sonochemical-coprecipitation preparation and applications in synthesis of 4Hchromene-3-carbonitrile scaffolds. UltrasonSonochem 2015, 26, 229–4. [42] Baghbanian SM, Farhang M. CuFe2O4 nanoparticles: A magnetically recoverable and reusable catalyst for the synthesis of coumarins via pechmann reaction in water. Synth Commun 2014, 44, 697–706. [43] Dandia A, Jain AK, Sharma S. CuFe2O4 nanoparticles as a highly efficient and magnetically recoverable catalyst for the synthesis of medicinally privileged spiropyrimidine scaffolds. RSC Adv 2013, 3, 2924–2934. [44] Cahyana AH, Saidah S, Ardiansah B. Nano CuFe2O4: A reusable magnetic catalyst for the synthesis of 2-amino-5,10-dioxo-4-styryl-5,10-dihydro-4H-benzo[g]chromene-3-carbonitrile via a one-pot multicomponent reaction. AIP Conf Proc 2017, 2023, 020075.

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Sanjay Paul, Asish Ranjan Das*

Chapter 6 Magnetic nanoparticle catalysis: a potential platform for the fabrication of C–C bond generation and oxidation reaction to achieve important structural motifs 6.1 Introduction In recent years, C–C bond forming reaction applying practical and sustainable chemistry has become an important and powerful chemical tool for the preparation of important scaffolds in natural products, pharmaceutical, agricultural, polymeric materials, and polyalkynylated molecules, etc. [1–9]. Oxidation reactions are extremely important to the academics and industry due to their potential for producing various fine chemicals and drugs. Usually, oxidation reactions are performed non-catalytically applying stoichiometric amount of oxidants (manganese and chromium oxides with higher valency) under strong acid or alkaline condition and hence generate huge toxic waste. Because of emergent environmental and economic concern, wide ranges of transition-metal-based catalytic protocols are developed for C–C bond forming reactions and oxidation reaction to improvise the experimental conditions. Although the C–C bond forming technology and oxidation reaction protocol have been extensively explored, the requirements of new methodologies are still demanding, especially for industrially and economically attractive issues like high yields, low catalyst loading, ease of handling and reusability. In this regard, these organic transformations with metal or metal oxide nanoparticle catalysts showed excellent efficacy due to their high surface-to-volume ratio and use of the nanocatalyst circumvents the problems of homogeneous catalysts such as catalyst recyclability and catalyst contamination in the product. However, the development of new recyclable and eco-friendly support is still necessary to minimize agglomeration of the nanoparticle catalysts for carboncarbon bond forming reaction under heterogeneous conditions. In this context, a large number of solid supports are reported to make the catalyst recovery process more efficient by reducing the agglomeration of NPs. Among the various solid supports, the metallic nanocomposites with magnetic supports are widely accepted in

*Corresponding author: Asish Ranjan Das, Department of Chemistry, University of Calcutta, Kolkata, e-mail: [email protected] Sanjay Paul, Department of Chemistry, Behala College, Kolkata, India https://doi.org/10.1515/9783110730357-006

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the arena of catalysis due to their specific surface area, high stability, and easy magnetic recovery [10–15]. In this chapter, applications of magnetic nanocatalysts for organic transformations such as C–C bond forming reaction and oxidation reaction are to be illustrated.

6.2 Magnetic nanoparticle catalyzed C–C bond forming reaction 6.2.1 Suzuki cross-coupling reaction Among the various cross-coupling reactions, Suzuki coupling is one of the greatest synthetic tools for C–C bond formation discovered in twentieth century [16–19]. Generally, in Suzuki coupling biaryl derivatives are synthesized by transition-metal-catalyzed cross-coupling of organoboron compounds with organohalides. Wide variety of catalysts has been introduced for the development of efficient and ecofriendly reaction conditions. Recently, magnetic nanocatalysts have attracted attention both in the academic and the industrial field for the Suzuki–Miyaura reaction. Gao et al. [20] prepared homogeneous superparamagnetic Pd nanoparticle catalysts through the immobilization of Pd-N heterocyclic carbene complexes on soluble magnetic nanoparticles. The magnetic Pd nanoparticle catalyst was very effective for the Suzuki coupling of aryl bromides and iodides (1) bearing electron-releasing and electron-withdrawing groups with phenylboronic acid (2) offering excellent yield of biphenyl derivatives (3) (Figure 6.1). The Pd–NHC complex immobilized on maghemite nanocatalyst showed very high stability in organic media even at higher reaction temperature. Comparative kinetic study of homogeneous superparamagnetic Pd nanoparticle catalysts and solid phase-Pd catalyst on Suzuki crosscoupling between 4-iodoacetophenone and phenylboronic acid demonstrated that the former nanoparticle-catalyzed reaction proceeded faster. Interestingly, more than 97% of Iron Oxide–Pd catalyst was recovered after completion of each reaction by applying external magnet.

Figure 6.1: Superparamagnetic Pd maghemite nanoparticle catalyzed Suzuki cross-coupling reactions.

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The same group also used core/shell super paramagnetic Pd-NHC catalyst for the Suzuki coupling reaction [21]. The core/shell superparamagnetic nanoparticle with crystalline γ-Fe2O3 core was synthesized by emulsion polymerization technique (Figure 6.2). A wide range of aryl iodides and bromides were successfully coupled with various arylboronic acids with an average isolated yield of 82% in the presence of nanoparticle-supported Pd catalysts. However, aryl chlorides were not screened under the developed protocol.

Figure 6.2: Synthesis of core/shell super paramagnetic Pd-NHC catalyst.

Yinghuai and his group [22] used commercially accessible magnetic nanoparticles for the preparation of ultra-small palladium (0) particles on magnetic nanoparticle support. The magnetic nanoparticles were enriched with phosphate functional groups by treating it with phosphorus oxychloride and neutralization with aqueous base. Subsequently, reaction with Pd(acac)2 with the phosphate functionalized magnetic support and reduction using glycol under argon atmosphere generated the palladium (0) particles on magnetic nanoparticle support. The catalyst system was very active in the Suzuki coupling of bromobenzene (4) with phenylboronic acid (2) and provided good yield of the product (3). Interestingly, the catalyst was effective both in organic as well as aqueous media at 80 °C (Figure 6.3). The authors also demonstrated that the developed catalyst systems showed greater activity than NHC-Pd complexes supported on magnetic nanoparticle. The catalyst was recovered by using external magnet and reused three times after drying the same with almost same catalytic efficiency. Although the pseudohomogeneous catalyst system was very effective in Suzuki cross-coupling reaction, the substrate scope was not investigated extensively by applying the catalyst.

Figure 6.3: Magnetic nanoparticle-supported palladium nanoparticles catalyzed Suzuki cross-coupling.

Nlate et al. [23] described the synthesis of metallodendron functionalized with dicyclohexyldiphosphino palladium nanoparticle. The grafting of the dendron was

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studied on core–shell γ-Fe2O3/polymer with various particle sizes (300, 200, and 500 nm). The dendron-functionalized magnetic catalyst was installed successfully towards the C–C cross-coupling haloarenes (5) with boronic acids (6) (Figure 6.4). Aryl iodides and bromides provided excellent yield of the coupling product (7) under the developed protocol. However, the catalyst showed lower activity for the coupling of phenylboronic acid with aryl chlorides and longer reaction time and greater catalyst loading were required to perform the reaction. Very low extent of Pd leaching after each reaction showed high stability of catalyst. The grafted catalyst was recovered very easily and can be reused 25 times with almost same catalytic activity.

Figure 6.4: Suzuki cross-coupling by dendron-grafted magnetic catalyst.

Thiel et al. [24] synthesized organic-inorganic hybrid nanocatalyst (Pd@SMP) by covalent grafting of palladium dichloride complex on magnetic nanoparticles. The Pd@SMP was applied successfully in the Suzuki reaction of aryl iodides and bromides (8) (Figure 6.5). The scope of the Suzuki coupling on a range of substrates was investigated in the presence of Cs2CO3 and 1 mol% of Pd@SMP catalyst at 80 °C temperature in dioxane media. The coupling of phenylboronic acid (2) with phenyl iodide and phenyl bromide provided biphenyl (9) in quantitative yield. However, phenyl chloride afforded very low yield (34%) of the coupling product. The influence of substituents at Ar-X on the reaction rate was also investigated by applying variety of aryl halides with electron-poor and electron-donating substituents.

Figure 6.5: Suzuki cross coupling in the presence of Pd@SMP.

Hanson et al. [25] reported the synthesis of Pd@Co/C-ROMPgel through the surface immobilization of ligand on Co-based nanoparticles. Norbornene tag (Nb-tag) was attached to the carbon coated cobalt nanoparticles by “click” reaction. Grafting of

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ROMPgel onto the magnetic nanobeads was subsequently achieved by metathesis polymerization reaction. Treatment of this PPh3-functionalized magnetic nanoparticles with Pd(OAc)2 formed the Pd-catalyst. The Suzuki-Miyaura cross-coupling of a wide range of aryl halides (10) with aryl boronic acids (11) occurred successfully using the Pd@Co/C-ROMPgel catalyst and the coupling products (12) were obtained in good to excellent yields (Figure 6.6). Electronreleasing and electron-poor groups on aryl bromides and iodides did not display noticeable effect on the product yield. However, aryl chlorides provided very low yield of the desired coupling product. Ferromagnetic character of metal core nanoparticles allowed the magnetic recovery of the catalyst system from reaction medium and the recovered nanoparticle was reused seven cycles with almost same catalytic activity.

Figure 6.6: Suzuki cross-coupling by nano- Pd@Co/C-ROMPgel catalyst.

Qian and co-workers [26] synthesized magnetically separable N-heterocyclic carbene (NHC) ligand-based palladium catalyst and utilized the catalyst for Suzuki–Miyaura cross-coupling reaction. Aryl halides (13) with diverse electronic environment efficiently reacted with variety of aryl boronic acids (14) coupling partners in the presence of palladium catalyst and K2CO3 base in aqueous alcoholic media (Figure 6.7). Electron-rich aryl halides showed lower reactivity compared to electron-deficient aryl halides. However, steric factor has great influence on Suzuki–Miyaura cross-coupling under the developed protocol. Coupling of 2,6-dimethylphenylboronic acid with bromobenzene provided low yield of coupling product (15) due to high steric hindrance.

Figure 6.7: Suzuki cross-coupling by magnetically separable palladium catalyst.

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Martínez-Olid et al. [27] synthesized and applied two different types of N-heterocyclic carbene based magnetic nanoparticle for Suzuki-Miyaura reaction. The synthesis of magnetically separable NHC complex based palladium catalyst was achieved by grafting onto the magnetic core/shell γ-Fe2O3/silica particles. Both the catalyst systems were active in coupling of a range of aryl halides (16) with phenylboronic acid (2) to obtain the coupled product (17) in the presence of K2CO3 as base (Figure 6.8). The bis(NHC) palladium complex on core-shell (γ-Fe2O3/silica) MNPs showed much better catalytic activity for Suzuki–Miyaura cross-coupling than analogous mono(NHC) compound. Interestingly, the bis-(NHC) complex catalyst was found to display more catalytic activity than corresponding homogeneous metal complex under mild conditions.

Figure 6.8: N-heterocyclic carbene based Pd magnetic nanoparticle-mediated Suzuki reaction.

Yuan and co-workers [28] utilized Schiff base palladium complex immobilized on magnetic polymer microspheres (Fe3O4/P(GMA-AA-MMA)-Schiff base-Pd) catalyst for the synthesis of biphenyl derivatives (20) by the coupling of aryl halides (18) containing both electron-rich and electron-poor substituents with phenylboronic acids (19) (Figure 6.9). Importantly, the catalyst was effective for the coupling of aryl chlorides under developed protocol. The magnetic support allowed the recovery of the catalyst with negligible palladium leaching and was reused seven times with almost same catalytic activity. Joshi et al. [29] prepared a novel Pd(II)-NHC complex of 1-[N-benzylacetamido]3-[1-(2-phenylsulfanylethyl)] benzimidazolium chloride ligand. The square planar geometry of the complex was characterized by X-ray crystallography. Furthermore, the authors also synthesized solid-supported magnetically recoverable Fe3O4@SiO2Pd(II)-NHC catalyst through the immobilization of the Pd(II) complex on the surface of the functionalized silica-coated magnetite nanoparticles. Both the homogeneous (Pd(II)-NHC complex) and heterogeneous catalysts (Fe3O4@SiO2-Pd(II)-NHC NPs) were effective for Suzuki coupling reactions of a range

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Figure 6.9: Fe3O4/P(GMA-AA-MMA)-Schiff base-Pd catalyzed Suzuki coupling.

of aryl/heteroaryl bromides with arylboronic acids in aqueous media in the presence of base. However, comparison of these two catalysts revealed that heterogeneous version has lower catalytic activity over its homogeneous counterpart but unfortunately Pd(II)-NHC complex deactivated after one catalytic cycle. Negligibly low Pd leaching of the Fe3O4@SiO2-Pd(II)-NHC NPs under the reaction condition demonstrated the high stability of the heterogeneous catalyst. Due to higher stability of the NPs, the catalytic activity was retained after seventh catalytic cycle. Nasrollahzadeh et al. [30] described the synthesis of nanosized Pd NPs@Fe3O4/ CS-AG microcapsules. The synthesized catalyst showed excellent activity for the coupling of various halogenoarenes (21) with boronic acids (2) to corresponding diphenyl derivatives (22) in good yield (Figure 6.10). Interestingly, coupling of aryl chlorides containing electron-donating and electron-poor substituents with phenylboronic acid were achieved by applying the nano-catalyst under the developed microwave condition in the presence of K2CO3 as base. Moreover, the catalyst also showed good reusability with an efficiency of 80% after the eighth catalytic run.

Figure 6.10: Nanosized Pd NPs@Fe3O4/CS-AG microcapsules catalyzed Suzuki coupling.

Instead of Fe3O4 nanoparticles, the application of nanoferrite (MFe2O4) is an alternative way to stabilize magnetically recoverable palladium catalysts. In this regard, Waghmode et al. [31] developed the synthesis of Pd immobilized NiFe2O4 nanoparticle as a magnetically retrievable catalyst for Suzuki coupling. The NiFe2O4/Pd0 catalyst was very efficient in the coupling of variety of aryl halides (23) with aryl and

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heteroaryl boronic acids (24) in DMF/H2O media (Figure 6.11). Aryl iodides and bromides were very reactive in the presence of the Pd catalyst. However, aryl chlorides were very less reactive and biphenyl derivatives (25) were obtained in very low yield at higher temperature and longer reaction time.

Figure 6.11: Pd immobilized NiFe2O4 nanoparticle catalyzed synthesis of biaryls.

6.2.2 Heck cross-coupling reactions The Heck coupling is another most effective and comprehensively used reaction for the formation of C–C bonds. In the Heck reaction, substituted alkenes are synthesized by the coupling of alkenes with aryl halides. Parallel to Suzuki coupling, magnetic nanoparticles has engrossed much attention to simplify the catalyst isolation and recycling and the product purification in the Heck reaction. He et al. [32] demonstrated Pd/Fe3O4 nanoparticle catalyzed Heck reaction of iodobenzene (26) with acrylic acid (27) (Figure 6.12) to obtain the cross-coupled product (28) in excellent yield. The authors synthesized Fe3O4 nanoparticle by conventional coprecipitation method and further modified the nanoparticles with 3-aminopropyl triethoxysilane (APTS). Immobilization of Pd(0) onto the APTS-coated Fe3O4NPs by the coordination of –NH2 provided the Pd/Fe3O4 catalyst (Figure 6.13). The catalyst separation could be achieved very easily by external magnet. However, significant loss of catalytic performance was detected from first to fifth catalytic cycle due to agglomeration of nanoparticles into big particles.

Figure 6.12: Pd/Fe3O4 nanoparticle catalyzed Heck reaction of acrylic acid with iodobenzene.

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Figure 6.13: Synthesis of Pd(0) immobilized on the APTS-coated Fe3O4 nanoparticles.

Jang and co-workers [33] reported fabrication of magnetic carboxylated polypyrrole nanotubes (MCPPy NTs). The consequent deposition of Pd nanocatalysts onto MCPPyNTs provided the Pd/MCPPy nano-catalyst. The nanocatalyst was successfully utilized for Heck reaction considering variety of aryl iodides with n-butylacrylate and styrene in the presence of tributylamine in DMA at 120 °C to obtain the coupled product in quantitative yield. The very high activity of Pd/MCPPy NTs for the Heck coupling may be due to the narrow size distribution and high surface area of the Pd nanocatalyst. The catalyst was highly stable and no morphological change was observed on reusing the catalyst up to five runs. In another report, Yinghuai et al. [22] used palladium(0) nanoparticles supported on phosphate functionalized magnetic nanoparticle catalyst for Heck coupling of bromo benzene (29) and styrene (30). The coupling reaction was performed in DMF media to increase the solubility of organic compounds and trans-stilbene (31) was isolated in 56% yield (Figure 6.14). However, the substrate scope of the magnetic nanoparticle catalyzed Heck reaction was not investigated extensively.

Figure 6.14: Magnetic nanoparticle-supported palladium (0) nanoparticles catalyzed Heck coupling of bromo benzene and styrene.

Kirschning et al. [34] described the synthesis of Pd(0) immobilized silica-coated iron oxide nanoparticle. In this report the authors demonstrated the use of magnetic nanoparticles as heatable media in an electromagnetic field capable to induce chemical reaction for the first time and the technique can be used in the inductive heating–mediated industrial processes (Figure 6.15). The Pd(0)-magnetic catalyst was successfully employed in the Heck coupling of aryl and heteroaryl iodides (32) with styrene (30) to obtain the coupled products (33) in good yields. However, during the Heck reactions, very small extent of Pd leaching was observed in ICP-MS analysis.

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Figure 6.15: Application of Pd(0) immobilized silica-coated iron oxide nanoparticle in Heck reaction.

Plucinski et al. [35] prepared three different palladium-based catalysts (Pd0, Pd (OAc)2, and Pd(PPh3)2(OAc)2) supported on Fe3O4 magnetic nanoparticles. All these three types of MNP-supported palladium environment were effective for the Heck reactions between substituted alkenes (35) and 4-bromobenzenes (34) to generate the coupled product, 36 in the presence of KOAc (Figure 6.16). However, the catalytic activity of MNP-Pd(0) and MNP-[Pd(OAc)2] retained their potential up to third catalytic cycle, but very sharp decrease of catalytic performance was observed after second run in case of MNP-[Pd(TPP)2(OAc)2]. The kinetic study for catalyst recycling experiment exhibited lowering of catalyst performance over each subsequent run. The decrease of catalytic activity after each cycle was due to palladium leaching from the catalyst which was primarily occurred due to the presence of aryl bromide.

Figure 6.16: Fe3O4-[Pd(OAc)2] catalyzed Heck coupling.

Varma and Polshettiwar [36] performed Heck coupling with nano-pine-Pd catalyst. The coupling between aryl halides (37) and various alkenes (38) was achieved in the presence of triethylamine as base under microwave irradiation to generate the cross-coupled product (39) in 85–90% yields (Figure 6.17). The catalyst was highly stable and negligible amount of Pd leaching from the catalyst was detected upon inductively coupled plasma atomic emission spectroscopic (ICP-AES) analysis of the reaction mixture.

Figure 6.17: Nano-pine-Pd-mediated Heck coupling.

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Yuan et al. [37] fabricated magnetically recoverable palladium catalyst (Fe3O4/P (GMA-MMA-AA)-Pd(0)) by immobilizing palladium nanoparticles onto the superparamagnetic composite microsphere surface (Figure 6.18). The catalyst exhibited high activity in promoting the Heck reaction of a wide array of aryl bromides and iodides (40) with acrylic acid (27) using DMF media in the presence of tributylamine as a base to obtain the corresponding coupled product (41) with Z-selectivity. However, prolonged reaction time and elevated temperature are required for the coupling of Ar-Br with acrylic acid and product was obtained in lower yield compared to Ar-I coupling partner. The catalyst was reused six times without any change of yield and selectivity of the coupled product under the developed protocol.

Figure 6.18: Heck coupling by Fe3O4/P(GMA-MMA-AA)-Pd(0).

Gao and co-workers [38] synthesized magnetic hybrid support of multicarboxylic HPG-grafted SiO2-layered iron oxide (Fe3O4/SiO2). Applying the grafted- HPGs as templates, Pd nanoparticles were directly grown on the surface of magnetic support with ultrasmall and almost monodisperse sizes to produce magnetically recoverable Fe3O4/SiO2/HPG-Pd catalyst. The Heck reaction of acrylic acid (27) and styrene (30) with bromobenzene and iodobenzene (42) were achieved successfully to obtain corresponding coupled products (28, 31) in excellent yields by applying the synthesized catalyst in the presence of NaOAc in DMF media at 140 °C (Figure 6.19). Styrene showed almost similar reactivity with Ar-Br and Ar-I coupling partner, but displayed greater reactivity than acrylic acid in Heck coupling reaction. Interestingly, the combined features of both Fe3O4 and HPG support allowed facile magnetic separation from the reaction mixture. The Fe3O4/SiO2/HPG-Pd catalyst was used repeatedly for four consecutive rounds with same catalytic performance. Coker et al. [39] described a biotechnological method for the preparation of PdNPs supported onto nanoscale biomagnetite. The magnetic support was prepared at ambient temperature in the presence of FeIII-reducing bacterium, Geobacter sulfurreducens. The biomagnetite support facilitated easy magnetic recovery of the catalyst and also increased catalytic performance by reducing agglomeration of the nanoparticles. The biomagnetite-Pd0 catalyst has been evaluated in the Heck couplingbetween iodobenzene and ethyl acrylate or styrene. The magnetic catalyst was highly active for the coupling of both olefins (ethyl acrylate or styrene) and ethyl cinnamate or stilbene; good yields were obtained within 90 and 180 min, respectively. Reported

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Figure 6.19: Pd-NCs catalyzed Heck reaction.

experimental results indicated that the Pd-coated biomagnetite catalyst displayed equal or superior activity over the colloidal palladium catalyst. Shen et al. [40] described synthesis of carbon encapsulated Au@Fe nanoparticles by the hydrothermal method and immobilized palladium NPs (12 nm) on this hydroxyl-terminated support. The synthesized catalyst system showed excellent activity in Heck coupling of Ph-Br/Ph-I (42) with methyl acrylate or styrene (43) (Figure 6.20). Under the developed protocol, styrene exhibited greater reactivity than the methyl acrylate and stilbene (44) was produced in higher yield. Again, phenyl iodide coupled at greater rate in the presence of the synthesized catalyst than the phenyl bromide and the corresponding cross-coupled products were achieved in shorter reaction time in case of phenyl iodide coupling partner. The magnetic nanoparticle catalyzed Heck coupling exhibited excellent E-selectivity. However, the E-selectivity was higher in case of coupling with methyl acrylate and acrylic acid than styrene. The catalyst can be easily recovered and reused ten rounds with same activity and selectivity.

Figure 6.20: Heck reaction by Pd/C@(Au@Fe).

Gao et al. [41] have investigated the catalytic behavior of superparamagnetic mesoporous NiFe2O4-supported palladium catalyst (Pd/NF300) for Heck coupling. Wide array of aryl bromides (45) coupled successfully with styrene (30) using Pd/NF300 as catalyst in the presence of Et3N as base in DMF media (Figure 6.21). The aryl bromides

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with electron-rich substituents were more reactive in the presence of the palladium catalyst and furnished the cross-coupled product (46) in high yields. However, the catalyst was unable to activate Ar-Cl bond for the Heck reaction and trace amount of stilbene was detected. The catalyst was found to be recyclable for the Heck coupling and a slight decrease of catalytic performance was observed after five cycles due to Pd leaching.

Figure 6.21: Heck reaction by mesoporous NiFe2O4-supported palladium catalyst.

Zhu and Diao [42] used magnetically separable Pd-NPs arrested on Fe3O4@C nanocomposites (Pd/MFC) for Heck coupling at 120 °C in DMF. Coupling of two alkenes, methyl acrylate and styrene (38) with Ar-I, Ar-Br and Ar-I (47) were tested. Aryl iodides and bromides were highly reactive in the presence of Pd/MFC catalyst and provided the cross-coupled product (48) in quantitative yields (Figure 6.22). Aryl chlorides displayed lowest reactivity under the developed protocol and gave low to modest yields. Moreover, no significant loss of catalyst performance was observed after fifth rounds of use of catalyst due to insignificant catalyst leaching.

Figure 6.22: Heck reaction by Pd/MFC.

In another report, Ma et al. [43] developed synthesis of palladium (0) catalyst supported onto amine-functionalized magnetite nanoparticles (Fe3O 4-NH2–Pd) (Figure 6.23). The performance of the synthesized catalyst has been evaluated in the Heck coupling of styrene and n-butyl acrylate (35) with a variety of aryl bromides and iodides (49) in the presence of K2CO3 at 130 °C temperature. Aryl bromides with electron-poor groups were more reactive in this coupling reaction and the coupled products (50) were obtained in quantitative yields in shorter reaction time. No significant change of catalytic performance was detected after fourth catalytic cycles. The recycling of the catalyst was performed by magnetic separation followed by washing with ethanol and finally drying before reuse.

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Figure 6.23: Synthesis and application of Fe3O4-NH2–Pd.

The research group of Zhang [44] employed Fe3O4–NH2–Pd (0) catalyst for Heck reaction of aryl halide with acrylic acid. The functionalized super-paramagnetic catalyst was prepared by immobilizing Pd nanoparticles onto the surface of magnetic Fe3O4–NH2 microspheres (Figure 6.24). Variety of aryl iodides and aryl bromides (51) were reacted successfully with acrylic acid (27) in DMF media in the presence of N,N-dicyclohexylmethylamine as base (Figure 6.25). Coupling product (52) of aryl iodides with acrylic acid were obtained in shorter time than coupling with aryl bromides. At the end of the reaction, the catalysts can be readily separated from the reaction mixture using a simple bar magnet. The catalyst can be recycled up to sixth rounds with same potency. However, the activity of the catalyst decreased gradually after sixth runs due to agglomeration of nanoparticles.

Figure 6.24: Synthesis of Fe3O4–NH2–Pd (0) catalyst.

Figure 6.25: Application of Fe3O4–NH2–Pd (0) catalyst in Heck reaction.

In another report, Varma et al. [45, 46] have described a Heck-type arylation reaction by utilizing Pd (II) catalyst supported onto dopamine-functionalized Fe3O4

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nanoparticles (Fe–Dopa–Pd) (Figure 6.26). The arylation of alkene was performed by using diaryliodonium salts as the arylating agent in the presence of 4.8 mol% Fe–Dopa–Pd catalyst in aqueous PEG-400 (1:1) media to obtain the cross-coupled products in 45–90% yields (Figure 6.27). The palladium catalyst appeared as highly effective for the coupling of various unreactive alkenes like allyl acetate and styrene (54) with diaryliodonium salts (53) to furnish excellent yields of the product (55). However, allyl alcohol showed lower reactivity in this Heck-type arylation reaction under the developed protocol. The nanocatalyst was recovered efficiently by magnetic separation technique with negligible Pd leaching.

Figure 6.26: Preparation of Fe3O4-DA-Pd(II) catalyst.

Figure 6.27: Heck reaction using Fe3O4-DA-Pd(II) catalyst.

Nagarkar et al. [47] prepared Pd–ZnFe2O4 nanoparticles for the Heck couplingbetween aryl halides (56) and activated alkenes (57) in the presence of Et3N as base in DMF at 120 °C (Figure 6.28). Variety of aryl bromides and iodides were coupled successfully with alkenes in the presence of 4.62 mol% catalyst to generate the crosscoupled product (58) in excellent yields. However, the coupling with aryl chlorides was not achieved applying the developed protocol.

Figure 6.28: Pd-ZnFe2O4 catalyzed Heck reaction.

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The same research group applied Pd–ZnFe2O4 catalyst for Heck type coupling of aryldiazonium tetrafluoroborates (59) with alkenes (60) (Figure 6.29) [48]. Various substituted arylated styrene or acrylate derivatives (61) were obtained in excellent yields under the base free cross-coupling reaction in water.

Figure 6.29: Pd-ZnFe2O4 catalyzed Heck type coupling of aryldiazonium tetrafluoroborates with alkenes.

Khalafi-Nezhad and Panahi [49] reported the functionalization of magnetic nanoparticles (MNs) by applying chlorodiphenylphosphine (ClPPh2) ligand as a recyclable phosphorus ligand to generate PFMN as (Figure 6.30). Subsequent complexation of PFMN ligand with Pd(OAc)2 provided the palladium-PFMN catalyst. The as-synthesized magnetic nanoparticle was used for Heck coupling of aryl chloride (62) with styrene and methyl/ethyl acrylate (63) in DMF at 120 °C to furnish the corresponding cross-coupled product (64) in excellent yields. The authors pointed out that the outstanding performance of catalyst in this Heck coupling of aryl chlorides was due to high dispersibility of the catalyst in used solvent and the presence of phosphorus ligands in the magnetic nanoparticles.

Figure 6.30: Heck reaction applying Pd-PFMN catalyst.

In another example of the application of magnetically retrievable catalyst, Khodaei and Dehghan demonstrated Fe3O4‐Pd@SF‐SBA‐15 nanocatalyst (Figure 6.31)-mediated Heck reaction [50]. In the presence of Et3N as base in DMF, various substituted iodo‐, bromo‐ and chlorobenzenes underwent coupling successfully with styrene at

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120 °C. In this cross-coupling reaction, a wide range of trans-stilbene derivatives were obtained with good yield in short reaction time. The catalyst was reused eight consecutive runs with unaltered efficiency and selectivity.

Figure 6.31: Fe3O4‐Pd@SF‐SBA‐15 Catalyst.

Research group of Jiang and Sun [51] fabricated Pd-Fe3O4 heterodimer composites using one-pot hydrothermal method. The synthesized catalyst showed excellent activity and stability for Heck coupling of styrene and iodobenzene in the presence of Et3N as base in DMF media. The recycling of the nanocomposite was achieved via magnetic separation, washing with ethanol and drying in vacuum without further purification. Catalytic performance of the nanoparticles decreased after recycling five times due to continuous leaching of Pd from the catalyst.

6.2.3 Sonogashira coupling reactions Sonogashira coupling is an important pathway for C–C bond formation in which cross-coupling between terminal alkynes and vinyl or aryl halides or triflates are performed in the presence of Pd-catalyst [52]. This reaction has been investigated extensively due to its widespread application in the production of natural products, pharmaceuticals and complex organic materials. Sun et al. [53] described the preparation of Pd/Fe3O4 magnetic nano-catalyst by wet impregnation of palladium nanoparticles onto superparamagnetic Fe3O4-NPs in KBH4 solution. The Pd/Fe3O4 catalyst was successfully applied for phosphine-free carbonylative Sonogashira coupling reaction between aryl iodides (65) and terminal alkynes (66) (Figure 6.32). The carbonylative coupling reactions were performed using Pd/Fe3O4 catalyst (Pd, 1.04 wt %) with Et3N as the base and allowing CO (g) with 2 MPa of pressure at 130 °C temperature in toluene. Various aryl iodides with a range of terminal alkyne as the coupling partners were screened to check the generality of the carbonylative Sonogashira reaction. Aryl iodides with electron-withdrawing or electron-donating group in different positions provided the alkynyl ketone derivatives (67) in good to excellent yields. Interestingly, the authors described that the Pd/Fe3O4 catalyst–mediated carbonylated Sonogashira reaction was a quasi-homogeneous catalytic process. Small amount of palladium species participates in the

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reaction by leaching from Pd/Fe3O4 catalyst and gets deposited partially or completely on the Fe3O4 support at the completion of the coupling reaction.

Figure 6.32: Pd/Fe3O4 catalyst–mediated carbonylated Sonogashira coupling reactions.

Jin et al. [54] demonstrated the synthesis of magnetically recoverable palladium catalyst (Pd-SiO2@Fe3O4) and described its application in the Sonogashira coupling reaction of aryl chlorides (68) with alkynes (69) in aqueous media without any copper co-catalyst (Figure 6.33). Various cross-coupled products (70) were obtained by the reaction of aryl chlorides with electron-rich and electron-poor groups using the palladium catalyst at 60 °C temperature whereas an elevated temperature (80 °C) was required to perform the coupling reaction of hindered chlorides. The catalyst was recovered very easily by the application of external magnetic field and interestingly the catalytic activity was retained completely even after ten re-use.

Figure 6.33: Pd-SiO2@Fe3O4 catalyzed Sonogashira coupling reactions.

Palladium complex arrested on pyridinylimine-functionalized cobalt ferrite nanoparticle was also examined for Sonogashira reaction. The nanocatalyst efficiently catalyzed the coupling reaction of aryl halides (71) with phenylacetylene (72) (Figure 6.34) without the phosphine ligands and provided the diphenylacetylene (73) as the principal product in excellent yield. Furthermore, catalyst was recovered by magnetic decantation method and reused several times.

Figure 6.34: Pd-CoFe2O4 catalyzed Sonogashira coupling reactions.

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The immobilized palladium complex was prepared by Phan and co-workers (Figure 6.35) [55]. The superparamagnetic CoFe2O4 NPs were prepared by microemulsion method, and on the surface of NPs Schiff-base was anchored to design an immobilized bidentate ligand. The functionalized CoFe2O4 NPs were then complexed with palladium acetate to obtain the immobilized Pd complex.

Figure 6.35: Preparation of Pd-CoFe2O4 nanoparticle.

A Pd(II)-supported Schiff base complex on super-paramagnetic Fe3O4 magnetic nanocatalyst was reported by Sardarian and co-workers (Figure 6.36) [56]. The immobilized Pd(II) complex was effective for the copper and phosphine ligand-free Sonogashira couplingbetween various aryl halides (74) and phenylacetylene (72). Excellent yield of expected coupled product (75) was observed in the presence of Pd(II) catalyst and NEt3 in DMF media. However, under the developed reaction conditions, moderate yields of the diphenyl acetylene derivatives were obtained in the coupling of chlorobenzene and phenylacetylenes.

Figure 6.36: Fe3O4@SiO2/Schiff base Pd (II) catalyzed Sonogashira coupling.

Zhang et al. [57] used magnetic polymer Pd-nanocomposite for Sonogashira coupling reaction. The magnetic Fe3O4/SiO2/P(GMA-co-EGDMA) nano composite was synthesized by the surface grafted copolymerization. The complexation between Pd(II) and imidogen groups or tertiary amine groups after the immobilization of the branched/linear polyethylenimine causes the formation of Pd2+ immobilized, Fe3O4/ SiO2/P(GMA-co-EGDMA) composite which on reduction leads to the fabrication of novel Fe3O4/SiO2/P(GMA-co-EGDMA)-Pd(0) nanocatalyst.

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The synthesized catalyst was used efficiently for the Sonogashira cross-coupling reaction between aryl halides and arylacetylene (Figure 6.37).Wide range of aryl halides (76) were successfully cross-coupled with various arylacetylenes (77) and provided good yield of desired product (78). Moreover, in this developed protocol of Sonogashira cross-coupling reaction only Fe3O4/SiO2/P(GMA-co-EGDMA)-Pd(0) nanocatalyst was used and the formation of 1,3-diyne by-product was successfully avoided in absence of CuI cocatalyst. Again, the nano-catalyst was reused eight times with almost same catalytic activity.

Figure 6.37: Fe3O4/SiO2/P(GMA-co-EGDMA)-PEI-Pd(0) nanoparticle catalyzed Sonogashira coupling.

Sobhani and co-workers [58] prepared palladium complex of bis(imino)pyridine supported on γ-Fe2O3@SiO2 magnetic nanoparticles (Pd-BIP-γ-Fe2O3@SiO2). The synthesized Pd-NPs revealed excellent activities in the Sonogashira coupling reaction (Figure 6.38). In the presence of 0.5 mol% catalyst, the best results were obtained with Et3N as the base in DMF media at 100 °C temperature. Wide range of aryl halides (79) with electron-rich and electron-poor substituents coupled with phenyl acetylene (72) to provide the corresponding diphenyl acetylene derivatives (80) in good to excellent yields.

Figure 6.38: Pd-BIP-γ-Fe2O3@SiO2-mediated Sonogashira coupling.

Asadi and co-workers [59] designed an elegant method for the synthesis of immobilized NNN Pd-complex-supported magnetic NPs. The reaction between chloro-functionalized magnetic Fe3O4@SiO2 nanoparticles (CPS-MNPs) and NNN ligand provided the CPS-MNPs-NNN ligand which underwent complexation with Pd(OAc)2 to afford the CPS-MNPs-NNN-Pd as the desired catalyst. The CPS-MNPs-NNN-Pd catalyst displayed excellent activity with K2CO3 in DMF/ H2O (1:2) media at 90 °C temperature in C–C bond forming reaction (Figure 6.39).

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Various substituted aryl and heteroaryl halides (81) successfully coupled with phenylacetylene (72) under the developed protocol and provided the desired Sonogashira coupled product (82) in excellent yields. Furthermore, the catalyst system was active for the Heck C–C coupling reaction.

Figure 6.39: CPS-MNPs-NNN-Pd-catalyzed Sonogashira coupling.

A green protocol for the synthesis of a Ni-based nanocatalyst supported on Fe3O4 was demonstrated by using Euphorbia maculata extract as the stabilizing and reducing agent [60]. The biosynthesized magnetic nanocatalyst showed excellent catalytic activity in Sonogashira coupling reactions between variety of aryl halides (83) and a broad range of aromatic and aliphatic alkynes (84) (Figure 6.40). The stability and recyclability of Ni-catalyst was tested in the preparation of diphenyl acetylene (85) from phenyl acetylene and iodobenzene. The catalyst was reused five fresh runs without any loss of activity.

Figure 6.40: Sonogashira coupling by Fe3O4@Ni nanoparticle.

6.2.4 Hiyama coupling reaction The Hiyama cross-coupling reaction is the palladium-catalyzed cross-coupling between organosilanes and organic halides to generate the biaryl derivatives. Although the organosilicon reagents are nontoxic, easily available and relatively less expensive, Hiyama cross-coupling reaction cannot replace Suzuki reaction to construct biaryl compounds due to lower reactivity of organosilicons and the necessity of fluoride ions to break the C-Si bond. Accordingly, reports on Hiyama reactions are very few, especially involving heterogenized catalyst systems. Magnetically recoverable nanoparticle catalyzed Hiyama coupling was first reported by Sreedhar and co-workers in 2011 [61]. The Pd/Fe3O4 nanoparticle was synthesized by immobilizing Pd(0) onto Fe3O4 nanoparticles by impregnation of Na2PdCl4

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and reduction with NaBH4.The catalyst was very much active in coupling of arylsiloxanes (86) with bromoarenes (87) under fluoride-free condition in the presence of NaOH base in aqueous media at 100 °C (Figure 6.41). A wide array of aryl bromides including sterically congested 2-bromomesitylene underwent coupling with variety of arylsiloxanes with electron-rich and electron-poor functional groups to generate the biaryl derivatives (88) in good yields. However, coupling with aryl iodides and chlorides are not demonstrated under the developed protocol. The Pd/Fe3O4 nanoparticle was recoverable from reaction medium and catalyst efficiency remained unaltered, even after five consecutive runs.

Figure 6.41: Hiyama coupling applying magnetically separable Fe3O4@Pd nanoparticle.

Zhang et al. [62] reported synthesis of silica-coated Fe3O4 magnetic nanoparticle-supported Pd catalyst for the Hiyama reaction (Figure 6.42). The preparation of the catalyst included the synthesis of SiO2@Fe3O4 via sol–gel method followed by the treatment with 2-(diphenylphosphino)ethyltriethoxysilane to generate phosphine-functionalized SiO2@Fe3O4. The reaction of this phosphine-functionalized SiO2@Fe3O4 with Pd(OAc)2 provided the SiO2@Fe3O4–Pd catalyst. A wide array of aryl iodides and bromides (90) with different electronic environments were coupled effectively with phenyltrialkoxysilanes (89) in the presence of magnetic Pd-catalyst to provide the corresponding biaryl derivatives (91) in excellent yields. Unfortunately, phenyltrimethylsilane did not respond to this coupling reaction under the developed protocol. Importantly, catalyst recovery was achieved easily by external magnet, and the isolated catalyst was reused ten times without any alteration in catalytic performance. In a similar work, Kim et al. [63] employed magnetic Pd–Fe3O4 heterodimer nanocrystals for the ligand-free Hiyama coupling of various trimethoxyphenylsilane with aryl iodides and chlorides in DMA media at 150 °C temperature to obtain the coupled product in excellent yields. Hajipour and Abolfathi [64] reported Ni(0)-catalyzed Hiyama cross-coupling reaction for the first time (Figure 6.43). For this, the authors designed and synthesized Nickel implanted on triazole-modified magnetic NPs through “click” reaction of azide-functionalized silica-coated magnetic nanoparticles (Fe3O4@SiO2) followed by immobilization of nickel nanoparticles. A wide range of biaryl derivatives were synthesized via Fe3O4@SiO2-Ni(0) nanoparticle catalyzed fluoride-free Hiyama coupling of aryl halides with trimethoxyphenylsilane (89). Interestingly, aryl chlorides and heteroaryl halides (92) were compatible with this Ni(0) catalyzed cross-coupling reaction to avail the desired product (93) in

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Figure 6.42: Hiyama coupling applying magnetically separable SiO2@Fe3O4–Pd catalyst.

good yield. The catalyst was recovered easily by external magnet. Comparison of SEM and TEM images of fresh catalyst with the recovered catalyst exhibited that the size and morphology remains almost unaltered after eight catalytic cycles.

Figure 6.43: Hiyama coupling using TF-SiO2@Fe3O4-Ni catalyst.

Karami et al. [65] demonstrated synthesis of Pd(II) nanocatalyst immobilized onto the surface of Fe3O4@SiO2/APTMS (APTMS: 3-aminopropyltrimethoxysilane) core-shell nanocatalyst. The surface morphology and composition of this Fe3O4@SiO2/APTMS/Pd (cdha)2 nanoparticles were characterized by TEM, SEM, XRD, and FT-IR analysis. The catalyst was successfully employed in fluoride-free Hiyama cross-couplingbetween various aryl halides and phenyltrimethoxysilane in the presence of NaOH/sodium dodecylsulfate (SDS) activating system in aqueous media at 100 °C temperature. Sobhani et al. [66] demonstrated the synthesis of water-dispersible magnetically recoverable Pd heterogeneous catalyst (Pd-γ-Fe2O3-2-ATP-TEG-MME) (Figure 6.44). The synthesized catalyst was active in coupling of various halobenzenes with triethoxyphenylsilane in the presence of Et3N in aqueous media at 80 °C. The hydrophilic triethyleneglycol

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allowed the scattering of catalyst in water and higher catalytic performance was achieved in aqueous media due to better contact between reactants and the catalyst.

Figure 6.44: Pd-γ-Fe2O3-2-ATP-TEG-MME.

In another work, Mansoori et al. [67] successfully utilized magnetic mesoporous Fe3O4@SiO2-SBA-supported NHC Pd(II) catalyst for the Hiyama cross-coupling reaction (Figure 6.45). Various electronically diverse aryl iodides and bromides underwent coupling with triethoxyphenylsilane in the presence of CsF in DMF media. Unfortunately, the developed protocol was not effective for Hiyama coupling of readily available and less expensive aryl chlorides with triethoxyphenylsilane (94). The catalyst exhibited good catalytic activity on the first run compared to the various palladium catalysts. The reusability study of the magnetic mesoporous catalyst was performed on the model Hiyama reaction between triethoxyphenylsilane and aryl iodide (95) to get the desired coupled product (96). After magnetic isolation from the reaction medium, the Pd(II) catalyst could be recovered for at least seven consecutive runs with no significant loss of catalytic performance.

Figure 6.45: Mesoporous Fe3O4@SiO2-SBA-supported NHC Pd(II)-catalyzed Hiyama coupling.

6.2.5 Stille coupling reaction Stille reaction is a palladium-catalyzed cross-coupling of organotin compounds with aryl halides and pseudohalides. It is a very important tool for C–C bond formation due to the easy availability of organostannanes and posing high air and moisture stability [68]. Moreover, Stille reaction has a widespread application in the synthesis of biaryl derivatives with pharmaceutical importance such as dynemicin and rapamycin [69]. One of the interesting features of this coupling reaction is the ability to couple allyl and vinyltin compounds with aryl halides. Unfortunately, very few Stille cross-coupling reactions are reported to date applying magnetic nanoparticle catalyst.

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The first report of Stille coupling using magnetically retrievable nanocatalyst was published by Jin et al. [54]. The authors synthesized Pd-SiO2/Fe3O4 catalyst by immobilizing triethoxysilyl-functionalized Pd complex onto the surface of SiO2/Fe3O4 nanoparticle (Figure 6.46). The catalyst showed excellent activity in Stille cross-coupling of electronically diverse aryl chlorides (97) with allyltributyltin (98) derivatives to generate the corresponding biphenyl derivatives (99) in almost quantitative yields (Figure 6.47). In addition, sterically hindered 2-chloro-1,3-dimethylbenzene coupled successfully with tributylphenylstannane. Interestingly, vinyl- and allylstannanes also coupled under the developed protocol to provide the desired coupled product in good yields (87% and 90% respectively). The catalyst was also effective for the reaction of unprotected 4-chlorophenol and 2-chloroaniline with organotin compound.

Figure 6.46: MNps-supported (β-oxoiminato)(phosphanyl) palladium synthesis.

Figure 6.47: Pd-SiO2/Fe3O4-catalyzed Stille cross-coupling.

In another report, Prasad and Satyanarayana [70] employed Pd/Fe3O4 nanocatalyst for coupling of various bromoarenes (100) with different organotin (101) reagents under ligand free condition in 1,4-dioxane media at 100 °C (Figure 6.48). Interestingly, the catalyst was successfully applied for coupling of sterically hindered arylbromide derivatives. The catalyst was very efficient to couple various aryl halides irrespective of the electronic environment and all of them underwent coupling smoothly to give good yield of the desired product (102). Notably, the Pd/Fe3O4 nanocatalyst can also be used for the coupling of tributyl(vinyl)- stannane (104) substrate with various aryl bromides (103) to obtain the corresponding styrene derivatives (105) in good yields.

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However, the coupling with less reactive aryl chlorides is not reported. The magnetic catalyst was recovered very easily by applying external magnet.

Figure 6.48: Stille cross-coupling using Pd/Fe3O4 nanocatalyst.

Ghorbani-Choghamarani et al. [71] used Fe3O4@PTA-Pd for Stille cross-coupling reaction (Figure 6.49). In the presence of Na2CO3, the desired products were formed in good to excellent yields. The palladium leaching investigations showed the high stability of the catalyst. The catalyst could be reused with constant activity.

Figure 6.49: Fe3O4@PTA-Pd nanocatalyst.

Nikoorazm and co-workers [72] developed Fe3O4@MCM-41@Pd-SPATB nanocatalyst by immobilizing S-propyl-2-aminobenzothioate complex of Pd(0) supported on functionalized magnetic nanoporous MCM-41, another example of magnetically retrievable catalyst system for Stille coupling (Figure 6.50). The magnetic nanoporous catalyst displayed very high activity in coupling of variety of aryl halides (106) with organo tin (107) compounds in the presence of K2CO3 as base at 80 °C temperature to provide corresponding biphenyl derivatives (108) in excellent yields. The amino- and thio-functionalized support stabilized the magnetic nanoporous catalyst and prevented Pd leaching during the course of the reaction. Furthermore, facile separation of the catalyst was enabled due to the presence of magnetic support.

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Figure 6.50: Fe3O4@MCM-41@Pd-SPATB catalyzed Stille coupling.

In another work, Koeckelberghs [73] applied Fe3O4/oleic acid/Pd nanocatalyst for Stille cross-coupling reaction. Unfortunately, very low yield of coupled product was detected in the presence of the Pd-catalyst under the developed reaction condition. The lack of formation of coupled product may be due to the fact that the Pd catalyst was degraded or poisoned by the tin-reagent.

6.2.6 Homocoupling reaction Another way of biaryl derivative synthesis is the homocoupling of arylboronic acids and aryl halides. A wide range of homogeneous catalysts with various transition metal ions have been utilized for such C–C bond forming reaction [74]. But homogeneous catalysis is associated with various drawbacks including nonreusability of catalyst, expensive, and environmental pollution from heavy metal ions. Thereby, in recent days, magnetically retrievable nanoparticle catalyzed homocoupling reactions attracted considerable attention from environmental and economic standpoint. Varma et al. [75] described the preparation of magnetically recoverable nanoferrite-anchored glutathione catalyst for the homocoupling of boronic acids under microwave irradiation in aqueous media (Figure 6.51). Nanoferrite-anchored glutathione showed superior activity in homocoupling reaction compared to glutathione-free ferrite catalyst (Fe3O4) in the optimization study. This was due to the inherent basicity of the nanoferrite glutathione system as compared to the parent ferrite. It noteworthy to mention that microwave irradiation had significant effect on catalytic activity in this C–C bond formation reaction. Negligible product conversion was detected when the homocoupling of phenylboronic acid (109) was performed under conventional heating condition. A variety of substituents, irrespective of their electronic environment (donating or withdrawing nature), coupled successfully in the presence of nanoferrite-anchored glutathione catalyst to give biphenyl derivatives (110) in excellent yield within a very short reaction time (45–60 min). However, low yield of the coupled product was isolated for the coupling of pyridyl and pyrazole boronic acid under the developed protocol.

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Figure 6.51: Nanoferrite glutathione catalyzed homocoupling of heteroarylboronic acids.

Li et al. [76] synthesized bimetallic hollow Fe–Pd nanospheres and successfully utilized the same for Ullmann homocoupling of aryl iodides in aqueous media (Figure 6.52). Homocoupling of various para-substituted aryl iodides (111) were achieved successfully utilizing the catalyst system in the presence of KOH and NaHCO3 at 363 K temperature to obtain the coupled products (112) in excellent yield.

Figure 6.52: Bimetallic hollow Fe–Pd nanosphere catalyzed Ullmann type coupling.

Zheng et al. [77] designed and synthesized Pd pincer catalyst embedded on the surface of MNps and employed the nanoparticle as an active catalyst for reductive coupling of various aryl halides (Figure 6.53). Aryl bromides and iodides (113) with electron-releasing and electron-withdrawing groups coupled successfully using K2CO3 in DMF media at 110 °C to obtain symmetric biaryl derivatives (114) in quantitative yield. Fe3O4 magnetic core played dual role in the coupling reaction. It acted as a magnetic support and as a reducing agent during the coupling reaction. The catalyst was separated very easily from the reaction mixture after completion of the reaction and was reused five times with almost same catalytic activity. However, the yield of the reductive homocoupling reaction decreased drastically after fifth run of the catalyst due to the drop of reducing ability of the catalyst.

Figure 6.53: Homocoupling of aryl halides by magnetic Pd-pincer complex.

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Kaboudin et al. [78] demonstrated a Cu(II)-catalyzed homocoupling of various arylboronic acids (115) to synthesize various symmetric biaryls (116) (Figure 6.54). For this coupling reaction authors synthesized magnetically recoverable and reusable Fe3O4MNps-supported Cu(II)-β-cyclodextrin catalyst (Fe3O4-β-CD-Cu). The catalyst was synthesized by a one pot reaction of FeCl2, FeCl3, and Cu (II)–β-cyclodextrin complex in the presence of NH4OH. The synthesized catalyst was thoroughly characterized by XRD, TEM, TGA, VSM, and FT-IR spectroscopy. Homocoupling reaction of various aryl- and heteroarylboronic acids were successfully achieved by applying the prepared catalyst system in DMF media to provide the corresponding coupled product in good yields. Interestingly, trans-2-phenylvinylboronic acid also underwent homocoupling reaction in the presence of copper catalyst to synthesize substituted 1,3-butadiene compound.

Figure 6.54: Fe3O4-β-CD-Cu catalyzed homocoupling of heteroarylboronic acids.

Dubey and Kumar [79] developed another strategy for homocoupling reaction. In this report palladium nanoparticles supported on polydopamine-layered iron oxide NPs (Pd/Fe3O4@PDA) was used as heterogeneous catalyst for Ullmann homocoupling of various arylboronic acids, aryl halides, and aryldiazonium salts using randomly methylated β-cyclodextrin in aqueous media. The recovery of the catalyst was achieved very easily using external magnetic field and recycled up to five runs without further activation of the catalyst.

6.3 Magnetic nanoparticle catalyzed oxidation reaction 6.3.1 Oxidation of alcohols The oxidation of alcohol is an essential organic transformation, because the oxidized carbonyl compounds are the fundamental chemicals as active intermediates in variety of organic synthesis [80]. Because of the industrial importance and environmental concern, application of magnetically retrievable nanoparticle catalysts

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in alcoholoxidation selectively is a hottest topic of current research. Oligosaccharide polymer raffinose is used as the support of magnetic nanoparticles. A heterogeneous nanocatalyst system (Fe3O4@raffinose-Cu2O) is developed by immobilizing Cu2O on the raffinose-based magnetic nanoparticles (MNPs) and was able to oxidize benzyl alcohols (117) to benzaldehydederivatives (118) using TBHP as the oxidant in acetonitrile under refluxing conditions (Figure 6.55) [81]. A range of benzyl alcohol with both electron-donating and electron-withdrawing groups was efficiently oxidized by the developed catalyst system to produce the corresponding benzaldehydes in good to excellent yields. Importantly, separation of the catalyst was achieved easily using an external magnetic field with negligible leaching of the Cu2O, and the recovered catalyst was reused for six cycles with negligible loss of catalytic performance.

Figure 6.55: Fe3O4@raffinose-Cu2O catalyzed oxidation of benzyl alcohols to benzaldehyde.

Another oxidation protocol was demonstrated by Varma and Polshettiwar [82] using dopamine-functionalized nanoferrite-supported Pd catalyst (NanoferritePd). Very strong coordination bond between Fe of nanoferrite and O of dopamine makes dopamine a robust anchor and consequently lowers the leaching of Pd making the catalyst highly stable. The nanocatalyst efficiently oxidized benzyl alcohols with electron-donating and electron-withdrawing substitutions, aliphatic alcohols and heteroaromatic alcohols (119) to their corresponding aldehydes (120) with excellent selectivity and high turnover number (TON) using H2O2 as the oxidant (Figure 6.56). The authors also described that aromatic and heteroaromatic alcohols showed high reactivity under the developed protocol but lower reactivity was observed for hexanol due to its volatile nature. Importantly, the nanoferrite-Pd catalyst showed very easy filtration free recoverability using an external magnetic field.

Figure 6.56: Nanoferrite-Pd-catalyzed oxidation of alcohols.

Wang et al. [83] synthesized nanoferrite-supported proline–palladium catalyst (SiO 2 @Fe 3 O 4 –Pro–Pd) by the complexation of Pd(II) with proline moieties of

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proline-modified MNPs (SiO 2 @Fe 3 O 4 -proline) with a loading of 0.106 mmol of palladium per gram. The developed catalyst was used for the alcoholoxidationto generate aldehydes and ketones (Figure 6.57). The catalyst oxidizes aryl alcohols with electron-donating and electron-withdrawing substituents, polycyclic aryl alcohols, heteroaryl alcohol, and aliphatic alcohols (121) to corresponding aldehydes (122) in good to excellent yields. The aliphatic alcohols showed lower reactivity and afforded lower yield of the oxidized product than aryl alcohols. Interestingly, secondary benzylic alcohols were also successfully oxidized to ketones in good yields. Moreover, the catalytic oxidation of cinnamyl alcohol to cinnamaldehyde revealed the excellent chemoselectivity of the reaction. This nanocatalyst SiO2@Fe3O4–Pro–Pd allowed very easy separation by external magnetic field and excellent recyclability.

Figure 6.57: SiO2@Fe3O4–Pro–Pd promoted oxidation of secondary benzylic alcohols.

The organic oxidant TEMPO-coated superparamagnetic nanoparticles was synthesized by introducing metal-oxide chelating phosphonates (Figure 6.58) and employed for alcohol oxidationreaction to the corresponding carbonyl compounds [84]. Investigation of substrate scope displayed that oxidation of primary and secondary alcohols with aromatic, heteroaromatic and aliphatic substitution exhibited good reactivities to provide good to excellent yield of the carbonyl analogues, under both acidic MnII/CuII protocol of Miniscioxidation [85] and basic Anelli [86] conditions. Interestingly, it was observed that alcohols with sulfur-containing heterocycles were oxidized chemoselectively to corresponding aldehydes and did not produce sulfoxide or sulfone derivatives. But lower aldehyde selectivity was noticed in case of 2-pyridinemethanol due to formation of the pyridinium salt and decreased rate of oxidation was observed. The catalyst system was also useful for the oxidation of hindered and unhindered secondary alcohols to their respective ketones. Furthermore, the leaching studies on the TEMPO-coated SPNs particle revealed the high constancy of the NPs under the highly acidic oxidizing conditions (Minisci conditions). This allowed the catalyst to be reused over twenty runs under the Minisci protocol of oxidation and eight times under Anelli protocol with high conversions and selectivities. A robust magnetically separable nanocatalyst, Fe3O4−Co MNP was synthesized and was explored as an excellent catalyst for oxidation of various primary and secondary alcohols (123) to analogous ketones (124) using excess TBHP as the oxidant (Figure 6.59) [87]. The oxidation of benzoin was also efficiently achieved by applying

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Figure 6.58: TEMPO-coated superparamagnetic NPs Synthesis.

the developed protocol without cleavage of carbon–carbon bonds. Interestingly, ICP– AES analysis of the filtrate (hot) showed negligible Fe and Co leaching after seven reaction cycles indicating high stability of the catalyst under the developed oxidation condition.

Figure 6.59: Fe3O4−Co MNP catalyzed oxidation of alcohols.

A similar heterogeneous catalyst system, Fe3O4-CoOx magnetite nanocatalyst was synthesized by wet impregnation method applying Fe3O4NPs and CoCl2.6H2O followed by reaction with NaBH4 [88]. The magnetic nanoparticle was explored for oxidation of 5-hydroxymethylfurfural (125) to the corresponding dicarboxylic acid derivative (126) with t-BuOOH as oxidant (Figure 6.60).

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Figure 6.60: Fe3O4-CoOx catalyzed oxidation of 5-hydroxymethylfurfural.

6.3.2 Epoxidation reaction Epoxidation of olefins is an important reaction as the epoxides are widely used for the generation of various valuable materials as well as intermediates in organic synthesis, fine chemicals and pharmaceuticals [89]. Recently, several magnetically recoverable nanocatalysts have emerged as attractive materials for epoxidation of olefins. Kooti et al. [90] prepared a heterogeneous catalyst consisting of phosphotungstic acid anchored imidazole functionalized silica layered cobalt ferrite nanoparticles (PTA/Si–imid@ Si–MNPs). Catalytic amount of PTA/Si–imid@ Si–MNPs efficiently catalyze epoxidation of olefins in the presence of tBuOOH as oxidant (Figure 6.61). The catalyst system is able to epoxidize a wide range of alkenes (127) including styrene, allyl phenyl ether, allylbenzene, cyclooctene, cyclohexene, and linear alkenes efficiently and selectively to corresponding epoxides (128) in good yields. The catalyst was very easily isolated from the reaction mixture by applying an external magnetic field and the recovered catalyst was successively recycled five times without any noticeable change of its catalytic performance.

Figure 6.61: PTA/Si–imid@ Si–MNPs catalyzed Oxidation of alkenes.

The oxidation of alkene was also performed by Mortazavi-Manesh and Bagherzadeh [91] using poly (methylacrylate)-coated magnetic nanoparticles containing a molybdenum Schiff base complex (MNP@PMA-SB-Mo). The polymeric functionalization offers the additional advantage of implanting more catalytic sites on the surface of nanoparticles. The MNP@PMA-SB-Mo catalyst was used in the epoxidation reaction in DCE media under refluxing condition in the presence of tBuOOH as oxidant (Figure 6.62).

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A wide range of olefins (127) including aromatic and aliphatic substituents were compatible in this MNP@PMA-SB-Mo catalyzed epoxidation reaction and provided excellent yields of the desired epoxide (128) within short reaction time.

Figure 6.62: MNP@PMA-SB-Mo catalyzed Oxidation of alkenes.

The molybdenum (VI)-catalyzed epoxidation reaction involves the coordination of tert-BuOO− anion to the molybdenum (VI) metal center after transferring one tertBuOOH proton to one oxo group of the dioxo complex. Subsequently peroxo oxygen transfer to the olefin takes place and thus generates the corresponding epoxide and tert-butyl alcohol as a byproduct. Another magnetically separable epoxidation catalyst, Ti−Fe3O4@MCM-41 (Ti-MS) was developed by postgrafting of titanium on the silica-magnetite composite [92]. The synthesized catalyst (Ti-MS) displayed excellent activity for the selective epoxidation of various aliphatic alkenes in the presence of tert-butyl hydroperoxide in toluene. The selectivity of epoxidation was not observed in case of styrene as the rearrangement of the styrene oxide occurred in the presence of acid sites of the Ti−Fe3O4@MCM-41 catalyst. Again, the selectivity decreases on changing the oxidant and the reaction media. On using H2O2 (30% aqueous) or O2, H2O2 (30% aqueous) mixture as oxidant in acetonitrile solvent provides almost 50:50 mixture of epoxide (130) and 1,2-diol (131)on the epoxidation of cyclooctene (129) (Figure 6.63). Investigation also revealed that the terminal C=C bonds are less prone to epoxidation than internal C=C bonds under the developed condition. The magnetically recoverable Ti-MS catalyst was separated quantitatively from the reaction mixture by simply using a bar magnet externally and was successfully reused for eight new set of reactions without any leaching of the catalyst.

Figure 6.63: Ti−Fe3O4@MCM-41 (Ti-MS) catalyzed oxidation of cyclooctene.

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6.3.3 Oxidation of sulfides and mercaptans Sulfoxides are important intermediates for synthesis of biologically active molecules, e.g., fine chemicals, chiral auxiliaries, and agrochemicals [93]. The selective oxidation of mercaptans to disulfides (sweetening) is a very important reaction in petroleum refineries as removal of harmful mercaptans is essential before end use [94]. Several magnetically retrievable nanocatalysts are developed for the oxidation of sulfides and mercaptans. Rostami et al. [95] reported N-Propylsulfamic acid-linked Fe3O4NPs (MNPs-PSA) catalyzed chemoselective oxidation of sulfides. MNPs-PSA catalyzed oxidation was optimized using methyl phenyl sulfide as the model compound in the presence of 30% H2O2 and a range of functional groups including aromatic, heteroaromatic, cyclic and heterocyclic sulfides (132) were successfully oxidized to sulfoxides (133) installing the MNPs-PSA catalyst under solvent free condition at room temperature (Figure 6.64). Interestingly, acid-sensitive and oxidation-prone functional groups like –COOCH3, –OH and –CHO were well tolerated in the sulfoxidation protocol showing excellent chemoselectivity. The catalyst was recovered very easily after the completion of the reaction by an external magnet and reused successfully several times with negligible catalyst leaching.

Figure 6.64: MNPs-PSA catalyzed selective oxidation of sulfides.

Peng et al. [96] prepared trisaminomethane–cobalt complex anchored on Fe3O4 magnetic nanoparticles (Fe3O4@PTMS–Tris–Co) via immobilization of trisaminomethane– cobalt (Tris–Co) complex on the Fe3O4 nanoparticle surface by covalent cross‐linking and investigated its catalytic activity for oxidation of sulfides. Various aromatic, aliphatic and heterocyclic sulfides (132) were successfully oxidized using Fe3O4@PTMS– Tris–Co catalyst in the presence of H2O2 (30%) as the oxidant (Figure 6.65). The Fe3O4 nanoparticle support of the catalyst enabled magnetic isolation of the NPs from the reaction mixture upon completion. Similarly, magnetically separable heterogeneous cobalt phthalocyanine catalyst (CoPcS@LDH@MNP) was synthesized by Jain et al. [97] via the immobilization of CoPcS onto the MgAl-LDH@MNP (LDH@MNP) by intercalation of CoPcS between the interlayers of LDH. The synthesized heterogeneous catalyst was employed for

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Figure 6.65: Fe3O4@PTMS–Tris–Co promoted oxidation of sulfides.

the oxidation of mercaptans (134) to their corresponding disulfides (135) applying molecular oxygen as the oxidant (Figure 6.66). Mercaptans with aromatic, aliphatic, and long chain aliphatic substitutions were successfully oxidized using the magnetically separable catalyst with almost quantitative yields of the product. In addition, the developed catalyst system is useful for the oxidation of mercaptans dissolved in kerosene for lowering the sulfur content of fuel oil.

Figure 6.66: CoPcS@LDH@MNP catalyzed aerobic oxidation of mercaptans.

Fe3O4@SiO2–NH2@Mn(III) nanoparticles was synthesized by immobilizing manganese(III) complex, [Mn(phox)2(CH3OH)2]ClO4 (phox = 2-(2′-hydroxyphenyl)oxazoline) onto silica-coated ferrite NPs through amino propyl linkage [98]. Synthesized nanoparticles have demonstrated unprecedented catalytic performance in selective oxidation of thiols (134) to disulfides (135) without the production of over-oxidized products (Figure 6.67). The Fe3O4@SiO2–NH2@Mn(III) nanoparticle catalyzed oxidation reaction was conducted in the presence of urea-hydrogen peroxide in CH2Cl2–CH3OH (1: 1) at room temperature. In broad scope, several thiols containing substituents were suitable for this oxidation reaction. The Fe3O4@SiO2–NH2@Mn(III) nanocatalyst was remarkably efficient and more selective catalyst in comparison to typical [Mn(phox)2(CH3OH)2]ClO4 complex. The catalyst was removed magnetically from the reaction mixture and used for six new runs without noticeable decrease in catalytic performance.

Figure 6.67: Fe3O4@SiO2–NH2@Mn(III) nanoparticle-mediated oxidation of thiols.

6.3.4 Oxidative amidation of alcohols The amide functionality has remarkable use in numerous research areas including organic synthesis, medicinal and biological chemistry [99]. Therefore, the development

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of efficient protocol for the formation of amide functionality has been one of the attractive fields of research during the past few decades. Heydari et al. [100] prepared magnetic Fe(OH)3@Fe3O4 nanoparticle for the direct oxidative amidation of alcohols (136) with amine hydrochloride (137) salts to obtain various primary, secondary and tertiary amide derivatives (138) in good yields (Figure 6.68). Benzyl alcohols with electron-donating groups showed lower reactivity than the electron withdrawing substitution in this Fe(OH)3@Fe3O4 catalyzed oxidative amidation reaction.

Figure 6.68: Oxidative amidation of alcohols by Fe(OH)3@Fe3O4 nanoparticle.

The oxidative amidation reaction starts with the formation of tert-butylperoxyl and tertbutoxyl radicals in the presence of Fe(OH)3@Fe3O4 catalyst (Figure 6.69). The tertbutylperoxyl and tertbutoxyl radicals then oxidize alcohol to aldehyde (II) via the intermediate radical I. Consequently, the amine produced from the neutralization of the amine salt with CaCO3 reacts with the aldehyde (II) to form the intermediate III. Finally, oxidation of III by FeIII-t-BuOOH catalytic system via the intermediate radical IV produces the desired amide derivative (Figure 6.70).

Figure 6.69: Decomposition of t-BuOOH by iron compounds.

The superparamagnetic nature of the catalyst allowed magnetic separation of the catalyst and the highly stable catalyst was reused for six runs with almost same catalytic activity.

6.3.5 Oxyphosphorylation Moghaddam et al. [101] prepared magnetically separable copper ferrite nanoparticle (CuFe2O4) by coprecipitation method and they used this as an efficient catalyst for the oxyphosphorylation of various styrene derivatives (139) by the reaction with dialkyl phosphonate (140) in acetonitrile medium in the presence of DTBP as the

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Figure 6.70: Plausible mechanism of oxidative amidation reaction.

oxidant (Figure 6.71). A wide range of styrenes with both electron-donating and electron-withdrawing substituents at the phenyl ring are tolerable for this oxidative phosphorylation reaction and provides corresponding β-ketophosphonates (141) in good to excellent yields.

Figure 6.71: Oxyphosphorylation reaction by magnetically separable copper ferrite nanoparticle.

The oxyphosphorylation starts with the copper ferrite catalyzed formation of phosphonyl radical (VII) from dialkyl phosphonate (VI) in the presence of DTBP (Figure 6.72). Subsequently, on reaction with phosphonyl radical VII and styrene V, alkyl radical VIII is formed which interacts with the hydroxyl radical to form β-hydroxyphosphonates IX. Finally, oxidation of IX takes place in the presence of DTBP to generate β-ketophosphonates (X).

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Figure 6.72: Plausible mechanism of oxyphosphorylation reaction.

6.3.6 Oxidation of silyl enolates Arai et al. [102] developed organic–inorganic hybrid magnetic nanobead (Cu-bpy HP-MB) catalyst system for the silyl enolate oxidationto corresponding α-hydroxy carbonyl compounds. The Cu-bpy HP-MB catalyst was initially evaluated in the aerial oxidation of silyl enolate 142 to provide hydroxy ketone 143 in 92% yields (Figure 6.73). Investigation of substrate scope revealed that both the cyclic and acyclic silyl enolates exhibited excellent activities to transform hydroxy ketones in good to excellent yields. Interestingly, it was observed that the application of amino-methylated polystyrene support on the surface of the magnetic beads showed lower catalytic activity in the oxidation reaction. Hence, it was inferred that organic–inorganic hybrid support onto the magnetic beads played vital role in this catalytic aerial oxidation of silyl enolates. In addition, Cu-bpy HP-MB catalyst is collected simply after completion of the reaction by the use of external magnetic field and was reused several times with almost same catalytic activity.

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Figure 6.73: Cu-bpy HP-MB catalyzed oxidation of silyl enolates to α-hydroxy carbonyl compounds.

6.3.7 Oxidation of aromatic amines to give azoxyarenes Azoxyarenes are widely used as therapeutic medicines, analytical reagents, dyes, and photosemiconductors [103]. Hence, the oxidation of aromatic amines (144)selectively to azoxyarenes (145) is a very important transformation in organic synthesis. Sharma and co-workers utilized a zinc based magnetic nanocatalyst for oxidation of aromatic amines. The silica encapsulated MNPs-supported Zn(II) catalyst (Zn(II)-AcPy@ASMNP) was prepared by grafting of 2-acetylpyridine through covalent bonding ontosilica@magnetite nanoparticles functionalized with amine group, followed by treatment with zinc acetate [104]. The standardization of the reaction condition was performed by employing aniline as the substrate in the presence of H2O2 as the oxidant and variety of electronically diverse anilines were successfully oxidized with high TON by applying the developed catalyst system (Figure 6.74). In addition, the magnetically recollected Zn(II)-AcPy@ASMNP catalyst was recycled for six runs without any appreciable change in its activity.

Figure 6.74: Oxidation of aromatic amines by Zn(II)-AcPy@ASMNP catalyst.

The Lewis acidity of the nanocatalyst plays vital role in this transformation. As indicated in Figure 6.75, in the presence of excess amount of hydrogen peroxide, the Zn (II) complex immobilized nanocatalyst provides electrophilic activation of H2O2 which in turn increases the electrophilic characters of the peroxide oxygen. The oxidation occurs by nucleophilic attack of –NH2 group of aniline (XI) to an active hydroperoxy intermediate, to form phenylhydroxylamine (XII). Subsequently, further oxidation of phenylhydroxylamine (XII), generates nitrosobenzene (XIII), which on condensation with aniline leads to the formation of azobenzene (XIV). Azobenzene

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(XIV) on further oxidization provides the desired oxidized product azoxybenzene (XV). The desired product can also be obtained by direct condensation of the two intermediates, phenylhydroxylamine and nitrosobenzene.

Figure 6.75: Plausible mechanism of oxidation of aromatic amines.

6.3.8 Benzylic and allylic C-H bonds to carbonyl compounds Among the magnetic nanoparticles, Fe3O4 MNPs have drawn great attention in organic synthesis due to their superparamagnetic behavior, low toxicity and biocompatibility. Zarghani and Akhlaghinia [105] prepared Fe3O4 magnetic nanoparticles by co-precipitation method and demonstrated their application as heterogeneous catalyst for oxidation of range of allylic and benzylic C-H bonds to the carbonyl compounds. Oxidation of a series of hydrocarbons (146) containing benzylic and allylic methylene groups were performed using Fe3O4 MNPs in the presence of tertbutyl hydroperoxide at 120 ᵒC, in sealed tube condition and the carbonyl compounds (147) were produced in excellent yields (Figure 6.76). After finishing the reaction, the NPs was separated magnetically from the reaction mixture and recycled for four consecutive runs. But a gradual loss of catalytic activity was observed after four successive runs. This loss of catalytic activity is attributed to the instability of nanoparticle under oxidizing conditions where Fe3O4 MNP is gradually oxidized to maghemite. Similarly, Gnanaprakasam and colleagues synthesized magnetically separable MnO2@Fe3O4 nanoparticle for the benzylic sp3 C-H oxidationof

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Figure 6.76: Oxidation of allylic C-H bonds by Fe3O4 MNPs.

ethers in the presence of TBHP as the oxidant to obtain ester derivatives (Figure 6.77) [106]. Interestingly, various hydrocarbons, carbocyclic and heterocyclic compounds with benzylic methylene groups (148) were also oxidized applying MnO2@Fe3O4 MNP catalyst under batch and continuous flow module and provided corresponding ketones or esters (149) in good yields. Moreover, the magnetic support enabled easy separation of the nanocatalyst from the reaction mixture.

Figure 6.77: MnO2@Fe3O4 MNP catalyzed benzylic sp3 C-H bond oxidation.

6.3.9 Oxidation of secondary amines to nitrones Nitrones have long been considered as convenient intermediates for the preparation of heterocyclic compounds. Among the various methods of preparation of nitrones, oxidation of secondary amines (150)selectively to nitrones (151) is the most striking as the secondary amines are readily available compared to other starting materials. In order to accomplish this transformation, Heydari’s group designed and synthesized superparamagnetic tungstophosphoric acid catalyst supported on γ-Fe2O3 nanocatalyst (γ-Fe2O3@SiO2–H3PW12O40) [107]. The synthesized magnetic nanoparticle successfully catalyzed the selective oxidation of various secondary amines to nitrones in the presence of H2O2 as the oxidant at room temperature (Figure 6.78). Secondary amines with electron-donating group showed greater reactivity in this developed oxidation protocol and provided the desired nitrone derivatives in excellent yields.

Figure 6.78: γ-Fe2O3@SiO2–H3PW12O40 catalyzed oxidation of secondary amines.

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The catalyst was collected simply by attaching an external magnet onto the reaction vessel and reused for four runs, maintaining the catalytic activity.

6.4 Conclusions With the growing economic and environmental concern, interest for the development of efficient and recoverable catalysts for organic transformations has been increasing. Along this line, magnetic NPs have appeared very important catalyst system for a wide array of catalytic transformations and attract the attention recently due to their unique properties. An efficient organic transformation has the characteristics of high selectivity, simplicity, and sustainability. Nanometer ranged magnetic nanoparticles allows very high activity and selectivity owing to their larger number of the active sites. Moreover, the most interesting feature of magnetic nanoparticles in organic reaction is their simple recovery after the reaction by applying external magnetic field without any filtration. This chapter includes the updates of this new window of organic synthesis with portrayal of range of magnetite-supported metal catalysts. The diverse application of magnetic catalysts are described for the reactions namely C–C bond forming reaction (Suzuki, Heck, Sonogashira, Hiyama, Stille, homocoupling reaction) and representative oxidation reactions.

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Nirjhar Saha, Asim Kumar, Antarlina Maulik, Asit K. Chakraborti*

Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions 7.1 Introduction Cross-coupling reactions are widely explored strategies for C-C and C-X (X = O, N, S) bond formation [1–5]. It can be defined as sigma bond forming reaction by the coupling between two identical (homo coupling) or different (hetero coupling) component with the loss of the functional group or particular moiety from the each of the molecule (coupling partner). The classical cross-coupling reactions leading to C-C bond formation include Heck [6–9], Sonogashira [10, 11], Ullman [12, 13], Stille [14–16], Negishi [17], Kumada [18, 19], and Suzuki [20–23] coupling reactions. Later, the cross-dehydrogenative coupling (CDC) via C-H bond activation was developed for C-C bond formation [24–27]. The widely explored cross-coupling reactions leading to C-N bond formation are the Chan-Lam [28, 29] and BuchwaldHartwig reactions [30–32] (Figure 7.1). For C-C bond formation, the Suzuki cross-coupling reaction is popular over the other coupling reactions due to certain advantages such as the ease of commercial availability of the coupling partners, versatility and chemical stability of the substrates, ease/feasibility in performing the reaction both under homogeneous (in non-aqueous/aqueous medium) and heterogeneous conditions, mild to moderate reaction temperatures, broad range of functional group tolerance, high regio- and stereoselectivity, ease of isolation and purification of the final product, lesser propensity of side product formation, and affording higher product yields. Transition metal–catalyzed bi-aryl synthesis via Suzuki cross-coupling methodology involves the reaction of boronic acids/esters with aryl halides in the presence of external ligand and base. The catalytic cycle of the Suzuki cross-coupling reaction starts with activation of the metal catalyst. In the presence of external ligand and the base, the M(II) is converted to its active form M(0), which participates in the oxidative addition step with the aryl halides to form the organopalladium intermediate. Base-promoted

*Corresponding author: Asit K. Chakraborti, School of Chemical Sciences, Indian Association for the Cultivation of Science (IACS), Jadavpur, Kolkata, West Bengal 700 032, India; e-mail: [email protected] Nirjhar Saha, Antarlina Maulik, Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research (NIPER), S. A. S. Nagar, Sector-67, Punjab 160 062, India Asim Kumar, Amity Institute of Pharmacy, Amity University, Haryana, Manesar 122 413, India https://doi.org/10.1515/9783110730357-007

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Figure 7.1: Various cross-coupling reactions.

transmetallation of the boronic acid via aryl anion generation forms the diarylpalladum complex, which subsequently undergoes base-assisted reductive elimination process to afford the desired bi-aryl product [33] (Figure 7.2). Suzuki cross-coupling reaction under heterogeneous catalysis is progressively replacing the traditional homogeneous catalysis protocol due to the sustainable impact of heterogeneous catalysis. The reported heterogeneous catalysis include metal catalyst supported on polymeric material or heterogeneous template, monometallic or bi-metallic nanoparticles, metal supported on magnetic template, heterogeneous and porous metal supports, metal organic frameworks, and metal catalyst entrapped into the nanostructures [34–37]. Among all of these approaches, magnetic nanocatalysis is the new trend in Suzuki cross-coupling reaction due to the easy and fast separation of the catalysts from the reaction mixture with the help of external bar magnet. The magnetic separation of the catalytic system is an efficient alternative to the time, solvent, and energy consuming separation protocols. The main objective of this chapter is to highlight the magnetically recyclable heterogeneous magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions reported during the last 10 years after a brief discussion about the recent developments on the Suzuki cross-coupling chemistry.

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Figure 7.2: Suzuki cross-coupling reaction and its mechanistic pathway [33].

7.2 Recent developments in the Suzuki cross-coupling reactions Various methodologies for Suzuki cross-coupling are reported till now [38–42]. In view of sustainable development of chemistry several methodologies that include the template supported heterogeneous catalysis [43], nanocatalysis [44], use of aqueous reaction medium [45–48], performing the reactions using non-conventional energy source such as irradiation with micro/ultrasonic-wave [49, 50] and visible light [51–55], and transition metal–free approaches [56–59]. The following discussion provides the update on the various developments in the Suzuki crosscoupling reactions since 2014. Chakraborti et al. [60] used for the first time benzazole tethered aryl bromides as a highly sterically hindered substrate for Pd-catalyzed Suzuki−Miyaura cross-coupling reaction with diverse aryl boronic acids that led to the identification of new therapeutic leads. The resultant 2-(2′-aryl)arylbenzazole moiety being a novel anti-inflammatory scaffold encouraged for further improvement in the context of sustainable development in terms of reducing reaction time, lowering catalyst load of the costly palladium (noble metal), etc. This led the same research group to devise palladium-nickel binary nanocluster as novel heterogeneous catalytic system for Suzuki–Miyaura coupling of sterically hindered benzazole tethered aryl bromides [61]. Coordination between the N atom of the benzazole moiety and the hard Ni site makes the Ni site electron rich. Transfer of the electron density from Ni to the larger and softer Pd site

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makes it more nucleophilic. The electronically enriched Pd oxidatively inserts into the C-Br bond to form the six-membered metallacycle, which is more stable compared to the five-membered metallacycles generated by the coordination-oxidative addition of either the PdNPs or the NiNPs alone (Figure 7.3). This type of intermediate stabilization is the reason for the better catalytic potential of the Pd-Ni binary nanocluster (NC) apart from the cooperative effect of the Pd and NiNPs in the binary Pd-Ni NC system. The heterogeneous NC could be recovered through cotton filtration method and recycled for five Suzuki catalytic cycles.

Figure 7.3: Plausible role of the Pd-Ni NC to catalyze the Suzuki cross-coupling reaction [61].

Recently it has been revealed that the binary Ni-Pd NCs was activating the C-O bond and efficiently catalyzing Suzuki−Miyaura cross-coupling reaction involving various heterocycle tethered sterically hindered aryl carbonate such as o-benzoxazole-tethered aryl ester, silyl ether, sulfonate, carbamate, and carbonate as the electrophilic coupling partners and aryl boronic acids as nucleophilic coupling partner [62]. Mechanistically it was proposed that the reaction proceeds via three distinct steps: (i) oxidative addition which involves addition of the M(0) to the C-O bond, (ii) trans-metalation of the aryl moiety of arylboronic acid to the complex formed through oxidative addition to the C-O bond, and (iii) reductive elimination to form the biaryl product and regenerate the M(0) as the active catalytic species [M = Ni or Pd center in the Ni-Pd NCs]. The bimetallic NCs of Ni-Pd works in unison. The Ni site of the Ni-Pd binary NCs undergoes coordination with the N atom of the oxazoline moiety whereas simultaneously the Pd coordinates with the oxygen atom of the carbonyl group of esters. The Ni center due to this coordination becomes electron rich (more nucleophilic), thereby activating the C-O bond via oxidative addition to generate metallacycle in which the carbonyl

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group acts as the directing group. Trans-metalation of the aryl moiety of the arylboronic acid followed by reductive elimination and regeneration of the catalyst (Ni-Pd NCs) synthesized the biaryl product (Figure 7.4). It was envisioned that water not only plays a role in the formation of MNPs [63], but also helps in the removal of the leaving group in the trans-metalation step. Further it was also proposed that water might play a role in the proteolytic removal of the ligand for regeneration of the NiPd NCs during the reductive elimination step.

Figure 7.4: Ni-Pd binary metallic nanocluster–catalyzed C-O bond activation [62].

The synthesis of 2-(2-aryl)arylbenzazoles using the Suzuki−Miyaura cross-coupling via the Pd-Ni NC-catalyzed C-O activation strategy has some distinct advantages in the context of sustainable chemistry development in view of the fact that the use of Ni as the co-catalyst reduces the load of costly Pd salts, use of phenolic hydroxyl-derived moiety as the leaving group of the electrophilic coupling partner in place of the corresponding bromides whose preparation would involve the use of hazardous brominating agent as the phenolic hydroxyl group can be efficiently installed in the aryl moiety through a novel aerial oxygen–mediated aryl hydroxylation via aryl C-H activation [64]. As nitroarenes could be better electrophilic coupling partner than aryl halides due to the strong electron withdrawing nature of nitro group, Suzuki coupling of nitroarenes with arylboronic acids via C-N bond cleavage has been reported (Figure 7.5). Presumably the nitro group facilitates the oxidative addition step of Suzuki catalytic cycle. The usage of ligands like BrettPhos is important for increasing yield of reaction. This reaction methodology is tolerant toward both EDG and EWG on nitroarenes, as established by 37 derivatives with yields of 36–81% [65].

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Figure 7.5: Suzuki−Miyaura coupling using nitroarene as substrate.

A cellulose Schiff base supported Pd(II) catalyst system has been used for Suzuki coupling reaction under mild and solvent-free conditions using microwave irradiation [66]. The catalyst showed excellent TON (19,800) and TOF (1,98,000) values. The catalyst could be reused up to eight cycles without any significant loss in activity. Both electron donating and electron withdrawing groups on phenyl or naphthalene backbone gave yields varying from 32 to 98% (Figure 7.6).

Figure 7.6: Cellulose Schiff base supported Pd(II) for synthesis of biaryls via Suzuki coupling reaction.

The effects of the substituent group position of the substrates on the yields of the Suzuki coupling reactions were observed to follow the order para > meta > ortho and the coupling of aryl halides with phenyl boronic acid was in the order Br > I > Cl. The catalyst is thermally stable, requires small loading, can be removed via filtration from the reaction media and can be used under oxygen atmosphere. In oxygen equilibrated aqueous solutions of Kolliphor EL, nanomicelle is formed with oxygen-free cores, which has been used as a green medium for carrying out Suzuki coupling reactions (Figure 7.7) [67]. Suzuki coupling reactions of aryl/heteroaryl bromides with boronate esters using Pd(dtbpf)Cl2 as catalyst in 2 wt percent water solution of Kolliphor EL

Figure 7.7: Suzuki coupling of Aryl/Heteroaryl bromides with boronic acid esters using a non-ionic surfactant.

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were performed under air at room temperature with the time period ranging from 2 to 12 h. Aryl and heteroaryl substrate like 2-thiophenes, 2-quinolines, and 2-napthalenes afforded the desired products in 80–97% yields. This micellar catalysis for Suzuki coupling forms an interesting green approach toward sustainable C-C bond formation reactions. The metal organic framework (MOF)-based Pd catalyst containing Pd (II) ions in its structure is used for one-pot Suzuki coupling reaction [68]. Initially the catalyst is prepared by treating 1,4-benzene diboronic acid with phenyl-substituted phosphine. Thus, the Pd NP containing MOF is made, which contains Pd(II) NP with phosphine as ligand. During the Suzuki coupling, this Pd NP gets reduced to Pd(0), when treated with aryl boronic acid and aryl halide at room temperature. The reaction works well with both the boronic acid and aryl halide bearing various electron withdrawing group (EWG) as well as electron donating group (EDG), giving 69–99% yield of the products (Figure 7.8). The catalyst can be filtered and reused up to 5 times without loss in activity. This methodology involving the use of nanoparticle in catalysis of Suzuki reaction is significant in the context of green chemistry. Nickel catalyst–mediated C-C bond formation via C-N bond cleavage of aliphatic amide derivatives is an exclusive approach. The process involves coupling of alkyl/ aryl/heterocyclic carboxamides with aryl/heteroarylboronate esters in the presence of the nickel catalyst and ionic liquids as ligand at 120 °C for 16 h (Figure 7.9). The ionic liquid plays the role of N-heterocyclic carbene (NHC) ligand precursor. The NHC ligand is required for the catalytic efficiency and the reaction condition tolerate heterocycles as substrate as well as epimerizable α-carbon containing amides. About 30 examples with aryl and heteroaryl substrate demonstrate the applicability of this reaction for formation of the product ketones in 54–90% yield. This catalyst system is very efficient for the synthesis of heterocyclic ketones which form important bio-scaffolds and have diverse applications [69]. Huang et al. [70] had developed the metal-free organocatalytic C(sp3)-C(sp2) Suzuki coupling involving benzyl chlorides and aryl boronic acids to synthesize di-aryl methanes (Figure 7.10). The exclusive chemoselectivity of alkyl over aryl halides is the key feature of this process. The reaction proceeds through a novel catalytic cycle: sulfonium salt, sulfur ylide, boron “ate” complex, 1,2-metalate shift, and protodeboronation. A single sulfide-based catalyst can activate both sp3 carbon as well as arylboronic acid via a zwitterionic boron “ate” intermediate, which undergoes a 1,2metallate shift to give Suzuki product from benzyl halides and aryl boronic acids. A series of biologically important diarylmethanes can be prepared. Suzuki coupling reaction using N,N′-bis(2,4,6-trimethylphenyl)ethane-1,2-diamine (BEDA), as a bidentate ligand of Pd(II) complex have been reported under mild conditions in the presence of green solvent [71]. The 25 examples with various EDW and EWD as substituents on the phenyl backbone have shown the synthetic applicability affording the desired products with yields varying from 80–96% (Figure 7.11).

Figure 7.8: MOF Pd NP as catalyst for Suzuki coupling.

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Figure 7.9: Suzuki coupling of aliphatic amides using nickel catalyst in the presence of NHC as ligand.

Figure 7.10: Metal-free sulfur catalyst–based Suzuki coupling [70].

Suzuki−Miyaura coupling reaction of thioureas or thioamides with aryl boronic acids was performed by in situ generation of Pd–carbene complexes from the desulfurization of the thio derivatives (Figure 7.12). This reaction results into the formation of a series of amidinium salts. The reaction proceeds initially with the Ag(I)-mediated desulfurization and subsequent generation of the Pd-NHC complex within the catalytic cycle. In the next step, trans-metalation of aryl anion with the Pd-NHC complex and reductive elimination of the Pd-salts generates the amidinium product. Reaction can be performed in open air and is tolerant to bromo-substituted aryl rings and

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Figure 7.11: Pd(II) precatalyst for Suzuki reaction of aryl chlorides under mild conditions.

compatible with both EDW and EWG substituents on the phenyl rings. The silver salt acts both as a desulfurizing agent (or thiophile) and Pd oxidizing agent, facilitating the PdII/Pd0/PdII catalytic cycle [72]. The overall outcome (product yield) of this reaction was found to be significantly dependent on the solvent and trifluoroethanol (TFE) proved to be the only effective reaction medium as other protic polar solvent such as tAmOH gave poor yield. Though the role of TFE has not been specifically mentioned in the above reported literature it may be presumed that TFE promotes the reaction through hydrogen-bond (HB)-assisted activation of the electrophilic coupling partner [73].

Figure 7.12: Palladium-catalyzed Suzuki−Miyaura coupling of thioureas or thioamides [72].

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A sustainable strategy for ligand-free synthesis of aryl ketones from amides via Suzuki cross-coupling via C-N bond cleavage has been reported (Figure 7.13) [74].

Figure 7.13: C-N bond cleavage-mediated Suzuki–Miyaura cross-coupling in aqueous medium at room temperature.

The reaction was carried out using Pd(OAc)2 as catalyst, H2O as solvent, at room temperature. The N-acyl succinimide, which is widely available and can be synthesized via non-toxic procedures, is used as the coupling partner of aryl trifluoroborate to form aryl ketones. Synthesis of variously substituted 26 ketones with moderate to good yield indicates that electron withdrawing substituents are better tolerated than electron donating substituents. The water used as the reaction medium could be reused up to four times by extracting the product in organic phase after each cycle with slight loss of yield. This green method of acylation via Suzuki coupling has an edge over the Friedel-Crafts acylation which requires harsh reaction conditions. It appears that the solvent plays important role in the progress of the reaction for which water was found to be the effective reaction medium and organic solvents such as MeCN, PhMe, or the water-PhMe (1:10) mixture did not work. While the use of water certainly addresses some green chemistry aspect of the protocol, however, its role remained to be addressed in the above report. In view of the understanding on the molecular level interaction of water as “ambiphilic (electrophile-nucleophile) dual activation” through a “co-operative hydrogen-bond network” proposed by Chakraborti et al. [75–77] that finds application in devising novel “all water” chemistries for the synthesis of bioactive heterocycles [78–80], drugs [80, 81], essential intermediates to generate therapeutic leads [82–84], and developing novel transition metal–free dioxygen activation protocol [85], the dependence of water as the best reaction medium in the above Suzuki coupling for diaryl ketone synthesis could also be due to HB-assisted activation of the N-acylsuccinimide, used as the electrophilic coupling partner, by water. A hydrophilic heterogeneous cobalt catalyst made up of chitosan-based Schiff base has been prepared and used for palladium- and fluoridefree Suzuki coupling [86]. Tagged triethylene glycol on the catalyst makes it easily dispersible in water so that it can lead to higher performance of the catalyst along with reusability (up to 6 times without loss of catalytic efficiency). The product can be extracted in organic solvents like ethyl acetate. Water has been used as green solvent in this reaction without requirement of any additive. The 12 examples show that substituents both as EDG and as EWG on the substrate benzene ring can be used to form the desired products with yield ranging from 80 to 93% (Figure 7.14).

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Figure 7.14: Hydrophilic heterogeneous cobalt catalyst supported on chitosan backbone for Suzuki coupling reaction.

The use of phase transfer catalysts always offers suitable advantage for various organic transformations in aqueous medium generally in enhancing the miscibility of organics in the aqueous medium but may also provide specific role to create micro reactor environment in accelerating the reaction rate [87–90]. Development of a Covalent Organic Framework (COF)-based phase transfer catalyst which is stable in aqueous system is a very interesting option for catalyzing C-C bond forming reactions like Suzuki coupling. The catalyst has been synthesized from a di-hydrazide decorated quaternary ammonium salt via Schiff base condensation with tri-formyl benzene (Figure 7.15). The heterogeneous nature of the catalyst was confirmed by leaching test. The catalyst could be recovered easily via centrifugation for recycling purpose to give product yields up to 90% after 10 runs/cycles. Good catalytic activity

Figure 7.15: A phase transfer composite catalyst for aqueous Suzuki−Miyaura coupling reaction.

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was exhibited for bromo- and iodo-benzene as substrates and 27 compounds were synthesized with 83–99% yields varied [91]. Pd-free Suzuki coupling reaction could be easily performed using cobalt phthalocyanine (CoPc) grafted NiO photocatalyst at room temperature under visible light irradiation (Figure 7.16) [92].

Figure 7.16: Photocatalytic Pd-free Suzuki coupling.

The CoPc in the hybrid photocatalyst acts as a photosensitizer allowing an uninterrupted flow of electrons to the conduction band of the NiO under visible illuminance. The transfer of electron from HOMO to LUMO takes place once the CoPc is excited under irradiation of light. Similarly, under the light irradiation, the charge separation occurred, and the photogenerated electrons are excited from the valance band (VB) to conduction band (CB) of NiO. The aryl boronate gets converted into the aryl radical cation by the photogenerated holes in the HOMO of the CoPc. However, the photoexcited electrons in the LUMO of CoPc are readily transferred to the conduction band of the NiO and reduce the aryl halide to the corresponding aryl anion radical. In a final step, the photogenerated aryl radical cation couples with aryl radical anion to form the corresponding biphenyl derivatives. The reaction was performed using phenylboronic acid having substitution in the phenyl ring with substituted aryl halides to generate biphenyl compounds. Different halogens on the phenyl backbone as well as other electron donating and withdrawing groups on the aryl moiety yielded the products in yields ranging from 36 to 95%. The catalyst could easily be recovered by filtering the reaction mixture through membrane filter (45 µ) and reused for six cycles without significant loss of product yield. This noble metal–free reaction has significance in the context of green chemistry. Ultrasound has been used to facilitate the supported Pd NPs-catalyzed Suzuki cross-coupling reaction performed in PEG as green solvent. A unique protocol was adopted to convert Pd(II) to Pd nanoparticles using Coleus amboinicus extract as the reducing agent. This nanocatalyst was used to explore ultrasound-mediated Suzuki coupling reaction with ultrasonic wave frequency of 40 kHz and power of 150 watt [93]. Aryl halides were reacted with aryl boronic acids for 30 min under ultrasonic wave irradiation to yield biphenyls (Figure 7.17). The catalyst could be reused for seven cycles without significant loss in product yield. The Pd-NHC complex has been employed as the catalyst for Suzuki coupling reaction via C-N bond cleavage involving aryl boronic acids and aryl amides (electrophilic partner) [94]. To develop a sustainable reaction methodology, authors

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performed the reaction in various solvents to select the best green reaction medium for this reaction. Varying various factors, it has been found that isopropyl acetate is the most efficient solvent in terms of reactivity as well as relative greenness. While using various alkyl, aryl, as well as heterocyclic amides or aryl/alkyl/heterocyclic boronic acids, the use of isopropyl acetate as the reaction medium has given yields ranging from 58 to 97% (Figure 7.18).

Figure 7.17: Ultrasound-mediated Suzuki coupling reaction by Pd NP.

Figure 7.18: Pd-NHC Complexed catalyzed Suzuki reaction in green reaction medium.

7.3 Magnetically separable nanocatalysts in Suzuki cross-coupling reaction With the growing interest and compelling need for sustainable and greener approaches that utilizes environmentally benign principles it was being felt to devise greener and more efficient catalyst system for various organic reactions. Metal nanoparticle (NPs) has emerged as one such alternative to the classical metal salt/ complex-based catalysts. The metal NPs have high surface to volume ratio which imparts unique physicochemical and catalytic properties which make them as efficient alternative for the homogeneous catalysts used as such or under immobilization [95–98]. Materials that exhibit magnetism show a response to an applied magnetic field and are labeled as magnetic materials. These magnetic materials are classified as ferromagnetic, paramagnetic, diamagnetic, antiferromagnetic, and ferromagnetic [99]. Magnetic nanoparticles (MNPs) are those metal NPs that show some response to an applied magnetic field [100]. Owing to its unique physicochemical and catalytic properties utilization of MNPs as efficient supports for

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catalysts is being vigorously investigated [101–104]. Figure 7.19 encompasses the various potential roles of transition metal–based magnetic nanocatalytic system.

Figure 7.19: Applicability of heterogeneous catalyst system.

Magnetic nanoparticles are material whose physicochemical properties can be manipulated using magnetic fields. Such particles commonly contain two components, a magnetic material, often iron, nickel and cobalt, and a chemical component that has functionality [105–108]. An astronomical surge in homogeneous transition metal nanoparticle-catalyzed cross-coupling reactions was found during the last decade. However non-recyclability, high cost, poisoning and loosing activity, difficulty in separating the residual metal from reaction mixture and less environment benignness are certain drawbacks that limit the applicability of homogeneous catalyst system [109–111]. These led to a compelling need of developing heterogeneous catalyst system supported on various innocuous and insoluble materials such as silica, grapheme, zeolites, alumina, and chitosan [112]. Figure 7.20 depicts the evolution of catalyst system for catalyzing various organic transformations over the year.

Figure 7.20: Degree of advancement in supported catalyst.

In the recent past there have been perpetual efforts to develop heterogeneous supported catalyst system based on transition metal complexes having magnetic property for the construction of C-C/C-heteroatom bond. These magnetic metal nanoparticles (MNPs) on heterogeneous support have large surface area to volume ratio and offer

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advantages acting as bridge between heterogeneous and homogeneous catalysts [113, 114]. Figure 7.21 depicts the magnetically separable nanocatalyst system.

Magnetic Het Heterometals

Support

Noble Metals

Magnetically Separable Semi-heterogenous catalyst System

Figure 7.21: Composition of heterogeneous magnetically separablenanocatalyst.

In the heterogeneous catalysis approach, the catalyst can be recovered from the reaction mixture through either filtration or magnetic decantation method. The filtration method is generally utilized for recycling the catalyst. Being susceptible to an external magnetic field, super paramagnetic NPs behave like a paramagnetic material in the presence of applied magnetic field but withdrawal of magnetic field immediately results into the disappearance of the net magnetic moment. Hence, the superparamagnetic NPs are not permanent magnet and can only be recovered/separated by inducing a magnetic moment into them through an external magnetic field. Magnetic decantation involves the application of external magnetic field in the form of a bar magnetic underneath the reaction vessel wall to accumulate the magnetically susceptible nanocatalyst at the wall, followed by decantation of the reaction mixture liquor, and washing the magnetically immobilized nanocatalyst for several times (Figure 7.22). This magnetically recovered nanocatalyst can be reused for the next catalytic cycle [115]. This methodology of heterogeneous catalysis is sustainable in terms of catalyst recycling, ease of separation of the catalyst system using cheap and easily available magnet, requirement of lesser amount of washing solvent, and the catalyst survival for more no. of catalytic cycles after the 1st recovery compared to other heterogeneous catalyst systems.

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Figure 7.22: Schematic diagram of magnetic separation and recycling of magnetic nanocatalysts.

7.3.1 Nanocomposite with non-magnetic metal catalyst supported on coated magnetic template Magnetically separable nanocomposite-catalyzed Suzuki cross-coupling reaction represent the heterogeneous catalysis approach [116, 117]. This nanocomposite can be assumed as the assembly of four crucial components such as magnetic template, coating material of magnetic template, coordinating ligand of the metal catalyst, and the transition metal catalyst involved in the Suzuki catalytic cycle (Figure 7.23). Each of these components plays its exclusive role. Magnetic template is the critical component of the nanocomposite as it enables the nanocatalyst to be separable under the influence of the external magnetic field and recyclable. The application of external magnetic field can easily separate out the catalytic system from the rest of the reaction mixture and the catalyst can be reused for next catalytic cycle or for another heterogeneous catalytic reaction. The coating material which envelops the magnetic template can be a polymeric material, silica-based small chain organosilicon compound, and derivatized heterocyclic compound. The coordinating or stabilizing ligand (extended part of the coating material), chelates with the active transition metal catalyst to maintain the active M(0) oxidation state of the transition metal. It also immobilizes the metal catalyst onto the surface of coated magnetic template. Transition metal catalyst of the nanocomposite is responsible for catalyzing the

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Figure 7.23: Transition metal salts or NPs anchored on magnetically susceptible template as magnetically nanocatalyst system.

Suzuki reaction of aryl halides and aryl boronic acids for synthesis of bi-aryls. The metal catalyst with zero oxidation state was found to be the active form for participating in the Suzuki catalytic cycle. As the coordination of the coordinating ligand of nanocomposite helps to maintain the M(0) oxidation state of the transition metal, most of the magnetically separable nanocomposite-catalyzed Suzuki cross-coupling reactions are external ligand-free reaction, which further adds to the sustainable advantages of the magnetically separable nanocomposite-catalyzed Suzuki cross-coupling reactions. The reported magnetic nanocomposite-catalyzed Suzuki reactions have been classified into the following four categories depending on the reaction conditions.

7.3.1.1 Reactions performed at room temperature (≤ 40 °C) As the reactions carried out at room temperature do not require any external energy source, it is considered as more sustainable approach than the reactions performed at moderate and high temperature. Suzuki cross-coupling reactions performed at room temperature [118–128] using the magnetically separable nanocatalyst systems are enlisted in Table 7.1. Earlier various research groups have carried out the Suzuki cross-coupling reactions in green solvents [129, 130]. Most of these room temperature reactions were carried out in the presence of sustainable reaction media such as the mixture of H2O, EtOH (1:1 v/v) (entry 1–7), H2O (entry 8), and EtOH (entry 9). Although the nanocomposites differ from each other in their structure and efficiency, the identical magnetic template (i.e., magnetite) was chosen for the development of

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the nanocomposite due to the ease of recycling through external magnetic field. The transition metal catalyst which participates in the catalytic cycle is considered as the active catalyst in this chapter. The active catalysts for the reactions under Table 7.1 are either Pd(II) or Pd(0) (generated from the precursor metal salts). Modified or crosslinked biopolymer (entry 1, 6) was used not only as coating material of magnetic template but also as the adsorptive support for the Pd NPs. In situ generated Schiff base formed from the aldehyde and amine was covalently tethered with magnetite template for its coating (entry 2). Various research groups have chosen SiO2-linked alkyl silane derivatives as coating material due to the ease of further derivatization of these coating materials with versatile coordinating ligands (entries 3, 7, 9). The polyphenolic natural products extracted from strawberry fruit can be used as coating material as well as bio reducing agent of the metal catalyst, and stabilizing ligand (entry 4). The hydroxyl groups of flavonoids and other phenolic compounds of the strawberry fruit extract reduce the Pd(II) to Pd(0) and also stabilize the generated Pd(0) NPs by coordinating to it. The endogenous biomolecule such as DNA was utilized as coordinating ligand for the metal (entry 5). Chelation properties of the phosphate backbone and DNA base pairs cooperate the Pd NPs to be grafted onto it. The triethylene glycol (TEG) functionalized imidazolinium ionic liquid was coated on silica modified magnetite nanoparticles to design and synthesize novel magnetically recoverable nanocomposite for Suzuki cross-coupling reaction (entry 8). The opposite ionic charge interaction between ionic liquid (IL) and anionic metal salt anchors the metal toward the surface of the nanocomposite. Diphenyl phosphine derivatized polyglycerols supported on SiO2 was chosen as the coating material of magnetite template to construct the magnetically recoverable nanocatalyst (entry 10, Table 7.1). Instead of aryl halides as coupling partner of the aryl boronic acid, Hajipour et al. used acyl halides to synthesize the bi-aryl ketones. Modified propylsilane backbone having acetylacetonate as metal stabilizing ligand was supported on magnetite NPs to construct the desired nanocomposite for Suzuki cross-coupling reaction (entry 11). Among all these enlisted reactions, the reaction of entry 10 proceeds relatively faster for all types of substituted substrates. The imidazolium-based ILs while had been used either for coating or as coordinating ligand of the metal-derived nanocatalyst may also render assistance during the course of the reaction as imidazoliumbased ILs have been reported as efficient organo-catalysts in promoting several organic reactions through their organo-catalytic abilities as ‘electrophile-nucleophile dual activation agent’ through a supramolecular assembly formed through a network of ‘hydrogen bond and charge-charge interaction’ involving the imidazolium C2 hydrogen of the IL, the anion of the IL and both the substrates (electrophile and nucleophile) demonstrated through identification of such supramolecular assemblies using various spectrometric techniques such as IR, NMR, MALDI-MS, and ESI-MS [131–136].

FeO

FeO

FeO

FeO

FeO

FeO

FeO

[]

[]

[]

[]

[]

[]

[]

SiO@propylsilane tethered ethylamine-functionalized ionic liquid

Pd () (. mol% Pd)

KCO ()

EtOH:HO (: v/v)

solvent

,,-triazole-based mesoionic carbene

NaPdCl ethylaminefunctionalized ionic liquid

NaPdCl Chitosan

DNA

NaOH ()

KCO ()

KCO (.)

KCO ()

NaCO (.)

EtOH:HO (: v/v)

EtOH:HO (: v/v)

EtOH:HO (: v/v)

EtOH:HO (: v/v)

EtOH:HO (: v/v)

– h

 min −  h

.– h

 min– h

.– h

.– h

– h

time

Reaction conditions

Schiff base of NaPO.HO EtOH:HO ‐Hydroxynapthaldehyde (.) (: v/v)

biguanidine

NaPdCl Strawberry fruit extract

Pd(OAc) Pd ()c (. mol% Pd)

Pd ()b (. mol% Pd)

SiO-NH@ glutaraldehyde cross- Pd (II) linked Chitosan (. wt%)

Aminopropylsilane supported on DNA

Strawberry fruit extract

Pd(OAc)

Pd () (. mol% Pd)

-triazolopropylsilane

PdCl

Pd () Pd(OAc) (. mol% Pd)

Pd (/II) (. mol% Pd)

Schiff base of modified APTES and ‐Hydroxynapthaldehyde

Biguanidine tethered chitosan polymer

Base (eqv.)

Precursor Coordinating ligand metal salts

template Coating

Active Catalyst (amount)

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic Triad for Suzuki cross-coupling reaction

Table 7.1: Reactions performed at room temperaturea (≤ 40 °C).

260 Nirjhar Saha et al.

FeO

FeO

FeO

[]

[]

[]

nSiO@pSiO (modified propylsilane)

SiO@HPG-OPPh

SiO@ (diphenylphosphino) ethyltriethoxysilane (Alkynyl bromide)

SiO@TEG derivatized Imidazolinium ionic liquid

Pd () (. mol% Pd)

Pd () (. mol % Pd)

Pd () (. mol% Pd)

Pd(II)d (.– . mol%)

Pd(OAc)

PdCl

Pd(OAc)

Acetylacetonate

OPPh

PPh

NaPdCl No ligand

KCO ()

KCO ()

NaPO ()

KCO (.)

PhMe

DMF:HO (: v/v)

EtOH

HO

– h

– min

 h

rt-reflux, Ar atmosphere, – h

For all the reactions of this table: the catalysts were recycled through magnetic separation and recycled upto 5–10 times without significant loss in catalytic efficiency, moderate (>50%) to excellent (>90%) yields of the products were obtained. Catalytic efficiency: bTON:400–960; cTON: 16,277–25,821, TOF: 8,000–96,074; dTON:425–4,000, TOF: 7–990.

a

FeO

[]

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7.3.1.2 Reactions at moderate reaction temperature (40–80 °C) Reactions which can’t be performed at room temperature are tried at moderate reaction temperature (40–80 °C) understanding the necessity of more thermal energy of the reaction. Magnetic nanocatalyst-promoted Suzuki cross-coupling reactions carried out at moderate temperature are enlisted in Table 7.2 [137–167]. Most of these reactions were carried out in the presence of environmentally benign reaction media such as H2O (entry 1–3, 7), polyethylene glycol (entries 4–6), EtOH (entries 8–11), and H2O-EtOH (1:1 v/v) (entry 12–20, 23). Although most of the nanocomposites were designed with Fe3O4 as the magnetic template, few templates include Fe2O3 (entries 2, 3, 17, 24, 29), CoFe2O4 (entry 8), NiFe2O4 (entry 12), and MgFe2O4 (entry 15) as the magnetic material. Functionalized alkylsilane derivatives with various stabilizing ligand were supported on coated magnetite nanoparticles (entries 1, 5, 6, 19, 22), ferric oxide nanoparticles (entries 2, 29), and nickel ferrite nanoparticles (entry 12) separately to construct the nanocomposite skeleton. Schiff base containing coating materials (entries 3, 5, 9, 12, 19, 27) are also of interest for various research groups due to the ease of in situ generation of imines at the surface of magnetic template and the capability of the imines to coordinate with the transition metal for stabilization of the NPs derived from these metal salts. Biomolecules such as carbohydrate (entries 4, 16, 18), amino acid (entry 11), and neurotransmitter (entries 30, 31) were used as coating material of magnetic template and also as the immobilizing platform for the metal catalyst. Although coating-free and ligand-free design and synthesis of nanocomposites (entries 8, 28) are preferred over others due to low cost and less time required for preparation, there is a possibility of destabilization of the metal catalyst in the absence of coordinating ligand. In the recent times, the bi-metallic nanocatalytic systems have been introduced for Suzuki reactions. The cooperative catalysis mechanism of the bi-metallic alloy have benefits for the regioselective bi-aryl formation due to facile oxidative addition step of the catalytic cycle as a result of selective coordination of the alloy with definite directing group. The Pd-Ni bi-metallic alloy was immobilized on the surface of magnetite template coated with cyclic acetal of mercaptosuccinic acid (entry 10). Free hydroxyl and carboxylic acid groups containing coating materials (entries 13, 14, 17) act as suitable stabilizing ligand without further requirement of additional ligand derivatization of the coating material. The NHCs serve as potential ligand in the transition metal complex used in various cross-coupling reactions [168, 169]. Keeping these applications in mind, the outer surface of the coating material precursor was functionalized with NHCs to construct the complete coating material (entries 16 and 29). A catalytic magnetic nanocomposite was developed by depositing a thin layer of a cross-linked hyperbranched pyridyl phenylene polymer (PPP) on the surface of mesoporous magnetic silica (Fe3O4−SiO2) followed by complexation with Pd species (entry 20). The interaction of Pd(OAc)2 with pyridine units of the polymer results in the formation of Pd(II) complexes which are evenly distributed through the PPP layer. For entry 22, the XPS analysis study suggests that the

Coordinating ligand

APTES derivative tethered with thiophenol-triethyleneglycolmonomethylether

γ-FeO

FeO

FeO

FeO

FeO

[]

[]

[]

[]

[]

Fuberidazole linked (-chloropropyl)trimethoxysilane (-CPTS)

APTES conjugated with -hydroxynapthaldehyde

Agarose

PEG-APTES-Schiff base (Co-Cu)

Bohemite-propylalkoxysilanetriethylenetetramine

FeO

[]

Pd ()f (. mol%)

Pd nanoparticle

Pd () (. mol% Pd)

Cu (II), Co (II) . mol% Cu, . mol% Co in the total nanocomposite

Pd () (. mol %)

Co(II) (. mol%) -aminothiophenol

triethylenetetramine

Pd(OAc)

KPdCl

Pd(OAc)

NaOH ()

Imine derivative of hydroxy- napthaldehyde

NaCO ()

KCO (.)

Fuberidazole

HO

HO

solvent

PEG-

PEG

PEG 

(continued)

 °C,  min– h

– °C, – min

 °C, .– h

 °C, .– h

 °C,  min– h

 °C,  min– h

Temp (°C), time

Reaction conditions

Base free HO

EtN ()

KOH ()

Agarose

PEG for Co(II), Co (OAc).HO Salicaldimine for Cu(II) Cu(OAc). HO

Pd(OAc)

CoCl.HO

Base (eqv.)

Precursor metal salts

Template Coating

Active catalyst (amount)

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.2: Reactions at moderate reaction temperaturee (40–80 °C). Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

263

Coordinating ligand

FeO

CoFeO

FeO

FeO

FeO

NiFeO

[]

[]

[]

[]

[]

[]

Schiff base of APTES and N- methyl-imidazole-carboxaldehyde

L-methionine

Cyclic acetal derivative of mercaptosuccinic acid

SiO-mSiO (Schiff base)

No coating

PTA coated

Pd (II) (. mol% Pd)

Pd ()i (. mol%)

Pd (), Ni ()h . mol% Pd . mol% Ni

Pd (II) (. mol% Pd)

Pd ()

Pd ()g

Pd(OAc)

Pd(OAc)

PdCl, Ni (NO)

Pd(OAc)

Pd(OAc)

Pd(OAc)

NaCO (.)

Schiff base of N-methylimidazole- carboxaldehyde

L-Methionine

Thiol group of mercaptosucccinic acid

KCO ()

KCO (.)

KCO ()

Imine derivative of KCO ,-diformyl--methylphenol ()

No ligand

N,N,N",N"NaCO tetraethyldiethylenetriamine (.) (TEDETA)- a tridentate ligand

Base (eqv.)

Precursor metal salts

Template Coating

Active catalyst (amount)

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.2 (continued)

reflux,  min min

 °C, – min

 °C, – h

reflux, – h

 °C, – min

Temp (°C), time

EtOH:HO  °C, (: v/v) .– h

EtOH

EtOH

EtOH

EtOH

HO or PEG-

solvent

Reaction conditions

264 Nirjhar Saha et al.

FeO

FeO

FeO

FeO

FeO

[]

[]

[]

[]

[]

MgFeO SiO-linked aminopropylsilane

[] Pd ()

Pd (II)

Pd () (. mg Pd/ mg NPs)

SiO@cross-linked hyperbranched pyridylphenylene polymer (PPP)

SiO--aminopropyl) trimethoxysilane (APTMS)salisaldehyde imine

Methyl salicylate linked chitosan

Humic acid

Pd(OAc)

Pd(OAc)

PdCl

PdCl

KPdCl

PdCl

Pd(OAc)

Pd(OAc) Pd (II)m (. mol% Pd)

Pd ()l (. mol%)

Pd()k (. mol% Pd)

Pd ()

Functionalized multi-walled carbon Pd ()j (.– mol%) nanotubes (MWCNT) tethered cross-linked chitosan (imidazolium NHC cross-linker)

Sepiolite

FeO

[]

PEG

FeO

[]

Pyridine moiety of PPP

Salicaldimine

Methyl salicylate

Humic acid

NaCO (.)

KCO ()

KCO ()

KCO ()

KCO ()

KCO ()

- isopropyl-methylcyclohexanone oxime and amine group of APTES N-heterocyclic carbene

KCO ()

KCO ()

Silanol group of sepiolite

PEG

 °C, – h

(continued)

EtOH:HO  °C, (: v/v)  min-  h

EtOH:HO  °C, (: v/v)  min

HO: EtOH (: v/v)

EtOH:HO  °C,  h, (: v/v) N atmosphere

EtOH:HO – °C, (: v/v) – h

EtOH:HO  °C, (: v/v)  min

EtOH:HO  min, (: v/v)  °C

EtOH:HO  °C,  h (: v/v) Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

265

Coordinating ligand

FeO

FeO

FeO

FeO

FeO

FeO

[]

[]

[]

[]

[]

[]

Pd () (. mol% Pd)

Pd(II) (. mol%)

SiO tethered Cryptand 

KPdCl

PdCl

PdCl

Pd () PdCl (. mol% Pd) Pd ()

Salep grafted polyglycidylmethacrylate-amino pyridine hybrid

NaPdCl

Pd () Pd(OAc) (. mol% Pd)

Pd nanoparticlen (. mol% Pd)

a. Catechol-salicylic acidformaldehyde (CSF, in molar ratio ::) b. thiourea-catechol-formaldehyde (TCF, in molar ratio :.:)

Diaminoglyoxime

SiO @diphenylphosphinoethylsilane

Citrate (as capping agent) @Polyethyleneimine

amino pyridine

Cryptand 

a. Catechol-salicylic acidformaldehyde (CSF, in molar ratio ::) b. thioureacatecholformaldehyde (TCF, in molar ratio :.:)

Amino groups of diaminoglyoxime

PPh

Polyethyleneimine

KCO ()

EtN (.)

KCO ()

KCO ()

KPO ()

KPO ()

Base (eqv.)

Precursor metal salts

Template Coating

Active catalyst (amount)

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.2 (continued)

 °C, . h

– °C, – h

Temp (°C), time

DMF:HO (: v/v)

 °C, .– h

DMF: HO  °C, (: v/v) – h

HO/DMF  °C, (: v/v) – min

EtOH:HO rt, (: v/v) .– h

MeOH

MeOH

solvent

Reaction conditions

266 Nirjhar Saha et al.

FeO

FeO

FeO

FeO

[]

[]

[]

[]

Dopamine

Dopamine-linked bipyridine through amide bond

N-propylsilane derivative of imidazole N-heterocyclic carbene

No coating

SiO-linked Schiff base of APTES and acetylacetonate

Pd (/II) ( mol%)

Pd (II)

Pd (. mol%)

Pd () (. wt% Pd)

PdCl

Pd(OAc)

PdCl

NaPdCl

Pd (II) Pd(OAc) (. mol% Pd)

Amine group of dopamine

bipyridine

NHC

No ligand

Imine derivative of acetylacetonate

KCO ()

KCO ()

NaCO ()

KCO ()

KCO ()

 °C, – h

anisole

dry toluene

DMF

 °C, – h

 °C, – h, Ar atmosphere

 °C,  h

DMA:HO  °C, (: v/v) .– h

DMF:HO (: v/v)

For all the reactions of this table: the catalysts were recycled through magnetic separation and recycled upto 3–10 times without significant loss in catalytic efficiency, moderate (>50%) to excellent (>90%) yields of the products were obtained. Catalytic efficiency: fTON; 31–35; gTON:1,143–1,216, TOF: 1,343–4,105; hTON:31,000–37,000, TOF: 37,000–233,000; i TON:870–990, TOF: 11–66; jTON: 40–970, TOF:5–485; kTON: 250–980; lTON: 282–350, TOF: 107–140; mTOF: 228–23,438; nTON: 485, TOF: 48.5.

e

FeO

[]

Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

267

268

Nirjhar Saha et al.

Pd(II) exists in the freshly prepared magnetic nanocatalyst whereas Pd(0) exists in the recovered catalyst. This change in oxidation state of freshly prepared and recovered nanocatalyst indicates the coordination of four PPh2 ligands of the coating material with the Pd salt during the course of the reaction. Polymeric solution of three different types of small molecules with a definite molar ratio was implicated both as coating material of the magnetic core and as chelating ligand for the transition metal derived NP for its stabilization (entry 24). This dual purpose of the coating material reduces the additional step of coordinating ligand attachment during the magnetic nanocomposite synthesis. Cryptand-22 (C22), an alkali earth metal preferred flexible macrocyclic N-and O-containing chelating ligand, was explored for coordination with the transition metal complex of the magnetic nanocomposites (entry 25). As the alkali metals are susceptible to C22, the base used in the reaction of entry 25 is an organic base. Stabilization of Pd nanoparticles on the surface of cross-linked polymeric amino pyridine modified salep generates a new polymeric coating material-transition metal assembly (entry 26).

7.3.1.3 Reactions at high temperature (> 80 °C) The reactions performed at higher temperature are less sustainable in terms of consumption of energy. Suzuki cross-coupling reactions performed at higher temperature in the presence of magnetic nanocatalyst are enlisted in Table 7.3 [170–185]. Most of the reactions were carried out in green reaction media such as isopropyl alcohol (entry 1), polyethylene glycol (entries 2, 9), water (entry 3–6), ethylene glycol (entries 7, 8), 1:1 (v/v) mixture of EtOH:H2O (entry 10). The SiO2 linked with functionalized alkyl silane derivatives and various stabilizing ligand were used as coating material of the magnetite nanoparticles (entries 1, 7, 8, 12, 13, 16). Carboxylic acid derivatives were covalently supported on the magnetic templates via acetal bond formation to construct the coating layer of the template (entries 2, 9). Oxime (entry 4) and Schiff bases (entry 14) generated from the reaction of aldehyde and amine can coat the magnetic template and the imine moiety of these coating materials can stabilize the metal catalyst through coordination. Amino acid lysine (entry 6) which served both as coating material and stabilizing ligand of the transition metal NPs in the catalyst was supported on the magnetite nanoparticles. Few magnetic nanocomposites without coating material were also reported (entries 11, 15). Due to the lone pair availability of the ring nitrogen of tetrazoles, it was implicated as coordinating ligand of the metal catalyst (entry 5). Natural products containing phenolic or catecholic and/or benzoquinone moiety play dual role of the coating material and coordinating ligand. For example, lignin supported Hibiscus rosasinensis extract acts as the coating material of magnetic template and concomitantly, the chemical constituent of Hibiscus rosasinensis extract (i.e., ortho-benzoquinone) coordinates with the metal NP in the catalyst to stabilize it (entry 10).

FeO

[]

FeO

FeO

FeO

[]

[]

[]

FeO

FeO

[]



FeO

[]

[]

Precursor metal salts

Coordinating ligand

SiO-linked propylsilane @functionalizedtriaminotriazine @EDTA

L-lysine

Lignosulfonate--Amino-Htetrazole

Dioxime derivatives

Nitrocarbon

Acetal of piperidine--carboxylic acid

SiO-linked propylsilane hyperbranced glycerol (HPG) esterified succinic acid

Ni () ( mol%)

Pd ()p (. mol%)

Pd (II)

Pd (II) (. wt% Pd)

Pd ()

Pd () (. mol%)

Pd () (. mol% Pd)

Lysine (Stabilizer)

KOH ()

NaCO (.)

Ethylene glycol

HO

HO

 Amino-H tetrazole KCO ()

HO

PEG

IPA

HO

KOH (.)

NaCO ()

KCO ()

solvent

(continued)

 °C,  h

reflux, – min

reflux, – h

Reflux, – s

 °C, . h

 °C, – min

– °C, – h

Temp (°C), time

Reaction conditions

KCO ()

Dioxime

Nitrocarbon

Piperidine

Carboxylic group of HPG modified succinic acid

EDTA Ni (OAc).HO

Pd(OAc)

PdCl

Pd(OAc)

PdCl

Pd(OAc)

PdCl

base (eqv.)

Active catalyst (amount)

template

Coating

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.3: Reactions at high temperature° (> 80 °C). Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

269

Precursor metal salts

Coordinating ligand

FeO

FeO

FeO

FeO

FeO iron oleate

FeO

[]

[]

[]

[]

[]

[]

Pd () ( mol%)

Pd () ( mol%)

Pd ()

Pd ()

SiO@aminopropylsilane Pd ()q functionalized polymer@imidazole

Silica@APTES (APTES = aminopropyltriethoxysilane)

No coating

Lignin-Hibiscus rosasinensis extract

Acetal of serine

Do

Ni(II) [ mol% Ni(II)]

PdCl

Pd(OAc)

Pd(acac)

PdCl

Pd(OAc)

Ni(OAc) NaCO ()

NaOCHOH ()

NHCs and chloride anion

APTES-silica

KCO ()

KCO ()

No ligand (olylamine NaCO (.) and oleic acid as stabilizer)

Ortho-benzoquinone KCO ()

Hydroxyl and amine group of serine

EDTA

base (eqv.)

Active catalyst (amount)

template

Coating

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.3 (continued)

NMP

DMF:HO (: v/v)

DME:HO (: v/v)

EtOH:HO (: v/v)

PEG

Ethylene glycol

solvent

 °C, .– h

 °C, – h

reflux,  h

 °C, – min

 °C, – min

 °C,  h

Temp (°C), time

Reaction conditions

270 Nirjhar Saha et al.

FeO

FeO

[]

[]

Mesoporous silica @imine functionalized amiopropylsilane (MCM--SB)

No coating

Schiff base of APTES and thiophene--carboxaldehyde

Pd (/II)s (. mol % Pd)

Pd-Au alloy (. mol%)

Ni(II)r

Pd(OAc)

PdCl HAuCl

NiCl.HO

KPO (.)

Schiff base of thiosemicarbazide, Schiff base of salicylaldehyde

KCO (.)

No ligand NaHCO (.) (PVP was used as stabilizer of the NPs)

Schiff base of thiophene-carboxaldehyde and PPh

DMF

DMF

Dioxane

 °C, – min

 °C, . h

 C, – h

°For all the reactions of this table: the catalysts were recycled through magnetic separation and recycled upto 5–10 times without significant loss in catalytic efficiency, moderate (>50%) to excellent (>90%) yields of the products were obtained. Catalytic efficiency: pTON: 1,087–12,375, TOF:3.40–275; q TON:93–116, TOF: 24–155; rTON: 278–314; sTON: 4.72–5.44, TOF: 4.02–68.06.

FeO

[]

Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

271

272

Nirjhar Saha et al.

7.3.1.4 Reactions under the influence of electromagnetic irradiation Considering the conservation of energy reactions that are performed above ambient temperature for longer time it becomes necessary to find alternative source of energy that would permit to reduce the overall time period of the reaction without any detrimental effect of the product yield. Toward this objective electromagnetic irradiation such as microwave and ultrasonic wave have marked their places as viable alternatives to the conventional/thermal energy in organic synthesis. The use of microwave and ultrasonic wave irradiation to accelerate organic reaction contribute to sustainability due to the less energy consumption, reduction of reaction time, less hazardous experimental setup, scope for faster production and supply of essential chemical entities, and less risk of lab accidents. The microwave irradiation in particular has gained popularity due to the above distinct advantages in performing organic reactions for the preparation of diverse organic compounds [186–188]. Therefore microwave and ultrasonic wave have been used to facilitate Suzuki cross-coupling reaction since the implication of microwave- and ultrasonic wave–assisted and magnetically recoverable nanocatalyst-catalyzed Suzuki cross-coupling reaction can be considered as additional advantages to the sustainability of the reaction methodology. Microwave- or ultrasonic wave–assisted, and magnetic nanocatalyst-promoted Suzuki cross-coupling reactions are enlisted in Table 7.4 [189–196]. All of these reactions were completed within 30 min producing high yield of the product and K2CO3 was used as base in these reactions. A few reactions require higher reaction temperature and higher power usage to conduct the reaction. The reactions under the entries 1–3 are considered most sustainable reactions of this table due to the following reasons: solvent-free condition, less reaction time and temperature, and ease of commercial availability of the coating material components. Coating-free and coordinating ligand-free magnetic nanocomposite was developed for the purpose (entry 7). The SiO2-linked functionalized propyl silane derivatives having various stabilizing ligand were supported on either the magnetite nanoparticles (entry 4) or nickel ferrite nanoparticles (entry 6). These two magnetic nanocatalysts were used separately for Pd (0)- and Ni(0)-catalyzed Suzuki cross-coupling reaction under ultrasonic wave irradiation. The glycoluril polymer cucurbit[7]uril was used for as both coating material and stabilizing ligand for the magnetite nanocomposite of entry 5. Apart from the application of the microwave and ultrasonic wave irradiation, a laser beam corresponding to a wavelength of 680 nm and optical power of 500 mW was utilized as energy source for the magnetic nanocomposite Fe3O4/Au/PEG/Pd-catalyzed Suzuki cross-coupling reaction (entry 8).

FeO

FeO

FeO

FeO

NiFeO

[]

[]

[]

[]

[]

Pd ()v (. mol%)

SiO-bis(-malonitrilepropylsilane

Cucurbit[]uril

SiO- propylsilane tethered O-phenylenediamine

Ni () ( mol% Ni)

Pd ()w (. mol% Pd)

Pd ()

Glutaraldehyde cross-linked Pd () chitosan-agar (.% w/w)

Schiff base of modified APTES and Pyrrole-carboxaldehyde

Pd (II)u (. mol%)

NiCl

Pd(OAc)

Pd(OAc)

PdCl

PdCl

Nitrile moiety of malonitrile group

Cucurbit[]uril-

O-phenylene diamine

Not mentioned

Nitrogen of Schiff base and pyrole moiety

KCO (.)

KCO

KCO (.),  mol% NaBH

KCO (.)

KCO (.)

KCO (.)

Sporopollenin- (APTES)-benzoyl pyridine Schiff base

NaPdCl -benzoyl pyridine Schiff base

FeO

[]

base (eqv.)

Precursor Coordinating metal ligand salts

Template Coating

Active catalyst (amount)

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.4: Reactions in presence of electromagnetic wave irradiationt.

rt,– mins under ultrasonication

 min, MW ( W).

 min, MW ( W)

 °C,  min, MW

Temp (°C), time

(continued)

– min, EtOH:  W irradiation HO (: v/v) with ultrasonic sound

 °C,  min, EtOH: MW HO (: v/v)

DCM: EtOH,

Solvent free

Solvent free

Solvent free

Solvent

Reaction conditions

Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

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FeO

[]

Au-PEG

No coating

Pd () (. mg Pd/mg of FeO/Au/PEG/Pd nanocomposites)

Pd () ( wt% Pd)

Pd(OAc)

PEG (Stabilizer)

Pd(acac) No ligand

KCO ()

KCO ()

Temp (°C), time

 h, EtOH: laser beam HO (: v/v)

 °C,  min, EtOH: MW,  W HO (: v/v)

Solvent

Reaction conditions

For all the reactions of this table: the catalysts were recycled through magnetic separation and recycled up to 5–10 times without significant loss in catalytic efficiency, moderate (>50%) to excellent (>90%) yields of the products were obtained. Catalytic efficiency: uTON: 40, TOF: 400; vTON:2,560–3,840, TOF: 25,600–39,200; wTON: 3,680, TOF: 7,360.

t

FeO

[]

base (eqv.)

Precursor Coordinating metal ligand salts

Template Coating

Active catalyst (amount)

Reagents

Magnetically separable nanocatalytic system

Entryref Catalytic triad for Suzuki cross-coupling reaction

Table 7.4 (continued)

274 Nirjhar Saha et al.

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7.3.2 Nanocomposite with non-magnetic metal catalyst and magnetic nanoparticle immobilized on organic or inorganic support Apart from the conventional approach of coating the magnetic component for designing of magnetic nanocomposites, a distinct strategy was adopted for designing of novel catalyst systems, in which there is no covalent or non-covalent interaction between the magnetic component and the active transition metal catalyst. In this novel strategy of designing the magnetic nanocomposites, an organic or inorganic nonmagnetic template, were used for adsorbing the magnetically susceptible superparamagnetic NPs and the transition metal catalyst surface at different position onto its surface. The active metal catalyst can be either coordinated to the stabilizing ligand supported on the organic/inorganic template (Figure 7.24A) or can be present as naked catalyst supported on the non-magnetic template (Figure 7.24B). Application

Figure 7.24: Nanocomposite with non-magnetic metal catalyst and magnetic nanoparticle. (A) Transition metal is linked to the organic or inorganic support through linking group and stabilized by coordinating ligand attached to it, (B) magnetic material and metal catalyst both are supported on organic or inorganic support.

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of the external magnetic field on the magnetic nanocomposite induces magnetic moment into the magnetic component of the nanocatalyst favoring the magnetic recycling step while the transition metal component of the nanocomposite catalyzes the Suzuki cross-coupling reaction. This type of magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions are enlisted in Table 7.5 [197–204]. Graphene-based organic non-magnetic templates were used as support for most of the nanocatalysts (entries 1–3, 6, 8). Inorganic non-magnetic templates are zeolite Y (entry 7) and MgFe calcined hydrotalcites (entry 5). The reactions of Table 7.5 are performed in green solvents at moderate to higher reaction temperature under microwave irradiation (entries 1, 2) or under conventional/thermal heating (entry 3–8). The iron oxide-based materials are chosen as magnetically susceptible component for the nanocomposites (except for entry 5). The coordinating ligand supported on the organic template stabilizes the metallic nanoparticles by preventing its aggregation. The Pd-Co bimetallic nanoparticles were immobilized on the Mg-Fe calcined hydrotalcites to synthesize the bi-metallic magnetic nanocatalyst for Suzuki cross-coupling reaction (entry 5).

7.3.3 Magnetic metal nanoparticle (active catalyst) supported on non-magnetic template It is being observed that bimetallic nanoparticle exhibits a synergistic effect [61, 62, 205, 206] and lead to exceptional catalytic performance by inducing an extra electric field to facilitate electron transport [207–209]. Bimetallic nanoparticles, composed of two different metal components, usually show a combination of the properties associated with monometallic counterparts and a significant enhancement of catalytic potential. Hence, heterobimetallic nanoparticle having a magnetically separable component would have significant advantages [210]. Recent studies have established that a hybrid structure combination of one noble and one non-noble metal like iron, cobalt, and nickel impart a synergistic effect and enhance the properties of individual atoms. One important aspect here is, if magnetic heterometals are alloyed into noble metals, we end up getting a separable and recyclable catalyst [211]. A terminology “semi-heterogeneous catalyst” or “quasi-homogeneous” (soluble heterogeneous) is being coined for these supported colloidal metallic nanoparticle system [95,212, 213]. These types of hybrid catalyst system having a magnetically separable nanocatalyst with unique physicochemical property have shown lots of promise in catalysis. The most important aspect related with MNP is that its isolation from the reaction mixture can be performed easily by simple magnetic separation, once the reaction is complete, thereby eliminating the catalyst filtration and centrifugation steps [214, 215]. A magnetically susceptible heterogeneous catalyst system such as metal nanoparticle (mono/bimetallic) supported on non-magnetic template has been explored and studied vigorously to catalyze Suzuki−Miyaura coupling reaction [216]. Table 7.6

FeO

FeO

FeO

FeO

Mg–Fe-CHT

FeO

NiFeO

FeO

[]

[]

[]

[]

[]

[]

[]

Coordinating Ligand

graphene

zeolite Y

GO

Mg-Fe Calcined Hydrotalcites

Irish moss

Graphene oxide

Graphene

Graphene oxide

Pd () (. wt % Pd)

Pd () (. mol% Pd)

Pd () (.– mol%)

Pd-Co bimetallic nanoparticle (. mol% Pd)

Pd (II) ( mol% Pd)

Pd ()y (.–. mol%)

Pd () ( mol%)

Pd () (. mol%)

PdCl

PdCl

PdCl

PdCl

Pd(OAc)

Pd(acac)

Pd (NO)

No ligand

No ligand

PAMPS (poly -acrylamido-methyl--propansulfonic acid)

No ligand

No ligand

acrylamide polymer

No ligand

No ligand

NaCO ()

KCO ()

KCO ()

KCO ()

KCO (.)

KCO(–)

KCO ()

KCO ()

HO: EtOH (: v/v)

EtOH:HO (: v/v)

EtOH:HO (: v/v)

HO

HO

HO

EtOH:HO (: v/v)

EtOH:HO (: v/v)

solvent

 °C, – min

 °C, .– h

 °C – h

 °C,  h

 °C,  h

– °C, – h

 °C,  min, MW

 °C,  min, MW,  W

Temp (°C), time

Reaction conditions

For all the reactions of this table: the catalysts were recycled through magnetic separation and recycled upto 5–10 times without significant loss in catalytic efficiency, moderate (>50%) to excellent (>90%) yields of the products were obtained. Catalytic efficiency: yTOF: 80–6,125, TON: 400–12,250.

x

Precursor metal salts

Base (eqv.)

Active Catalyst (amount)

Magnetic component

Support

Reagents

Magnetically separable nanocatalytic system

Catalytic triad for Suzuki cross-coupling reaction

[]

Entryref

Table 7.5: Reactions catalyzed by magnetic nanocatalyst having magnetic nanoparticle immobilized on organic or inorganic support.x Chapter 7 Magnetic nanocomposite-catalyzed Suzuki cross-coupling reactions

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Table 7.6: Suzuki−Miyaura reaction performed by magnetically recyclable hetero-bimetallic nanoparticle. Entry

Catalyst

Condition

Substrate scope

Reference



Pd-Ni alloy

one-pot wet chemical route

Ar-Br

[, ]



Pd/NiFeO

Wet impregnation of Pd on NiFeO and subsequent Reduction by H gas

ArBr, ArI

[]



Pd()–FeO

Solvothermal reaction of PdCl with FeCl.HO in the presence of PVP

ArBr, ArI

[]



Ni-Pd/G

sequential reduction of nickel (II)acetate and palladium(II) bromide in oleylamine (OAm) and trioctylphosphine (TOP)

ArX, HetArX X = Cl, Br, I



Pd–ZnFeO

surface adsorption of Pd(OAc) on ZnFeO magnetic nanoparticles

ArI, ArBr,Ar-Ar

[]



Pd-Co/G

Chemical reduction of Co (OAc).HO and PdCl(II)

ArI, HetArI, ArBr

[]



Fe@Pd/C

Sequential reduction of FeCl and PdCl with NaBH

ArX (X = Cl, Br, I)

[, ]



Ni-Pd/CB

Immobilization of NaPdCl on Ni/CB

ArX(X = Cl, Br, I)

[, ]



Pd/FeO/G

Microwave (MW)-assisted reduction of palladium and ferric nitrates in the presence of graphene oxide (GO) Nanosheets

ArBr, ArCl

[]



FeO@SiO-Pd

Chemical coprecipitation of FeCl and FeCl with functionalized SiO

ArI, ArBr

[]



Pd NPs@FeO/ CS-AG Microcapsules

Immobilization of PdCl on to homogenous solution of chitosan/agar/FeO

ArI, ArBr

[]



Pd–Co@Mg–Fe–CHT

Co-precipitation of Pd-Co on calcined Hydrotalcites (CHT)

ArX (X = Cl, Br, I)

[]



FeO-lignosulfonate @-amino-H-tetrazole -Pd(II)

Conjugating lignin with FeO nanoparticles followed by tethering with -amino-Htetrazole

ArI

[]

[, ]

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Table 7.6 (continued) Entry

Catalyst

Condition

Substrate scope

Reference



Pd NPs@Kao/FeO/Pyr

Adsorption of Pd on FeO loaded Schiff base modified kaolin

ArX (X = Cl, Br, I)

[]



MNPs@ SiONH@Pd(dpa)Cl

Anchoring [Pd(dpa)Cl] with mPEG

ArX

[]

encompasses some of the notable works carried out by magnetically separable heterobimetallic NPs catalyzed Suzuki−Miyaura reaction. A Pd-Ni-based “magnetically separable quasi-homogeneous” nanoalloy was developed as catalyst which showed flexible composition when synthesized via a onepot chemical route (Figure 7.25).

Figure 7.25: Pd-Ni nanoalloy-catalyzed Suzuki−Miyaura reaction in a quasi-homogeneous reaction.

This being an example of palladium/non-noble metal alloy catalyst shows more efficient catalytic activity compared to monometallic palladium nanoparticles. By virtue of their excellent superparamagnetic nature these heterobimetallic nanoalloy catalysts reduce the catalyst loading of noble metals. It was observed that the Pd-Ni alloys exhibits better catalytic activity than individual palladium nanoparticles. The heterobimetallic alloy catalyst system was recycled for five consecutive cycles without loss of activity. Figure 7.26 entails the plausible mechanism proposed for this conversion. The proposed mechanism as shown above outlines the role of leached nanoparticle from the bulk [217]. It was proposed that after the oxidative addition of aryl halide on to the Pd-Ni nanoparticle surface, the palladium (II) and nickel (II) molecular species leached into the solution and enter the catalytic cycle, generating the biaryl product and at the end re-formation of the nanoparticles takes place. It has been proposed that leaching took place most likely from the surface of nanoparticle which takes part in the catalytic cycle [218]. The Pd supported on surface-modified nano NiFe2O4-catalyzed Suzuki−Miyaurareaction has been explored extensively during the last decade. The Pd-Ni bimetallic combination exhibits a synergistic interaction. It has been reported in various studies that the electron get transferred to

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Figure 7.26: Schematic representation of the mechanism for the Miyaura−Suzuki coupling reaction catalyzed by Pd50Ni50 NPs [218].

nickel ion from palladium, so the nickel ion (+2 O.S.) easily gets reduced to nickel (0) in such conditions [221]. A magnetically recoverable palladium supported on nickel ferrite (Pd/NiF2O4) catalyst system was developed. The catalyst was exhibiting magnetic property and was easily removed from the reaction mixture by applying an external magnetic field after completion of the reaction. The synthesized catalyst was used for Suzuki−Miyaura coupling reaction and the biaryl products were obtained in good to excellent yields. The catalyst system was recycled for several times without significant loss of catalytic activity. Figure 7.27 denotes the general reaction conditions. It has been proposed that the colloidal Pd suspension is the active catalyst species in this heterogeneous reaction.

Figure 7.27: Pd/NiFe2O4-catalyzed Suzuki−Miyaura reaction.

The nickel ferrite (NiFe3O4) activates Pd surface and helps in the formation of Pd colloids in solution as well as leads to redeposition of palladium on the support after the

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completion of reaction. So, the colloidal palladium that is getting leached from the Pd/NiFe3O4 system into the solution is providing the catalytic site for the reaction. All the analytical methods adopted for the purpose validate the formation of the claimed catalyst. Some salient features associated with the reactions are the use of water as a co-solvent, no requirement of additives and ligand [219]. Li et al. has synthesized a magnetic Pd(0)-Fe3O4 nanocomposite via a solvothermal reaction of PdCl2 with FeCl2 · 4H2O in the presence of PVP. The Fe3O4 nanocrystal acts as the support for the Pd NPs and facilitates the uniform dispersion of Pd from its surface. This helps in the enhancement of catalytic activity. The synthesized nanocatalyst showed very good catalytic activity for the coupling reaction and different of biaryl products were obtained (Figure 7.28).

Figure 7.28: Pd/Fe3O4 nanocomposite catalyzed Suzuki−Miyaura reaction.

The catalyst exhibits good magnetic property and was recovered and recycled for 10 consecutive cycles without significant loss of catalytic activity. Presumably the DMF in the presence of polyvinyl pyrrolidine (PVP), used for the synthesis of Pd-Fe3O4, was also acting as reducing agent [236]. The capping agent PVP coats the Fe3O4 nanocrystal surface via hydrophobic interaction thereby allowing the Pd to disperse on to the surface. The FeCl2 and PdCl2 were used as precursor of the active catalyst system. The Fe2+ was reducing the Pd2+ and was getting oxidized in turn. The Fe3+ generates Fe3O4 which acts as a support for Pd. A red shift was observed in the Raman spectra of the catalyst system which indicates that Pd NPs to be attached and dispersed on the ferrite surface. However, during the reaction TBAB was used as stabilizer. Figure 7.28 entails the general reaction condition [220]. The graphene supported hetero-bimetallic nanoparticles are a subject of intense study now a days. The G–Ni/Pd NPs has emerged as a novel class of catalyst for efficiently catalyzing the Suzuki–Miyaura coupling reactions. Metin et al. [221] synthesized a monodisperse Ni/Pd core/shell nanoparticles (NPs) supported on Graphene. Synthesis of NPs was carried out by a tandem reduction of nickel (II) acetate and palladium(II) bromide in oleylamine (OAm) and trioctylphosphine (TOP). Various analytical and mechanistic studies established the core/shell structure of the NPs. It was observed that TOP is essential for the formation of core shell Ni/Pd NPs as it helps in the reductive decomposition of Ni-TOP and Pd-TOP. The graphene monolayer was prepared by ultra-sonification and was further utilized as support for Ni/Pd NPs. Figure 7.29 is the generalized representation of this reaction.

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Figure 7.29: G–Ni/Pd (Ni/Pd = 3/2) catalyzed Suzuki–Miyaura cross-coupling reaction of various aryl halides with phenyl boronic acid.

The photoelectron spectroscopy revealed that it is the Ni/Pd = 3/2 which is the active species in the catalytic cycle. The DMF/H2O mixture is advantageous as it causes good dispersion of G in DMF leading to monodispersed NP formation and water provides high solubility to the arylboronic acids. After the completion of reaction, the NPs were separated by use of a simple magnet [222]. Singh and co-workers developed a Pd incorporated ZnFe2O4 super paramagnetic catalyst by ultrasound-assisted co-precipitation without using additional stabilizing surfactant and capping agents (Figure 7.30). The EDAX analysis of the catalyst system revealed that the ratio of iron/zinc in ZnFe2O4 is 2.05 which is in well concurrence with the earlier literature reports. The prepared catalyst was then evaluated for its catalytic activity for Suzuki−Miyaura reactions and excellent yields were obtained with various substituted phenyl boronic acid and aryl halides. This establishes the versatility and applicability of the catalyst system. The magnetic properties of the synthesized catalyst Pd–ZnFe2O4 as investigated by PPMS indicate it to be a superparamagnetic in nature. Owing to its magnetic property the catalyst system was easily separated and recycled for five consecutive cycles without significant loss of catalytic activity. The ICP-MS elemental analysis of the reaction mixture aliquot after each cycle indicates leaching of the palladium from the active site i.e. surface of Pd–ZnFe2O4. It was found that after the fifth cycle the concentration of Pd from the ZnFe2O4 surface is reduced gradually and at the same time the turnover number is also getting decreased for the Suzuki−Miyaura reaction. Figure 7.30 denotes the general reaction for Pd–ZnFe2O4catalyzed Suzuki−Miyaura reaction. The reactions were carried out under ligand-free and aerobic conditions [223].

Figure 7.30: Suzuki−Miyaura reaction of various aryl halides with phenylboronic acid–catalyzed by Pd–ZnFe2O4.

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In 2014, Feng et al. [224] developed Pd-Co heterobimetallic nanoparticles (NPs) supported on G using chemical reduction methodology. Various analytical study revealed the Pd-Co NPs are having alloy structure and the XPS study indicates the presence of both the Pd(+2) and Pd(0) states. Further, the peaks corresponding to Pd 3d5/2 and Pd 3d3/2 were observed along with those corresponding to the Co 2p3/2 and Co 2p1/2. This clearly indicated a synergistic interaction between Pd and Co. The catalyst was found to catalyze the Suzuki−Miyaura coupling reaction efficiently and the diversely substituted products were obtained with excellent yields (Figure 7.31).

Figure 7.31: Pd-Co/G-catalyzed Suzuki coupling reactions.

It has been proposed that the cobalt acts as dopant here and the G provides an enhanced surface area leading to a more active sites of Pd available for the catalysis to take place. The catalyst system was recycled for five consecutive cycles and due to its magnetic nature the separation after each cycle of recyclability was easily performed (Figure 7.31). Tang et al. have developed a novel core-shell Fe-Pd heterobimetallic NPs supported on carbon. The NPs were prepared by sequential reduction method. The synthesized catalyst was used for Suzuki−Miyaura coupling reaction and excellent catalytic activity was observed (Figure 7.32) [225]. Various substituted biaryl derivatives were synthesized with excellent yield using the catalyst system. It was observed that in presence of external magnetic field the catalyst system was exhibiting excellent magnetism. This helps in the recovery/separation of the catalyst with the help of a magnet after completion of the reaction. The Fe-Pd/C heterobimetallic NPs were recycled for five consecutive cycles with minor variation of the catalytic efficiency.

Figure 7.32: The Suzuki reaction of aryl halides with phenylboronic acid using Fe@Pd/C catalyst.

The XPS spectra demonstrates presence of two types of Fe species Fe(0), Fe 2p3/2 and oxidized Fe(+2), Fe 2p3/2. In addition, two peaks of Pd(0) Pd 3d5/2 and Pd 3d3/2 were observed. The annular dark-field scanning transmission electron microscopy (STEM) image of the catalyst system reveals that Pd is dispersed on to the surface of carbon

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and being partially covered on the outer layer of Fe-core. The core shell structure of the NPs was established by Argon sputtering experiments. This indicates that the catalyst system has a core of made of Fe and a shell of Pd surrounding it. The catalyst system offers several advantages such as low cost, long-term stability, high catalytic activity and easy recovery, and use of water as eco-friendly reaction medium. Vulcan XC-72R is a commercial variety of carbon black (CB) and it has significant advantages such as large surface area, high electrical conductivity, suitable pore structure, and relatively low cost. Xia et al. has utilized this Vulcan XC-72R carbon black as catalyst support for developing a magnetically separable core-shelllike Ni-Pd catalyst [226]. Suzuki−Miyaura coupling reaction was performed using this catalyst and desired coupling product was obtained with good to moderate yield (Figure 7.33). The NPs were synthesized by a sequential reduction strategy. It was proposed that the enhanced catalytic efficiency of the catalyst system for the Suzuki−Miyaura reaction is due to the synergistic effect between the NPs and their unique core shell like structure. The addition of Ni metal leads to magnetic separation of the catalyst after the completion of reaction. The HAADF-STEM (High-angle annular dark-field-scanning transmission electron microscope) image of the NPs indicates that the Ni core is partially surrounded by Pd nanoparticles.

Figure 7.33: Ni0.02Pd0.05/CB catalyst in Suzuki−Miyaura coupling reaction.

Interestingly it was observed that the carbon black that was used is exhibiting a π-π stacking interaction with the aromatic alkyl halide used as the coupling partner. This helps the reactants in accessing the catalyst surface efficiently. Some notable advantages of the protocol are that the coupling reaction was performed in air under mild conditions using a relatively environmentally benign solvent system and does not require ligands or protective atmosphere [227]. Hassan et al. [228] prepared a Pd/Fe3O4 nanocatalyst system which is supported on a graphene nanosheet (Pd/Fe3O4/G) via microwave (MW)-assisted reduction of palladium and ferric nitrates by hydrazine hydrate in the presence of graphene oxide (GO) nanosheets. Elazab et al. demonstrated that the synthesized catalyst works very well for both Suzuki and Heck coupling reactions and exhibits excellent catalytic activity (Figure 7.34) [229]. The reduced graphene oxide under microwave irradiation in presence of a reducing agent has a significant role to play in the reaction. The MW causes a non-equilibrium dielectric heating which creates defects sites on GO. The Pd-Fe3O4 nanoparticles anchor into these defect sites which acts as

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centers of nucleation. The presence of Fe3O4 in the catalyst system imparts magnetic properties which lead to smooth separation and easy recycling of the catalyst. The advantages of this protocol include the magnetic property associated with the catalyst system that leads to its easy recovery, recyclability of the recovered/separated catalyst up to ten times, mild reaction conditions, and use of a relatively greener solvent system.

Figure 7.34: Pd/Fe3O4/G-catalyzed Suzuki−Miyaura coupling reaction.

Khazaei et al. [230] has developed a magnetically recoverable nano-Fe3O4@SiO2 supported Pd(0) catalyst system. Interestingly an environmentally benign and natural source derived rice-husk was used as a support for stabilization of palladium nano-particles in this study. The other most notable development in this work was the use of CaO derived from the waste egg cell as natural solid base for the Suzuki– Miyaura coupling reaction. The catalyst was found to be effectively catalyzing Suzuki– Miyaura reaction (Figure 7.35). The Transmission Electron Microscopy (TEM) of the Fe3O4@SiO2-Pd catalyst reveals that the Pd NPs are well dispersed throughout the surface of the Fe3O4@SiO2 having average diameter of 10–30 nm. The nanocatalyst system owing to the presence of Fe3O4 possessed magnetic property and was readily recovered by using an external magnet and recycled several times without significant loss of catalytic activity.

Figure 7.35: Pd/Fe3O4/G-catalyzed Suzuki−Miyaura coupling reaction.

Baran et al. [231] have prepared a palladium nanoparticle (Pd NPs) stabilized on magnetically separable chitosan/agar microcapsules (Pd-NPs@Fe3O4/CS-AG microcapsules).

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Various analytical techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry, and inductively coupled plasma-optical emission spectrometry (ICP-OES) validated the formation of the Pd NPs stabilized on chitosan/ agar microcapsules. The particle size was found to be in the range of 28–39 nm. The catalytic activity of the prepared nanocatalyst was further evaluated for Suzuki−Miyaura reactions and excellent yields were obtained with various substituted phenyl boronic acid and aryl iodide (Figure 7.36). The catalyst system was recycled for 10 consecutive cycles without any significant loss of activity. The ICP-OES analysis after each cycle of reusability showed that a very less amount (≤ 1%) of palladium was leached out from the Pd NPs@Fe3O4/CS-AG microcapsules. This methodology offers some notable advantages such as magnetic recoverability and recyclability, solvent-free, relatively less expensive catalyst system, greener support system for Pd NPs, and use of microwave irradiation as non-conventional energy source.

Figure 7.36: Pd NPs@Fe3O4/CS-AG microcapsules-catalyzed Suzuki−Miyaura reaction.

Dong et al. [232] developed a novel Pd-Co nanocatalyst supported on Mg–Fe–CHT (calcined hydrotalcites). The catalyst was prepared by first co-precipitating palladium and cobalt with MgCl2. 6H2O to generate Pd-Co-HT and subsequently calcination was done in muffle furnace at 500 °C to get Pd–Co–CHT. The synthesized catalyst exhibits very good catalytic reactivity for Suzuki−Miyaura reactions, and products with excellent yields were obtained with various substituted phenyl boronic acid and substituted aryl halides (Figure 7.37). After the completion of reaction, the catalyst was recovered by either simple filtration or magnetic separation. The catalyst was subsequently recycled for three consecutive cycles without significant loss of activity. The inductively coupled plasma (ICP) analysis revealed that the concentration of Pd and Co in Pd–Co–CHT catalyst is 0.49 and 0.67 wt %, respectively. Some notable advantage of this protocol is use of water as greener solvent, less catalyst loading (0.009 mol %), open air reaction, and high Turn over Number (TON 10,761). Nasrollahzadeh et al. [233] developed a novel strategy to prepare magnetic lignosulfonate-supported Pd-complex Fe3O4-lignosulfonate@5-amino-1H-tetrazolePd(II) (FLA-Pd). 5-Amino-1H-tetrazole was tethered with the calcium lignosulfonatemagnetite. The catalyst FLA-Pd showed high activity for phosphine ligand-free Suzuki−Miyaura coupling reaction in aqueous media. The reaction methodology offers several advantages such as use of biopolymer calcium lignosulfonate as support for the immobilization of the 5-amino-1H-tetrazole-Pd(II) complex, water as solvent, phosphine-/ligand-free, and reusability of the catalyst. Figure 7.38 denotes the general

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Figure 7.37: Pd/Co–Mg–Fe–CHT (Hydrotalcites)-catalyzed Suzuki−Miyaura reaction.

reaction conditions of this protocol. The magnetic hysteresis loop of the prepared catalyst establishes its magnetic behavior. Therefore, the catalyst was easily recovered by using an external magnet after each cycle. The catalyst was recycled up to seven cycles with slight loss of activity as reflected in the product yields. The TEM and FESEM images of the recycled and recovered FLA-Pd catalyst showed no morphological change.

Figure 7.38: FLA-Pd-catalyzed Suzuki−Miyaura coupling reaction.

Baran et al. [234] developed a support of Fe3O4 embedded Schiff base modified kaolin and palladium NPs were anchored on it. The Schiff base modification of the kaolin was carried out by mixing pyrrole-2-carboxyldehyde in a solution of 3-aminopropyltriethoxysilane (APTES) modified Kaolin/Fe3O4. Subsequently, the PdCl2 was stirred in this resulting reaction mixture and the Pd NPs@Kao/Fe3O4/Pyr catalyst was obtained. The catalyst was used in Suzuki−Miyaura reaction and excellent catalytic activity was observed (Figure 7.39). After the completion of reaction, the catalyst was easily separated by an external magnet. The catalyst was recycled for 10 consecutive runs and at the 10th run the yield obtained was 89% as compared to 99% at the first run. The ICP analysis was carried out after the recycling test which recorded very less leaching (1.1 wt%) of Pd NPs from the Kao/Fe3O4/Pyr support. This methodology has advantages like less catalyst loading, high catalytic activity, no use of chemical agent for the synthesis of the catalyst, and magnetically recyclable catalyst. An invigorating Pd(II)-based polyfunctional magnetic nanocatalyst was developed by tethering a hydrophilic monomethyl ether poly(ethylene glycol) (mPEG) functional group containing Pd(2,2′-dipyridylamine)Cl2 on to the surface of silica-coated magnetic nanoparticles having amino-functionality [235]. The most indispensable aspect

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Figure 7.39: Pd NPs@Kao/Fe3O4/Pyr-catalyzed Suzuki−Miyaura reaction.

of this work is that the prepared nanocatalyst mimics the acts of amphiphilic artificial metalloenzyme. The mPEG chains present on [Pd(dpa)Cl2] and the nanomagnetic core, mimicking as protein renders it as artificial enzymes acting as biomimetic artificial enzyme. This palladium catalyst was easily separated from the reaction mixture using an external magnetic field and was subsequently recycled for 15 consecutive cycles without significant loss of activity. It was proposed that due to the amphiphilic nature, the catalyst system provides a favorable interaction between the hydrophilic boronic acids and hydrophobic aryl halides which improves the catalytic efficiency and enhances the rate of reaction in aqueous media. The photoelectron spectroscopy (XPS) reveals a Pd(II)/Pd(IV) pathway for the catalyst cycle. Figure 7.40 represents the general reaction conditions of this catalyst system.

Figure 7.40: Suzuki–Miyaura coupling of aryl halides with aryl boronic acids employing MNPs@SiO2-NH2@Pd(dpa)Cl2.

7.4 Metal or template magnetic but no magnetic separation Some notable works regarding magnetically active metal nanoparticle-catalyzed Suzuki−Miyaura reaction are outlined in the Table 7.7. However, the metal nanoparticles are not magnetically separated/recycled after the completion of the reaction. A novel and highly efficient palladium-based catalyst Pd–DKTS–APSG was developed by grafting diphenyl diketone–monothiosemicarbazone (DKTS) on silica gel covalently followed by metalation with palladium chloride. This represents organic–inorganic hybrid material. The synthesized catalyst was used for Suzuki– Miyaura cross-coupling reaction for evaluation of its catalytic activity and afforded

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Table 7.7: Magnetic nanocatalysts having magnetic metal or template but no magnetic separation was reported. Entry Catalyst

Preparation

Precursor metal salt

Condition

Substrate Reference scope



Pd–DKTS– APSG

Grafting of diphenyldiketone– Monothiosemicarbazone on silica gel followed by metallation with palladium chloride

PdCl

KCO, DMFHO

Ar-X (X = Cl, Br, I)

[]



Ni-MWCNTs “Click” reaction of azide- NiCl.HO KPO, Dioxane, functionalized  °C,  h Nanotube with propargyl alcohol followed by immobilization of nickel nanoparticles

Ar-X (X = Cl, Br, I)

[]



Pd–Ni/ZrO One-step impregnationreduction Using NaBH as reducing agent

Ar-X (X = Cl, Br, I)

[]



Pd-Ni/TiO

One-step impregnationreduction Method

NiCl.HO KCO, EtOH: HO,  °C, h

Ar-X (X = Cl, Br, I)

[]



Ni-NPs

Mechanochemical reduction Ni(OH) using hydrazine

Ni(OH)

KCO DMFHO, reflux

Ar-I

[]



Ni-NPs

Ultrasound-Assisted Hydrazine Reduction Method

NiI

KCO, DMF, Ar-I  W ultrasonication

[]

PdCl

NaCO, EtOH: HO = :, N atmosphere,  °C,  h

diverse set of various substituted E-stilbenes with excellent yields. Various substituted phenyl vinyl boronic acid and substituted aryl halide were taken for the cross-coupling reaction. The synthesized catalyst was well characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), energy dispersive X-ray fluorescence (ED-XRF), BET surface area analysis, solid-state 13C CP-MAS (Cross Polarization-Magic angel Spinning) NMR spectroscopy, atomic absorption spectroscopy (AAS), and elemental analysis. The notable features of the reaction include high turnover frequency, use of microwave irradiation as non-conventional energy source, low catalyst loading, mild reaction conditions, high selectivity for Estilbenes, easy of recovery and reusability of the catalyst and devoid of Pd-leaching from the heterogeneous catalyst surface. All these render the present protocol environmentally benign. It has been proposed that the DKTS acts as a ligand in the

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cross-coupling reaction and the S and N-atom acts as ϭ-donors and stabilize the Pd in +2 oxidation state in the catalytic cycle as well as the steric bulk of the two aryl groups present in DKTS facilitates the reductive elimination step. The catalyst was recovered through filtration after first/each run, dried, and recycled/reused for six consecutive cycles without any appreciable loss of catalytic activity. Figure 7.41 depicts the general reaction conditions of this catalyst system [237].

Figure 7.41: Pd–DKTS–APSG-catalyzed Suzuki cross-coupling reaction of various aryl halides with phenyl vinyl boronic acid.

In the past few years, the click reaction–based covalent modification of the surface of carbon nanotube (CNT) has become a popular approach [238]. A novel reusable and efficient heterogeneous nickel nanocatalyst stabilized by triazole-functionalized carbon nanotube was developed using “click reaction.” Initially, the azidefunctionalized nanotube reacts with propargyl alcohol followed by immobilization of the nickel nanoparticles. The such-prepared heterogeneous Ni nanocatalyst exhibits enhanced and more effective catalytic activity for Suzuki−Miyaura cross-coupling reaction (Figure 7.42) [239]. The catalyst was found to be very efficient and versatile as various substituted haloarenes reacted with phenylboronic acid giving rise to biphenyl products with good to excellent yield. The catalyst was well characterized by various analytical techniques such as SEM-EDX, TEM, XRD, and IR. The TEM image of the nanocatalyst indicates the Ni-NPs are spherical in shape and are monodispersed having particle size in the range of 8 nm. The catalyst was recovered and recycled for 7 consecutive cycles, and it was observed that up to fifth cycle there was no noticeable loss of activity. The SEM image of the catalyst recovered after seventh cycle, however, reveals that the morphology of the catalyst remains unchanged as compared to the freshly prepared catalyst.

Figure 7.42: Ni/MWCNT-catalyzed Suzuki cross-coupling reaction of various aryl halides with phenylboronic acid.

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An efficient ZrO2-supported bimetallic Pd-Ni nanoparticle was synthesized via impregnation–reduction method. The prepared bimetallic catalyst was found to be efficiently catalyzing the Suzuki−Miyaura cross-coupling reaction and showed excellent functional groups tolerance (Figure 7.43) [240]. Almost quantitative conversion of a series of arylboronic acids and bromobenzene derivatives into final cross-coupled product was observed under mild conditions and in the absence of ligand. Area-selected scanning electronic microscope (SEM) and energy dispersive spectroscopy (EDS) mappings of the catalyst established the formation of a homogeneous Pd-Ni NPs. The prepared bimetallic nanocatalyst was having a homogeneous and well-dispersed matrix which favors a smooth flow of electron from the surface. The catalyst optimization revealed that 1:4 ratio of Pd/Ni is optimum for the reaction. The catalyst was recycled for four consecutive cycles without noticeable loss of catalytic activity.

Figure 7.43: Pd-Ni NP-catalyzed Suzuki cross-coupling reaction of various aryl halides with phenylboronic acid.

A catalyst system comprising monodispersed Pd-Ni bimetallic nanoparticles supported on titanium oxide nano powder surface was prepared by impregnation-reduction method. The nanocatalyst exhibited enhanced catalytic activity as compared to TiO2 supported monometallic nanocatalyst in Suzuki−Miyaura cross-coupling reaction (Figure 7.44) [241]. A catalyst ratio of 2.95 for Pd- Ni was found to be perfect for exhibiting optimum synergistic interaction. The bimetallic nanocatalyst system was having excellent functional group tolerance and various substituted products with excellent yield were obtained. The TEM analysis of the prepared Pd-Ni/TiO2 catalyst revealed that the bimetallic nanoalloy of Pd-Ni was dispersed uniformly on the surface of TiO2 and is spherical in shape with mean diameter of 2 nm. The XPS spectra reveals Ni to be in +2 oxidation state as peaks corresponding to Ni 2p3/2 and 2p1/2 was observed at 856.2 and 874.0 eV, whereas the palladium was found in both Pd(+2) and Pd(0) oxidation states. Subsequently, it was observed that even after the four consecutive recyclability study the catalytic activity was retained. The HRTEM image of the recovered catalyst after the fourth cycle indicates to have identical FCC (face-centered cubic) Pd-Ni alloy lattice structure as compared to fresh sample and having 220 lattice structures with interlayer spacing of 0.127 nm.

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Figure 7.44: Pd-Ni/TiO2-catalyzed Suzuki cross-coupling reaction of various aryl halides with phenylboronic acid.

A mechanochemical pre-treatment reduction using hydrazine was explored for the synthesis of Ni NPs [242]. It was proposed that mechanochemical treatment led to a phenomenon ‘cold welding’ of the solid particle leading to agglomeration. The mechanochemical treatment increases the specific surface area of the solid particle, thereby providing a relatively higher density of active sites. Preparation of the nanocatalyst was carried out by reaction of pre milled Ni(OH)2 with hydrazine dissolved in ethanol. It was observed that the non-milled nickel hydroxide particles were not forming a uniform dispersion. Use of 10 min of milling, however, significantly enhanced the yield of the cross-coupling reaction. The SEM images of the milled particle were found to have relatively rounded contours with varying range. The synthesized nanocatalyst was tested for its activity for Suzuki−Miyaura crosscoupling reaction of iodobenzene and phenylboronic acid and the product obtained with excellent yield (Figure 7.45) [243].

Figure 7.45: Suzuki−Miyaura reaction of phenylboronic acid and iodobenzene catalyzed by pretreated nickel NPs.

An ultrasonication-mediated synthesis of Ni NPs using hydrazine as the reducing agent was carried out. Sonication was able to maintain an average primary crystallite size of 7–8 nm. The prepared nanocatalyst was used for Suzuki−Miyaura cross-coupling reaction, and its catalytic property was evaluated under both the conventional as well as the ultrasonic methods. The catalyst prepared under ultrasonication method exhibited better catalytic activity. The highest catalytic activity was observed for nanoparticles prepared under low-power (30 W) continuous sonication. Temperate Programmed Desorption (TPD) studies of the prepared catalyst were carried out to ascertain the acid-base properties of the prepared nanoparticles. This study revealed that the sample prepared by 30 W–100% ultrasonication method is having minimum

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basicity 0.032 mmol CO2/g and total acidity of 0.257 mmol NH3/g. The acidity and basicity of the prepared catalyst samples were determined by mild (30 W output power) and continuous ultrasound treatment. Total acidity of the sonically prepared nanoparticles was found to be ten times higher. It is well established fact that the Lewis acid additives promote oxidative addition steps in cross-coupling reactions. So, it was proposed that the surface Lewis centers available at the close vicinity of the nickel atoms led to the formation of the organonickel species by weakening the bond between aryl carbon and iodine. The synthesized nanocatalyst was subjected for evaluation of their catalytic activity in Suzuki−Miyaura reaction (Figure 7.46) [244]. The reaction was carried out in three different solvents such as DMF, DMSO, and toluene with water content of 20% v/v and the yield of the final products were compared. It was observed that toluene was the best solvent as it gave the product with 95% yield in 24 h as compared to DMSO (81%, 24 h) and DMF (76%, 24 h).

Figure 7.46: Ni-NP-catalyzed Suzuki−Miyaura reaction of phenylboronic acid and iodobenzene.

7.5 Conclusions Owing to the compelling need of developing relatively greener and environmentally benign methodologies to access bioactive heterocycles and pharmaceutical intermediates the magnetically active nanocatalyst-catalyzed synthetic transformations are very much of use now a days. The last decade has witnessed a paradigm shift in the heterogeneous catalyst-mediated organic transformations. The magnetic nanocomposite has emerged as dawn of a new era of catalysis and has added a new perspective in heterogeneous catalysis. Due to several inherent advantages such as smooth and easy recycling, high surface area resulting in high turnover number (TON), relatively uniform dispersion, and outstanding stability magnetic nanocatalysts are being extensively utilized for Suzuki cross coupling reactions. The nanoscale support to the immobilized catalyst not only increases the active catalytic sites but also imparts a smooth transfer of electrons from the catalyst surface which is very much evident in heterobimetallic magnetic nanocomposites. It can be assumed that the coming decade or so would witness more such interesting advancements in heterogeneous catalysis.

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Yadavalli Venkata Durga Nageswar*, Jayathirtha Rao Vaidya

Chapter 8 Ni nanoparticles-mediated synthesis of various heterocycles 8.1 Introduction Heterocyclic scaffolds are privileged biologically active entities as they are widely distributed in nature and these are present in alkaloids, vitamins, and in many other essential natural products. The heterocyclic moieties represent different potential medicinal activities, such as antiviral, antibacterial, antifungal, antitumor, anti-inflammatory, antidiabetic, antidepressant, etc. [1–4]. Many protocols are reported every day globally on heterocycles synthesis, and many researchers are passionately working in this evergrowing arena. Catalysts are a group of chemicals that enable any chemical reaction to progress, comparatively at a faster rate, under alternate conditions without involving themselves in the reaction. Catalysts play a key role in the design, development, and improvement of different chemical processes for synthesizing a broad spectrum of medicinally useful chemical entities, such as natural products, medicinally functional molecules, and other industrially important compounds. These are of two types – homogeneous and heterogeneous catalysts. Homogeneous catalysts work in the same phase as reactants, making them useful, as the interaction between reactants and catalysts in the same phase is more facile, efficient, and convenient for the fruitful completion of the reactions. The disadvantage, however, is the difficulty in removing, separating, and recovering the catalyst from the reaction mixture, after the reaction. In the case of heterogeneous catalysts, the catalyst is in a different phase compared to the reactants, and hence the recovery becomes easy. Among the varied spectrum of heterogeneous catalysts, nanocatalysts have dominated both academic and industrial research due to the potential advantages, mainly in view of their size and high surface area-to-volume ratio [5]. Moreover, they

Acknowledgments: VJR Thank CSIR-New Delhi for Emeritus Scientist Honor. *Corresponding author: Yadavalli Venkata Durga Nageswar, Retired Chief Scientist, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad, Telangana 500007, India, e-mail: [email protected] Jayathirtha Rao Vaidya, Emeritus Scientist, FluoroAgro Chemicals Department, CSIR-Indian Institute of Chemical Technology and AcSIR-Ghaziabad, Uppal Road, Tarnaka, Hyderabad, Telangana 500007, India https://doi.org/10.1515/9783110730357-008

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are eco-friendly, sustainable, cost-effective, and stable entities, with an average size of 1–100 nm. Among nanocatalysts, magnetically retrievable catalysts have gained much prominence because of their easy separation and excellent recyclability. They can be easily modified and tailored, as well as their stability and efficacy can be enhanced according to the needs. Solid-supported magnetic nano catalysts obtained by doping nanocatalysts with magnetic solid supports have gained prominence as they can be easily separated, quantitatively collected, and recycled many times without significant activity and efficacy loss [6]. Among metal catalysts, nickel has many commonalities with both palladium and platinum [7]. Given the facile oxidative addition and the ready access to multiple oxidation states, nickel makes it possible to be employed in a broad range of chemical reactions as an efficient catalyst. Moreover, nickel catalysis contributed to the growth of organic transformations in the recent decades. Several nickel complexes are applied in reactions as catalysts. Among the different nickel catalysts, the present review focuses on the recent applications of nickel nanoparticles (Ni NPS) in synthesizing a wide range of heterocyclic derivatives. The physical and chemical properties of these Ni-MNPS can be ascertained by employing a series of techniques, such as FT-IR, vibrating sample magnetometer (VSM), X-ray diffraction (XRD), Transmission electron microscopy (TEM), High resolution transmission electron microscopy (HR-TEM), inductive coupled plasma-atomic emission spectroscopy (ICP-AES), Energy -dispersive X-ray spectroscopy (EDS), and thermo -gravimetric analysis (TGA). In general, the size, shape, magnetic property, and surface characteristics of the particles, along with the presence of any impurities as well as surface coatings, can be known from the above techniques.

8.2 Recent literature in the application of nickel nanoparticles as catalysts in the formation of heterocyclic derivatives Saini et al. [8] developed an eco-friendly facile method for preparing spiro-fused heterocycles (4) via microwave-assisted three-component Biginelli-like reaction by the condensation of urea (2), barbituric acid (1), and diversely substituted aryl aldehydes (3) in the absence of any solvent, catalyzed by a mild Lewis acid, nickel chloride hexahydrate (Figure 8.1). The authors also observed that cobalt chloride hexahydrate was also generally effective for this protocol.

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Figure 8.1: Preparation of spiro-fused heterocycles.

Sachdeva et al. [9] described an efficient multi-component one-pot synthesis of novel spiro and condensed indole derivatives (8,10), involving substituted isatins (5), an activated methylene reagent (6,9), and 2-thioxo-4-thiazolidinone (7), catalyzed by reusable NiO nanoparticles (NiO-NPS), in both conventional heating and microwave irradiation conditions, following the Knoevenagel condensation and Michael addition reactions (Figure 8.2). The reaction mechanism for the formation of the products was discussed and explained by the authors.

Figure 8.2: NiO-NPS-assisted synthesis of spiro and condensed indole derivatives.

A library of diversified heterocyclic molecules (13,16,19,21) was obtained by the construction of C-N, C-O (Figure 8.3a), and C-C (Figure 8.3b) bond formation from easily available nitrogen and oxygen nucleophiles (12,14,17), aryl/heteroaryl halides (11,15,20), and aryl boronic acids (18), using the new nickel ferrite nanoparticlescatalyzed ligand-free reaction strategy [10]. Excellent functional group tolerance, chemoselectivity, recyclability of the catalyst, magnetic retrievability of NiFe2O4-NPS, and high yields were noted as features of this protocol. During the study, the authors screened different solvents, bases, and evaluated the suitability of the ligands for this reaction. Even though all these C-C, C-N, and C-O cross-coupling reactions were achieved by the NiFe2O4 NPS catalysis under ligand -free conditions; other conditions being the same, the reaction times differed. The scope of the study was extended to include a range of aromatic/ heteroaromatic amines, substituted phenols, aryl halides, boronic acids, and coumarin derivatives.

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Figure 8.3a: Selected representation of C-N and C-O coupling reactions.

Figure 8.3b: Selected representation of C-C coupling reactions.

Singh et al. [11], in their research paper, explained the regioselective synthesis of pyrazoloquinolinone (26) and triazolo quinazolinone derivatives (24), obtained by the condensation reaction between dimedone (22), aryl aldehydes (3), and 3-amino1H-1,2,4-triazole (23) or 3-amino-5-methyl-2H–pyrazole (25), employing the magnetically retrievable nickel nanoparticles as catalyst (Figure 8.4). The authors conducted a series of tests to standardize the optimum quantity of Ni nanoparticles required for the reported reaction. They also explained the mechanistic aspects of the Ni-catalyzed reaction. A facile preparative solvent- free method for polyhydroquinoline derivatives (29) by the condensation of dimedone (22), ethyl acetoacetate (27), ammonium acetate (28), and aromatic aldehydes (3) under micro-wave irradiation conditions, catalyzed by recyclable magnetic nickel ferrite nano particles (NiFe2O4 MNPS), was reported by Ahankar and co-authors [12] (Figure 8.5). For the same batch of reactions, Fardood

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Figure 8.4: Synthesis of pyrazoloquinolinones and triazoloquinozolinone derivatives.

et al. [13] employed Ni0.35Cu0.25Mg0.4Fe2O4 as an efficient magnetic nanocatalyst in microwave-assisted conditions. The efficiency of the catalytic activity of the prepared catalyst was checked and compared with several previously reported catalysts. The recyclability data is also presented. The protocol was examined for various aromatic aldehydes carrying electron-donating as well as electron-withdrawing groups on the aromatic ring and also for heterocyclic aldehydes. Ni-Cu-Mg ferrite nanoparticles were studied for their structural properties by FT-IR, XRD, and SEM. It was revealed that they possess an average particle size of 19 nm, with a cubic phase.

Figure 8.5: Synthesis of polyhydroquinoline derivatives.

Tang et al. [14] successfully developed a synthetic strategy for tetrahydro quinoline derivatives (32) by the visible light-mediated direct cyclization of tertiary anilines (30) with maleimides (31), employing reusable, inexpensive, and efficient Nickel (II) oxide surface-modified titanium dioxide (TiO2/NiO) as a photocatalyst. The catalyst can be easily recovered and used nine times due to its high thermal stability and photochemical stability. (Figure 8.6).

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Figure 8.6: Heterocyclic derivatives synthesis catalyzed by TiO2/NiO/hv.

The catalyst was prepared by chemisorptions-calcination cycle techniques to form tiny NiO clusters on commercial TiO2 Degussa P25. Later, the catalyst was studied using ICP, HR TEM, and XRD. It was observed that the protocol was tolerant toward N-aryl/alkyl maleimides and N,N-dimethyl anilines having electron-donating as well as electron-withdrawing groups. Recyclability data and reaction mechanism were presented in the research paper. A series of 1,8-dioxo-octahydroxanthenes (33) (Figure 8.7) and 1,8-dioxo hexahydro acridines (34) (Figure 8.8) were synthesized via the one-pot three-component reaction strategies by Maripi et al. [15] using a catalytic amount of magnetically separable and reusable nickel-cobalt ferrite nanoparticles-Ni0.5Co0.5Fe2O4. The authors prepared the catalyst following the chemical sol-gel co-precipitation method from the cobalt nitrate, iron nitrate, and nickel nitrate reactants.

Figure 8.7: Nickel-cobalt ferrite MNPS-promoted synthesis of 1,8-dioxooctahydroxanthenes.

Figure 8.8: Ni0.5Co0.5Fe2O4MNPS catalyzed synthesis of 1,8-dioxohexahydroacridines.

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While optimizing the reaction, the authors examined the feasibility of different solvents as well as various catalysts. They concluded that the ideal conditions were ethanol/water mixture under reflux and 20 mol% Ni0.5Co0.5Fe2O4NPS. Bhattacharjee et al. [16] described an eco-friendly aqueous synthesis of diversely substituted heterocyclic derivatives-pyrazolylpyrimidinetriones (37) (Figure 8.9a) and bis(heterocyclyl)methanes (39) (Figure 8.9b) at room temperature, employing magnetically recoverable Ni-nano particles. Pyrazolyl pyrimidinetriones were prepared by a three-component reaction between barbituric acid (36), pyrazolone (35), and several aromatic aldehydes (3). Bis(4-hydroxy-2H-chromen-2-ones) (39) and bis(3-hydroxy-5,5dimethyl cyclohex-2-enone) derivatives (40) were prepared by reacting aromatic aldehydes with active methylene compounds like 4-hydroxy coumarin (38) and dimedone (22). The reactions of heterocyclic and aliphatic aldehydes were also explored in this study. Synthesized compounds showed moderate antibacterial activity.

Figure 8.9a: Synthesis of pyrazol-4-yl-methyl-pyrimidine-2,4,6(1H,3H,5H)-trione derivatives.

Figure 8.9b: Preparation of bis(heterocyclyl)methanes derivatives.

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Kalhor et al. [17] developed Ni (II) ion stabilized on zeolite –Y(NNZ), a highly non-porous efficient catalyst, and examined its catalytic efficiency in the preparation of 3-benzimidazolyl-1,3-thiazolidin-4-one derivatives (43) by the condensation reaction between thioglycolic acid (42), 2-aminobenzimidazole (41), and aromatic aldehydes (3) in the presence of ethanol as solvent, in ambient reaction conditions (Figure 8.10).

Figure 8.10: Synthesis of N-benzimidazole-1,3-thiazolidinone derivatives.

While optimizing the reaction conditions, the authors examined the catalyst loadings, solvents, reaction times, and other criteria, and observed that 10% w/w catalyst load, ethanol solvent, and room temperature are optimum conditions. During recyclability studies, the authors observed that the recycled catalyst could be used five times effectively without a loss of activity. The authors also presented a comparison of the different synthetic methods employing various catalytic and reaction conditions in the preparation of thiazolidin-4-one derivatives. The reaction mechanism was discussed by the authors in their research paper. An aqueous medium one-pot eco-friendly methodology for preparing 1H-tetrazoles (46,47), catalyzed by recyclable Ni(OH)2 nanoparticles, was successfully explored by Halder et al. [18]. The products were obtained by the reaction of aldoximes (44) and sodium azide (45) in water, in mild conditions. The authors screened the different solvents and other reaction parameters before optimizing the protocol, which was extended to include aliphatic, aromatic, and heterocyclic aldoximes (Figure 8.11).

Figure 8.11: Ni(OH)2-mediated synthesis of various tetrazoles.

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This versatile protocol was reportedly compatible with both electron-donating as well as electron-withdrawing groups. The possible reaction mechanism and comparative studies with literature-reported methods were presented in the paper. Krishnaveni et al. [19] prepared phase-pure and size-controlled NiO nanoparticles in a cost-effective eco-friendly way, employing a new capping agent, quercetin, by the hydrothermal crystallization method, and studied the nanoparticles using TG-DSC, XRD, SEM, FE-SEM, HR-TEM, EDAX, and other analytical methods. The authors executed the N-arylation of electron-deficient pyrrole and indole derivatives (49,51) with diversely substituted aryl boronic acids (48), employing the prepared NiO nanoparticles in the presence of K2CO3 (51), affording C-N coupled products (50,52) (Figure 8.12).

Figure 8.12: Synthesis of N-arylated pyrroles and indoles using NiO catalyst.

Das and co-authors [20] demonstrated an unprecedented highly selective preparation of E-configured vinyl arenes (55), employing a range of methyl heteroarenes (53) with primary alcohols (54) at excellent yields, and catalyzed by a simple nickel catalyst system, stabilized by the readily available nitrogen ligands (Figure 8.13).

Figure 8.13: α-Olefination of methyl-N-heterocyclics.

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The authors established 2.5 mol% NiBr2 with 3.0 mol. 1,10-phenanthroline as a suitable catalyst to provide the highly selective E-substituted olefins. A variety of alkyl-substituted N-hetero arenes, such as pyridine, pyrazine, benzoxazole, 4-methyl quinoline, and 2-methyl quinolone, were employed as reactants, apart from alkyl and benzyl alcohols. It was established that the transformations proceeded through enamine intermediates or de-aromatization of 2-alkyl heteroarenes with the generation of water and dihydrogen as byproducts, as indicated by the deuterium-labeling experiments and mechanistic studies. The authors explored the selective synthesis of drug molecules and complex natural products having E-olefinated functionalities. A series of 2,5-disubstituted pyrroles (58) and 2,3,5-tri substituted pyrroles (60) were synthesized by Alanthadka et al. [21] via the double dehydrogenative coupling of secondary alcohols (59) and β-amino alcohols (56), employing inexpensive NiBr2 catalyst in combination with 1,10-phenanthroline, t-BuOK as a base and toluene as a solvent, following the proper optimization studies (Figure 8.14). The authors studied the general applicability of the protocol using several amino alcohols bearing alkyl, aryl, and benzyl groups as well as a series of electronically different secondary alcohols bearing functional groups, such as o-Me, m-OMe, p-OMe, p-Cl, p-Br, p-Et, and naphthyl groups. As claimed by the authors, the protocol was tolerant to amino alcohols, including a variety of aryl, alkyl, methoxy, and halide substituents. They postulated plausible

Figure 8.14: Synthesis of substituted pyrroles.

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mechanistic pathways with the help of deuterium-labeling studies, detection of water generation, and quantitative determination of hydrogen gas evolution. Selective synthesis of mono and disubstituted benzimidazoles (62,63), and quinoxalines (65) was reported by Bera et al. [22] via nickel-catalyzed dehydrogenative coupling of primary alcohols (54) and ethylene glycol (64) with aromatic diamines (61) (Figure 8.15). During the study, they examined the effectiveness of the different nickel compounds in combination with several ligands and concluded that the combination of NiCl2 and 1,10-phenanthroline was ideal for the reaction. Among bases, t-BuOK was selected. The present catalytic protocol was described to be tolerant to aryl, methoxy. trifluromethyl, halides, thiophene, pyridine, and furfural moieties.

Figure 8.15: Synthesis of 1,2-disubstituted benzimidazoles, 2-substituted benzimidazoles and quinoxalines.

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Deuterium-labeling experiments and the quantitative determination of hydrogen gas were conducted to establish the mechanistic aspects of the reaction. Rajender et al. [23] described the one-pot synthesis of pyrano-[2,3-d]-pyrimidines (67) (Figure 8.16a) by the condensation reaction of variously substituted arylaldehydes (3) with ethyl cyanoacetate (9) and barbituric acid (66), as well as the preparation of 1,8-dioxo-octahydroxanthenes (68) (Figure 8.16b) by the reaction of aryl/ heterocyclyl aldehydes (3) with dimedone (22). The authors prepared nickel NPS@Ndoped TiO2 after obtaining the nanoparticles of Ni by the chemical reduction of nickel acetate, and loading them over nitrogen-doped titania. Furthermore, the catalytic efficiency of NiNPS@N-dopedTiO2 was explored and established by applying it in the above two protocols. Before applying them as catalysts, the authors studied their elemental composition and surface morphology utilizing SEM-EDX, and the internal morphology and the size of the nanostructures were examined using TEM analysis. TGA, FTIR, and XPS were also conducted to know more about the thermal stability, presence of nitrogen, and the oxidation state. During optimization studies, the authors examined the behavior of the different solvents, and the influence of the reaction time and temperature conditions on the protocol. Under similar reaction conditions, a comparative study was conducted between TiO2 nanospheres, the present catalyst, NiNPS, N-doped TiO2 nano spheres, and the catalyst-free conditions. Recyclability studies and the reaction mechanism were also presented in the paper.

Figure 8.16a: Ni-NPs-N-dopedTiO2-mediated synthesis of pyrano[2,3-d]pyrimidines.

Figure 8.16b: Ni-NPs-N-dopedTiO2-mediated synthesis of 1,8-dioxo-octahydroxanthenes.

Geedkar and co-authors [24] reported a novel series of pharmacologically active benzo[d]imidazo [2,1-b] thiazole (71) scaffolds from 2-amino benzothiazole (69),

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several aryl aldehydes (3), and phenylacetylene derivatives (70) via a coupling reaction assisted by the multi-walled carbon nanotubes embraced with nickel ferrite (NiFe2O4CNTs) magnetic nanoparticles as a heterogeneous catalyst in the presence of PEG as a green solvent in aerobic conditions. The catalyst was studied using powder X-ray diffraction, FT-IR, Raman spectroscopy, VSM-TGA-DTA-DTG analyses, and other relevant studies (Figure 8.17). The authors explained the mechanistic pathway for the formation of these derivatives.

Figure 8.17: Synthesis of benzo[d]imidazo[2,1-b]thiazole derivatives.

Bains [25] developed a homogeneous, phospine-free, inexpensive nickel catalyst for obtaining a series of benzazole derivatives (72,73,75) by the reaction of aromatic diamine (61) /2-amino(thio)phenol (74), and alcohols (54) via alcohol oxidation, imine formation, ring cyclization, and dehydrogenative aromatization sequence in relatively mild conditions. Square planar-shaped nickel complex, derived from Ni(OAC)2 and azophenolate ligand, was air-stable isolable and molecularly defined entity. The mechanistic pathway is governed by the hydrogen atom transfer (HAT) principle. The scope of the protocol was expanded to include a diverse set of benzyl alcohols and heterocyclic alcohols, such as 2-furan methanol and 2-pyridine methanol. The protocol was also tolerant toward diversely substituted bis-amines. HAT-promoted amine dehydrogenative mechanism was explained (Figure 8.18). Kohzadian [26] reported a facile and efficient preparation of bis(pyrazoly)methanes (77) via the one-pot multi-component reaction of phenyl hydrazine (76), ethyl acetoacetate (27), and variously substituted aryl aldehydes (3) in ethanol solvent, catalyzed by nickel-guanidine complex and immobilized on MCM-41 (MCM-41@Gu@Ni), a reusable nanocatalyst. Nano-sized MCM-41 was prepared using the sol-gel method with TEOS (tetra ethyl ortho silicate) as silicon source, CTAB (cetyl trimethyl ammonium bromide) as the template, and NaOH as pH control agent (Figure 8.19). The mesoporous MCM-41 was modified by 3-chloro propyl trimethoxy silane to obtain MCN41@n-PrCl. Guanidine was immobilized onto the surface of MCN-41@n-PrCl using guanidine nitrate. Finally, MCN-41@n-Pr-guanidine@Ni was made by immobilizing Ni onto MCN-41@nPr-guanidine using Ni(NO3)2.6H2O in ethanol as the Ni source. Recyclability studies and the reaction mechanism were accounted in the paper. Results of the present catalyst for factors like turnover efficiency (TOF) and turnover number,

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Figure 8.18: Synthesis of 2-disubstituted benzimidazoles, 1,2-disubstituted benzimidazoles, 2-substituted benzoxazoles, and 2-substituted benzothiazoles.

reaction yield, time, and the reaction temperature in making a model product were compared with those of other known catalysts. TEM, FE-SEM, and XRD were used to study the catalyst.

Figure 8.19: MCM-41-@Gu@Ni-assisted synthesis of bis(pyrazolyl)methanes.

A simple cost-effective NiBr2/1,10-phenanthroline recyclable heterogeneous catalytic system was employed by Shee et al. [27] to prepare a series of quinoxalines (65) from both 1,2-diamines and 2-nitro anilines (74). The catalyst was studied by TEM, XPS,

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and PXRD. While optimizing the reaction, several bidentate and tridentate nitrogen-based ligands (5 mol%), in combination with anhydrous NiBr2(5 mol%), were examined in the presence of Cs2CO3. 1,10-Phenanthroline was reported to have shown promising results. Other nickel sources were also screened for the protocol. For the present dehydrogenative coupling reaction, a 1:2 ratio of 1,2–diamino benzene (74) and 1,2-propane diol (78) in the presence of 5 mol% NiBr2 /Phen and 0.75 equivalent of CsOH-H2O was observed to be ideal. The scope of the methodology was extended to include several 2-nitro anilines and various 1,2-diols (Figure 8.20).

Figure 8.20: NiBr2-1,10-Phen-mediated synthesis of substituted quinoxalines.

Moavi et al. [28], in their research paper, described a novel biological route to prepare nickel oxide nanoparticles (NiO NPS), employing marine microalgae extract as a reducing and coating agent. The authors also further utilized these biogenic NiO NPS to effectively catalyze the one-pot synthesis of pyrido-pyrimidine derivatives (80) in a water medium, free of any toxic reagent, in shorter reaction times, obtained by the reaction of ammonium acetate (28), arylaldehydes (3), thiobarbituric acid (79), and 4-hydroxycoumarin (38). The reusability of NiO NPS was further effected by isolating them at the end of the reaction using an external magnet and using them in consecutive reactions. They reported that even after several successive cycles, the morphology of the recovered nano-scaled NPS did not exhibit any significant change. The generality of the catalytic efficiency was examined by applying the reaction for a library of variously functionalized aldehydes in similar reaction conditions (Figure 8.21). Khojastehnezhad and his research group [29] designed and developed a new magnetically separable and reusable catalyst containing ferric hydrogen sulfate supported on silica-coated nickel ferrite nanoparticles, NiFe2O4@SiO2-FHS. After proper studies employing different analytical techniques, the authors applied this nano composite catalyst to the synthesis of 1,8-dioxo decahydroacridines (34) in solvent-free conditions (Figure 8.22). The catalyst could be recovered by a simple decantation technique and reused five times. The efficacy of the catalyst was compared with the results obtained from other solid-acid catalysts. The authors also suggested a possible reaction mechanism.

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Figure 8.21: Synthesis of substituted tetracyclic pyridopyrimidine derivatives.

Figure 8.22: NiFe2O4@SiO2-FHS-mediated synthesis of 1,8‐dioxodecahydroacridine derivatives.

A facile one-pot [2 + 3] cycloaddition of several nitrile derivatives (81) with sodium azide (45), providing 5-substituted-1H-tetrazoles (47) and catalyzed by magnetically retrievable recyclable nickel ferrite nanoparticles, was described by Abrishami and research group (Figure 8.23) [30].

Figure 8.23: NiFe2O4-NPs-mediated synthesis of tetrazoles.

A series of spiro [indole-3,2′-pyrrole]-2,5′ (1H,1′H)-diones (83) was synthesized from arylamines (12), acetylene dicarboxylates (82), and isatin derivatives (5) from a three-component condensation reaction, by Meghyasi and co-authors [31]. They successfully utilized eco-friendly NiFe2O4 nanoparticles as a recyclable heterogeneous catalyst under ethanol reflux conditions. The authors examined the various solvents, like water, ethanol, DMF, and acetonitrile as well as catalysts such as

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ZnCl2, NiCl2, NiO, FeCl3, Fe3O4, piperidine, and NiFe2O4NPS, during optimization of the reaction conditions. From the recycling data it was concluded that the catalyst can be efficiently used five times (Figure 8.24).

Figure 8.24: NiFe2O4 -NPs-assisted synthesis of spiro[indole-3,2′-pyrrole]-2,5′(1H,1′H)-diones.

Dabholkar and co-workers [32] developed a facile, efficient, and eco-friendly synthesis of biologically active 1H-pyrazole[1,2-b] phthalazine-5,10-diones (84,86) by a one-pot four-component reaction of phthalimide (83), differently substituted aromatic aldehydes (3,85), hydrazine monohydrate (82), and malononitrileor ethyl cyanoacetate (6,9), catalyzed by NiFe2O4NPS, as a heterogeneous recyclable base catalyst, which was prepared by a simple co-precipitation method. (Figure 8.25). The catalyst was efficiently employed for the reaction for up to five runs. During model studies, the authors screened solvents, like water, ethanol, methanol, DMF, and DMSO, in dry conditions. The effect of the catalyst quantities was also assessed on the model reaction in the optimization studies. Several biologically promising 4-arylquinolin-2(1H)-one derivatives (90)/arylcoumarin derivatives (92) were synthesized by Borhade et al. [33] via a simple onepot three-component reaction of the various substituted 2-iodo aniline derivatives (87) / 2-iodo phenol (91) with substituted arylhalides(89) and acrylic acid (88), catalyzed by reusable Pd/ NiFe2O4 in DMF medium in the presence of triethylamine base (Figure 8.26). Moghaddam et al. [34] efficiently used nickel ferrite nanoparticles as a recyclable heterogeneous catalyst for a neat, eco-friendly, and one-pot four-component preparation of pyrrole derivatives (95) from nitromethane (94), 1,3-dicarbonyl compounds (93) and differently substituted aryl amines (12), and aryl aldehydes (3). The catalyst was successfully recovered with the aid of an external magnet and was reused nine times without a loss of significant catalytic activity. The reaction mechanism was also depicted in the publication. The catalyst was prepared from iron nitrate and nickel nitrate in a basic medium by the co-precipitation method (Figure 8.27). Lu and co-authors [35] demonstrated the eco-friendly preparation of novel magnetic NiFe2O4-supported glutamate-copper catalyst and after studying the catalyst employing FT-IR, XRD, EDX, TEM, SEM, XPS, and VSM techniques, applied it in the

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Figure 8.25: NiFe2O4NPS-mediated synthesis of 1H-pyrazolo[1,2-b]phthalazine-5,10-dione derivatives.

Figure 8.26: Pd-NiFe2O4-mediated synthesis quinolone and coumarin derivatives.

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Figure 8.27: NiFe2O4-NPs-assisted synthesis of highly-substituted pyrroles.

synthesis of a library of 1,4-disubstituted-1,2,3-triazoles (97,99,100) via one-pot three-component aqueous phase click reactions of non-activated terminal alkynes (70), sodium azide (45), and several azide precursors, including benzyl chloride (98), epoxides (96), and aryl boronic acids (48). Initially, the catalyst, NiFe2O4-glutamate-Cu, was prepared by a three-step process – a) synthesis of NiFe2O4 NPS by the chemical precipitation method from NiCl2and FeCl3, b) anchoring L-glutamic acid on the surface of NiFe2O4NPS in MeOH c) followed by the addition of Cu(OAc)2 and NaBH4. While optimizing the click reaction, parameters such as solvents, kinds of catalysts, and different catalyst concentrations were examined. CuSO4.5H2O, Cu(OAc) 2H 2O, nano Fe 3O 4 , nano-CoFe2O 4 , nano-CuFe2O 4, nano CuFeO2, CuCl, and CuI were some of the catalysts studied for the reaction, apart from NiFe2O4glutamate-Cu. Ease of catalyst separation, recoverability, and reusability were also examined by the authors. The scope of the activity of various epoxides and alkynes was evaluated (Figure 8.28). The catalyst was recovered and reused successfully in ten runs without a significant loss of activity.

Figure 8.28: NiFe2O4-Glu-Cu-assisted synthesis of substituted triazoles.

Payra et al. [36] developed an efficient and eco-friendly regioselective approach for the synthesis of novel 2–alkoxy-3-aryl imidazo[1,2-a]pyridines (104) via magnetically retrievable and reusable nano NiFe2O4 -catalyzed microwave-assisted

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sequential one-pot three-component reactions between β-nitrostyrenes (102), 2-amino pyridines (101), and variously substituted alcohols (103), in solvent-free aerobic conditions. The authors claim that this is the first method to produce alkoxy-functionalized imidazo pyridine derivatives (Figure 8.29). The results of the study were also compared with conventional heating conditions. During optimization, the authors screened different nano-catalysts, like CuFe2O4,CoFe2O4, MnFe2O4, Fe3O4, and NiO NPS, apart from NiFe2O4 NPS.

Figure 8.29: Synthesis of 2-alkoxyimidazopyridine derivatives involving NiFe2O4 catalyst.

Different control experiments were conducted to study the mechanistic aspects. From the data, it was concluded that the Ni site in NiFe2O4 NPS accelerated the initial selective aza-Michael addition, and the Fe sites in ferrite promoted the oxidative imination, followed by the displacement of the nitro group by the alkoxy group. The plausible reaction mechanism was explained in the publication. Poor-Heran and co-workers [37] prepared the highly substituted 1, 4-dihydropyrano [2, 3-c] pyrazole derivatives (105) by the reaction of phenyl hydrazine (76) 1,3dicarbonyl compounds (28), substituted arylaldehydes (3), and mnonitrile (6) in a solvent-free one-pot four-component condensation reaction, catalyzed by NiFe2O4 NPS, a magnetically retrievable reusable heterogeneous catalyst (Figure 8.30). The catalyst was reused five times without any loss of activity.

Figure 8.30: Synthesis of highly substituted 1,4-dihydropyrano[2,3-c]pyrazole derivatives.

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8.3 Conclusions An effort has been made to review the recent research papers where nickel nanoparticles are employed to catalyze the chemical reactions, resulting in the formation of heterocyclic scaffolds. The authors of this article sincerely appreciate the contributions made by the authors of the original communications cited herein. The readers are advised to go through these papers for detailed information about the research works. All figures are redrawn and are representative.

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Yogesh B. Wagh, Yogesh A. Tayade, Dipak S. Dalal*

Chapter 9 Sulfonic acid functionalized magnetic nanocatalysts in organic synthesis 9.1 Introduction The magnetic nanocatalyst is one of the most demanding areas of catalysis and has been widely used in many sectors of chemical manufacturing. The magnetic nanocatalysts functionalized by sulfonic acid–catalyzed organic transformations can be classified as one of the most important aspects of catalysis primarily due to its ability in recycling [1]. However, compared to the homogenous acid catalyst is related to many difficulties in the industry like wastewater production involved during neutralization of acid by washing, tedious purification processes of products, equipment corrosion and recycling problems. The recovery and reusability of the catalyst are the two most important features for many catalytic processes. Hence, one efficient way to overcome the problem of homogeneous catalysts is the heterogenization of active catalytic molecules, creating a heterogeneous catalytic system. In contrast, the recovery of the most heterogeneous catalysts from the final reaction systems requires a filtration or centrifugation step and/or a tedious workup. For this purpose, by applying magnetic supports and an external magnet on magnetic nanocatalyst can be easily recovered and subsequently reused in another cycle without a significant decrease in their activity (Figure 9.1). Magnetic nanocatalysts have been showing many applications in nanotechnology, productions of biofuels from biomass, wastewater treatment, bioelectrocatalysis, environmental applications [2], and sustainable applications in green chemistry [3] and organic synthesis.

Acknowledgments: The first author Dr. Y. B. Wagh is grateful to Council of Scientific & Industrial Research (CSIR), New Delhi, India for the postdoctoral research fellowship under CSIR-RA [Award No. 09/728(0036)/2019 (EMR-I)]. Dr. Y. A. Tayade thankful to Principal, Dhanaji Nana Mahavidyalaya, Faizpur, India, for their kind support. Prof. D. S. Dalal is thankful to UGC and DST for financial grants under UGC-SAP-DSA-I program and FIST-DST to School of Chemical Sciences, KBC NMU, Jalgaon. *Corresponding author: Dipak S. Dalal, School of Chemical Sciences, Kavayitri Bahinabai Chaudhari North Maharashtra University, Jalgaon, (M. S.) 425 001, India, e-mail: [email protected] Yogesh B. Wagh, School of Chemical Sciences, Kavayitri Bahinabai Chaudhari North Maharashtra University, Jalgaon, (M. S.) 425 001, India Yogesh A. Tayade, Department of Chemistry, Dhanaji Nana Mahavidyalaya, Faizpur, (M. S.) 425503, India https://doi.org/10.1515/9783110730357-009

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Figure 9.1: The recovery of magnetic nanocatalyst by an external magnet.

Nevertheless, it is still a challenge to develop solid acid catalysts with high catalytic performance and stability that are as cost-effective as sulfuric acid. In the last decades, magnetic nanoparticles have received excessive attention from researchers due to their potential use in catalysis [4], biomedical applications [5], magnetic resonance imaging [6], magnetic fluids [7], and conservational remedy [8].

9.2 Magnetic nanoparticles (MNPs) as catalysis “Catalysis” is the terminology coined by Swedish chemist Jone Jacob Berzelius in 1836, which means a substance awakens dormant affinities by its mere presence [9]. Catalysis can be categorized into homogenous catalysis and heterogeneous catalysis. Heterogeneous catalysis is more favored over homogenous catalysis because the former is beneficial for simple isolation, ease of purification, and high recycling cost [10]. MNPs have been widely used as catalysts in organic synthesis is due to their exclusive properties like high surface area, simple synthesis, outstanding stability, easy separation, convenience in recycling by using a simple external magnet, and preventing loss of catalyst during the separation steps. Furthermore, magnetic separation is a green procedure that saves time, energy, and solvents, which support the industrial application of these nanostructured catalysts [11]. The heterogeneous catalysts compared with homogeneous to showed a decrease in the catalytic efficiency and selectivity due to the low surface area. This drawback can be overcome by using nanoparticles as heterogeneous supports. Nanoparticulate supports can be used to efficiently bridge the gap between homogeneous and heterogeneous catalysts by retaining the favorable features of both systems [12]. Most nanoparticles (NPs) are heterogeneous catalysts, boosting the goals of the synthetic chemist to co-align green chemistry approaches [13]. Nanocatalysts have a particle size in the nanometer scale and thus have a large surface area, which enables the

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interaction of chemical reactants via cooperative activation to bring them in closer proximity with each other [14]. Magnetic nanoparticles are particles that commonly consist of magnetic elements such as iron, nickel, and cobalt and their chemical compounds. Among magnetic NPs, iron oxide NPs are a class of magnetic material with excellent performance. Currently, Fe3O4 nanoparticles, used as magnetite nanoparticles, are attracting increasing interest due to their unique properties, including a large surface-to-volume ratio, superparamagnetism, low toxicity, biocompatibility, and their potential applications in various fields [15]. The immobilization of sulfonic acid groups on magnetic nanoparticles for use as recyclable and solid acid catalysts. Various sulfonic acids were reported for catalytic activity and high stability. The general strategy involved during preparation of the magnetic nanoparticle supports with sulfonic acid and then coated with silica, providing an inert barrier between the metal oxide core and surface functional groups. The hybrid organic/inorganic, magnetic, solid acid catalysts were characterized via various techniques like FTIR, titration, XRD, and TEM. The active sulfonic acid-supported NPs catalysts were easily recovered in the presence of an external magnetic field and exhibited good recyclability. Sulfonic acid catalysts grafted with inorganic support were observed to exhibit higher activities than those grafted on the silica-coated magnetic nanoparticles for the synthesis of heterocycles on organic chemistry [16].

9.3 Scope of sulfonic acid functionalized MNPs in organic synthesis According to green chemistry, MNP-catalyzed organic transformations are the safest reactions, which do not affect the environment. Some of the important applications of sulfonic acid-functionalized magnetic nanocatalyst have also been demonstrated for various organic transformations.

9.3.1 Synthesis of spirochromene derivatives Hazeri et al. [17] prepared a new magnetic nanocatalyst (CoFe2O4@amino-2-naphthol-4-sulfonic acid) and they studied its application for the synthesis of various spirochromenes (5) and (6) from the reactions of 1,3-dicarbonyl compounds (1), ethyl cyanoacetate or malononitrile (2) and acenaphthoquinone (3) or substituted isatins (4) in aqueous ethanol at 40 °C (Figure 9.2). This catalyst was synthesized by dissolving FeCl3.6H2O and CoCl2.6H2O in water under an inert atmosphere in the presence of KOH and refluxed conditions for 3 h at 14 °C to form magnetic CoFe2O4. Then this prepared suspension of CoFe2O4 nanoparticles was taken in ethanol under sonication and treated with 1-amino-2-naphthol-4-

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Figure 9.2: ANSA-CoFe2O4 catalyzed synthesis of spirochromene.

sulfonic acid and refluxed the mixture for 24 h under N2 atmosphere to get the final ANSA-CoFe2O4 nanoparticles. The detailed process for the preparation of magnetic nanocatalyst ANSA-CoFe2O4 is shown in Figure 9.3.

Figure 9.3: Preparation of ANSA-CoFe2O4.

9.3.2 Synthesis of spiropyran derivatives Zolfigol et al. [18] reported a method for the preparation of sulfonic acid functionalized 1,4-diaza-bicyclo[2.2.2]octane (DABCO)-based magnetic nanoparticle Fe3O4 [Fe3O4@SiO2@Pr-DABCO-SO3H]Cl2. Then, this catalyst was examined for the synthesis of spiropyran (7) derivatives via one-pot three-component reactions of isatins (4), malononitrile (2), and 1,3-dicarbonyl compounds (1) (Figure 9.4) in water. The notable features of this method are the high yield of all products, shorter reaction times, and the reusability of the catalyst.

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Figure 9.4: One-pot synthesis of spiropyran derivatives.

9.3.3 Synthesis of spirooxindoles Nasseri et al. [19] prepared silica-supported sulfonic acid immobilized cobalt ferrite magnetic catalyst (CoFe2O4/SiO2/SO3H) by simple technique (Figure 9.5). Initially, the mixture of Co(NO3)2 · 6H2O and Fe(NO3)3 · 9H2O was dissolved in distilled water, and then the solution was placed in ultrasonication. After 20 min, aqueous NaOH was added to the mixture dropwise, and the mixture was heated at 80 °C for 2 h. In the next step, the pre-prepared CoFe2O4 MNPs were dispersed in ethanol under ultrasound-assisted conditions. After 20 min, water, TEOS (tetraethyl orthosilicate), and ammonia solution were added under constant stirring. Then obtained CoFe2O4@SiO2 nanocomposite dispersed for 30 min in the ultrasonicate instrument with dichloromethane. Finally, chlorosulfonic acid was added dropwise to the whole mixture was then stirred for 5 h to form the desired nanocatalyst (CoFe2O4@SiO2@SO3H).

Figure 9.5: Preparation of catalyst CoFe2O4@SiO2@SO3H.

This prepared catalyst was further characterized by using different techniques like FT-IR, XRD, EDX, VSM, TEM, SEM, and TGA. After this study, Nasseri et al. [19] used (CoFe2O4/SiO2/SO3H) nanoparticles as an efficient catalyst for the synthesis of spirooxindole derivatives (9) via one-pot three-component reactions of malononitrile (2), dimedone (8), and isatins (4) under refluxed conditions at 80 °C in aqueous ethanol (1:1) (Figure 9.6). A simple catalytic process, high magnetic properties, easy separation of the catalyst with a permanent magnet, and the application of low-cost and readily available precursors are some of the major advantages of this developed

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protocol. The products were obtained in high yields within a very short reaction time which represents the high catalytic activity of the catalyst.

Figure 9.6: Synthesis of spirooxindole derivatives catalyzed by CoFe2O4@SiO2@SO3H.

On the other hand, Khalafi-Nezhad et al. [20] used anchoring 3-sulfobutyl-1-(3 propyltriethoxysilane) imidazolium hydrogen sulfate onto the surface of silica-coated Fe3O4 nanoparticles AIL@MNP as an effective catalyst for the synthesis of newer spirooxindole derivatives (Figure 9.7). With the combination of nano-support features as well as flexible imidazolium linkers, this catalyst can act as a “quasi-homogeneous” catalyst to effectively catalyze the one-pot synthesis of spirooxindoles (11) by three-component reaction of a wide variety of substituted isatins (4), 1,3-dimethyl-2-amino uracil (10), and different 1,3-dicarbonyl groups (1) such as barbituric acid, thiobarbituric acid, and dimedone as nucleophiles under mild conditions in water with excellent yields. The attractive features of these nanoparticles are reusability and quickly recovered by using an external magnet.

Figure 9.7: Synthesis of spirooxindoles by using magnetic nanoparticle supported dual acidic ionic liquid.

9.3.4 Synthesis of spiro[indeno[1,2-b]quinoxaline derivatives Shaterian et al. [21] achieved the efficient synthesis of novel spiro[indeno[1,2-b]quinoxaline (16) derivatives via the four-component condensation of amines (15), ninhydrin (13), isatoic anhydride (12), and о-phenylenediamine (15) derivatives catalyzed by (3-oxo-[1,2,4]triazolidin-1-yl)bis (butane-1-sulfonic acid) supported on γ-Fe2O3 as novel heterogeneous magnetic nanocatalyst (Figure 9.8). The nanocatalyst was

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characterized by X-ray diffraction (XRD), FT-IR, vibrating sample magnetometry (VSM), Field Emission Scanning Electron Microscopy (FE-SEM), and thermal analysis (TGA-DTG). The nanoparticles covered by (3-oxo-[1,2,4]triazolidin-1-yl) bis(butane-1sulfonic acid) showed enhanced catalytic performance in the preparation of spiro[indeno[1,2-b]quinoxaline derivatives in excellent yields. In addition, this method has advantages: mild conditions, high yields, easy workup in which the catalyst can be easily separated from the reaction mixture by an external magnet, recycled, and reused several times without a noticeable decrease in catalytic activity.

Figure 9.8: Synthesis of spiro[indeno[1,2-b]quinoxaline derivatives in presence γFe2O3@oxotriazolidinsultone as nanocatalyst.

9.3.5 Synthesis of monospiro-2-amino-4H-pyran Shafiee et al. [22] reported the synthesis of two series of monospirooxindoles derivatives by using propane-1-sulfonic acid-modified magnetic hydroxyapatite nanoparticles. In first series synthesis of isatins-based spirooxindoles (17) from the one-pot reaction of isatins (4), malononitrile (2), and 1,3-dicarbonyl compounds (1) in water. In another series, synthesis of ninhydrin-based spirooxindoles (18) taken by three components coupling of ninhydrin (13), malononitrile (2), and 1,3-dicarbonyl compounds (1) in water by using a catalytic amount of propane-1-sulfonic acid-modified magnetic hydroxyapatite nanoparticles in H2O (Figure 9.9). The advantages of this work are that water is used as a solvent and easily separates nanocatalyst from the reaction mixture by an external magnet. The catalyst could be easily recycled for more than five times without loss of activity.

9.3.6 Synthesis of polysubstituted pyridines Highly substituted pyridine (22) derivatives were prepared by the reaction of aromatic aldehydes (19), acyclic or cyclic ketones (21), malononitrile (2), and ammonium acetate (20) in the presence of the catalyst Fe3O4@g-C3N4-SO3H under the ultrasonic condition [23] in water via one-pot multi-component approach (Figure 9.10). The reported method is highly cheap and environmental friendly.

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Figure 9.9: Synthesis of spirooxindoles by using propane-1-sulfonic acid-modified magnetic hydroxyapatite nanoparticles.

Figure 9.10: Synthesis of 2-amino-3-cyanopyridine derivatives catalyzed by Fe3O4@γ-C3N4-SO3H.

For the catalyst preparation, initially the melamine (23) was heated in a muffle furnace at 550 °C for 4 h to form γ-C3N4 nanosheets (24). Then after the Fe3O4@γ-C3N4 was prepared from γ-C3N4 at 80 °C with in 30 min in the presence of ammonia. After all, Fe3O4@γ-C3N4 (25) was treated with chlorosulfonic acid in dry CH2Cl2 at room temperature and stirred the solution for 10 h to get the desired catalyst Fe3O4 @γ-C3N4-SO3H (26) (Figure 9.11)

9.3.7 Synthesis of indeno[1,2-b]pyridines Ziarani et al. [24] reported a magnetically separable acidic catalyst Fe3O4@SiO2@PrSO3H for the synthesis of indeno[1,2-b]pyridines (29) via multi-component reaction between substituted benzaldehyde (19), 1,3-indandione (28), acetophenone (27), and ammonium acetate without solvent at 80 °C (Figure 9.12) The catalyst Fe3O4@SiO2@PrSO3H was fabricated by the reaction between FeCl2. 4H2O and FeCl3.6H2O to form Fe3O4 nanoparticles. Later, a Fe3O4 NPs surface was coated with silica to form silica-coated magnetic nanoparticles. After the SiO2-coated,

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Figure 9.11: Preparation of Fe3O4@γ-C3N4-SO3H catalyst.

Figure 9.12: Fe3O4@SiO2@Pr-SO3H catalyzed synthesis of indeno[1,2-b]pyridine derivatives.

Fe3O4 NPs were treated with (3-mercaptopropyl)trimethoxysilane. Finally, the desire catalyst Fe3O4@SiO2@PrSO3H was formed by the oxidation of -SH group in the presence of solvent H2O2/MeOH (Figure 9.13).

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Figure 9.13: Preparation of Fe3O4@SiO2@PrSO3H nanoparticles.

9.3.8 Synthesis of 2-amino-3-cyano pyridine derivatives In another study, Yarie et al. [25] developed a recyclable urea-based nano-magnetic catalyst Fe3O4@SiO2@(CH2)3-urea-benzimidazole sulfonic acid. The structure of the given catalyst was determined by FTIR, XRD, EDX, SEM, TEM, VSM, and thermogravimetric analysis/differential thermal analysis. After complete surface characterization of preprepared catalyst, then, the catalytic performance of Fe3O4@SiO2@(CH2)3-ureabenzimidazole sulfonic acid was successfully inspected toward the multi-component synthesis of 2-amino-3-cyano pyridine (31) derivatives through the reaction of aromatic aldehydes (19), methyl isopropyl ketone (30), malononitrile (2), and ammonium acetate under solvent-free conditions at 70 °C (Figure 9.14). The progress of the reactions was checked by using TLC, and after the completion of the reaction, the catalyst can be separated by adding hot methanol. The catalyst remains insoluble in that mixture. Finally, the nanocatalyst can be recovered from the reaction mixture by using an external magnetic system.

Figure 9.14: Synthesis of 2-amino-3-cyano pyridines by using Fe3O4@SiO2@(CH2)3-ureabenzimidazole-SO3H.

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9.3.9 Synthesis of 2,4,6-triarylpyridines derivatives Amoozadeh et al. [26] found nano titania-supported sulfonic acid (n-TSA) to be a highly efficient heterogeneous nanocatalyst for the solvent-free synthesis of 2,4,6-triarylpyridines (32) through the one-pot three-component reaction of acetophenones (27), aryl aldehydes (19), and ammonium acetate (20) at 110 °C under solvent-free condition (Figure 9.15). This method has many valuable advantages like simplicity, high product yield, easy workup, and reusability of the catalyst at least six times without substantial reduction in its catalytic activity.

Figure 9.15: Synthesis of 2,4,6-triarylpyridine derivatives by n-TSA.

9.3.10 Synthesis of 1,4-dihydropyridine derivatives Foroughifar et al. [27] reported the novel magnetic solid acid catalyst N-propylbenzoguanamine sulfonic acid stabilized on silica-coated nano-Fe3O4 particles (Fe3O4/SiO2N-propyl benzoguanamine-SO3H) for the synthesis of 1,4-dihydropyridine derivatives (35) in a one-pot pseudo-four-component condensation reaction of barbituric acid (33), aromatic aldehydes (19), and aniline (34) in ethanol at 50 °C (Figure 9.16). The progress of all the reactions was followed by the TLC technique. When the completion of the reaction, the resultant mixture was cool to room temperature, and the catalyst was easily separated by using an external magnet. The prepared nanoparticles were characterized by different techniques like FTIR, FE-SEM, TEM, EDXA, XRD, TGA, DTA, VSM, AAS, and ICP-MS. The heterogeneous catalyst can be quickly recovered by an external magnet and can be reused for further runs without significant loss of its catalytic activity.

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Figure 9.16: Synthesis of 1,4-dihydropyridine derivatives using novel MNP–NPBG–SA as a catalyst.

9.3.11 Synthesis of 1,4-dihydropyridines derivatives by Hantzsch reaction Zeynizadeh et al. [28] reported the immobilization of sulfonic acid on silica-layered nickel ferrite, NiFe2O4@SiO2@SO3H as sulfonated Ni-nanocatalyst for the Hantzsch synthesis of dichromeno-1,4-dihydropyridines (38) via one-pot condensation reaction of 4-hydroxycoumarin (36), and aromatic aldehydes (19) and aqueous ammonia (37) at 70 °C in water (Figure 9.17). Correspondingly, Zeynizadeh et al. also reported Hantzsch synthesis of diester 1,4-dihydropyridines (40) by the reaction of ethyl acetoacetate (39) and aromatic aldehydes (19) and aqueous ammonia (37) in water (Figure 9.18).

Figure 9.17: Hantzsch synthesis of dichromeno-1,4-DHPs catalyzed by NiFe2O4@SiO2@SO3H-MNPs.

Figure 9.18: Synthesis of 2,4,6-triarylpyridine derivatives by n-TSA.

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All the reactions were carried out within 10–100 min to afford the products in high to excellent yields. The reusability of sulfonated Ni-nanocatalyst was studied by seven consecutive cycles without the significant loss of catalytic activity. This prepared nanocatalyst was further characterized by using FT-IR, SEM, EDX, XRD, and VSM analyses. Compared to earlier reported methods, the given method showed remarkable advantages in terms of mild reaction conditions, using water as an ecofriendly solvent, high stability, easy separation of the magnetic nanocatalyst from the reaction mixture, and high yield of products.

9.3.12 Synthesis of polysubstituted tetrahydropyridines and dihydropyrimidinones Maleki et al. [29] reported a new biopolymer cellulose-based magnetic heterogeneous catalyst, MgFe2O4/cellulose/SO3H nanocomposite as an excellent catalyst in two multi-component syntheses of polysubstituted tetrahydropyridines (42) as well as dihydropyrimidinones (44) under solvent-free conditions. The synthesis of substituted tetrahydropyridines (42) takes place via a 5-CR in the presence of the magnetic nanocatalyst under solvent-free conditions at 80 °C (Figure 9.19). First, β-ketoester (41), aromatic amine (34) to form enamines, and then aromatic aldehyde (19) was added and stirred for a suitable time under solvent-free conditions at 80 °C. After completion of the reaction, ethyl acetate was added to the reaction mixture for dissolving the product. Subsequently, the nanocatalyst can be removed easily by an external magnet.

Figure 9.19: One-pot synthesis of tetrahydropyridines by using MgFe2O4/cellulose/SO3H nanocatalyst.

Maleki et al. [29] also synthesized dihydropyrimidines (44) via multi-component reaction of β-ketoester (41), aromatic aldehyde (19), and urea (43) in the presence of the nanocatalyst under solvent-free conditions at 90 °C (Figure 9.20). This nanocomposite is well characterized by FTIR TGA, DTG DSC, X-ray diffraction, and energy-dispersive X-ray, field-emission scanning electron microscopy. This nanocatalyst can be recovered and reused several times without significant loss of catalytic activity.

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Figure 9.20: One-pot synthesis of dihydropyrimidines by using MgFe2O4/cellulose/SO3H nanocatalyst.

9.3.13 Synthesis of dihydropyrano[2,3-c]pyrazoles derivatives Safari and Ahmadzadeh [30] developed a method for the synthesis of zwitterionic sulfamic acid-functionalized nanoclay (MMTZSA) via the functionalization of montmorillonite K10 as a link with 3-aminopropyltriethoxysilane as a linker and chlorosulfonic acid as an SO3H source (Figure 9.21).

Figure 9.21: Synthesis of zwitterionic sulfamic acid-functionalized nanoclay (MMTZSA) nanoparticles.

The physical and chemical properties of zwitterionic nano clay were characterized by using SEM, TGA, DTA, XRD, FT-IR spectroscopy, elemental analysis, and Hammett

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acidity function techniques. Then after the catalytic activity of MMT-ZSA was investigated in the synthesis of dihydropyrano[2,3-c]pyrazoles (46) derivatives via the multi-component reaction between β-keto ester (41), phenylhydrazine (45), malononitrile (2), and aromatic aldehyde (19) derivatives under solvent-free conditions (Figure 9.22).

Figure 9.22: Synthesis of pyrano[2,3-c]pyrazoles using MMT-ZSA under solvent-free conditions.

9.3.14 Synthesis of pyranopyrazole compounds Ghorbani et al. [31] published the preparation of magnetic catalyst by the reaction of silanol groups, on the surface of silica-coated Fe3O4 magnetic nanoparticles, with (3-chloropropyl) triethoxysilane followed by hexamethylenetetramine and chlorosulfonic acid. The catalytic activity of Fe3O4@SiO2-HMTA-SO3H-MNPs was investigated in the synthesis of pyranopyrazole (49) compounds by the reaction of aromatic aldehydes (19), malononitrile (2), dimethyl acetylenedicarboxylate (47), and hydrazine hydrate (48) at room temperature under solvent-free condition (Figure 9.23). All the reactions were monitored by TLC, and after completion of the reaction, the nanomagnetic catalyst was separated by an external magnet. All the products were purified by recrystallization from EtOH and found the excellent yield of the products and short reaction time. Ghorbani et al. [31] observed that electron-donating substituents on phenyl rings make the reaction slower and lead to a lower yield than that of corresponding products compared to electron-withdrawing substituents.

Figure 9.23: Preparation of pyranopyrazole derivatives catalyzed by Fe3O4@SiO2-HMTA-SO3H.

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9.3.15 Synthesis of 4,4′-(arylmethylene)-bis(1H–pyrazol-5-ol) and pyrano[3,2-c]pyrazole derivatives Zolfigol et al. [32] reported the preparation method for nano-magnetic solid acid catalyst with a thiourea-based acidic ionic liquid tag with sulfonic acid. The synthesis method involves the stepwise synthesis of this heterogeneous catalyst has been shown in Figure 9.24. To cover the surface of Fe3O4 nanoparticles, different amounts of double-distilled water, ethanol, ammonia, and tetraethylorthosilicate were added to as-prepared Fe3O4. Silica-covered Fe3O4 was then mixed with (3-chloropropyl)-triethoxysilane in toluene under reflux as well as under nitrogen blanketing. The obtained solid was functionalized with thiourea and then with chlorosulfonic acid.

Figure 9.24: Fe3O4 particles with thiourea-based acidic ionic liquid tag.

Then after, Zolfigol et al. [33] used the same catalyst in the formation of 4,4′-(arylmethylene)-bis(1H–pyrazol-5-ol) (51) and pyrano[3,2-c]pyrazoles (52) derivatives applying 3-methyl-1-phenyl-1H-pyrazol-5(4H)-ones, aromatic aldehydes, and malononitrile as starting materials. Synthesis of 4,4-(arylmethylene)bis(1H–pyrazol-5-ol) (51) derivatives by the reaction of arylaldehydes (19) and 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (50) under solvent free conditions in the presence of nanocatalyst (Fe3O4@SiO2@(CH2)3-thioureadioxide-SO3H/HCl) 90 °C (Figure 9.25). On the other hand, the same catalyst was also used in the synthesis of pyrano[3,2-c]pyrazole (51) by using aromatic aldehydes (19, 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (50) and malononitrile (2) at 90 °C under solvent-free conditions (Figure 9.26).

Figure 9.25: Synthesis of 4,4′-(arylmethylene)bis(1H–pyrazol-5-ol) derivatives.

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Figure 9.26: Synthesis of pyrano[3,2-c]pyrazoles under mild and solvent‐free conditions.

9.3.16 Synthesis of dihydropyrano[2,3-c]pyrazole and 4Hchromene derivatives Abbasabadi et al. [34] found the method for the preparation of magnetic nanocatalyst Fe3O4-supported propane-1-sulfonic acid-grafted graphene oxide quantum dots (Fe3O4@GOQD-O-(propane-1 sulfonic acid)). This newly prepared Fe3O4@GOQD-O(propane-1-sulfonic acid) nanocomposite was structurally well-established by different analytical techniques and vibrating sample magnetometer (VSM) analyses. The catalytic performance of this nanocomposite was exhibited in the one-pot synthesis of dihydropyrano[2,3-c]pyrazole (53) by the reaction of aldehyde (19), 3-methyl-1-phenyl-2-pyrazolin-5-one (50), and malononitrile (2) in deionized water at room temperature (Figure 9.27). After completion of the reaction, the hot ethanol was added to the reaction mixture, and after 5 min of stirring, the catalyst was separated by using an external magnet.

Figure 9.27: Synthesis of 1,4-dihydropyrano[2,3-c] pyrazole derivatives catalyzed by Fe3O4@GOQDO-(propane-1-sulfonic acid) in deionized water.

In the same direction, Abbasabadi et al. [34] also synthesized the 4H-chromene (54) derivatives by using the reaction of aromatic aldehyde (19), dimedone (8), malononitrile (2), and prepared nanocatalyst Fe3O4@GOQD-O-(propane-1-sulfonic acid) in deionized water at room temperature (Figure 9.28). The attractive features of this work are excellent yields of the products in short reaction time, ease to handle, and reusability of catalyst up to five times without loss of catalytic activity.

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Figure 9.28: Synthesis of 4H-chromenes by using nanocatalyst Fe3O4@GOQD-O-(propane-1sulfonic acid).

9.3.17 Synthesis of 4H-benzo[b]pyrans and dihydropyrano[c] chromenes Shafee and co-workers [35] prepared the magnetically inorganic-organic hybrid nanocatalyst supported on hydroxyapatite-encapsulated γ-Fe2O3([γ-Fe2O3@Hap-Si(CH2)3AMP]). This was prepared from the reaction of magnetic hydroxyapatite [γ-Fe2O3@HAp] with aminopropyltrimethoxysilane, 2-hydroxybenzaldehyde, and NaBH3CN. Then they used this catalytic system for the synthesis of 4H-benzo[b]pyrans and dihydropyrano[c] chromenes. The prepared catalyst was used for the synthesis of 2-amino-5-oxo-4-aryl4,5-dihydropyrano[3,2-c]chromene-3-carbonitriles (55) by using aromatic aldehyde (19), malononitrile (2), and 4-hydroxycoumarin (36) in water under refluxed conditions (Figure 9.29). Then after completion of the reaction, the reaction mixture was cooled at room temperature and then diluted with ethyl acetate. Finally, the given magnetic nanocatalyst was easily separated from the reaction mixture with an external magnet. Then after this series, γ-Fe2O3([γ-Fe2O3@Hap-Si(CH2)3-AMP]) catalyst was also utilized for the synthesis of tetrahydrobenzo[b]pyran derivatives (56) by the reaction of aromatic aldehyde (19), malononitrile (2), and dimedone (8) in water under refluxed conditions (Figure 9.30). In all cases, products showed excellent yield, and the pre-prepared nanocatalyst was easily recovered without loss of activity by using an external magnet and reused easily.

Figure 9.29: Synthesis of 2-amino-5-oxo-4-aryl-4, 5-dihydropyrano [3,2-c]chromene-3-carbonitriles.

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Figure 9.30: Preparation of 4H-benzo[b]pyrans and dihydropyrano[c]chromenes.

9.3.18 Synthesis of pyrano[2,3-d] pyrimidines derivatives Sajjadifar and Gheisarzadeh [36] demonstrated isatin-SO3H coated aminopropyl modified magnetic nanoparticles (Fe3O4@APTES@isatin-SO3H) as an effective magnetic nanocatalyst for the synthesis of pyrano[2,3-d] pyrimidines (57) derivatives via threecomponent reactions of various aromatic aldehydes (19), malononitrile (2), and barbituric acid (33) in aqueous ethanol (1:1) under refluxed conditions (Figure 9.31). The XRD patterns of the synthesized nanoparticles showed nine typical peaks at 2θ = 30.5°, 34.7°, 25.1°, 43.1°, 53.6°, 57.8°, 62.5°, and 74.6°. The obtained result showed very broad peaks, which indicated the ultra-fine nature and small crystallite size of the nanoparticles. Furthermore, the magnetic properties of the nano Fe3O4@APTES@isatin-SO3H were taken by vibrating sample magnetometric technique and showed magnetization saturation was found as 26.5 emu/g.

Figure 9.31: Synthesis of pyrano[2,3-d] pyrimidines derivatives by using nanoFe3O4@APTES@isatin-SO3H.

9.3.19 Synthesis of 2-amino-3-cyano-1,4,5,6-tetrahydropyrano [3,2-c]quinolin-5-ones and 5-oxo-dihydropyrano[3,2-c] chromenes Azarifar et al. [37] reported the synthesis of magnetic 1-naphthalenesulfonic acidgrafted graphene oxide (Fe3O4-GO-naphthalene-SO3H) via a three-step procedure. The structure of this nanographene oxide was fully studied by X-ray diffraction, energy-

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dispersive X-ray, vibrating sample magnetometer, scanning electron microscopy, FTIR, Raman spectroscopy, and TGA analytical techniques. The catalytic efficiency of these nanoparticles as explored in one-pot three-component reaction between aldehydes (19), malononitrile (2), and 4-hydroxycoumarin (36)/4-hydroxyquinolin-2(1H)one (59) in water for the synthesis of 5-oxo-dihydropyrano[3,2-c]chromenes (58) and 2-amino-3-cyano-1,4,5,6-tetrahydropyrano-[3,2-c]quinolin-5-ones (60) respectively (Figure 9.32). The advantages of this work on the previous report are high yields of the products in short reaction time, easy preparation of the catalyst, and the catalyst can be easily separated simply by using a magnet and reused for six fresh runs without significant loss of activity.

Figure 9.32: Synthesis of 2-amino-5-oxo-4-phenyl-4H,5H-dihydropyrano[3,2-c]chromene-3carbonitriles and 2-amino-3-cyano-1,4,5,6-tetrahydropyrano[3,2-c]quinolin-5-ones.

9.3.20 Synthesis of 1,4-dihydro-pyrano[2,3-c]pyrazoles/ tetrahydrobenzo[b]pyrans/4H-chromenes Azarifar et al. [38] developed the magnetic acidic catalyst in which titanomagnetite nanoparticles were functionalized with sulfonic acid groups from Fe3-xTixO4@SO3H nanoparticles. The structure of the catalyst was studied by taking infrared, energy-dispersive X-ray (EDX), and scanning electron microscopy analyses. The catalytic activity of these sulfonic acid–functionalized titanomagnetite Fe3-xTixO4@SiO3H-MNPs is examine for the synthesis of 4H-chromenes (61) by the reaction of dimedone (8), aromatic aldehyde (19), and malononitrile (2) in aqueous EtOH (1:1) under refluxing conditions (Figure 9.33). In continuation of this work, the given catalyst was also used in the synthesis of 1,4-dihydropyrano[2,3-c]pyrazoles (62) by the reaction of aldehyde (19), malononitrile (2), and 3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (50), under stirring at 105 °C (Figure 9.31). The progress of all the reactions was monitored by thinlayer chromatography (TLC). After completion of the reaction, the resulted reaction mixture was cooled to room temperature, diluted with hot ethanol (5 mL), and stirred for 10 min. Finally, the catalyst was recovered by using an external magnetic field.

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Figure 9.33: Synthesis of 4H-chromenes and pyrano[2,3-c]pyrazoles.

9.3.21 Synthesis of 2-thioxopyrido[2,3-d]pyrimidines derivatives Mamaghani et al. [39] was prepared a heterogeneous nanocatalyst γ-Fe2O3@HAp-SO3H and explore its catalytic properties for synthesis of 2-thioxopyrido[2,3-d]pyrimidines (64) from 3-(6-amino-1,2,3,4-tetrahydro-4-oxo-2-thioxopyrimidin-5-yl)-3-oxopropanenitrile (63) and arylaldehydes (19) in DMF at 120 °C (Figure 9.34).

Figure 9.34: Synthesis of thioxopyrido[2,3-d]pyrimidine-6-carbonitriles.

To explore more applications of this nanocatalyst, the catalyst γ-Fe2O3@HAp-SO3H were also used in the synthesis of 2-thioxo-2,3,7,10-tetrahydropyrido[2,3-d:5,6-d’]dipyrimidine-4,6(1H,5H)-diones (67) by the reaction between 6-amino-2-(alkylthio)-pyrimidine-4,6(1H,5H)-dione (65), 1,3-diethyl-2-thioxodihydro pyrimidine-4,6(1H,5H)-dione (66) and substituted arylaldehydes (19) in ethanol at reflux condition (Figure 9.35).

Figure 9.35: Synthesis of 2-thioxo-2,3,7,10-tetrahydropyrido[2,3-d:6,5-d’]dipyrimidine-4,6(1H,5H)diones.

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9.3.22 Synthesis of pyrano[2, 3-d] pyrimidinone derivatives A synthesis of greener catalyst Fe3O4@SiO2@-FSA has been reported by Heydari et al. [40] and used as an efficient catalyst for the synthesis of arylidene malononitriles/arylidene ethylcyanoacetate (68) by the reaction of aldehydes (19) and malononitrile/ethyl cyanoacetate (2) in the presence of water as green solvent at room temperature (Figure 9.36).

Figure 9.36: Knoevenagel reaction.

After the successful result obtained for the Knoevenagel reaction, the author also employed the catalyst for condensation reaction between aromatic aldehydes (19), malononitrile (2), and barbituric acid (33) to form the desired product pyrano[2,3 d] pyrimidines (69) in aqueous medium at 50 °C (Figure 9.37).

Figure 9.37: Synthesis of pyrano[2,3,d] pyrimidinone derivatives.

9.3.23 Synthesis of arylamine substituted chromeno[4, 3-b] pyrrol-4(1H)-ones The coumarin derivatives containing fused indole and pyrrole moieties are showed several types of biological activities [41, 42]. Pramanik and co-workers [43] have been developed a sustainable and solvent-free protocol for the synthesis of chromeno[4,3b]pyrrol-4(1H)-one (72) from aryl glyoxal (70), arylamines (34), and 4-aminocoumarine (71) catalyzed by magnetic nanocatalyst Fe3O4@SiO2-SO3H at 60 °C (Figure 9.38). Inspiring from the above results, the catalyst Fe3O4@SiO2-SO3H was further employed to synthesize methylene nitrile substituted chromeno[4,3-b]pyrrol-4(1H)-ones

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Figure 9.38: Fe3O4@SiO2-SO3H-catalyzed arylamines substituted chromeno[4,3-b]pyrrol-4(1H)-ones synthesis.

(73) from aryl glyoxal (70), malononitrile/ethyl cyanoacetate (2), and 4-aminocoumarine (71) with similar reaction conditions (Figure 9.39).

Figure 9.39: Fe3O4@SiO2-SO3H catalyzed synthesis of methylene nitrile substituted chromeno [4,3-b]pyrrol-4(1H)-ones.

Furthermore, chromeno[4,3-b]pyrrol-4(1H)-one derivative (73) undergoes intramolecular cyclization takes place under reflux condition without catalyst to form the newer product indolo[3,2-c]coumarin derivatives (74) (Figure 9.40).

9.3.24 Sonogashira and Heck cross-coupling reactions The Sonogashira coupling is a palladium-copper catalyzed reaction between aryl halides and terminal acetylenes. This coupling reaction is one of the most significant and widely used sp2-sp C–C bond formation reactions to form diaryl-substituted acetylenes (77) [44]. Firouzeh et al. [45] described a very straightforward method for Sonogashira coupling reaction (Figure 9.41) between the aryl halide (75) and terminal alkynes (76) in the presence of base triethylamine and magnetic-Fe3O4@SiO2–T/Pd nanoparticles catalyst in DMF at 130 °C to form the product with good yields. The author additionally reported the Fe3O4@SiO2–T/Pd catalyzed Heck coupling reaction (Figure 9.42) to synthesize the alkene (79) products under a similar condition at 120 °C via C–C bond formation takes place between aryl halide (75) and alkenes (78).

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Figure 9.40: Synthesis of indolo[3, 2-c]coumarin derivatives.

Figure 9.41: Fe3O4@SiO2–T/Pd catalyzed Sonogashira reaction.

Figure 9.42: Fe3O4@SiO2–T/Pd catalyzed Heck reaction.

9.3.25 Synthesis of piperazinyl-quinolinyl fused benzo[c]acridine derivatives Gengan et al. [46] developed a sulfonic acid functionalized boron nitride nanomaterials-based catalyst BN-Pr-SO3H and explore its applications for the synthesis of Knoevenagel- and Michael-type reactions to synthesize new piperazinyl-quinolinyl fused acridine (81) derivatives from 2-(4-methylpiperazin-1-yl)quinoline-3-carbaldehyde (80), dimedone (8), and aromatic amines (34) under microwave irradiation at 140 °C (Figure 9.43).

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Figure 9.43: Synthesis of 3, 3-dimethyl-9-(2-(4-methylpiperazin-1-yl)quinolin-3-yl)-3,4,9,10tetrahydroacridin-1(2H)-one derivative.

9.3.26 Synthesis of benzo[c]acridine-8(9H)-ones and 2-amino4H-chromenes Recently, in 2021, Behbahani and co-worker [47] synthesized sulfonic acid functionalized melamine based on magnetic nanocatalyst (γ-Fe2O3@Si-(CH2)3@melamine@butyl sulfonic acid) and studied its catalytic applications for the synthesis of benzo[c]acridin-8(7H)-ones (83) by the reactions between substituted aldehydes (19), dimedone (8), and 1-naphthylamine (82) in ethanol at 60 °C (Figure 9.44).

Figure 9.44: γ-Fe2O3@Si-(CH2)3@melamine@butyl sulfonic acid catalyzed synthesis of 7,10,11,12tetrahydrobenzo[c]acridine-8(9H)-ones.

Encouraged from the above results, the author also investigated the catalytic properties of nano γ-Fe2O3@Si-(CH2)3@melamine@butyl sulfonic acid for the synthesis of 2amino-4H-chromenes (84) in excellent yields (Figure 9.45) by the reaction of aryl aldehydes (19), malononitrile (2), and dimedone (8) under the same optimized conditions. The catalyst was synthesized by the treatment of magnetic FeCl2 and FeCl3 in the presence NH4OH to form Fe3O4 nanoparticles. The Fe3O4 nanoparticles were heated at 200 °C to form γ-Fe2O3 nanoparticles. These Fe2O3 nanoparticles were reacted with (3-chloropropyl)-trimethoxysilane in dry toluene for 48 h under refluxed conditions to form γ-Fe2O3@Si-(CH2)3Cl nanoparticles. Later this was treated with melamine in solvent DMSO and base K2CO3 for 24 h at 100 °C to get the γ-Fe2O3@Si-(CH2)3@melamine nanoparticles. These nanoparticles were sonicated for 30 min in dry THF, and

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Figure 9.45: Synthesis of 2-amino-4H-chromenes.

then 1,4-butane sultone was added to it and refluxed conditions to get the desired magnetically separable nanocatalyst γ-Fe2O3@Si-(CH2)3@melamine@butyl sulfonic acid (Figure 9.46).

γ -Fe2O3

γ-Fe2O3@Si-(CH2)3@mel@-butyl sulfonic acid

Figure 9.46: Synthesis of γ-Fe2O3@Si-(CH2)3@melamine@butyl sulfonic acid nanoparticle.

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9.3.27 Synthesis of 3,4-dihydropyrimidine-2-[1H]thione derivatives Foroughifar et al. [48] reported the synthesis of novel L-proline N-sulfonic acid-functionalized magnetic nanoparticles (MNP-L-proline-SO3H). Initially, Fe3O4@L-proline magnetic nanoparticles (MNP-L-proline) were obtained by the co-precipitation method, henceforth chlorosulfonic acid (ClSO3H) was covalently grafted on the surface of magnetic nanoparticles. This new nano-magnetic solid acid catalyst was characterized by using XRD, TGA, VSM, SEM, and EDAX analyses. The catalytic performance was cheeked in one-pot synthesis of 3,4-dihydropyrimidine-2-[1H]-thione derivatives (87) by the reaction of thiourea (85), β-ketoester (86), and diverse aromatic aldehydes (19) under the solvent-free condition at 100 °C (Figure 9.47). The main advantages of this newly developed protocol are high yields, short reaction times, easy workup of procedure, and reusability of the catalyst without any significant loss in its catalytic activity.

Figure 9.47: Synthesis of 3,4-dihydropyrimidine-2-(1H)-thiones.

9.3.28 Synthesis of dihydropyrimidinones Azarifar et al. [49] described a simple and straightforward procedure for the one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones and 3,4-dihydropyrimidin-2(1H)-thiones (89) using Fe3−xTixO4@SO3H MNPs as an efficient and recyclable heterogeneous magnetic nanocatalyst under solvent-free conditions at 100 °C. The synthesis of 3,4-dihydropyrimidin-2(1H)-ones (89) was accomplished by the cyclization reaction between aldehyde (19), urea (43), methyl or ethylacetylacetate (89) under the influence of the prepared catalyst at 100 °C. While synthesis of 3,4-dihydropyrimidin-2(1H)-thiones (90) was carried out by the reactions between aldehydes (19), thiourea (85), methyl or ethylacetylacetate (39) in the presence of the nanocatalyst at 100 °C. The catalyst was also used for the synthesis of a series of novel 3,4-dihydropyrimidin-2(1H)-ones (88) by the reaction of aromatic aldehyde (19), acetophenone (27), and urea (43) at 100 °C under solvent-free conditions (Figure 9.48). This sulfonic acid-functionalized titanomagnetite (Fe3−xTixO4@SO3H) nanoparticle was prepared by grafting sulfonic acid groups on Fe3−xTixO4@SO3H nanoparticles. These nanoparticles were characterized by

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various physicochemical techniques such as FTIR, SEM, TGA, energy-dispersive X-ray analysis, and vibrating sample magnetometer (VSM) analyses. This nanocatalyst could easily be separated from the reaction mixture by using an external magnet, recycled, and reused several times with no significant loss of catalytic activity.

Figure 9.48: Titanomagnetite@SO3H-catalyzed synthesis of 3,4-dihydropyrimidin-2(1H)-ones/ thiones.

9.3.29 Biginelli reaction Moghanian et al. [50] prepared and well-characterized stable and heterogeneous catalysts, i.e., bis(p-sulfoanilino)triazine functionalized magnetic silica nanoparticles (MNPs-BSAT). The dihydropyrimidinone (91) and octahydroquinazolinone derivatives (90) have gained immense importance in organic as well as in medicinal chemistry due to their significant biological efficacies. Moghanian et al. [50] explored the catalytic activity of MNPs-BSAT for the Biginelli reactions via the condensation of 1,3-dicarbonyl compounds like ethyl acetoacetate (39) or dimedone (8), urea (43), or thiourea (85), and a wide variety of aromatic or heteroaromatic aldehydes (19) under solvent-free conditions (Figure 9.49). The attractive feature of this newly prepared catalyst is the superparamagnetic properties that are confirmed by VSM analysis. Moreover, it was found that the catalyst is stable under the different reaction conditions and can be readily separated by using a magnet and reused without any significant loss of its catalytic activity even after sixth runs.

9.3.30 Synthesis of 3,4-dihydropyrimidin-2(1H)-one/thiones derivatives Zamani et al. [51] prepared Fe3O4 nanoparticle coated 3-mercaptopropanoic acid (Fe3O4/SMPA) through a simple in situ method The synthesized nanocatalyst was used for the synthesis of 3,4-dihydropyrimidin-2(1H)-one/thiones (92) derivatives via the Biginelli reactions of various aldehydes (19) with ethyl acetoacetate (39) and urea (43) or thiourea (85) (Figure 9.50). Aldehydes with electron-withdrawing or

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Figure 9.49: Synthesis of dihydropyrimidinone and octahydroquinazolinone derivatives via Biginelli reaction.

electron-donating substituents afforded the corresponding products in excellent yields. The main advantages of this protocol are easier catalyst preparation, a high level of reusability, and simple reaction workup.

Figure 9.50: Synthesis of 3,4-dihydropyrimidin-2(1H)-one/thiones.

9.3.31 Synthesis of pyrido[2,3-d]pyrimidine derivatives Zolfigol et al. [52] reported [Fe3O4@SiO2@(CH2)3S-SO3H] as a magnetically separable catalyst by mixing 3-(trimethoxysilyl)-1-propanethiol as dropwise with as-synthesized silica-coated magnetic nanoparticles. Then sonication and dropwise mixing of chlorosulfonic acid gives [Fe3O4@SiO2@(CH2)3S-SO3H] catalyst. Then, this catalyst was used in the synthesis of pyrido[2,3-d]pyrimidine (94) derivatives by using various aldehydes (19), malononitrile (2), and 2,4-diamino-6-hydroxypyrimidine (93) (Figure 9.51). In addition, precisely the same procedure was applied for the synthesis of 2amino-4,7-dioxo-5-phenyl-3,4,5,6,7,8-hexahydropyrido[2,3-d]pyrimidine-6-carbonitrile (96) derivatives with only one exception in which methyl or ethyl cyanoacetate (95) was used instead of malononitrile (Figure 9.52).

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Figure 9.51: Synthesis of pyrido[2,3-d]pyrimidine derivatives by using [Fe3O4@SiO2@(CH2)3S-SO3H] catalyst.

Figure 9.52: Synthesis of 2-amino-4,7-dioxo-5-phenyl-3,4,5,6,7,8-hexahydropyrido[2,3-d] pyrimidine-6-carbonitrile.

9.3.32 Synthesis of hexahydroquinolines Azarifar et al. [53] reported the preparation of N-(3-silyl propyl) diethylene triamine N, N’, N’’-tri-sulfonic acid (SPDETATSA), which was embedded on magnetic Fe3−x TixO4 nanoparticles. These nanoparticles exhibited high catalytic activity as novel magnetically recyclable acid nanocatalyst in the synthesis of a diverse range of hexahydroquinolines (97) through one-pot tandem reactions of dimedone (8), ammonium acetate (20), aldehyde (19), and malononitrile (2) and nanocatalyst Fe3xTixO4@SPDETATSA MNPs in ethanol at 75 °C (Figure 9.53)

Figure 9.53: Synthesis of hexahydroquinoline derivatives.

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In another work, Badalkhani et al. [54] also reported the preparation of sulfamic acidfunctionalized Fe3-xTixO4 magnetic nanoparticles (MNPs). This catalyst was prepared by the reaction of Fe3-xTixO4 MNPs with 3-chloropropyltrimethoxysilane, imidazolidine-2,4-dione and chlorosulfonic acid. The catalytic ability of this nanocatalyst was shown in one-pot four-component condensation reaction between aromatic aldehydes (19), dimedone (8), alkyl acetoacetates (39), and ammonium acetate (20) in order to form various hexahydroquinoline (98) derivatives (Figure 9.54).

Figure 9.54: Synthesis of a range of hexahydroquinolines.

9.3.33 Synthesis of aryl benzo[α]xanthenone derivatives Shariati et al. [55] reported a new class of solid acid catalysts as organic-inorganic MCM-41 mesoporous magnetite nanoparticles (MNPs@MCM-41) with large density of sulfonic acid groups by a co-condensation method. The surface area and textural properties of synthesized nanoparticles were improved through the coupling of inorganic and organic components by template synthesis on the magnetite nanoparticle surface. Initially, Fe3O4 was coated with alkylsulfonic acid groups by means of a onestep simple synthesis approach involving tetraethoxysilane (TEOS) and (3-mercaptoalkyl) trimethoxysilane in the presence of CTAB as a template and then oxidation of thiol groups to sulfonic acid (Figure 9.55). The synthesized nanocomposites was used as a catalyst for the one-pot three-component synthesis of aryl benzo[α]xanthenone derivatives (101) by the reaction of benzaldehyde (19), naphthol (100), and 1,3cyclohexadione (99) under solvent-free conditions at room temperature (Figure 9.56). A plausible mechanism of catalyst working in the reaction is shown in Figure 9.57.

9.3.34 Synthesis of 1,8-dioxodecahydroacridines Zolfigol group’s investigate [56] the catalytic activity of Fe3O4@TiO2@O2PO2(CH2) NHSO3H for the synthesis of 1,8-dioxodecahydroacridines (102) via the condensation reaction between dimedone (8), aromatic aldehydes (19), and ammonium acetate (20) under mild and solvent-free reaction conditions (Figure 9.58). The reaction progress was monitored by TLC using a solvent system n-hexane and ethyl acetate.

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Figure 9.55: Fe3O4@MCM-41 supported sulfonic acid.

Figure 9.56: Synthesis of benzo[α]xanthenone derivatives catalyzed by Fe3O4@MCM-41-SO3H.

In this work, after completion of reaction, the catalyst was separated by adding hot EtOH into the reaction mixtures to form resultant 1,8-dioxodecahydroacridines products. The undissolved Fe3O4@TiO2@O2PO2(CH2)NHSO3H nano-magnetic catalyst was easily separated from the reaction mixture by utilizing an external magnet.

9.3.35 Synthesis of 2H-indazolo-[2,1-b]phthalazine-1,6,11-trione derivatives Javanshir et al. [57] reported the SO3H-functionalized mesoporous silica nanoparticles (SO3H-FMSM) and used as nanocatalyst for the synthesis of 2H-indazolo-[2,1-b]phthalazine-1,6,11-trione (104) derivatives via one-pot three-component condensation reaction of 2,3-dihydrophthalazine-1,4-dione/phthalhydrazide (103), dimedone (8), and aromatic aldehydes (19) at 110 °C under solvent-free conditions (Figure 9.59). The main features of this method are the high yield of product in shorter reaction time and recyclability SO3H-FMSM nanopartciles for four runs without any significant decrease in catalytic activity.

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Figure 9.57: Possible mechanism for synthesis of benzo[α]xanthenone derivatives.

Figure 9.58: Synthesis of 1,8-dioxodecahydroacridines.

9.3.36 Synthesis of 2H-indazolo[2,1-b]phthalazine-triones Zhang et al. [58] developed an efficient method for the preparation of magnetic CoFe2O4 chitosan sulfonic acid nanoparticles (CoFe2O4-CS-SO3H). They applied for one-pot, fourcomponent synthesis of 2H-indazolo[2,1-b]phthalazine-triones (106) by condensation of

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Figure 9.59: Synthesis of 2H-indazolo[2,1-b]phthalazine-1,6,11-triones by SO3H-FMSM.

various aldehydes (19) phthalic anhydride (105), 1,3-cyclohexanedione (8), and hydrazinium hydroxide (48), under solvent-free conditions (Figure 9.60).

Figure 9.60: One-pot four-component synthesis of 2H-indazolo[2,1-b]phthalazine-triones.

In catalytic reactivity study of CoFe2O4@SC-SO3H, for the model reaction of phthalic anhydride, hydrazinium hydroxide, 5,5-dimethyl 1,3-cyclohexanedione, and 4-chlorobenzaldehyde. Zhang et al. examined different solvents like EtOH, Toluene, CH3CN, AcOEt, H2O, PEG 400, glycerol, [bmin]BF4, and [bmin]PF6 on the model reaction. Among all trials, the high yield was found only in case of ethanol. Furthermore, the catalyst quantity was also screened under optimized conditions and found that 0.5 mol% (10 mg) catalyst showed maximum conversation. When the quantity of catalyst used as 0.4 mol%, then the reaction would proceed incompletely, while when the higher quantity of catalyst (more than 0.5 mol%) showed no positive effect on the overall yield of the product. Therefore, based on the optimized results, the most suitable reaction conditions was CoFe2O4-SC-SO3H (0.5 mol%, 10 mg) as the catalyst under solvent-free conditions at 80 °C. The magnetic nanocatalyst can easily recover by applying an external magnet and recycled several times without significant loss of its catalytic activity.

9.3.37 Synthesis of diindolyloxindole derivatives Hassani et al. [59] reported the synthesis of diindolyloxindole derivatives (108) by the coupling reaction of indole (107) and isatins (4) in an aqueous medium (Figure 9.61)

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by using sulfonic acid supported on ferrite–silica superparamagnetic nanoparticles (Fe3O4@SiO2@SO3H). This nanocatalyst was prepared in three simple steps: preparation of colloidal iron oxide magnetic nanoparticles (Fe3O4 MNPs), coating of silica on Fe3O4 MNPs (Fe3O4@SiO2), and incorporation of sulfonic acid as a functional group on the surface of Fe3O4@SiO2 nanoparticles (Fe3O4@SiO2@SO3H).

Figure 9.61: Synthesis of diindolyloxindole derivatives.

9.3.38 Synthesis of 2,4,5-trisubstituted phenanthroimidazoles 2,4,5-Trisubstituted phenanthroimidazole (110) were synthesized by the reaction between 9,10-phenanthraquinone (109), aromatic aldehydes (19), and ammonium acetate (20) in the presence of nanocatalyst SBA-Pr-SO3H in acetic acid under the reflux condition (Figure 9.62) [60].

Figure 9.62: SBA-Pr-SO3H-catalyzed synthesis of 2,4,5-trisubstituted phenanthroimidazoles.

9.3.39 Synthesis of 1-substituted-1H-1,2,3,4-tetrazoles Akhlaghinia and co-worker [61] carried out the reaction of primary amines (15), triethyl orthoformate (111) and 1-butyl-3-methylimidazolium azide (112) in the presence of catalyst Fe3O4@WO3-EAE-SO3H to form product 1-substituted-1H-1,2,3,4-tetrazoles (113) at 60 °C in the aqueous phase (Figure 9.63).

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Figure 9.63: 1-substituted-1H-1,2,3,4-tetrazoles synthesis catalyzed by Fe3O4@WO3-EAE-SO3H.

9.3.40 Ritter reaction The Ritter reaction is acid-catalyzed reaction between substituted olefin or alcohol and nitriles to form amides [62]. Gawande et al. [63] prepared the magnetically retrievable nanocatalyst-Fe-OSO3H and applied it on Ritter reaction for the amide (116) synthesis from alcohol (114) and nitrile (115) under the solvent free condition at 90 °C (Figure 9.64) with good yields of products.

Figure 9.64: Synthesis of amide derivatives.

After successful preparation of amide derivatives, the nanocatalyst-Fe-OSO 3H was employed in one-pot multi-component reaction between 4-fluorobenzaldehyde (117), piperidine (118), and TMSCN (119) to form 2-(4-fluorophenyl)-2-(piperidin-1-yl) acetonitrile (120) at room temperature (Figure 9.65).

Figure 9.65: Preparation of 2-(4-fluorophenyl)-2-(piperidin-1-yl) acetonitrile.

The catalyst was also used for the preparation of 3,3-dimethyl-9-phenyl-3,4-dihydroacridin-1(2H)-one (122) by the reaction of 2-amino benzophenone (121) with 5,5-dimethyl-1,3-cyclohexandione (8) under microwave irradiation at 130 °C (Figure 9.66). The Nanocat-Fe-OSO3H was fabricated by reaction of Magnetite (Fe3O4) neat with chlorosulfonic acid. The reaction mixture was shaken for 30 min, then a brown solid of magnetic sulfonic acid (Nanocat-Fe-OSO3H) was formed (Figure 9.67).

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Figure 9.66: Synthesis of 3,3-dimethyl-9-phenyl-3,4-dihydroacridin-1(2H)-one.

Figure 9.67: Synthesis of nanocat-Fe-OSO3H.

9.3.41 Synthesis of alkyl levulinates from the esterification of levulinic acid In 2015, Guo et al. [64] reported the preparation of nanohybrids Si(Et)Si-Pr/ArSO3H and used as nanocatalyst for the synthesis of alkyl levulinates (methyl 4-oxopentanoate) (125) by the esterification of levulinic acid (123) with ethanol (124) (Figure 9.68) and ethanolysis of furfural alcohol (126) and ethanol (124) to obtained ethyl 4-oxopentanoate (125) (alkyl levulinates) (Figure 9.69).

Figure 9.68: Synthesis of alkyl levulinates.

Figure 9.69: Ethanolysis of furfural alcohol.

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9.3.42 Synthesis of 2-substituted benzimidazoles Benzimidazoles derivatives show various biological activities like antivirals and anti-HIV [65–67]. A simple, convenient, and one-pot protocol was developed for the efficient synthesis of 2-substituted benzimidazoles (127) from o-phenylenediamine (14), and aromatic aldehydes (19) in the presence of catalyst sulfonic acid functionalized silica and oxidant oxygen in DCM at room temperature (Figure 9.70) [68].

Figure 9.70: Sulfonic acid functionalized silica-catalyzed benzimidazoles synthesis.

9.3.43 Friedlander quinoline synthesis Quinoline derivatives are extensive famous for showing various pharmacological properties like LTD4 Receptor Antagonists [69], inflammatory diseases [70] and various significant role in tyrosine kinase (PDGF-RTK) activity [71]. Heydari et al. [72] reported an efficient catalyst SO3H-functionalized ionic liquid (TSIL) for the synthesis of a series of quinoline (130) derivatives from 2-aminoaryl ketones (128) and β-ketoesters/ketones (129) in an aqueous medium at 70 °C (Figure 9.71). After completion of reaction, the final product quinoline could be conveniently separated from catalyst by simple filtration. The separated catalyst could be easily recycled and reused five times without loss of much activity.

Figure 9.71: Synthesis of quinoline derivatives.

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9.3.44 Production of acetals Acetalization is one of the most significantly used acid-catalyzed reactions employed for the preparation of acetals from various carbonyl compounds. Acetals are helpful for many industrial purposes, such as solvents, perfumes, and intermediates [73]. Abbasi et al. [74] prepared an environmentally safe MIL-101(Cr)-SO3H catalysts and studied its application toward the synthesis of acetals (132) derivatives from aromatic aldehydes (19) and ethylene glycol (131) at 90 °C (Figure 9.72).

Figure 9.72: MIL-101(Cr)-SO3H catalyzed acetalization reaction.

The plausible mechanism for acetalization of benzaldehyde is shown in Figure 9.73. First, the protonation of the carbonyl group (19) takes place by the catalyst. Then, after the nucleophilic attack of O atom of the ethylene glycol takes place on intermediate (133) to get the compound 134. Later on, deprotonation of 134 was carried out to form the corresponding hemiacetal (135). Then the hemiacetal (135) was again protonated to form the compound 136. After removing the water molecule, from oxonium ion (137) was formed. The reaction of 137 with the second hydroxyl group of ethylene glycol to form 138 and undergoes subsequent deprotonation to give desire acetal (132).

Figure 9.73: Plausible mechanism for preparation of acetals.

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9.4 Conclusions Sulfonic acid-functionalized magnetic nanocatalyst holds a wide range of scope in organic synthesis as an efficient, ecofriendly, magnetically separable, and reusable catalyst. As a result, in the last decade, tremendous research and methods were published to offer a variety of heterocyclic scaffolds under various reaction conditions. In this direction, sulfonic acid-functionalized magnetic nanoparticles offer alternative support to silica-based materials due to simple preparation, non-energy-intensive method for recovery by external magnet and high rate of reusability. The silica coating provides an inert barrier to adverse interactions between the core of the metal oxide and surface functionalizations. This chapter summarizes the applications of sulfonic acid-functionalized magnetic nanocatalysts that are useful in synthesizing Oxygen and nitrogen-containing heterocycles like spirooxindoles, 2-amino-3-cyano pyridine, pyrano-pyrazole, pyrano-pyrimidines, hexahydroquinolines, xanthenone, acridines, and diindolyloxindole. This magnetic catalyst also plays a crucial role in the C–C bond formation reactions such as Sonogashira and Heck reactions. The catalyst has been applicable for Knoevenagel condensation, Michael addition, Hantzsch Pyridine synthesis, Friedlander synthesis, Biginelli reaction, Acetalization reactions, Ritter reaction, esterification reaction, and ethanolysis reactions. The sulfonic acid functionalized magnetic nanocatalyst exhibited comparable or better activities to other commercially available sulfonic acid catalysts.

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Manavi Yadav, Sriparna Dutta, Anju Srivastava, Rakesh K. Sharma*

Chapter 10 Silica-coated magnetic nanocatalysts as efficient green catalysts for organic synthesis 10.1 Introduction The year 2015 marks a renaissance in the history of global movements when the entire world seemed to come on a common consensus of working toward a sustainable and waste-free planet through the adoption of the 2030 Agenda of Sustainable Development Goals (SDGs) [1]. The framework comprised 17 universal aspirational goals outlined by the various member countries of the U.N. which would serve as the blueprint to protect the planet, improve health and education, put an end to poverty, spur economic growth, and ensure peace and prosperity all over. Underpinning the 17 goals were the 169 specific targets which the world leaders globally agreed to work upon and accomplish within a span of 15 years from the date of the launch of the SDGs. Just a year down the line and appreciably striking advancements have been accomplished in this regard. In particular, the chemical industries have contributed enormously toward making the SDGs a reality by powering the manufacture of almost 95% essential goods via energy generation [2]. There are many ways in which chemists are working to support global sustainable development considering the fact that chemistry has a broad reach into human health, economy, and environment. According to the American Chemistry Council, “the business of chemistry is worth 486 billion dollar enterprise which is about 26% of the global chemical product and the key of the nation’s economy” [3]. Today, we have an access to a broad spectrum of products that are safe, sustainable, and, most importantly, environmentally sound. Thankfully, green chemistry has helped in the furtherance of the SDGs by providing the necessary impetus required for sustainable manufacturing by allowing the efficient use of raw materials, elimination

*Corresponding author: Rakesh K. Sharma, Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007 Manavi Yadav, Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007 Sriparna Dutta, Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007; Hindu College, Department of Chemistry, University of Delhi, Delhi 110007 Anju Srivastava, Hindu College, Department of Chemistry, University of Delhi, Delhi 110007 https://doi.org/10.1515/9783110730357-010

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of waste, and the use of nontoxic materials. It is extremely encouraging to note that chemical entrepreneurs, researchers, and other relevant stakeholders have started realizing their responsibility toward providing society with safer goods. Catalysis “the pillar of green chemistry” has always remained at the heart of countless chemical protocols and often credited for shaping the modern world [4]. By drastically reducing the waste generation and ensuring greater selectivity, catalysis has paved the pathway toward sustainability. Currently, 90% of the chemical products are manufactured using catalysts [5]. Undeniably, all the accomplishments would not have been possible with the aid of solely noncatalytic processes. The success stories of drugs such as letermovir is a perfect example of state-of-the-art approaches to sustainable commercial manufacturing processes in the pharmaceutical industry that underwent rapid boost up in terms of yield by more than 60% and reduction in raw material cost by almost 93% by employing a better catalyst [6]. Many of the catalysts developed so far have shown promising efficacy in meeting some of the key SDGs including (i) zero hunger, (ii) good health and well-being, (iii) clean water and sanitation, (iv) affordable and clean energy, (v) industries, innovation, and infrastructure, (vi) responsible consumption and production, and (vii) climate action. Today, it is well accepted that catalysis is a key tool for green chemistry and green chemistry is the much needed key to combat the environmental challenges and move toward a sustainable tomorrow. Fundamentally, catalysts work by lowering the activation energy and providing an alternative energetically favorable pathway [7]. It is well known that the utilization of Pd-based catalysts for C–C and C–heteroatom bond formation had completely changed the landscape of organic synthesis, and the discoverers of these important coupling reactions had been conferred noble prizes. These were the homogeneous catalysts which showed superlative activity but were not easily recoverable from the reaction medium. The tedious separation as well as problems associated with metal contamination was considered as limiting factor for the large-scale industrial applicability of these catalysts. Thus, slowly efforts were directed toward developing and using heterogeneous catalysts (wherein the catalyst and reactant would be in different phases) which were readily recoverable. But challenges associated with their activity made it imperative to rethink and revisit the existing processes. Soon, the creative mind of the researchers led them to amalgamate the fascinating attributes of homogeneous and heterogeneous catalysts through the immobilization of the homogeneous metal complexes onto the surface of readily recoverable solid support matrices. This was addressed as the “heterogenization approach” [8]. A number of economically viable and environmentally benign catalysts have been designed so far using the principles and virtues of this approach. The frequently employed examples of support materials include graphene oxide, [9] carbon nanotubes, [10] silica, [11, 12] zeolites, [13] titania, [14] and zirconia [15]. However, with the advent of nanomaterials, a complete revolution in the field of green catalyst design occurred, and consequently there was a pragmatic shift in focus toward the use of nanomaterials

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(nanoparticles (NPs), nanosheets, and nanocubes) as preferred choice of solid support [16]. The reasons were obvious in view of the increased surface-area-to-volume ratio owing to nano-sized dimensions which resulted in enhanced activity and the possibility to tailor the properties of the nanomaterials as per applicability via effective surface engineering techniques. Apparently, nanotechnology provides the necessary tools for the design of an ideal, green catalyst that is inclusive of the 3 R’s (reduce, reuse, and recycle) – the goals for sustainable manufacturing [17–19]. In the most preferred list of nanosupports, Fe3O4 NPs which invariably form the most important class of magnetic NPs (MNPs) have emerged as the support of choice owing to their superlative superparamagnetism that assists in ready separation, economic viability, ease of synthesis using inexpensive precursors, outstanding stability to withstand diverse reaction conditions, low toxicity, and ease of modification to tune its catalytic applicability and tether it with diverse linkers [20]. The introduction of the magnetically retrievable nanocatalysts has opened new horizons for promoting greener catalysis. The ensuing section throws light upon bare MNPs – their role as catalytic materials and the need for further surface engineering, especially as far as their protection is concerned using inorganic silica.

10.2 Magnetically separable nanocatalysts Magnetically driven separation has entranced the field of catalysis by allowing the greener recovery of the catalytic species [21]. Here, the term “greener” implies (i) no catalyst loss which is often encountered in the traditional separation methods; (ii) rapid recovery using only an external magnet which is brought into the vicinity of the reaction vessel, (iii) energy efficiency, (iv) no requirement of additional solvents, and (v) no generation of any other organic residue [22].

10.2.1 Bare MNPs Until now, various types of MNPs are known which include pure metal-based (Fe, Co), alloys (CoPt3, FePt), and spinel-type ferromagnets (AB2O4 where A = Mg, Mn, Co, Fe and B = Fe), yet Fe3O4 NPs have spread their magical essence showing widespread utility for catalytic applications owing to a very high magnetization saturation value [23]. Fe3O4 invariably forms an important class of iron oxide NPs comprising both Fe2+ and Fe3+ ions and possess an inverse spinel crystal structure. Considerable attention has been devoted toward the synthesis of the MNPs which include physical, chemical, and biological methods [24, 25]. Table 10.1 summarizes the methods along with their merits and demerits. Yet, for synthesizing high-quality MNPs, chemical methods

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surpass other methods. Some of the classical commercial methods have been briefly discussed below: (a) Co-precipitation method: This is one of the most facile and convenient strategy to synthesize MNPs from aqueous ferrous/ferric salt solutions by adding base under inert conditions at room temperature or high temperature. In this method, the composition, size, and morphology depend upon the type of salts used, Fe2+/Fe3+ ratio, ionic strength of the media, pH of the solution, and reaction temperature. One of the best features of this method is that once all the parameters are fixed, the amount of resultant MNPs are completely reproducible; however, this technique results in polydisperse MNPs [26]. (b) Hydrothermal/solvothermal method: In this method, the MNPs are synthesized in reactors or autoclaves with temperature above 200 °C and pressure greater than 2,000 psi. The reaction is performed in aqueous media and there are two major approaches, namely hydrolysis and oxidation/neutralization of mixed metal hydroxides. The two routes are very similar, except that ferrous salts are used in the former. The factors affecting product in this method are solvent, time, and temperature. It was observed that the particle size of MNPs increases under high water content and prolonged reaction time. In this method, the particle size is controlled through rate of nucleation and grain growth that is further dependent on reaction temperature while keeping the other parameters constant. At higher temperatures, nucleation is faster than grain growth and results in reduced particle size, while prolonging reaction time aids grain growth. Yet, this method is usually not employed due to problems like reproducibility, difficulty in controlling size, higher reaction temperatures, and pressure requirements [27]. (c) Thermal decomposition: This technique results in small-sized monodisperse particles and involves the thermal decomposition of organometallic compounds in high-boiling organic solvents containing stabilizers. Parameters such as surfactant, solvent, reaction time, and temperature strongly influence the size and shape of the MNPs [28]. (d) Microemulsion: This method comprises oil–water microemulsions involving different salts, where a thermodynamically stable dispersion exists between the immiscible components. The microdomains of these liquids are stabilized by surfactants. When the two alike microemulsions having reactants are mixed together, the microdroplets continuously coalesce and break apart, leading to the formation of microreactors encompassing homogeneous mixtures of the two metal salts. On adding solvents, such as ethanol or acetone, to these microemulsions, the precipitate can be easily recovered by filtration or centrifugation of the mixture. This technique produces oblong- and spherical-shaped MNPs. However, the yield is poor and the method employs appreciable amounts of solvents. Also, the MNPs obtained are of varying size and shape and is thus not considered an effective method for synthesizing MNPs [29].

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Table 10.1: Synthetic techniques for MNPs with their advantages and disadvantages. Method Chemical

Physical

Advantages

Disadvantages

Co-precipitation

Simple and large-scale synthesis

Time consuming and consist impurities

[]

Hydrothermal

Good crystallinity

High temperature and pressure requirements

[]

Thermal decomposition

Good yield and controllable Employs toxic solvents size

[]

Microemulsion

Thermodynamically stable

Poor yield

[]

Chemical reduction

Safe and simple

Creates environmental pollution

[]

Sonochemical

High crystallinity, narrow size distribution

Poorly understood mechanism

[]

Sol–gel

Good crystallinity

Longer reaction time, employs toxic solvents

[]

Microwave

Rapid and safe

Homogeneous nucleation

[]

Ball milling

Simple and widely employed, results in fine powder

Possibility of product contamination

[, ]

Laser evaporation

Cost-effective process, nonpolluting

Energy intensive and expensive laser equipment

[, ]

Wire explosion method

Clean and safe

Possibility of product contamination

[]

Environmental friendly, clean, and efficient

Poor dispersion of NPs

[]

Biological Use of microorganisms

References

Amongst all the techniques, the co-precipitation is the simplified and preferred approach for synthesizing MNPs. However, thermal decomposition method is considered better in terms of morphology and size control. Till date, these two techniques are popular and best studied as they can be used for scaling up the synthesis of MNPs.

10.2.2 Silica: the ultimate choice as the coating/protecting agent Bare MNPs do not show promising capability as catalytic materials themselves owing to their tendency to undergo, agglomeration which is unavoidable due to the existence of van der Waals interaction, short interionic distance, and high surface energy [44, 45]. Moreover, the bare MNPs possess high chemical activity, and thus

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easily undergo oxidation in air, and this is more prominent when MNPs are made up of pure metals, alloys, or smaller size particles, resulting in loss of dispersibility and magnetism. Also, on exposure to other harsh chemical reagents, their original structure gets altered, causing a drastic effect on their magnetic properties. Besides, an exposure to biological environments has also shown a drastic effect on their properties; the MNPs start biodegrading under certain biological conditions, causing cytotoxicity. In order to render them suitable for catalytic transformations, the bare MNPs require protection which is accomplished with the aid of certain coating agents/stabilizers. For this, a straightforward method employs protection by an impenetrable layer that does not allow oxygen to reach the surface of MNPs. This can be executed by making use of suitable protecting agents including SiO2, ZrO2, TiO2, polyethylene glycol, polystyrene, and chitosan [46]. Consequently, a core-shell structure is obtained where the MNPs act as core and the outer shell is coated by protecting agents, thereby insulating the core from the surroundings. Usually, the protective strategies include either chemical anchoring or physical adsorption of agents on the outer surface of MNPs to form layers that creates sterical repulsion which further balances the forces acting on MNPs. In general, two approaches have been employed for the purpose of imparting stability to the MNPs: (i) Coating with organic polymers/surfactants (ii) Coating with inorganic shells which can be pure metal (Au, Ag) based or oxides (silica, titania) or inorganic carbon compounds (carbon nanotubes, graphene nanosheets) To date, various polymers having functional groups like phosphates and carboxylic acid have been explored for coating such as poly(aniline), poly(alkylcyanoacrylates), poly(pyrrole), polyesters, and poly(methylidene malonate) [47–51]. Amongst them, biocompatible polymers are extensively used for drug-targeting studies and as MRI contrast agents. However, these provide a thin layer coating which is not adequate for preventing oxidation of these highly reactive particles. In fact, the single or even double layers of these polymeric surfactants are unstable in air and readily leaches under acidic media, thereby resulting in the loss of magnetization of the MNPs. Moreover, the polymer-coated MNPs are unstable at higher temperatures. Thus, polymeric coating is considered insufficient for these highly reactive MNPs and urges the need for methods that can truly protect MNPs. In this regard, silica coating certainly tops the list of best coating agents due to the fascinating benefits it offers over the others (Figure 10.1) [52]. This includes screening of magnetic dipolar attraction between the particles, preventing agglomeration, favoring dispersion in reaction media, protecting against leaching under acidic conditions, and providing abundant silanol functionality for further modification with organic moieties. Table 10.2 summarizes several synthetic strategies adopted for coating MNPs with silica.

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Figure 10.1: Merits of silica coating. Table 10.2: Synthetic approaches for silica coating onto MNPs. Method

Reagents used

Reaction conditions

References

Stöber method Ethanol, TEOS, NHOH

TEOS hydrolysis in presence of base

[]

Microemulsion Ethanol, TEOS, oil phase

Base-catalysed hydrolysis followed by polymerization reaction of TEOS in microemulsion

[]

Aerosol pyrolysis

Methanol, Fe(NO).HO

Calcination at  °C

[]

In situ formation

TEOS, CTAB

In situ synthesis of MNPs in the pores of presynthesized silica

[]

Mixing of NPs with concentrated NHOH, sonication for ∼ min followed by addition of . M Igepal CO- in cyclohexane (Aldrich,  mL) and TEOS, sonication ∼ min and stirring

[]

Reverse TEOS, NHOH, . M microemulsion Igepal CO- in cyclohexane

Arc-discharge method

Fe and SiO powders (:), Copper as anode, tungsten as cathode evacuated chamber containing H: Ar (:) pressure

[]

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Table 10.2 (continued) Method

Reagents used

Reaction conditions

Silylation

Silylating agent Water:ethanol (:),  °C, stirring -aminoethylaminopropyltrimethoxysilane (AEAPS) for coating, citric acid

Forced hydrolysis

Water/ethanol mixture, Fe (III)acetylacetonate, sodium dodecylsulfate, TEOS,  °C

– °C, stirring, obtained NPs are thermally reduced under H atmosphere

References []

[]

10.2.3 Surface functionalization of SMNPs To enhance the properties of SMNPs, modification of active sites is performed via covalent grafting on the silanol moiety. Besides improving the desirable features of the synthesized SMNPs, this functionalization also helps in controlling the interaction between chemical groups and NPs, which is essential for its effective utilization in diverse fields. Use of strong acids and oxidizing agents for direct modification of MNPs is usually avoided as this would deteriorate the particles and their magnetic behavior. Thus, silica functionalization followed by further modification is the preferred route, and to perform this several organic reagents are used, of which p-aminophenyl trimethylsilane (APTS), 3-aminopropyltriethoxysilane (APTES), and mercaptopropyltriethoxysilane are considered as promising candidates as they further facilitate binding with metals, ligands, polymers, and biomolecules by providing amino and sulfhydryl terminals [17].

10.3 SMNPs supported The rich surface chemistry of silica-coated magnetic NPs (SMNPs) and the prospects of utilization of a diverse variety of organosilane precursors for generating the silica shell around the magnetic core have spread a radical revolution in the catalysis sector [52]. By allowing the possibility to control the interactions between the SMNPs and other chemical reagents (functionalizing agents/ligands/metals), desired requirements for profitable exploitation of these materials as solid support for the preparation of heterogenized catalysts can be met. To illustrate how the surface engineering through the tethering of various moieties including metals/bimetals, metal organic frameworks (MOFs), ionic liquids (ILs), and solid acids have helped in the fabrication of catalyst that lie within the premises of the 3 R’s, specific examples are projected that have been categorized as under:

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10.3.1 Metals Since metals are the active catalytic species, attempts have been directed toward the incorporation of metals/metal complexes onto the surface of the SMNPs either via dispersion (wet-impregnation/mixing approach) or using a covalent immobilization strategy for the fabrication of SMNP-supported catalysts. Dispersion using the wet-impregnation approach simply involves the dissolution of the active metal precursors (metal salts) in aqueous solution followed by addition of the metal solution to catalyst support with/without using a suitable reducing agent. This approach usually results in the physisorption of the active component (metal/bimetals) on the support. Varma et al. exemplified the dispersion one-pot synthetic technique in the fabrication of a magnetic silica-supported copper catalyst (Cu@SiO2@Fe3O4 NPs) which was subsequently applied in catalyzing the aqueous Ullman-type amination reaction under MW irradiation conditions (Figure 10.2) [61]. The synthesis of the catalyst was achieved by first stirring a solution containing Fe(II) and Fe(III) salts in water followed by addition of 25% ammonia solution and heating the contents in water bath at 50 °C. To this, under vigorous stirring conditions, tetraethyl orthosilicate (TEOS) was added and continually stirred for a period of 18 h after which the supernatant liquid was decanted and fresh water was added. To this fresh water containing solution, CuSO4 was added and once again the stirring was continued for another 24 h. The catalyst, as fabricated, could be used successfully for accelerating the amination of aryl halides in aqueous medium as a benign solvent under MW irradiation conditions with good yield.

Figure 10.2: Synthesis of Fe3O4@SiO2@Cu nanocatalyst and its applicability in catalyzing the aqueous Ullman-type amination reaction.

Moving a step further, a double core-shell structured magnetic mesoporous Fe3O4@SiO2@mSiO2 microspheres with immobilized palladium NPs (Fe3O4@SiO2@mSiO2– Pd(0) were subsequently utilized for the Suzuki coupling reaction by Li and coworkers (Figure 10.3) [62]. While the inner shell helped in imparting protection to the magnetic core, the outer shell provided high specific surface area for the loading of the Pd NPs.

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Further, it was striking to note that the Pd NPs were not only present on the outer surface but also within the mesopores which worked effectively toward increasing the product yield of 4-chloroacetophenone to 93.77% and also displayed good recyclability up to six times without any appreciable loss in its catalytic activity (Figure 10.4).

Figure 10.3: Preparation of Fe3O4@SiO2@mSiO2-Pd(0) catalyst. Adopted with permission from ref. [62].

Figure 10.4: Investigation of the reusability of immobilized Fe3O4@SiO2@mSiO2–Pd(0) catalyst for catalyzing Suzuki coupling reaction between iodobenzene and phenylboronic acid (a) and between chlorobenzene and phenylboronic (b), respectively. Adopted with permission from ref [62].

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In a quest to explore the morphology-guided catalytic activity, Kim et al. [63] also developed core-shell structured Fe3O4/SiO2/Ag nanocubes using a novel sonochemical approach and exploited the catalytic efficacy of these nanocubes in the reduction of 4-nitroaniline (Figure 10.5). While the Tem micrographs revealed that the Ag NPs of 10–20 nm size were decorated onto the surface the Fe3O4@SiO2 support, XRD pattern of the final nanocatalyst divulged the appearance of peaks of pristine Fe3O4 with an additional peak appearing at 2θ = 38° and 64.3° which could be indexed to fcc structure of Ag. Further the EDS mapping image indicated the spatial distribution of the elements Fe, O, Si, and Ag, which authenticated the formation of the catalyst (Figure 10.6). The most fascinating aspect of this catalytic protocol was that the silver catalyst could be reused for at least 15 times without any loss in its performance.

Figure 10.5: Schematic diagram illustrating the synthesis process of (a) the silver-decorated silicacoated iron oxide (Fe3O4/SiO2/Ag) nanocubes and (b) the silver-decorated silica (SiO2/Ag). Adopted with permission from ref [63].

However, it must be noted that from the viewpoint of industrial applicability, especially in light of sustainability, the simple attachment of metals via weak van der Waals forces is not desirable as they might undergo leaching from the support and result in metal contamination issues. The alternative and a preferred approach is the covalent immobilization of metal–ligand complexes onto the surface of a solid support material which is usually accomplished by reacting bidendate ligand with a functionalized support containing groups which can be tethered with the ligand, followed by metalation. This methodology results in the formation of robust catalysts with negligible or no leaching of the active catalytic species as also evidenced by various analyses (GC-MS

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Figure 10.6: EDS mapping analysis of Fe3O4/SiO2/Ag nanocubes. Adopted with permission from ref [63].

and AAS). Sharma et al. [64] have developed several competent recyclable catalysts comprising silica-coated magnetite nanosupport which were first functionalized using APTES and thereafter reacted with either N,N- or N,O-donor-containing ligands which were further metalated using different metal salts (Figure 10.7). The catalysts showed promising applicability in coupling, oxidation, reduction, and multicomponent reactions. As an illustrious example, a core-shell-structured SMNP based organic–inorganic hybrid copper nanocatalyst was designed and fabricated using the strategic principle of covalent grafting wherein 2,2ʹ bipyridine ketone ligand was

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immobilized on the surface of amine-functionalized SMNPs (ASMNPs). The copperbased magnetite nanocatalyst exhibited promising potential in boosting the tandem oxidative cyclization of amines, furnishing a broad spectrum of oxazole moieties.

Figure 10.7: Synthetic pathway for obtaining Cu-BPy@Am-SiO2@Fe3O4 core-shell nanocatalyst and its application in furnishing oxazoles via tandem oxidative cyclization between 1,3 dicarbonyls and benzylamines. Adopted with permission from ref [64].

10.3.2 Solid acid catalyst Since decades, solid acid catalysts (solids possessing acidic properties on virtue of acid functionalities present on their surfaces, serving as Lewis or Bronsted acid sites) have been playing a key role in catalyzing a number of industrially significant organic reactions including Friedel Crafts, esterification, and hydrolysis. The reasons are quite obvious in light of their ability to completely replace and rephase the existing acid catalyzed pathways that rely on the use of toxic and corrosive acids (e.g., H2SO4 and AlCl3) and are often associated with safety as well as waste issues.

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Moreover, the solid acid catalysts also exhibit ease of separation, greater selectivity and furnish products with higher purity and outstanding catalytic reusability. Thus, efforts are on way toward the development of newer solid acid nanocatalysts to promote environmentally friendly catalytic processes. Recently, SMNPs have gained prominence as an appropriate support material for the synchronized anchoring of sulfonic acids and heteropoly acid (HPA) groups[Keggin type H3PW12O40 (HPW) species], as well documented in the work by Hosseini and Farahani [65]. The catalyst was prepared using a covalent grafting strategy wherein the SMNPs were functionalized with 3-aminopropyl groups and further reacted with 1,4-butane sultone followed by the acidification with dilute sulfuric acid or phototungstic acid. The catalyst exhibited dense distribution of Fe and Si elements which formed the core components (Figure 10.8). As prepared, HPW–ampsul–SMNPs and H–ampsul–SMNP solid acid catalysts showed great efficacy in catalyzing the esterification of acetic acid with butanol and acetalization of benzaldehyde with ethylene glycol. Owing to the synergism of both the active sites, the immobilization of HPW on ampsul–SMNPs resulted in the improvement of its catalytic activity in the acetalization reaction up to 96% in 3 h.

Figure 10.8: Fabrication of the catalyst. Adopted from ref [65].

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The recyclability experiment conducted clearly divulged that the catalysts could be reused continually for five runs without any appreciable loss in activity. Another magnetically retrievable solid acid catalyst was prepared by Maleki et al. [66a] by immobilizing phototungstic acid (H3PMo12O40) on the surface of silica-coated mixed ferrite NiFe2O4 NPs (Figure 10.9). On account of fascinating features such as high saturation magnetization, excellent chemical stability, and notable permeability, NiFe2O4 was chosen as the magnetite core material. While, on the other hand, the prime reason for utilizing phosphomolybdic acid as the HPA was in view of it commercial viability, environmental benignness, high activity, and selectivity in the catalysis of acidmediated reactions. On supporting this HPA onto the surface-engineered encapsulated magnetite support, all the desirable traits of both the support material as well as the acid could be combined in a single platform. The catalyst could be successfully utilized for the one-pot synthesis of tri- and tetra-substituted imidazoles, and it emerged as a clear winner in the race of several catalytic protocols reported previously in light of the advantages offered such as excellent yield percentage, operational simplicity of the synthetic procedure, short reaction times, simple work-up procedure, and the use of greener and sustainable reaction conditions. Amongst the striking attributes, the most appealing feature of the protocol was the catalytic reusability up to 10 continual runs.

Figure 10.9: (a) Preparation of NiFe2O4@SiO2–H3PMo12O40 (NFS–PMA), (b) synthesis of 2,4,5trisubstituted imidazoles and 1,2,4,5-tetrasubstituted imidazoles using NFS–PMA catalyst.

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The demand for developing a simple, versatile, and greener strategy for the acid catalyzed synthesis of 1-amido- or 1-aminoalkyl-2-naphthols that form a core component of biologically significant drug molecules urged Moghanian and coworkers [66b] to fabricate sulfanilic acid–functionalized SMNPs (Figure 10.10). The Fe3O4 NPs were prepared using the co-precipitation technique while the silica encapsulation was achieved utilizing the prospects of the well-known Stober’s method. The core-shell NPs (Fe3O4@SiO2) were subsequently reacted with 3-chloropropyltriethoxysilane (CPTS), which were covalently bound to the free hydroxyl groups of the silica-decorated magnetite nanosupport. The CPTS-functionalized Fe3O4@SiO2 NPs were finally reacted with sulfanilic acid to form the sulfanilic acid–functionalized silica-coated magnetite NPs (MNPs–PhSO3H). Mechanistically, the catalyst participated in this reaction by activating the carbonyl group of the aldehyde moiety and exhibited good functional group tolerance in the concerned reaction by rendering good to excellent yields in cases of both electron withdrawing as well as donating groups present on the aldehyde moiety within short reaction times. Also, the MNPs–PhSO3H could be recovered readily with the aid of a magnet and subsequently reused continually for about six runs.

Figure 10.10: (a) Synthetic strategy for obtaining sulfanilic acid–functionalized silica-coated nano-Fe3O4 particles; (b) application of the catalyst in expediting the synthesis of 1-amido- and 1-aminoalkyl naphthols.

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10.3.3 Metal organic frameworks MOFs, comprising spatially assembled organic linkers and inorganic secondary building units, have emerged as an ultimate class of crystalline materials that are blessed with ultimate porosity and significantly high surface area (extending beyond 6,000 m2/g). Although MOFs have played a phenomenal role in advancing the frontiers of science in diverse multidisciplinary areas, catalysis is one of those distinct areas that has witnessed escalating interest of researchers owing to the prospects of designing complex hybrid catalytic materials for a plethora of organic reactions. The fascinating properties of MOFs that render them catalytically active include presence of coordinatively unsaturated metal centers, spatially separated active sites, functionalized ligands that facilitate in initiating reactions, and sites available for loading the metals within the pores for enabling synergistic catalysis. Additional benefits of tuning the structure of MOFs via post-synthetic modifications that permit the incorporation of additional catalytic functionalities render them proficient for challenging organic reactions. Despite offering a myriad of advantages, bare/pristine MOFs suffer from a few bottlenecks that impede their large-scale applicability. First, there are stability issues especially in case of realistic applications where materials might be subjected to acidic, basic, or moist conditions. Second, their separation remains challenging which makes it imperative to design readily separable and highly stable catalysts. Fortunately, the synergistic integration with magnetic components (mostly SiO2@Fe3O4 NPs and SiO2@CoFe2O4 NPs) has allowed the design of such kind of highly desirable robust catalytic systems [67]. The SMNPs also add to the catalytic competence as they themselves possess a large surface-area-to-volume ratio, economic viability, lesser toxicity, high stability (devoid of aggregation and oxidation issues as a result of the silica coat), and superparamagnetism exhibiting enormously large saturation magnetization values that enable quick and facile separation from the reaction vessel. As described earlier, the magnetic assisted separation offers an appealing alternative route to the traditional time-consuming filtration and centrifugation techniques. The silica-coated magnetic MOFs have displayed promising advantages in boosting a number of industrially significant organic transformations including C–H activation, oxidation, and reduction in terms of excellent turnover numbers (TONs), good product yield, high selectivity, and remarkable durability by showing the capability to withstand diverse reaction conditions. Cho et al. [68] developed Fe3O4@SiO2@MOF-199 composites wherein a copper hydroxide [Cu(OH)2] shell was grown on the surface of the Fe3O4@SiO2 NPs which later on transformed into a Cu-BTC framework on addition of H3BTC ligand. The synthetic strategy for the preparation of the magnetic MOF has been depicted in Figure 10.11. Using this catalyst, binuclear N-fused scaffolds 8,11-dimethoxybenzo [4,5]-imidazo-[2,1-a]isoquinolines and 6,9-dimethoxybenzo[4,5]-imidazo-[1,2-a]pyridines could be synthesized under microwave irradiation conditions. The beauty of

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this transformation lied in the ability of the products to undergo oxidation in the presence of ceric ammonium nitrate in acetonitrile/H2O or HBr/FeCl3 in H2O that provided a ready access to binuclear isoquinoline- and pyridine-fused benzimidazole-4,7-diones in good to high yield. Interestingly, the MOF composite offered a striking platform to obtain important bioreductive quinone-based drugs. (a)

(b)

(c)

Figure 10.11: (a) Synthetic route for obtaining Fe3O4@SiO2@MOF-199 catalyst, (b) Fe3O4@SiO2@MOF-199-catalyzed synthesis of binuclear isoquinoline- and pyridine-fused benzimidazoles, (c) further oxidation of substituted benzimidazoles to substituted benzimidazole4,7-diones.

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Exploiting the prospects of a covalent immobilization approach, Sharma et al. designed a magnetically responsive and thermally stable copper isophthalate-based MOF framework [CoFe2O4@SiO2@NH2@Cu(5-NIPA)] that displayed unparalleled catalytic performance in the synthesis of 2-substituted benzimidazoles obtained using the greener multicomponent C–H functionalization approach [69]. Highly dispersed and spherical CoFe2O4 NPs could be obtained using the solvothermal strategy which were encapsulated within a silica shell and functionalized using APTES and finally these functionalized NPs were anchored onto the 3-D copper isophthalate MOF using the covalent anchoring strategy (Figure 10.12). Figure 10.13 also illustrates the SEM and TEM micrographs of the desired stages of catalyst. The catalyst played a pivotal role in rendering 2-substituted benzimidazole moieties with remarkable yield, appreciable TON and good recyclability up to eight consecutive runs. Through a plausible mechanistic pathway, the authors proposed that the cooperative interaction between CoFe2O4@SiO2@NH2@Cu(5-NIPA) and reactants worked majestically in providing a ready access to 2-substituted benzimidazoles and the yield was reported to be higher

Figure 10.12: Synthetic pathway for CoFe2O4@SiO2@NH2@Cu(5-NIPA) catalyst (b) application of this catalyst in the multicomponent oxidative coupling reaction. Adopted with permission from ref [68].

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than the literature precedents. The approach replaced the use of toxic solvents with water which is demarcated as a greener solvent.

Figure 10.13: (a) TEM image of synthesized CoFe2O4 nanoparticles, (b) TEM image of CoFe2O4@SiO2, (c) SAED pattern of CoFe2O4, (d) SEM image of CoFe2O4, (e) SEM image of Cu(5-NIPA) MOF, and (f) SEM image of hybrid magnetic CoFe2O4@SiO2@NH2@Cu(5-NIPA). Adopted with permission from ref. [68].

With an idea to integrate the novel properties of Cu-BTC MOF framework with the virtues of magnetically responsive support material, a series of Cu-BTC@SiO2@Fe3O4 composites with variable mass percentage of copper was developed by Li and coworkers using an ultrasonic assisted methodology [70]. The TEM and SEM images of the Fe3O4, SiO2@Fe3O4, Cu-BTC, and MCC-4 catalysts provided valuable insights into the morphology of the materials, demonstrating a viable change from regular octahedron shape of Cu-BTC to spherical shaped Cu-BTC@SiO2@Fe3O4 (Figure 10.14). The catalytic applicability of the MOF encapsulated SiO2@Fe3O4 NPs was subsequently investigated in the Pechmann condensation reaction between 1-naphthol and ethyl acetoacetate that led to the synthesis of coumarin, an oxygen heterocycle, blessed with potent bioactivity (Figure 10.15). In the series of the developed MOFs, the 50.8%

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Figure 10.14: SEM and TEM images for Fe3O4, SiO2@Fe3O4, Cu-BTC, and MCC-4 catalysts. (A) TEM of Fe3O4; (B) TEM of SiO2@Fe3O4; (C) SEM of Cu-BTC; (D) TEM of Cu-BTC; (E), (F), (G), and (H) SEM of MCC-4; (I), (J), (K), and (L) TEM of MCC-4. (C), (D), (H), and (L): 120 min; (E) and (I): 30 min; (F) and (J): 60 min; (G) and (K): 90 min. Adopted with permission from ref [69].

Figure 10.15: Cu-BTC@SiO2@Fe3O4 catalyzed Pechmann condensation reaction.

Cu-BTC@SiO2@Fe3O4 rendered the desired product with remarkable conversion (96%) and selectivity (98%). The protocol displayed ease of catalyst separation and notable reusability up to five consecutive cycles, maintaining consistency in catalytic efficiency. An interest in the synthesis of α-amino nitriles using the multicomponent Strecker’s approach wherein aldehydes (or ketones), amines, and trimethylsilyl cyanide (TMSCN) are used as substates led Mostafavi et al. to design a porous magnetic Fe3O4/ MIL-101(Fe) material comprising MIL‐101(Fe) MOF framework integrated with amino‐ modified Fe3O4@SiO2 NPs (Figure 10.16). The catalyst was prepared through an in situ solvothermal methodology wherein the MOF grew over the surface-engineered MNPs. The authors efficiently showed that the various catalytic moieties unified within a

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Figure 10.16: (a) Fabrication of Fe3O4/MIL-101(Fe) composite. (b) Fe3O4/MIL-101(Fe) mediated strecker reaction between aldehydes, amines, and TMSCN. (c) Fe3O4/MIL-101(Fe) mediated strecker reaction between ketones, amines, and TMSCN. Adopted with permission from ref [67].

single architecture worked syngergistically in intensifying the catalytic function, as unveiled by excellent results indicating the accomplishment of a broad range of targeted products under mild reaction conditions. The reason behind the superlative catalytic performance could be attributed to large surface area, accurate pore size, and readily accessible highly active Lewis acidic sites.

10.3.4 Ionic liquid Ionic liquids (ILs) belongs to a class of organic compounds that can be used as both solvents and catalysts due to their outstanding features such as excellent solubility, high conductivity, easy tunability, good thermal stability, low vapor pressure, and reusability [71, 72]. Even though they possess many advantages, several problems are associated with them like handling difficulties due to air and moisture sensitivity, high viscosity, expensive nature, and troublesome recovery, which limits their practical utility. Thus, to exploit ILs in a manner that is both economically and environmentally

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safe, immobilization of ILs on SMNPs offers one of the best solutions. In this regard, several catalysts have been synthesized, some of which will be discussed in this section [73]. Recently, Zheng et al. reported a CO2 cycloaddition reaction using SMNP-supported IL catalyst [74]. In this work, first a series of alkylimidazolium chloride precursors were synthesized by quartenization of N-alkylimidazole. Then, SMNPs were reacted with these to produce immobilized ILs (Figure 10.17). The as-synthesized catalyst was used to carry out the cycloaddition reaction under low CO2 pressure at 140 °C and good yields were observed in a few hours. Several other MNP-based ILs were tested for this reaction but due to the long alkyl side chain of imidazolium ring, the yield for cyclic carbonates was low. Also, the nanocatalyst was easily recycled and reused up to 11 runs without loss in its performance. On a similar note, another imidazolium functionalized IL having [TiCl5]– was grafted on SMNPs and exploited in cycloaddition of CO2 and epoxides [75]. The reactions were performed in an autoclave with low CO2 pressure at 100 °C in the presence of small catalytic amount and the products were obtained in high yield. Besides, the catalyst was reported to be reusable up to five consecutive cycles.

Figure 10.17: Formation of cyclic carbonates with MNP-ILs.

Another significant area for researchers has been the usage of waste cooking oil for biodiesel production as it not only less expensive than fresh oils but also employing waste as a resource. But, these waste oils contain considerable amount of free fatty acids and water which hamper catalyst activity. These cases therefore require an acid that can catalyze both esterification and transesterification reactions so that free fatty acids can be converted to produce biodiesel. In this respect, Liang developed a solid acid catalyst by copolymerizing acidic 4-vinylpyridinium IL with double-bondmodified SMNPs and utilized this for generating biodiesel from waste cooking oil (Figure 10.18) [76]. The reaction employed low catalyst amount and less reaction time and displayed excellent activity with 99% yield. Also, the magnetic nature of the catalyst assisted in easy recovery from the media and prevented catalyst loss that might have occurred because of dissolution of IL in polar layer of glycerol.

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Figure 10.18: Synthesizing acidic IL supported on SMNP for biodiesel production.

A tungstate-based IL was fabricated by encapsulating MNPs in multi-layered crosslinking IL polymer consisting of tungstate anions (PILW-SMNP). Figure 10.19 depicts the synthesis of the PILW-SMNP catalyst [77]. The catalytic efficacy was explored for the oxidation of benzyl alcohols, sulfides, and alkenes into benzaldehydes, sulfones and epoxides, respectively, using H2O2 as an oxidant. It was reported that the catalyst afforded the products in excellent yields under milder conditions and shorter times. Moreover, the catalyst was reused up to 10 cycles without significant loss in its performance. In addition, variety of tungstate-based SMNPs supported ILs catalysts for promoting oxidation of alcohols, and sulfides have been developed that displayed promising results under simple reaction conditions. In recent times, Li et al. [78] fabricated a series of magnetically retreivable aza crown ether complex cation ILs. Figure 10.20 depicts the immobilzation of ILs on SMNPs via the formation of C–N bond between carbon atom of the carrier and Natom of the aza-crown ether. The synthesized catalyst was then deployed for synthesizing bisindolyl-methanes and Hantzsch reaction. Good yields were reported for the desired products under mild conditions. In comparison to other acid-based catalysts, this catalyst exhibited good activity and also possessed advantages of effortless magnetic recovery. Also, the catalyst was recyclable and displayed good activity for up to five consecutive runs. Figure 10.21 depicts TEM images and elemental mapping images of the catalyst where X = HSO4.

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(a)

(b)

Figure 10.19: (a) Synthesis of PILW-SMNP catalyst, (b) applications of PILW-SMNP catalyst in the oxidation of benzyl alcohols, sulfides, and alkenes into benzaldehydes, sulfones and epoxides. Adopted with permission from ref [77].

10.4 Conclusion and future outlook The modern era has witnessed an exponential upsurge in the interest pertaining to the fabrication of silica-encapsulated magnetite nanocatalysts that have exhibited promising efficacy in boosting key organic transformations. From the accomplishments so far in this field, it is certain that nanotechnology has been able to provide new wings to the imagination of the researchers focusing on the design and development of green and sustainable catalysts. By being able to amalgamate

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Figure 10.20: Magnetically retrievable aza-crown ether complex cation ILs supported by SMNPs for synthesizing bisindolylmethanes and Hantzsch reaction.

Figure 10.21: (a and b) TEM micrographs and (C-H) elemental mapping images of catalyst with X = HSO4. Adopted with permission from ref [78].

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the desirable traits of high activity, selectivity, and recyclability, the core-shell magnetically responsive catalysts documented in literature have succumbed to qualify as green nanocatalysts. In this chapter, we have given due consideration to a diverse variety of SMNP-supported catalysts including SMNP-supported metals/bimetals, ILs, MOFs, COFs, and solid acids in order to enable the readers to understand their potentiality as efficient green catalysts. Special emphasis has been directed toward illustrating the strategies adopted for the synthesis and modification (coating and functionalization) of the MNPs that assist in the designing of potent nanocatalysts well suited for diverse challenging organic reactions such as coupling, oxidation, cyclization, reduction, and many more. Considering the fact that the SDGs have very few years left to be met, intensive efforts will be required toward designing more competent catalysts that can meet the global environmental challenges substantially. It is here where the silica-encapsulated magnetite nanocatalysts can make a real difference and contribute enormously in this regard by allowing their profitable use in real-time applications including eradication of toxic organic contaminants and CO2 capture. However, understanding the dynamics and mechanistic aspects will be highly essential for tailoring these nanostructured catalysts for enhanced results as they might undergo changes in structure and electronic states in the due course of the reaction. Nonetheless, undeniably, these encapsulated magnetically retrievable have offered a promising platform to the future chemists for developing new generation catalytic systems.

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Manmeet Kaur, Anu Priya, Arvind Singh, Aditi Sharma, Gurpreet Kaur, Bubun Banerjee*

Chapter 11 Multicomponent synthesis of biologically promising pyrans and pyran annulated heterocycles using magnetically recoverable nanocatalysts 11.1 Introduction Pyrans and pyran annulated heterocycles are very common among the naturally occurring bioactive compounds. Figure 11.1 represents a glimpse of naturally occurring bioactive compounds bearing pyran or pyran annulated skeleton [1–7]. Various commercially available drugs such as zanamivir, laninamivir, epicalyxin G, epicalyxin F, and calyxin F are also made of pyran or pyran annulated heterocyclic skeletons [2]. Recently, a number of synthetic pyran derivatives were also found to possess significant biological efficacies that include antibacterial, antituberculosis, anticancer, antitubulin, antirheumatic, and antimicrobial activities (Figure 11.2) [8–15]. After realizing the immense importance of pyrans and pyran annulated scaffolds, a huge number of protocols were reported under various reaction conditions. Majority of these reported methods are following one-pot multicomponent strategies. Multicomponent reaction strategies are regarded as one of the valuable tools for synthesizing structurally diverse organic compounds [16–20]. Scientists are constantly trying to design less toxic and atom-efficient reaction pathways. This is now well established that one-pot multicomponent reaction strategies possess so many advantages which include atom as well as energy efficiency, operational simplicity, reduction in the number of workup steps, and easy workup procedure [21–26]. Therefore, it is not required to mention that by implementing multicomponent reaction strategy, one can achieve some of the goals of “green and sustainable chemistry.”

Acknowledgments: The authors are thankful to Prof. Gurmail Singh, vice-chancellor, Akal University, for his wholehearted encouragement and support. Dr. Banerjee is grateful to Akal University and Kalgidhar Trust (Baru Sahib, Himachal Pradesh, India) for providing the laboratory facilities. *Corresponding author: Bubun Banerjee, Department of Chemistry, Akal University, Talwandi Sabo, Bathinda, Punjab 151302, India, e-mail: [email protected] Manmeet Kaur, Anu Priya, Arvind Singh, Aditi Sharma, Gurpreet Kaur, Department of Chemistry, Akal University, Talwandi Sabo, Bathinda, Punjab 151302, India https://doi.org/10.1515/9783110730357-011

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On the other hand, during organic synthesis, the catalyst plays an important role [27–32]. Scientists are always trying to find out more efficient catalytic systems to enhance the reaction rates. It is well established that the heterogeneous catalysts are much more advantageous than the homogeneous catalysts [33]. Reusability of the heterogeneous catalysts makes protocols environmentally benign as well as cost-effective. The reusability of the catalyst become easier if the material itself is magnetic in nature. In that case, after completing the reaction, the magnetic catalyst can be recovered quantitatively by using a simple external magnet. And after processing, the same materials can be recycled for several runs with almost equal efficiency. Moreover, nano-sized catalysts are generally found to be more effective due to the large active surface area than the bulk form of the same material [34]. Transition metal–based nanomaterials are highly useful as they can mimic metal surface activation as well as provide nano-sized support systems [35, 36]. Under this background, magnetically recoverable heterogeneous nanocatalysts are becoming highly attractive and find many applications for diverse organic transformations [37–40]. In this chapter, we have discussed some recently reported magnetic nanocatalyzed one-pot multicomponent synthesis of biologically promising pyrans and pyran annulated heterocycles under greener conditions.

11.2 Synthesis of fully functionalized 4H-pyrans Very recently, in 2020, Kargar et al. [41] prepared cobalt-centered cellulose nanofiber-functionalized magnetic nanoparticles [Fe3O4@NFC@Co(II)]. The synthesized nanoparticle was well characterized by using infrared spectroscopy (IR), vibrating sample magnetometer (VSM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDAX), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), and thermal gravimetric analysis (TGA) techniques. By using a catalytic amount of this synthesized nanomaterial, they have synthesized a series of fully functionalized 4H-pyran derivatives (4) via one-pot three-component reactions of aromatic aldehydes (1), malononitrile (2), and ethyl acetoacetate (3) in water at 55 °C (Figure 11.3). All the reactions were completed just within 20 min and afforded the desired products in excellent yields. After completion of the reaction, the used catalyst was recovered quantitatively and recycled for four successive runs without any significant loss in product yields. Chitosan-based nanocopper ferrite was also found as an efficient magnetically recoverable heterogeneous catalyst for the synthesis of various 4H-pyran derivatives (4) in ethanol at room temperature (Figure 11.3) [42]. Desired products were formed in excellent yields though the time required by using this catalyst was little bit longer (90 min). Starting from various aromatic aldehydes (1),

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Figure 11.1: Pyran annulated naturally occurring bioactive compounds.

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Figure 11.2: Pyran annulated synthetic compounds having significant biologically efficacies.

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malononitrile (2), and ethyl acetoacetate (3), another series of fully functionalized 4H-pyran derivatives (4) were also synthesized by using a catalytic amount of silicacoated magnetic NiFe2O4 nanoparticle-supported H14[NaP5W30O110] in ethanol under ultrasound-assisted conditions at room temperature (Figure 11.3) [43].

Figure 11.3: Magnetic nanocatalyzed three-component synthesis of fully functionalized 4H-pyrans.

11.3 Synthesis of pyran annulated heterocycles 11.3.1 Synthesis of 2-amino-3-cyano-tetrahydro-4H-chromenes In 2016, Maleki and Azadegan [44] prepared silica-supported iron oxide (Fe3O4@SiO2) as efficient heterogeneous magnetic nanoparticles which were well characterized by Fourier transform infrared spectroscopy (FT-IR), XRD, SEM, EDX, and TEM analyses. Using a catalytic amount of the same nanoparticles, they carried out a one-pot threecomponent condensation reaction between various aromatic aldehydes (1), malononitrile (2) and dimedone (5) or 1,3-cyclohexanedione (5a), which afforded the corresponding 2-amino-3-cyano-tetrahydro-4H-chromenes (6) in excellent yields at room temperature in ethanol (Figure 11.4). All the reactions were completed within 135 min. After completion of the reaction, the magnetic nanocatalyst was recovered quantitatively by using a bar magnet and reused further for five successive runs without any significant loss in its catalytic activities. Next year, the same group also synthesized another reusable novel diamine-functionalized silica-supported magnetic nanocatalyst

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viz. Fe3O4@SiO2@propyltriethoxy-silane@o-phenylendiamine. This newly prepared catalyst was found to be more efficient for the same batch of the reaction, and thus synthesis of 2-amino-3-cyano-tetrahydro-4H-chromenes (6) was achieved within just 15 min (Figure 11.4) [45]. Very recently, in 2020, the same group also carried out the same batch of reactions using a low-cost robust biopolymer xanthum gum–supported Fe3O4 as an efficient magnetically separable nanocatalyst which produced the desired 2-amino-3-cyano-tetrahydro-4H-chromenes (6) in excellent yields (Figure 11.4) [46]. For the same batch of reactions, Maleki et al. [42] employed another magnetic nanocatalyst, that is, chitosan-based magnetic nanocopper ferrite and synthesized a series of 2-amino-3-cyano-tetrahydro-4H-chromenes (6) in excellent yields in ethanol at room temperature (Figure 11.4).

Figure 11.4: Magnetic nanocatalyzed three-component synthesis of 2-amino-3-cyano-tetrahydro4H-chromenes.

Javidi et al. [47] synthesized a series of 2-amino-3-cyano-tetrahydro-4H-chromenes (6) in excellent yields via one-pot three-component reactions of aldehydes (1), malononitrile (2), and dimedone (5) under ultrasound-assisted conditions by using silica-supported iron oxide–immobilized phosphomolybdic acid [Fe3O4@SiO2–imid– PMAn] as an efficient magnetic nanocatalyst in water (Figure 11.5). All the reactions were completed within just 14 min. Using the catalyst, the reactions required longer times under conventional refluxed conditions (up to 50 min). After completion of the reactions, the catalyst was recovered by using an external bar magnet and was recycled for eight successive runs without any significant loss in its catalytic efficiency. Mohammadi et al. [48] employed silica-supported iron-based guanidine-

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functionalized polyacrylic acid nanoparticles as an heterogeneous magnetically recoverable catalyst for the synthesis of 2-amino-3-cyano-tetrahydro-4H-chromenes (6) from the reactions of substituted benzaldehydes (1), malononitrile (2), and dimedone (5) in aqueous medium at 70 °C (Figure 11.6). The catalyst was recovered simply by using a bar magnet and reused six times without any significant decrease in the product yields. Iron-based inorganic–organic hybrid magnetic nanocatalyst was used by Khoobi et al. [49] for the efficient synthesis of 2-amino-3-cyano-tetrahydro4H-chromenes (6) in water under refluxed conditions (Figure 11.7). After completion of the reaction, the catalyst was recovered easily simply by employing a bar magnet and reused for 10 successive runs with almost equal efficiency.

Figure 11.5: Ultrasound-assisted magnetic nanocatalyzed synthesis of 2-amino-3-cyano-tetrahydro4H-chromenes.

Figure 11.6: Magnetic nanocatalyzed aqueous mediated synthesis of 2-amino-3-cyano-tetrahydro4H-chromenes at 70 °C.

Figure 11.7: Magnetic nanocatalyzed aqueous mediated synthesis of 2-amino-3-cyano-tetrahydro4H-chromenes under refluxed conditions.

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11.3.2 Synthesis of 2-amino-3-cyano-dihydropyrano[3,2-c] chromenes Mohammadi and Sheiban [48] synthesized a series of 2-amino-3-cyano-dihydropyrano[3,2-c]chromenes (8) via one-pot three-component reactions of aromatic aldehydes (1), malononitrile (2), and 4-hydroxycoumarin (7) in the presence of a catalytic amount of silica-supported guanidine-polyacrylic acid functionalized iron oxide nanoparticles (Fe3O4@SiO2–guanidine–PPA) in water at 70 °C (Figure 11.8). The catalyst was recovered easily and recycled further without any decrease in product yields. Iron-based inorganic–organic hybrid magnetic nanomaterial was also found as an efficient nanocatalyst for the synthesis of 2-amino-3-cyano-dihydropyrano[3,2c]chromenes (8) in excellent yields starting from the same batch of reactions in water under refluxed conditions (Figure 11.8) [49]. All the reactions were completed within just 10 min. Under this catalytic system, along with various substituted benzaldehydes, heteroaryl aldehydes also produced the desired products with equal efficiency. Very recently, in 2020, Lati et al. [50] employed silica-coated sultone-functionalized iron oxide nanoparticles (Fe3O4@SiO2–Sultone) as an efficient nanocatalyst to carry out aqueous-mediated one-pot three-component reactions of various aromatic aldehydes (1), malononitrile (2), and 4-hydroxycoumarin (7) under refluxed conditions which afforded the corresponding 2-amino-3-cyano-dihydropyrano[3,2-c]chromenes (8) in excellent yields (Figure 11.8). Silica-supported iron oxide immobilized phosphomolybdic acid [Fe3O4@SiO2–imid–PMAn] nanocatalyst also efficiently carried out the same batch of reactions both under refluxed as well as ultrasound-assisted conditions in aqueous medium (Figure 11.8) [47]. By using this magnetically recoverable nanocatalyst, a total of 22 different 2-amino-3-cyano-dihydropyrano[3,2-c]chromene derivatives (8) were synthesized. Under ultrasound-assisted conditions, all the reactions were completed within just 12 min although under refluxed conditions the same reactions took little bit longer times (40 min). Aldehydes with electron-donating as well as electron-withdrawing substituent underwent smoothly and afforded the desired products in excellent yields. All the products were isolated pure just by simple filtration. No column chromatographic purification procedure was required. After completion of the reactions, the catalyst was recovered easily by using a bar magnet and recycled for eight further runs.

11.3.3 Synthesis of pyrano[2,3-d]pyrimidine derivatives In 2018, Sajjadifar et al. [51] prepared 1-methyl imidazole-based ionic liquid-stabilized silica-coated Fe3O4 nanoparticles. The prepared nanocatalyst was well characterized by using various techniques such as FT-IR, XRD, SEM, differential scanning calorimetry-thermogravimetry analysis (DSC-TGA), and VSM. Using a catalytic amount of this

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Figure 11.8: Magnetic nanocatalyzed three-component synthesis of 2-amino-3-cyanodihydropyrano[3,2-c]chromenes.

magnetic nanocatalyst, they synthesized a series of pyrano[2,3-d]pyrimidine derivatives (10) via one-pot three-component reactions of substituted benzaldehydes (1), malononitrile (2), and barbituric acid (9) or thiobarbituric acid (9a) in ethanol under refluxed conditions (Figure 11.9). Magnetic nanocatalyst was recovered and recycled for five successive runs with equal efficiency. Next year, Maleki et al. [52] used copperated polyvinyl alcohol–functionalized iron oxide nanoparticles (PVA@Fe3O4-Cu) as an efficient magnetic nanocatalyst for the synthesis of another series of pyrano [2,3-d]pyrimidine derivatives (10) in water at room temperature (Figure 11.9). Aldehydes with electron-donating as well as electron-withdrawing substituents underwent smoothly and produced the desired products in excellent yields. All the products were isolated purely just by simple filtration. Khazaei et al. [53] carried out one-pot three-component reactions of aromatic aldehydes (1), malononitrile (2),

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and N,N-dimethylbarbituric acid (11) in the presence of a catalytic amount of nanoZnFe2O4, which afforded a series of pyrano[2,3-d]pyrimidine derivatives in excellent yields under solvent-free conditions at 75 °C (Figure 11.10). All the reactions were completed within 30 min.

Figure 11.9: Magnetic nanocatalyzed three-component synthesis of pyrano[2,3-d]pyrimidine derivatives.

Figure 11.10: Nano-ZnFe2O4 catalyzed synthesis of pyrano[2,3-d]pyrimidines under neat conditions.

11.3.4 Synthesis of dihydropyrano[2,3-c]pyrazoles Maleki et al. [43] synthesized a series of dihydropyrano[2,3-c]pyrazoles (15,16) via three-component reactions of aromatic aldehydes (1), malononitrile (2), and 3methyl-1-phenyl-1H-pyrazol-5(4H)-one (13) or 3-methyl-1H-pyrazol-5(4H)-one (14) in the presence of a catalytic amount of silica-coated magnetic NiFe2O4 nanoparticlessupported H14[NaP5W30O110] in ethanol under ultrasound-assisted conditions at room temperature (Figure 11.11). The developed protocol was so efficient that all the reactions were accomplished just within 10 min. Four-component synthesis of 6amino-3-methyl-4-aryl-2,4-dihydropyrano[2,3-c]pyrazole-5-carbonitriles (16) was

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achieved from the reactions of aromatic aldehydes (1), malononitrile (2), ethyl acetoacetate (3), and hydrazine hydrate (17) by using zinc ferrite as an efficient heterogeneous magnetic nanocatalyst under solvent-free conditions at 80 °C (Figure 11.12) [54]. After completion of the reaction the magnetic nanocatalyst was collected by using a simple bar magnet, and the recovered catalysts were further used without any loss in its catalytic efficiency. Very recently, in 2020, Kargar et al. [41] employed a novel biocompatible core-shell magnetic nanocatalyst (Fe3O4@NFC@Co(II)) for the same batch of reactions in water at 55 °C and obtained the corresponding dihydropyrano[2,3-c]pyrazole derivatives (16) in excellent yields (Figure 11.12). In 2014, Das and his research group [55] synthesized another series of carboxylate-functionalized dihydropyrano[2,3-c]pyrazole derivatives (19) via one-pot four-component reactions of hydrazine or aryl-substituted hydrazines (17a), ethyl acetoacetate (3), dialkyl but-2-ynedioate (18), and malononitrile (2) or ethyl 2-cyanoacetate (2a) in the presence of a catalytic amount of nanocopper ferrite in aqueous medium at 60 °C (Figure 11.13). After completion of the reaction, the nanocatalyst was recovered quantitatively and recycled for several runs without any significant reduction in the products’ yields.

Figure 11.11: Ultrasound-assisted magnetic nanocatalyzed three-component synthesis of dihydropyrano[2,3-c]pyrazoles.

11.3.5 Synthesis of benzo[a]pyrano[2,3-c] phenazines Ghorbani-Choghamarani et al. [56] prepared spinel-type magnetic nanoferrite (FeAl2O4) which was well characterized by using FTIR, XRD, EDS, SEM, BET, and VSM techniques. Using this newly synthesized nanomaterial as catalyst, they synthesized a series of benzo[a]pyrano[2,3-c]phenazine derivatives (22) via one-pot four-component

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Figure 11.12: Magnetic nanocatalyzed four-component synthesis of dihydropyrano[2,3-c]pyrazoles.

Figure 11.13: Magnetic nanocatalyzed four-component synthesis of 3-methyl-1,4-dihydropyrano[2,3c]pyrazole derivatives.

reactions of o-phenylenediamine (20), 2-hydroxynaphthalene-1,4-dione (21), substituted benzaldehydes (1), and malononitrile (2) in polyethelene glycol as solvent at 100 °C (Figure 11.14). This developed protocol has many advantages that include short reaction times, high yields, easy workup procedures, and most importantly reusability of the catalyst at least for four successive runs without any significant loss of its activity. A plausible mechanism of this conversation is shown in Figure 11.15.

Figure 11.14: Magnetic nanocatalyzed four-component synthesis of benzo[a]pyrano[2,3-c] phenazines.

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Figure 11.15: Plausible mechanism for the synthesis of benzo[a]pyrano[2,3-c] phenazines.

11.3.6 Synthesis of 2-amino-4H-pyrano[3,2-h]quinolines Pyranoquinoline skeleton is common in naturally occurring compounds [57–59]. Many synthetic pyranoquinoline scaffolds showed significant biological efficacies that include antimicrobial [60, 61], antitumor [62], anti-HIV [63], antimalarial [64], and antischistosomal [65] activities. In 2016, Bandaru et al.[66] developed a microwave-assisted protocol for the efficient synthesis of a series of 4-phenyl-4H-pyrano [3,2-h]quinoline derivatives (24) from the one-pot three-component reactions of aromatic aldehyde, malononitrile (2) or acetonitrile (2b), and 8-hydroxyquinoline (23) using magnetically separable nanocobalt ferrite (CoFe2O4) as a green catalyst in ethanol (Figure 11.16). Nanomagnetic CoFe2O4 was synthesized by sol–gel method, and this was well characterized by using FT-IR, SEM, XRD, and TEM techniques. Magnetic nanocatalyst was recovered and reused for five further runs without any loss in its catalytic activities.

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Figure 11.16: Magnetic nano-CoFe2O4-catalyzed three-component synthesis of 2-amino-4H-pyrano [3,2-h]quinolines.

11.3.7 Synthesis of 5-amino-2-aryl-chromeno[4,3,2-de][1,6] naphthyridine-4-carbonitriles In 2015, Dandia et al. [67] reported a facile protocol for the synthesis of 5-amino-2phenylchromeno[4,3,2-de][1,6]naphthyridine-4-carbonitrile derivatives (26) from the one-pot pseudo five-component reaction between one equivalent of 2-hydroxyacetophenone (25), one equivalent of aromatic aldehydes (1), and two equivalents of malononitrile (2) using iron oxide nanoparticles as an efficient heterogeneous magnetically recoverable catalyst under microwave-irradiation in aqueous medium at 70 °C (Figure 11.17). A variety of aldehydes with diverse substituents afforded the desired products in excellent yields within just 25 min. A plausible mechanism of this conversation is shown in Figure 11.18.

Figure 11.17: Magnetic nanocatalyzed three-component synthesis of chromeno[1,6]naphthyridines.

11.4 Spiro pyrans 11.4.1 Synthesis of spiro[chromene-4,3ʹ-indoline] derivatives In 2019, Mohammadian and Akhlaghinia [68] prepared taurine-functionalized β-cyclodextrin immobilized nanomagnetic calcined oyster shell (Fe3O4/COS@β-CD-SO3H NPs).

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Figure 11.18: Plausible mechanism for the synthesis of chromeno[1,6]naphthyridines.

After characterized these nanomaterials, they successfully utilized this newly prepared nanocatalyst for the efficient synthesis of a series of spiro[chromene-4,3ʹ-indoline] derivatives (28) via one-pot three-component reactions of substituted isatins (27), 1,3-cyclohexanedione derivatives (5 or 5a) and malononitrile (2) or ethyl 2-cyanoacetate (2a) in water at 50 °C (Figure 11.19). All the reactions were completed within just 22 min. Products were isolated in excellent yields just by simple filtration. The nanocatalyst was recovered easily by using a simple bar magnet. It is noteworthy to mention that the same nanocatalyst was equally efficient even after eight runs. On the same year, Nasseri and his group [69] also prepared another series of spiro[chromene-4,3ʹ-indoline] derivatives (28) from the reactions of five substituted isatins (27), 1,3-cyclohexanedione derivatives (5 or 5a), and malononitrile (2) using a catalytic amount of sulfonic acid–immobilized cobalt ferrite magnetic nanoparticles (CoFe2O4@SiO2@SO3H) as catalyst in aqueous ethanol at 80 °C (Figure 11.20).

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Figure 11.19: Magnetic nanocatalyzed three-component synthesis of spiro[chromene-4,3ʹ-indoline] derivatives.

Figure 11.20: Sulfonic acid immobilized cobalt ferrite magnetic nanocatalyst catalyzed synthesis of spiro[chromene-4,3ʹ-indoline] derivatives.

11.4.2 Synthesis of spiro[indoline-3,4ʹ-pyrano[3,2-c]chromene] derivatives A catalytic amount of sulfonic acid–immobilized cobalt ferrite nanoparticles (CoFe2 O4@SiO2@SO3H) were also found efficient to catalyze the reactions of substituted isatins (27), 4-hydroxycoumarin (7), and malononitrile (2) which afforded the corresponding of spiro[indoline-3,4ʹ-pyrano[3,2-c]chromene] derivatives (29) in excellent yields in aqueous ethanol at 80 °C (Figure 11.21) [69]. Before use, the prepared catalyst was characterized by using various techniques such as FTIR, EDX, XRD, TEM, SEM, VSM, and TGA analyses. After completion of the reaction, the employed catalyst was recovered by using a simple magnet and recycled up to five runs without any significant loss in its catalytic efficiency.

Figure 11.21: Synthesis of spiro[indoline-3,4ʹ-pyrano[3,2-c]chromene] derivatives.

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11.4.3 Synthesis of spiro[pyrano-indene-1,3-dione/ acenaphthylene] derivatives In 2019, Nasab and Safari [70] successfully modified silica-supported magnetic NiFe2 O4 nanoparticles with melamine (NiFe2O4@SiO2@Melamine). They used [3-(2,3-epoxypropoxy)propyl]trimethoxysilane as a cross-linker to bond melamine on the surface of silica-supported nickel ferrite nanoparticles. The synthesized nanocatalyst was well characterized by using various techniques such as FT-IR, XRD, SEM, VSM, TGA, and EDX. Using a catalytic amount of this newly prepared nanomaterial, they synthesized a series of structurally diverse spiro[pyrano-indene-1,3-dione] derivatives such as spiro[chromene-4,2ʹ-indenes] (32), 2ʹ-amino-1,3,5ʹ-trioxo-1,3-dihydro5ʹH-spiro[indene-2,4ʹ-pyrano[3,2-c]chromene]-3ʹ-carbonitrile (33), spiro[indene-2,5ʹpyrano[2,3-d]pyrimidines] (34), 6ʹ-amino-3ʹ-methyl-1,3-dioxo-1,3-dihydro-1ʹH-spiro [indene-2,4ʹ-pyrano[2,3-c]pyrazole]-5ʹ-carbonitrile (35), and 2ʹ-amino-1,3,5ʹ-trioxo-1,3dihydro-5ʹH-spiro[indene-2,4ʹ-indeno[1,2-b]pyran]-3ʹ-carbonitrile (36) from one-pot three-component reactions of 1H-indene-1,2,3-trione (30), malononitrile (2) or ethyl 2-cyanoacetate (2a), and various C–H-activated acids like 1,3-cyclohexanediones (5,5a), 4-hydroxycoumarin (7), barbituric acid or thiobarbituric acid derivatives (9,11), 3-methyl-1H-pyrazol-5(4H)-one (14), and 1H-indene-1,3(2H)-dione (31), respectively, in ethanol under refluxed conditions (Figure 11.22). Under the same optimized reaction conditions, spiro[acenaphthylene-1,4ʹ-chromenes] (38) were synthesized in excellent yields from the reactions of acenaphthylene-1,2-dione (37), malononitrile (2), and dimedone (5) or 1,3-cyclohexanedione (5a). Synthesis of 2ʹ-amino-2,5ʹ-dioxo2H,5ʹH-spiro[acenaphthylene-1,4ʹ-pyrano[3,2-c]chromene]-3ʹ-carbonitrile (39) was achieved from the reaction of acenaphthylene-1,2-dione (37), malononitrile (2), and 4-hydroxycoumarin (7). Using barbituric acid derivatives (9,11) as the C–H activated acids, they synthesized spiro[acenaphthylene-1,5ʹ-pyrano[2,3-d]pyrimidine] derivatives (40) in excellent yields. This developed catalytic system was also found efficient for the reaction between acenaphthylene-1,2-dione (37), malononitrile (2), and 3-methyl-1H-pyrazol-5(4H)-one (14) which afforded the corresponding 6ʹ-amino-3ʹmethyl-2-oxo-1ʹH,2H-spiro[acenaphthylene-1,4ʹ-pyrano[2,3-c]pyrazole]-5ʹ-carbonitrile (41) in excellent yield (Figure 11.23). After completion of the reaction, the used nanocatalysts were recovered easily by using a bar magnet and then recycled six times without any notable loss in its catalytic activities.

11.4.4 Synthesis of 4H-chromenes and 5H-pyrano[3,2-c] chromenes Amine-functionalized silica-supported iron oxide nanoparticles were prepared by Maleki and Azadegan [71]. They utilized the magnetic nanomaterials for the efficient synthesis of 2-amino-4H-chromene-4-carboxylates (42) from the one-pot three-component

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Figure 11.22: Synthesis of structurally diverse fused spiro[-pyrano-indene-1,3-dione] derivatives.

reactions of dialkyl but-2-ynedioate (18), malononitrile (2) or ethyl 2-cyanoacetate (2a), and dimedone (5) or 1,3-cyclohexanedione (5a) in ethanol at 60 °C (Figure 11.24). All the desired products were isolated in excellent yields. The used magnetic nanocatalyst was recovered easily and recycled for six further runs. Under the same optimized conditions, they were also able to synthesize 5H-pyrano[3,2-c] chromenes (43) in excellent yields starting from dialkyl but-2-ynedioate (18), malononitrile (2) or ethyl 2-cyanoacetate (2a), and 4-hydroxycoumarin (7) (Figure 11.24).

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Figure 11.23: Synthesis of structurally diverse fused spiro[pyrano-acenaphthylene] derivatives.

11.5 Conclusions In this chapter, we have summarized the recent developments related to the synthesis biologically promising structurally diverse pyrans or pyran annulated heterocyclic scaffolds by using various magnetically recoverable heterogeneous magnetic nanocatalysts under diverse reaction conditions. These reported methods are very much advantageous in terms of sustainability. In all the cases, the used catalysts were recovered by using a simple bar magnet and recycled several times without any significant loss in its catalytic activities.

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Figure 11.24: Fe3O4@SiO2–NH2 catalyzed synthesis of 4H-chromenes and pyrano[3,2-c]chromene derivatives.

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Author list Volume 1 Dr. Bablee Mandal Department of Chemistry Surya Sen Mahavidyalaya Siliguri 734004 India [email protected] Prof. Basudeb Basu Department of Chemistry Cotton University Guwahati 781001 India [email protected] [email protected] Brindaban C. Ranu School of Chemical Sciences Indian Association for the Cultivation of Science Jadavpur, Kolkata 700032 India [email protected] Laksmikanta Adak Department of Chemistry Indian Institute of Engineering Science and Technology Shibpur, Botanic Garden Howrah 711103 India Nirmalya Mukherjee School of Chemical Sciences Indian Association for the Cultivation of Science Jadavpur, Kolkata 700032 India Tubai Ghosh Department of Chemistry Jadavpur University Kolkata 700032 India

https://doi.org/10.1515/9783110730357-012

Nahid Ahmadi School of Mahdiyeh Shahed Education of Zanjan Zanjan 45186-17981 Iran Ali Ramazani Department of Chemistry Faculty of Science University of Zanjan Zanjan 45371-38791 Iran [email protected], [email protected] Mohammadreza Shokouhimehr Department of Materials Science and Engineering Research Institute of Advanced Materials Seoul National University Seoul 08826 Republic of Korea Email: [email protected] Ho Won Jang Department of Materials Science and Engineering Research Institute of Advanced Materials Seoul National University Seoul 08826 Republic of Korea Abhijeet Singh, Trisha Ghatak, Shalini Agarwal Department of Chemistry Faculty of Science University of Delhi Delhi 110 007 India Ramendra Pratap Department of Chemistry Faculty of Science University of Delhi Delhi 110 007 India

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Author list

Mahendra Nath Department of Chemistry Faculty of Science University of Delhi Delhi 110 007 India Email: [email protected]

Yadavalli Venkata Durga Nageswar Retired Chief Scientist CSIR-Indian Institute of Chemical Technology Uppal Road, Tarnaka Hyderabad 500007 Telangana, India Email: [email protected]

Sanjay Paul Department of Chemistry Behala College Kolkata, India Asish Ranjan Das Department of Chemistry University of Calcutta Kolkata, India Email: [email protected]

Jayathirtha Rao Vaidya Emeritus Scientist FluoroAgro Chemicals Department CSIR-Indian Institute of Chemical Technology and AcSIR-Ghaziabad Uppal Road Tarnaka Hyderabad 500007 Telangana, India

Nirjhar Saha Department of Medicinal Chemistry National Institute of Pharmaceutical Education and Research (NIPER) S. A. S. Nagar, Sector 67 Punjab 160 062 India Asim Kumar Amity Institute of Pharmacy Amity University Haryana, Manesar 122 413 India Antarlina Maulik Department of Medicinal Chemistry National Institute of Pharmaceutical Education and Research (NIPER) S. A. S. Nagar, Sector 67 Punjab 160 062 India Asit K. Chakraborti School of Chemical Sciences Indian Association for the Cultivation of Science (IACS) Jadavpur, Kolkata, West Bengal 700 032, India; Email: [email protected] / [email protected]

Yogesh B. Wagh School of Chemical Sciences Kavayitri Bahinabai Chaudhari North Maharashtra University Jalgaon 425 001 (MS) India Yogesh A. Tayade Department of Chemistry Dhanaji Nana Mahavidyalaya Faizpur 425503 (MS) India Dipak S. Dalal School of Chemical Sciences Kavayitri Bahinabai Chaudhari North Maharashtra University Jalgaon 425 001 (MS) India Email: [email protected] Manavi Yadav Green Chemistry Network Centre Department of Chemistry University of Delhi Delhi 110007, India

Volume 1

Sriparna Dutta Green Chemistry Network Centre Department of Chemistry University of Delhi Delhi 110007, India And Hindu College Department of Chemistry University of Delhi Delhi 110007, India Anju Srivastava Hindu College Department of Chemistry University of Delhi Delhi 110007, India Rakesh K. Sharma Green Chemistry Network Centre Department of Chemistry University of Delhi Delhi 110007, India Manmeet Kaur Department of Chemistry Akal University, Talwandi Sabo Bathinda Punjab 151302, India Anu Priya Department of Chemistry Akal University, Talwandi Sabo Bathinda Punjab 151302, India

Arvind Singh Department of Chemistry Akal University, Talwandi Sabo Bathinda Punjab 151302, India Aditi Sharma Department of Chemistry Akal University, Talwandi Sabo Bathinda Punjab 151302, India Gurpreet Kaur Department of Chemistry Akal University, Talwandi Sabo Bathinda Punjab 151302, India Bubun Banerjee Department of Chemistry Akal University, Talwandi Sabo Bathinda Punjab 151302, India Email: [email protected]/ [email protected]

437

Index (UHP) 62 1,10-phenanthroline 61 1,2-metallate shift 247 1,3-dicarbonyl compounds 181 1,4-dihydropyridine 343 1,8-dioxodecahydroacridines 363 2-amino-3-cyano pyridine 342 2-amino-3-cyano-tetrahydro-4H-chromenes 416 2-amino-4H-chromenes 357 3,4-dihydropyrimidin-2(1H)-ones 359 3,4-dihydropyrimidine-2-[1H]thione 359 4H-benzo[b]pyrans 350 acetals 371 acetylthiophene (AcTp) 81 Agenda of Sustainable Development Goals 379 agglomeration 111 alcohol oxidation 220–221 aldoximes 316 alkoxycarbonylation 21 alkyl levulinates 369 ambiphilic (electrophile-nucleophile) dual activation 251 ammonium acetate 312, 323 amphiphilic nature 288 annellation 185 aqueous reaction medium 243 aryl boronic acids 311, 317, 327 aryl C-H activation 245 asymmetric synthesis 157 azoxyarenes 230 barbituric acid 310, 315, 320 benzimidazoles 370 benzo[a]pyrano[2,3-c]phenazine 421 Benzyl-Br3 59 bi-aryl synthesis 241 Biginelli reaction 360 bi-metallic magnetic nanocatalyst 276 bimetallic nanocatalyst 291 bimetallic nanoparticle 276–278 bimetallic nanoparticles 276, 281, 291 bi-metallic nanoparticles 242 binary Ni-Pd NCs 244 biologically active 169 biomedicine 157 Boehmite 90 https://doi.org/10.1515/9783110730357-013

calcinated 159 capping agent 281 Carbon dots 107 carbon-heteroatom bond formation 2 catalysis 157 catalysts 141 catalytic activity 293 catalytic systems 157, 162 cation sensors 157 centrifugation 161 Chagas disease 38 chelating 111 chemoselectivity 24 chemo-selectivity 161 chitosan 59, 99 chitosan-based nanocopper ferrite 412 choline chloride 77 CMC 110 C-N bond cleavage 253 C-O activation strategy 245 C-O bond activation 245 coating material 257, 259, 262, 268, 272 cobalt ferrite 97 cobalt phthalocyanine 56 Conclusions 372 copper ferrite 161, 166, 172, 182 co-precipitation 72, 174 core-shell 73 cotton filtration method 244 Covalent Organic Framework 252 cross-coupling reactions 162, 241 crystallographic sites 158 Cyclodextrins 83 data storage 157 defect sensors 157 dehydrative condensation 174 DES 76 desorption 165 diaryl ethers 19 dicyclohexylcarboxamide 166 dihydropyrano[2,3-c]pyrazole 349 dihydropyrano[2,3-c]pyrazole derivatives 421 dihydropyrano[2,3-c]pyrazoles 347 dihydropyrano[c]chromenes 350 diindolyloxindole 366 dimedone 312, 315, 320

440

Index

dioxygen activation 251 diphenyl diselenide 38 disulfanes 64 disulfides 49 Dopamine sulfamic acid 56 electron withdrawing group 247 electrophile-nucleophile dual activation 259 enantioselectivity 157 energy dispersive X-ray 163 environmental remediation 157 environmentally benign 285 Epoxidation 223 ethylene glycol 319 external magnet 287 external magnetic field 256–257, 259, 276, 280, 283, 288 ferrimagnetic oxides 157 ferrites 157 ferromagnetic 159–160 Friedlander synthesis 5 garnet 157 graphene oxide 101 green chemistry 157, 247 green solvent 247 Hantzsch synthesis 344 Heck coupling 198–207 Heck coupling reaction 355 Hercynite 112 heterobimetallic 276, 279, 283, 293 heterogeneous 241, 243, 252, 255–257, 276, 280, 289–290, 293 Heterogeneous catalysis 52, 242–243, 256–257, 293 heterogeneous catalyst system 255, 276 heterogeneous magnetic nanocomposite 242 heteropoly acid 393 hexahydroquinolines 362 Hiyama cross-coupling 211–214 homocoupling reaction 217–219 homogeneous catalyst system 255 HPG 88 HRTEM 120See hydrogen-bond (HB)-assisted activation 250 hydroxy coumarin 315

imidazolium-based ILs 259 Irish moss 104 leaching test 252 L-lysine 111 loss of catalytic 291 maghemite 52 maghemite-Cu nanocatalyst 31 magnetic anisotropy 160 magnetic clay 64 magnetic decantation 256 magnetic metal nanoparticles 255 magnetic nanocatalytic system 255 magnetic nanocomposite 141 magnetic nanocomposites 268, 275, 293 magnetic nanoparticles 52, 157, 254–255 magnetic resonance imaging 157 magnetic separation 53 magnetic template 242, 257–258, 262, 268, 275–276 magnetically recoverable 412 magnetically recovered nanocatalyst 256 magnetically separable 256, 258, 276, 279, 284–285 magnetically tunable colloidal photonic crystals 157 magnetite 52 magnetization 159 maleimides 313–314 mechanochemical treatment 292 mercaptans 55 mesoporous polyaniline 27 metabolites 185 Metal nanoparticle 254 metal organic framework 247 metal-free organocatalytic 247 metalloenzyme 288 Metal-organic frameworks 103 micro/ultrasonic-wave 243 microemulsion 382 microemulsion technique 161 micro-fluidics 157 microwave irradiation 272 MNPs 72 monometallic 279 Montmorillonite 63 morphological change 287

Index

Multi-component reactions 170 multi-component strategies 161 multicomponent synthesis 412 N- and C-alkylations of anilines 5 nanoalloy 279, 291 nanocatalysis 52, 243 nanocatalyst 52, 293 nano-catalysts 157 nanocomposite 241, 257–259, 262–263, 272, 276, 281, 293 nanocrystal 281 nanocubes 381 nanomaterials 412 nanoparticles 381 nanosheets 381 Nanotechnology 51 N-arylation 4 nitroarenes 141 nitroaromatics 141 nitrones 232 nitrostyrenes 328 non-magnetic templates 276 N-sulfonylamidines 6 nucleation 285 O-allylation of phenols 21 O-arylation of phenols 18 octahedral 158 of 2H-indazolo-[2,1-b]phthalazine-1,6,11trione 364 OPSF 74 optical filters 157 organic synthesis 157, 163 organic transformations 157, 186 organo-catalysts 259 organo-chalcogenides 24 organonickel species 293 organopalladium 241 oxidation 191–192, 219–233 oxidation of sulfides 225–226 oxidative addition 165 oxidative amidation 227–228 oxidative phosphorylation 228 paramagnetic 159 Pd nanoparticles 253, 268, 284 Pd(II) catalyst 246 Pd(II) complex 247

441

Pd–carbene complexes 249 Pd-NHC complex 249 Pd-Ni bi-metallic alloy 262 Pd-Ni binary nanocluster 244 peptidomimetics 49 peroxymonosulfate 105 PGMA 78 phase transfer catalysts 252 phenyl boronic acid 246 phenyl hydrazine 321, 328 phenyl selenyl bromide 37 phenylacetylene 321 photocatalyst 253 phototungstic acid 392 poly-substituted pyrroles 7 POSS 85 Preyssler 75 protein folding 50 protodeboronation 247 pyrano[2,3-d]pyrimidine 419 pyrano[2,3-d]pyrimidine derivatives 23 pyranocoumarins 22 pyranopyrazole 347 pyrans and pyran annulated heterocycles 411 pyrazolone 315 quinoline 370 R. tinctorum 111 Raman spectra 281 recyclability 52, 286 redox condensation 176 reduce, reuse, and recycle 381 Reduction 141 regio-controlled cascade reaction 172 reusability 163–164, 180, 184, 186, 251 Ritter reaction 368 room temperature 247, 251, 253, 258, 262 scanning transmission electron microscopy 283 Schiff base 246 self-assembled monolayers 64 separation 128 silyl enolate oxidation 229 sintering temperature 157 sol-gel approach 160 sol-gel auto combustion method 164 sonochemical method 159

442

Index

Sonogashira coupling 207–211 Sonogashira coupling reaction 355 spinel 157 spinel ferrites 62 SPIONs 79 spirochromenes 335 spirohexahydropyrimidines 179 spirooxindole 337 spiropyran 336 spiropyrimidines 178 Stille reaction 214 sub lattice 158 superparamagnetic 256, 275, 279, 282 superparamagnetic NPs 256, 275 super-paramagnetism 381 supramolecular assemblies 259 Suzuki coupling 192–194, 196–198 Suzuki cross-coupling reaction 241 Suzuki-Miyaura 243–246, 249–250, 252, 276, 278–288, 290–293 Suzuki-Miyaura reaction 279 synergistic effect 276 synthesis of 1,2,3-triazole 11 synthesis of 1,3-thiazolidin-4-ones 36 synthesis of 1,4-dihydropyridine 11 synthesis of alkynyl chalcogenides 40 synthesis of quinazolinones 14

tetraethyl orthosilicate 174, 387 tetrahedral 158 thermal decomposition 72, 382 thio-arylation 27 thioglycolic acid 316 tissue specific targeting 157 trifluoroethanol 250 tryptophan 107 turnover 282, 289 turnover number (TON) 108 Ullman-type coupling reaction 17 ultrasonication method 171 ultrasound 253–254, 282, 289, 293 vinyl 93 vinyl arenes 317 visible light 243 WAXRD 122 WGSR 106 XRD 123