Sustainable and Functional Redox Chemistry [69] 9781839162466

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Sustainable and Functional Redox Chemistry

Green Chemistry Series Editor-in-chief: James H. Clark, Department of Chemistry, University of York, UK

Series editors: Catherine Birch, AgriFood X Limited, UK Graham Bonwick, AgriFood X Limited, UK George A. Kraus, Iowa State University, USA Andrzej Stankiewicz, Delft University of Technology, The Netherlands Peter Siedl, Federal University of Rio de Janeiro, Brazil

Titles in the series: 1: 2: 3: 4: 5: 6: 7: 8: 9:

The Future of Glycerol: New Uses of a Versatile Raw Material Alternative Solvents for Green Chemistry Eco-Friendly Synthesis of Fine Chemicals Sustainable Solutions for Modern Economies Chemical Reactions and Processes under Flow Conditions Radical Reactions in Aqueous Media Aqueous Microwave Chemistry The Future of Glycerol: 2nd Edition Transportation Biofuels: Novel Pathways for the Production of Ethanol, Biogas and Biodiesel 10: Alternatives to Conventional Food Processing 11: Green Trends in Insect Control 12: A Handbook of Applied Biopolymer Technology: Synthesis, Degradation and Applications 13: Challenges in Green Analytical Chemistry 14: Advanced Oil Crop Biorefineries 15: Enantioselective Homogeneous Supported Catalysis 16: Natural Polymers Volume 1: Composites 17: Natural Polymers Volume 2: Nanocomposites 18: Integrated Forest Biorefineries 19: Sustainable Preparation of Metal Nanoparticles: Methods and Applications 20: Alternative Solvents for Green Chemistry: 2nd Edition 21: Natural Product Extraction: Principles and Applications 22: Element Recovery and Sustainability 23: Green Materials for Sustainable Water Remediation and Treatment 24: The Economic Utilisation of Food Co-Products 25: Biomass for Sustainable Applications: Pollution Remediation and Energy 26: From C–H to C–C Bonds: Cross-Dehydrogenative-Coupling 27: Renewable Resources for Biorefineries 28: Transition Metal Catalysis in Aerobic Alcohol Oxidation 29: Green Materials from Plant Oils

30: Polyhydroxyalkanoates (PHAs) Based Blends, Composites and Nanocomposites 31: Ball Milling Towards Green Synthesis: Applications, Projects, Challenges 32: Porous Carbon Materials from Sustainable Precursors 33: Heterogeneous Catalysis for Today’s Challenges: Synthesis, Characterization and Applications 34: Chemical Biotechnology and Bioengineering 35: Microwave-Assisted Polymerization 36: Ionic Liquids in the Biorefinery Concept: Challenges and Perspectives 37: Starch-based Blends, Composites and Nanocomposites 38: Sustainable Catalysis: With Non-endangered Metals, Part 1 39: Sustainable Catalysis: With Non-endangered Metals, Part 2 40: Sustainable Catalysis: Without Metals or Other Endangered Elements, Part 1 41: Sustainable Catalysis: Without Metals or Other Endangered Elements, Part 2 42: Green Photo-active Nanomaterials 43: Commercializing Biobased Products: Opportunities, Challenges, Benefits, and Risks 44: Biomass Sugars for Non-Fuel Applications 45: White Biotechnology for Sustainable Chemistry 46: Green and Sustainable Medicinal Chemistry: Methods, Tools and Strategies for the 21st Century Pharmaceutical Industry 47: Alternative Energy Sources for Green Chemistry 48: High Pressure Technologies in Biomass Conversion 49: Sustainable Solvents: Perspectives from Research, Business and International Policy 50: Fast Pyrolysis of Biomass: Advances in Science and Technology 51: Catalyst-free Organic Synthesis 52: Hazardous Reagent Substitution: A Pharmaceutical Perspective 53: Alternatives to Conventional Food Processing: 2nd Edition 54: Sustainable Synthesis of Pharmaceuticals: Using Transition Metal Complexes as Catalysts 55: Intensification of Biobased Processes 56: Sustainable Catalysis for Biorefineries 57: Supercritical and Other High-pressure Solvent Systems: For Extraction, Reaction and Material Processing 58: Biobased Aerogels: Polysaccharide and Protein-based Materials 59: Rubber Recycling: Challenges and Developments 60: Green Chemistry for Surface Coatings, Inks and Adhesives: Sustainable Applications 61: Green Synthetic Processes and Procedures 62: Resource Recovery from Wastes: Towards a Circular Economy 63: Flow Chemistry: Integrated Approaches for Practical Applications 64: Transition Towards a Sustainable Biobased Economy

65: 66: 67: 68: 69:

Transportation Biofuels: Pathways for Production: 2nd Edition Challenges in Green Analytical Chemistry: 2nd Edition CO2-switchable Materials: Solvents, Surfactants, Solutes and Solids Green Toxicology: Making Chemicals Benign by Design Sustainable and Functional Redox Chemistry

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Green Chemistry Series No. 69 Print ISBN: 978-1-83916-246-6 PDF ISBN: 978-1-83916-482-8 EPUB ISBN: 978-1-83916-483-5 Print ISSN: 1757-7039 Electronic ISSN: 1757-7047 A catalogue record for this book is available from the British Library r The Royal Society of Chemistry 2022 All rights reserved Apart from fair dealing for the purposes of research for non-commercial purposes or for private study, criticism or review, as permitted under the Copyright, Designs and Patents Act 1988 and the Copyright and Related Rights Regulations 2003, this publication may not be reproduced, stored or transmitted, in any form or by any means, without the prior permission in writing of The Royal Society of Chemistry or the copyright owner, or in the case of reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of the licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to The Royal Society of Chemistry at the address printed on this page. Whilst this material has been produced with all due care, The Royal Society of Chemistry cannot be held responsible or liable for its accuracy and completeness, nor for any consequences arising from any errors or the use of the information contained in this publication. The publication of advertisements does not constitute any endorsement by The Royal Society of Chemistry or Authors of any products advertised. The views and opinions advanced by contributors do not necessarily reflect those of The Royal Society of Chemistry which shall not be liable for any resulting loss or damage arising as a result of reliance upon this material. The Royal Society of Chemistry is a charity, registered in England and Wales, Number 207890, and a company incorporated in England by Royal Charter (Registered No. RC000524), registered office: Burlington House, Piccadilly, London W1J 0BA, UK, Telephone: þ44 (0) 20 7437 8656. For further information see our website at www.rsc.org Printed in the United Kingdom by CPI Group (UK) Ltd, Croydon, CR0 4YY, UK

Preface Redox (reduction and oxidation) is a key element in a wide variety of fields of chemistry, ranging from synthesis, reactions to materials, and analytical applications. Due to the recent and future demand for sustainable development goals (SDGs), chemists are becoming conscious of sustainability in synthesis and of functionality in applications. Therefore, redox chemistry toward sustainability and functionality is an essential idea for nextgeneration chemistry to be shared among researchers in both academia and industry. In this context, a research group of young redox chemists (Amaya, Ooyama, Inagi, Mitsudo, Nokami, Shimakoshi, Shimizu) started under the support of the Chemical Society of Japan in 2016. Through activities such as international symposiums, we explored possibilities of sustainable and functional redox chemistry involving international young and energetic researchers. Some of them and their collaborators kindly contributed to the book project. This book is part of the RSC Green Chemistry series and aims to share hot topics in redox chemistry sustainability and functionality. In Part 1, Sustainable Redox Reaction, the chapters summarize recent developments in redox reactions and synthesis toward sustainability (e.g. green but powerful electrosynthesis, novel electrode processes). In Part 2, Sustainable Redox Catalysis, chapters describing recent progress in redox catalysis (electrocatalysis, photoredox reactions, synergy of electrosynthesis and metal catalysis) are presented. Part 3, Functional Redox System, is composed of chapters describing the applications of redox systems for functionality such as organic batteries, photofunctionality, redox-active polymeric materials, chiral electrodes, and electrogenerated chemiluminescence. I believe that this collection of chemical processes and systems with redox chemistry as a core technology inspires readers toward SDGs.

Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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I am very thankful for the time spent and the energy contributed by all authors and their collaborators, as well as very thankful for the support given by the RSC team (especially Drew Gwilliams and Liv Towers). Shinsuke Inagi Department of Chemical Science and Engineering Tokyo Institute of Technology Yokohama, Japan

Contents Part 1 Sustainable Redox Reaction Chapter 1 Redox-mediated Electrochemical Cyclization Reactions Zheng-Jian Wu and Hai-Chao Xu 1.1 1.2

Introduction Radical Cyclization Reactions 1.2.1 Cyclization Reactions of Heteroatomcentered Radicals 1.2.2 Cyclization Reactions of Carbon-centered Radicals 1.3 Halide-mediated Ionic Cyclization Reactions 1.4 Conclusion Acknowledgements References Chapter 2 Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids Alessia Petti and Kevin Lam 2.1 2.2 2.3 2.4

Introduction Background of the Kolbe Electrolysis Background of Non-Kolbe Electrolysis Recent Advances in the Electrolysis of Carboxylic Acids 2.4.1 Kolbe Intramolecular Cyclisation 2.4.2 Hofer–Moest Synthesis of Isocyanates 2.4.3 Hofer–Moest Synthesis of Orthoesters

Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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3

3 4 4 14 19 24 24 24

29

29 30 31 32 32 38 40

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2.4.4 2.4.5

Electrochemical Methoxylation Electrochemical Decarboxylation of Malonic Acid Derivatives 2.5 Recent Advances in the Electrolysis of Carboxylic Acid Derivatives 2.5.1 Electrochemical Deprotection of Aromatic Esters 2.5.2 Electrochemical Deoxygenation of Diphenylphosphinates 2.6 Future Perspectives 2.7 Conclusion Abbreviations Acknowledgements References Chapter 3 Novel Electrolytic Processes Koichi Mitsudo 3.1 3.2

Introduction Parallel Batch Systems Used for Electroorganic Synthesis 3.2.1 Parallel Batch Systems Using the Cation Pool Method 3.2.2 Parallel Batch Processes for Electrosynthesis 3.3 Combinatorial Flow System for Electroorganic Chemistry 3.3.1 Flow Electrochemistry 3.3.2 PEM Reactor 3.4 Bipolar Electrochemical System 3.5 Conclusion References Chapter 4 A Sugar Machine Hirofumi Endo, Md Azadur Rahman and Toshiki Nokami 4.1 4.2

Introduction Electrochemical Generation of Glycosylation Intermediates 4.2.1 Generation of Glycosyl Triflate Intermediates 4.2.2 Generation of Glycosyl Sulfonium Ion Intermediates

46 49 51 51 52 53 54 55 55 55 60

60 61 61 61 63 63 68 72 75 76 80

80 82 82 84

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4.3

Development of a Method for Automated Electrochemical Solution-phase Synthesis of Oligosaccharides 4.3.1 Proof of Principle of One-pot Iterative Glycosylation 4.3.2 Demonstration of Automated Electrochemical Assembly of Oligosaccharides 4.4 Synthesis of Biologically Active Oligosaccharides 4.4.1 Synthesis of TMG-chitotriomycin 4.4.2 Synthesis of Myc-LCOs 4.5 Synthesis of 1,2-trans Glycosidic Linkages of Hexoses via Automated Electrochemical Assembly 4.6 Synthesis of Cyclic Oligosaccharides via Automated Electrochemical Assembly 4.7 Conclusion Acknowledgements References

86 86 88 89 89 92 93 96 97 97 97

Part 2 Sustainable Redox Catalysis Chapter 5 Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species Toru Amaya 5.1 5.2

Introduction Oxovanadium(V)-induced Intermolecular Selective Oxidative Cross-coupling between Boron and Silyl Enolates 5.3 Oxidative Cross-coupling between Various Boron and Silyl Enolates 5.4 Oxovanadium(V)-catalyzed Oxidative Cross-coupling between Boron and Silyl Enolates under O2 as a Terminal Oxidant 5.5 Conclusion Abbreviations Acknowledgements References

Chapter 6

101

101

105 107

110 116 116 117 117

Mediated Electron Transfer in Electrosynthesis: Concepts, Applications, and Recent Influences from Photoredox Catalysis 119 ´jek Robert Francke and Michal Ma 6.1

Introduction

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6.2

Concepts and Applications 6.2.1 Direct and Indirect Electrosynthesis 6.2.2 The Catalytic Current 6.2.3 Redox Catalysis and Chemical Catalysis 6.2.4 In-cell- and Ex-cell-mediated Transformations 6.3 Approaches Toward Facilitating Mediator Recycling 6.3.1 Ionically Tagged Mediators 6.3.2 Polymediators 6.3.3 Mediator-modified Electrodes 6.4 Mediators in Photoelectrochemical Synthesis 6.4.1 Transformations at Photoelectrodes 6.4.2 Sequential Activation of Substrates by Electro- and Photochemistry 6.4.3 Enhancing Mediator Reactivity with Light 6.5 Conclusions Acknowledgements References

Chapter 7 Synergy of Electrochemistry and Asymmetric Catalysis Yi-Min Jiang, Yi Yu, Zhaojiang Shi, Yi-Lun Li, Hong Yan and Ke-Yin Ye 7.1 7.2

Introduction Substrates as the Redox Entities in Electrochemical Asymmetric Catalysis 7.3 Catalysts as Redox Entities in Electrochemical Asymmetric Catalysis 7.4 Both Substrates and Catalysts as the Redox Entities in Electrochemical Asymmetric Catalysis 7.5 Conclusion Acknowledgements References Chapter 8 Alternative Approaches for Scalable Artificial Photosynthesis via Sustainable Redox Processes Han Sen Soo 8.1 8.2

Introduction Nonfood Biomass Oxidation 8.2.1 Photocatalytic Nonfood Biomass Oxidation 8.2.2 Electrocatalytic and Photoelectrocatalytic Nonfood Biomass Oxidation

120 121 121 123 128 132 132 133 136 136 138 140 142 146 148 148 154

154 156 161 166 172 172 172

175

175 178 178 184

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8.3

Synthetic Polymer Oxidation 8.3.1 Heterogeneous Photocatalytic Oxidation of Synthetic Polymers 8.3.2 Homogeneous Photocatalytic Oxidation of Synthetic Polymers 8.4 Photosynthetic and Photocatalytic Reduction by Metal Halide Perovskites 8.5 Conclusions and Outlook Acknowledgements References

Chapter 9 Bioinspired Catalyst Learned from B12-dependent Enzymes Hisashi Shimakoshi 9.1

Introduction 9.1.1 B12 (Cobalamin)-dependent Enzymes 9.1.2 Catalyst Design for B12-dependent Enzyme-inspired Reactions 9.2 Photo-driven Molecular Transformation 9.2.1 Heterogeneous Catalyst System 9.2.2 Esters and Amides Formation Coupled with Dehalogenation 9.2.3 Visible Light-driven Catalytic System 9.2.4 B12-inspired Hydrogen Production and Alkene Reduction 9.2.5 Homogeneous Catalyst System 9.2.6 Cross-coupling Reactions 9.2.7 B12–BODIPY Dyad System 9.2.8 Catalysis of B12 Without Photocatalyst 9.3 Summary and Outlook Acknowledgements References

188 189 192 196 200 201 201 207

207 208 210 210 210 213 214 216 217 218 220 221 222 223 223

Part 3 Functional Redox System Chapter 10 Redox-active Molecules and Their Energy Device Application Akihiro Shimizu 10.1 10.2

Introduction Organic Active Materials for Li-ion Batteries 10.2.1 Basic Concepts 10.2.2 Capacity Increase

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229 230 230 230

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10.2.3 Cyclability Increase 10.2.4 Voltage Increase 10.3 Organic Active Materials for Redox Flow Batteries 10.3.1 Aqueous Electrolyte 10.3.2 Nonaqueous Electrolyte References Chapter 11 Redox-active Polymeric Materials Naoki Shida and Shinsuke Inagi 11.1 11.2

Introduction Conjugated Polymers 11.2.1 Doping of Conjugated Polymers 11.2.2 Oxidative and Reductive Electropolymerization 11.2.3 Electrochemical Polymer Reaction 11.2.4 Two- and Three-dimensional Conjugated Polymers 11.3 Nonconjugated Polymers with Redox-active Units 11.3.1 Polymers with Redox-active Units in the Side Chain 11.3.2 Block Copolymers with Redox-active Units 11.3.3 Polymeric Materials Mimicking Metalloproteins 11.3.4 Redox Units at the Periphery of Dendrimers 11.3.5 Redox-active Inorganic Polymers 11.4 Conjugated Polymers with Redox-active Moieties 11.5 Conclusion References

Chapter 12 Chiral Metal Electrodes for Enantioselective Analysis, Synthesis, and Separation Chularat Wattanakit and Alexander Kuhn 12.1 12.2

12.3

Background Elaboration of Chiral Metal Electrodes 12.2.1 Adsorption of Chiral/Achiral Molecules on Metal Surfaces 12.2.2 Binding of Chiral Ligands to Metal Surfaces 12.2.3 Controlled Cutting of Bulk Metals 12.2.4 Chiral Molecular Imprinting Applications of Chiral Metal Electrodes 12.3.1 Enantioselective Analysis

232 241 242 242 244 246 249

249 250 250 250 252 259 259 261 261 263 265 266 268 270 270

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274 275 276 276 277 278 283 283

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12.3.2 Asymmetric Synthesis 12.3.3 Electrochemical Separation 12.4 Conclusion and Perspectives Acknowledgements References Chapter 13 Fluorescent Sensors for Water Yousuke Ooyama 13.1 Introduction 13.2 PET-based Fluorescent Sensors 13.3 PET/FRET-based Fluorescent Sensors 13.4 PET/AIEE-based Fluorescent Sensors 13.5 SFC/AIEE-based Fluorescent Sensors 13.6 ICT-based Fluorescent Sensors 13.7 Fluorescent Sensor-doped Polymer Films 13.8 Conclusion Acknowledgements References Chapter 14 Photoredox Chemistries of Cyclometalated Ir(III) Complexes Youngmin You Photoinduced Electron Transfer of Cyclometalated Ir(III) Complex 14.2 Electronic Structures of Cyclometalated Complexes of Ir(III) 14.3 Sensory Applications of Intramolecular Photoinduced Electron Transfer of Ir(III) Complexes 14.4 Photoredox Catalysis Based on Intermolecular Photoinduced Electron Transfer of Ir(III) Complexes 14.5 Outlook Acknowledgements References

287 289 290 290 291 300

300 302 308 312 316 319 326 328 328 328

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Chapter 15 Electrogenerated Chemiluminescence in Functional Redox Chemistry Elena Villani and Shinsuke Inagi 15.1

Introduction

331 337

338

346 356 356 357

359

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15.2

Fundamentals of ECL: Mechanisms of Light Generation 15.2.1 Annihilation ECL 15.2.2 Coreactant ECL 15.3 Applications of ECL in Molecular Electrochemistry 15.3.1 Novel ECL Reaction Systems 15.3.2 ECL for Imaging Applications 15.3.3 ECL of Organic Systems 15.3.4 Aggregation and Crystallization-induced Emission in ECL 15.4 Conclusions and Future Directions Acknowledgements References Subject Index

360 360 362 364 364 366 366 369 371 371 371 375

Part 1 Sustainable Redox Reaction

CHAPTER 1

Redox-mediated Electrochemical Cyclization Reactions ZHENG-JIAN WU AND HAI-CHAO XU* State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China *Email: [email protected]

1.1 Introduction More than 90% of common organic compounds contain rings.1,2 As a result, the search for efficient means for the construction of cyclic structures has been constantly pursued in the field of organic synthesis. Organic electrochemistry employs electric current to drive organic synthetic reactions and is attracting renewed interest in the past decade.3,4 Since electrons do not produce reagent-related waste and are among the cheapest reagents for chemical synthesis, organic electrochemistry holds great promise in developing green and sustainable synthetic methods.5 While the majority of the reported organic electrosynthetic reactions rely on direct electrolysis, indirect electrolysis with mediators has been increasingly explored, especially in the past few years.6–9 The use of mediators not only allows the reactions to proceed under mild electrode potentials to reduce energy consumption and increase selectivity10,11 but also significantly expands the scope of organic electrosynthesis to many redox inactive compounds through atom transfer catalysis12 or C–H bond activation.13–16 Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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In addition, electrolysis with a redox mediator allows the generation of radical intermediates in the bulk solution away from the electrode surface to avoid electrode passivation and reduce their local concentration.9 Key to the success of indirect electrosynthesis is the development of mediators that can function under electrochemical conditions. Efforts in the past few years have significantly expanded the list of mediators, some of which are listed in Scheme 1.1A, leading to the rapid development of indirect electrosynthesis. The mediators promote electrochemical reactions through an outer-sphere or inner-sphere mechanism (Scheme 1.1B). In the latter case, a transient adduct is formed between the mediator and substrate either after or before electron transfer on the electrode. In this context, many redox-mediated electrochemical cyclization reactions have been disclosed for the synthesis of various hetero- and carbocycles and will be the focus of this chapter (Scheme 1.1C). These reactions proceed mainly through radical or ionic cyclization to forge the ring structures.

1.2 Radical Cyclization Reactions Radical cyclization reactions are effective for the synthesis of ring structures because of the versatile reactivity of radical species and the possibility for cyclization cascades.17,18 In this context, several redox strategies have been developed for the electrochemical generation of various heteroatom- and carbon-centered radical species. These reactive species react to form several classes of hetero- and carbocycles by cyclization onto the tethered p-systems, 1,5-hydrogen atom transfer, or intermolecular addition to alkenes or alkynes to induce cyclizations.

1.2.1

Cyclization Reactions of Heteroatom-centered Radicals

Nitrogen-centered radicals (NCRs) are attractive intermediates for the construction of C–N bonds.19–22 These reactive species are commonly produced through the cleavage of a weak N–heteroatom bond.23 The current trend is to generate NCRs from stable and easily available N–H precursors.24–28 The Xu group reported in 2014 an early example of redox-mediated electrochemical generation of NCRs from anilides using 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) as the mediator (Scheme 1.2A).29 Mechanistically, the anilide 1d is deprotonated by hydroxide generated at the cathode and then oxidized to amidyl radical 3 via single electron transfer (SET) by anodically generated oxoammonium salt (TEMPO1). Intermediate 3 undergoes 5-exo-trig cyclizations to give carbon-centered radical 4, which is trapped with TEMPO to generate the final aminooxygenation product 2d. A similar aminooxygenation reaction was later achieved using a continuous flow electrochemical microreactor by Wirth and coworkers.30 Although the use of TEMPO as the mediator limits the reaction to aminooxygenation,31 this work proves that redox catalysis can be an effective strategy in developing electrochemically driven radical reactions. Importantly, the

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solutions because of its reduced stability and oxidation potential in these solvents. As a result, organic mediators such as 7 or 6 are used for the cyclizations of 12 and 24, respectively.34,38 The use of redox catalysis allows the formation of heterocyclic products that are oxidized at lower potentials than the starting anilides. Direct electrolysis can also be employed to promote NCR formation and cyclizations but often requires an increase in oxidation potential from substrate to product.39–42 Otherwise, further oxidation of the product can occur.43 Aza-Wacker-type cyclization reactions, which are commonly achieved with Pd-catalysis, are attractive transformations for the preparation of N-heterocycles.44 To expand the scope of these types of cyclization reactions and avoid the use of noble metals, several alternative methods based on radical cyclizations have been developed.32,45–49 For example, the Xu group has developed radical-based aza-Wacker-type cyclization reactions of anilides employing Cu-catalysis in the presence of stoichiometric hypervalent iodide as the terminal oxidant.45 The same group has also developed similar types of cyclization reactions through direct electrolysis without using any catalysts or external chemical oxidants.46,47 While these transformations are effective for the cyclization of various unactivated alkenes, relatively high electrode potentials are needed to generate the radical intermediates. Hu and coworkers have developed electrochemical aza-Wacker-type cyclizations of anilides employing a catalytic amount of Cu(II) salt as the mediator.50 These reactions are conducted in a divided cell to avoid the cathodic reduction of the Cu(II) salt to copper. The Xu group has recently achieved cobalt-catalyzed electrochemical aza-Wacker-type cyclizations of various di-, tri-, and tetrasubstituted alkenes in an undivided cell (Scheme 1.4).32 Besides function as an electron transfer mediator for amidyl radical formation, the cobalt catalyst (5) also serves as a hydrogen atom transfer agent to convert the carbon radical 32 to the alkene product. The low oxidation potential of the cobalt-based catalyst ensures high functional group tolerance of the electrocatalytic method. Other easily oxidized nucleophiles such as sulfonyl hydrazine (29f) and oxime (29e) are also suitable for electrocatalytic cyclization. The oxidation of oximes produces iminoxy radicals that are reactive to both oxygen and nitrogen atoms.51 The electrocatalytic oxidation of oximes to iminoxy radicals can also be accomplished with TEMPO as the catalyst.52 Halides are widely employed mediators for organic electrosynthesis and have been utilized for the generation of NCRs from acidic amides or sulfonamides.53–55 The anodic oxidation of iodide and bromide generates the corresponding dihalogen, which reacts with the nitrogen anions to produce N–halogen species (Scheme 1.5). These intermediates are converted to NCRs through the cleavage of the N–halogen bond facilitated by heating, cathodic reduction, or light irradiation. Chen and coworkers have reported that the electrolysis of N-aryl sulfonamide 33 in the presence of a catalytic amount of iodide-afforded carbazole 35 (Scheme 1.6A).56 On the other hand, Zheng and coworkers have disclosed that the electrooxidation of N-acetoxy amide 36 with 1 equiv.

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inner-sphere pathway to produce the key radical intermediate 55. The weak S–O bond of the adduct 54 allows its homolytic cleavage at rt. Unlike the anilide-derived NCR, which reacts on the nitrogen atom, thioamidyl radicals such as 55 undergo cyclization reactions on the sulfur atom. The cyclization reactions of thioamides can also be achieved in the absence of mediators in batch71 or continuous flow electrochemical microreactors72–74 but require higher electrode potentials. While P-centered radicals have been extensively studied for the synthesis of organophosphorus compounds,75–77 electrochemical methods for the generation of these intermediates have remained underdeveloped.78 Suga and Mitsudo and coworkers reported an electrosynthesis of five- and sixmembered phosphacycles with DABCO (1,4-diazabicyclo[2.2.2]octane) as the mediator via phosphinyl radical cyclization (Scheme 1.9). The anodic oxidation of DABCO produces amine radical cation 58, which abstracts a hydrogen atom from diarylphosphine oxide 56a to generate P-centered radical 59. The latter undergoes cyclization and rearomatization to generate the final phosphacycle 57a.

1.2.2

Cyclization Reactions of Carbon-centered Radicals

Redox catalysis is also an effective strategy for the electrochemical oxidation of 1,3-dicarbonyl carbon compounds to electrophilic carbon-centered radicals.32,79–82 Like the electrocatalytic generation of NCRs from anilides, deprotonation of the acidic C–H precursor with base generated at the cathode produces carbanions that can be oxidized by the catalyst to carboncentered radicals (Scheme 1.10A). This method has been applied for the synthesis of 3-fluorinated 2-oxoindoles 62 through the dehydrogenative cyclization of malonate amides 61 (Scheme 1.10B).79 3-Alkyl 2-oxoindoles are also accessible with this strategy under modified conditions.80 Cobalt salen complex 68 has been employed as a catalyst by the Xu group for the intramolecular allylic alkylation reactions (Scheme 1.10C). The electrocatalytically generated carbon-centered radical 66 undergoes cyclization onto the tethered alkene to generate alkyl radical 67. The latter loses an H atom to the Co(II) catalyst to produce the alkene product 65. Note that these types of cyclizations are traditionally achieved with stoichiometric metal salts such as Cu(OAc)2 and Mn(OAc)3.83 Radical addition to alkynes and alkenes generates new carbon-centered radicals, which can undergo cyclizations with appropriately positioned p-systems. The Xu group has reported that difluoromethylsulfonylhydrazine 70 is an easily available precursor for difluoromethyl radical (Scheme 1.11).84,85 The electrochemical oxidation of difluoromethylsulfonylhydrazine 70 with ferrocene as the catalyst generates difluoromethyl radical through oxidative dehydrogenation to diazene 75 followed by its decomposition. The addition of difluoromethyl radical to alkyne 69 initiates a radical cascade cyclization to produce fluorinated dibenzazepine 71.

Redox-mediated Electrochemical Cyclization Reactions

19

oxidants such as N-chlorosuccinimide (NCS) can also promote the same reaction but not I2. Based on these observations, the authors propose that iodide is oxidized on the anode to generate I1, which promotes the oxidation of sulfinic acid 77a to sulfonyl radical 79. The addition of this radical to alkynones 76a followed by cyclization and rearomatization generates the indanone product 78a. On the other hand, Zeng and coworkers have shown that electrochemically generated bromine reacts with aryl and alkyl sulfinates 84 to generate the sulfonyl bromide 86, which undergoes heat-induced homolytic bond scission to generate sulfonyl radical 87 (Scheme 1.12B).88 The addition of 87 to acrylate amides 83 generates sulfonated 2-oxoindoles 85. The use of sodium trifluoromethylsulfonate 91 (Langlois reagent) produces trifluoromethylated 2-oxoindoles 92 (Scheme 1.12C). This latter reaction has also been achieved with MnBr2 as the mediator by Mo and coworkers.89 The Lin group has developed a series of Mn-catalyzed electrochemical transformations employing easily available heteroatom and carbon nucleophiles as the radical precursors.90 In one example, they achieved the electrocatalytic synthesis of chlorotrifluoromethylated pyrrolidines 94 by cyclization of enyne 93 (Scheme 1.13).91 Here, CF3SO2Na and [MnII]–Cl are oxidized on the anode to generate CF3 radical and [MnIII]–Cl. Regioselective addition of CF3 radical to the alkene moiety of 93 followed by cyclization produces vinyl radical 96. The latter reacts with [MnIII]–Cl to produce the final pyrrolidine product. Manganese salt has also been employed to promote the cyclization of N-substituted 2-arylbenzoimidazoles 97 by Lei and coworkers (Scheme 1.14).92 They propose that anodic oxidation of alkylboronic acids 98a generates alkyl radical 100, which reacts with MnII to produce MnIII complex 101. The latter reacts with 97a to initiate radical cyclization to ultimately generate the cyclized product 99a.

1.3 Halide-mediated Ionic Cyclization Reactions Anodically generated electrophilic halogen species have been employed for the promotion of ionic cyclizations of alkenes, ketones, and arenes. Hypervalent iodine species, which are common reagents in organic synthesis, can be generated in situ through anodic oxidation of iodobenzenes.93–96 Zeng and coworkers have reported that electrolysis of 2-vinylanilide 104 in alcoholic solvents in the presence of 0.5 equiv. of nBu4NI produced indoline 105 (Scheme 1.15A). The cyclization of the iodonium intermediate 106 to generate 3-iodoindoline 107 is proposed as the key step for this process. Analogous electrochemical cyclization reactions have been reported by Wang and coworkers for the synthesis 3-azido (109)97 and 3-aminoindolines (111)98 (Scheme 1.15B). Anodic oxidation of iodobenzenes in the presence of Et3NxHF (x ¼ 3 or 5) generates the corresponding difluoroiodobenzenes.99 Waldvogel and

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Scheme 1.15

Chapter 1

Halide-mediated electrochemical cyclizations of alkenes and indoles.

coworkers employed 4-iodotoluene 114 as the mediator to promote the fluorocyclization of N-allylcarboxamides 112 to 2-oxazolines 113 (Scheme 1.15C).100 The authors proposed that the difluoroiodotoluene 115 generated on the anode reacted with the alkene moiety of 112a to produce iodonium species 116, which cyclized on the amidyl oxygen atom to afford hypervalent iodine species 117. The latter was converted into the final oxazoline product after nucleophilic substitution with fluoride. Vincent and coworkers have developed an electrochemical halocyclization of tryptamine, tryptophol, and tryptophan derivatives 118 for the synthesis of 3-haloindolines 119 with MgBr2 or MgCl2 as the halogen sources (Scheme 1.15D).101 The mechanism for the bromination reaction involves

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1.4 Conclusion The past few years have witnessed tremendous progress in mediated electrosynthesis. The application of this strategy in cyclization reactions has led to the development of exciting methodologies for the preparation of various heterocycles and carbocycles. The development of new redox mediators and the discovery of new mechanisms for the mediated activation of small molecules have significantly expanded the scope of electrosynthesis in ring constructions. Most of these electrochemical transformations are oxidative reactions but do not require stoichiometric sacrificial chemical oxidants, providing sustainable entries into ring structures. Future efforts in expanding the repertoire of mediators will further increase the synthetic utility of electrochemistry in promoting cyclization reactions.

Acknowledgements The authors acknowledge the financial support of this research from NSFC (No. 21971213) and Fundamental Research Funds for the Central Universities.

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CHAPTER 2

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids ALESSIA PETTI AND KEVIN LAM* Department of Pharmaceutical, Chemical and Environmental Sciences, School of Science, University of Greenwich, Chatham Maritime, Chatham, Kent, ME4 4TB, UK *Email: [email protected]

2.1 Introduction The electrolysis of carboxylic acids can be dated as early as 1832,1 when Faraday firstly studied the electrochemical behaviour of aqueous acetic acid solutions. However, it was Hermann Kolbe 15 years later who recognised ethane and CO2 as the main products of this transformation.2 This initial foray has been followed by many other electrochemical developments such as the Tafel’s rearrangement of acetoacetic esters,3 the Simons production of fluorocarbons,4 the Monsanto adiponitrile process,5 or the Shono oxidation.6 Despite these noticeable contributions, organic electrochemistry has been placed at the bottom of a synthetic chemist’s toolbox for many years.7 The lack of standardised equipment, prohibitive costs, and the possibility to replicate the same reactions using chemical reagents, even if toxic and expensive, are among the main reasons for this negligence. More recently, the advent of easy-to-use and affordable electrochemical setups8 in combination with the increasing need for more sustainable synthetic practices has contributed to a rediscovery of the field. In line with this renewed Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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interest, the electrolysis of carboxylic acids has gained considerable attention throughout the last few years.9,10 These inexpensive and readily available building blocks can be electrolysed and converted into high-valueadded products, with a range of applications in natural product chemistry, medicinal chemistry, material chemistry, etc.3 Before illustrating the most recent developments in the field, a brief overview on the Kolbe and nonKolbe processes will be given to the reader.

2.2 Background of the Kolbe Electrolysis Known as the earliest example of electrochemical C–C bond formation,11 the Kolbe electrolysis takes place when a carboxylate is oxidised at the anode to form the corresponding carboxyl radical (see Scheme 2.1). If R is an alkyl, a decarboxylation rapidly occurs, generating the alkyl radical R . If R is an aryl, as we shall discover later, the outcome of the transformation is quite different.12 By modifying the reaction conditions, R can undergo different reaction pathways. Indeed, the alkyl radical can react with itself to form homodimers (see Scheme 2.1, pathway a).13–16 This approach proved to be useful for extending carbon chains17,18 and in natural product synthesis.19,20 More recent applications include the homodimerisation in sono-emulsified systems,21 C–C coupling using solid-supported bases,22 or cycloalkane-based thermomorphic systems.23 When an additional carboxylic acid (R2COOH) is introduced in excess in the cell, the Kolbe electrolysis leads to the formation of heterodimers as final products (see Scheme 2.1, pathway b).24 This type of unsymmetrical radical sp3–sp3 cross-coupling has found interesting applications in the synthesis of insect pheromones.16,25–28 However, the presence of the co-acid in excess results in the unavoidable formation of its homodimer as the primary side product. For this reason, the method is mainly limited to co-acids giving volatile or easily removable homodimers. Interestingly, when an unsaturated acid undergoes a Kolbe electrolysis in combination with a co-acid (R2COOH), a first decarboxylation followed by a subsequent intramolecular cyclisation allows the formation of an electrochemically generated carbon-centred radical (ECCR). The latter then reacts intermolecularly with R2 to form cyclic

Scheme 2.1

Possible applications of the Kolbe electrolysis.

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids

31

products. This process, also known as Kolbe intramolecular cyclisation tandem reaction (see Scheme 2.1, pathway c), has been widely adopted to assemble valuable polycyclic structures.16,29,30 A series of experimental parameters have proven to have a significant impact on Kolbe decarboxylation.31 Usually, the reaction takes place when a partially neutralised methanolic solution of the carboxylate is electrolysed in an undivided electrochemical cell equipped with Pt electrodes. Other electrode materials were demonstrated to be detrimental to the outcome of the transformation, due to competitive overoxidation of the newly formed alkyl radicals on and near the electrode surface.32 High current densities (Z100 mA cm2) have also proven to favour the process since they guarantee prompt adsorption of the negatively charged carboxylate on the electrode surface and therefore ensure high concentrations of the radicals ready to recombine among themselves. The presence of foreign anions (e.g. NO3, ClO4) and cations (e.g. Fe21, Co21, Mn21) should be avoided, as they seem to interfere with the formation of the carboxylate layer. When this happens, lower yields of the Kolbe products are recovered as a consequence of the competitive oxidation of the alcoholic solvent at the anode.33 Nevertheless, alkali metal hydroxides or alkoxides proved experimentally to be the most suitable bases for deprotonating the carboxylic acid, while ensuring a good conductivity to the reaction medium. The best results in terms of yield are achieved in the presence of a neutral or weakly acidic environment. When more acidic media are chosen, a higher percentage of radicals gets converted into the corresponding carbocations. Lastly, higher percentages of Kolbe products are formed when running the reaction at room temperature, whereas increasing and decreasing the temperature leads to degradation and lower mass conversions,32 respectively. When deviating from the optimal conditions described herein, a series of non-Kolbe products can be generated.

2.3 Background of Non-Kolbe Electrolysis The Kolbe-produced alkyl radical can be further oxidised into the corresponding carbenium ion when platinum electrodes are switched for inexpensive graphite electrodes.16,34,35 The latter can then undergo a b-Helimination to form an alkene (see Scheme 2.2, pathway a). Alternatively, if the electrolysis is performed in a nucleophilic solvent such as water or alcohols,36 the newly formed carbenium can be trapped, leading to the formation of alcohols and ethers, respectively (see Scheme 2.2, pathway b). This kind of transformation is known as Hofer–Moest decarboxylation. Other commonly employed nucleophilic partners for carbocation include trifluoromethyl,37 olefins,38 carboxylic acids,39 and nitriles.40 Another possible fate for the carbenium involves a reaction with the unreacted carboxylate to generate esters (see Scheme 2.2, pathway c).33 Coming back to the factors that may facilitate the formation of the carbenium ion, the use of graphite electrodes has been demonstrated to be crucial for the success of the reaction.41 A possible explanation for that lies in the increased

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids 30

33

` et al. to synthesise five- and sixapproach has been used by Marko membered carbo and heterocycles via electrogenerated carbon-centred radicals. However, when an aromatic acid replaces the unsaturated aliphatic acid, the outcome of the reaction is dramatically different. Aromatic carboxylates were believed for a long time to be inert towards Kolbe electrolysis.35 Indeed, they exhibit a higher oxidation potential than their aliphatic counterparts (1.9 V vs. saturated calomel electrode SCE for benzoate and 1.24 V vs. SCE for acetate).49 Once the corresponding aroyloxy radical is generated, its decarboxylation does not occur at temperatures below 120–130 1C.50–52 Nonetheless, this different reactivity can be turned into an advantage. When a benzoic acid derivative, bearing a suitably positioned unsaturation, is electrolysed under Kolbe conditions, the electrogenerated oxygen-centred radical can cyclise intramolecularly, leading to the formation of the corresponding phthalide as well as a new carbon-centred radical (see Scheme 2.4). The latter can then react with an alkyl radical coming from the concomitant Kolbe decarboxylation of an aliphatic co-acid. This method has been used by Lam49 and coworkers to rapidly access a range of a-functionalised phthalides starting from unsaturated benzoic acid derivatives. The electrolysis was carried out in an undivided electrochemical cell under standard Kolbe conditions using an excess of aliphatic co-acid. The use of electron-deficient alkenes facilitated the radical cyclisation and provided good yields of the final product. Indeed, when 2-vinylbenzoic acid was submitted for the electrolysis conditions, only traces of the phthalide were recovered along with several degradation products. More noticeably, functionalities such as esters, amides, olefins, and halide were all well tolerated by the new methodology. The nature of the co-acid was also demonstrated to influence the fate of the electrolysis, with primary acids giving the best results in terms of yield, while easily overoxidisable secondary and tertiary acids failed in affording the alkylated phthalides. Interestingly, no decrease in yield was observed when the reaction was scaled up to 2 g. A possible mechanism for Kolbe lactonisation is suggested (see Scheme 2.4). Due to the difference in oxidation potentials between an aromatic and aliphatic carboxylate, it is reasonable to assume that the reaction starts with the anodic decarboxylation of the co-acid to form an alkyl radical, which is then responsible for the oxidation of the aromatic carboxylate. At this stage, the resulting aroyloxy radical cyclises through a classical 5-exo-trig mechanism, affording the phthalide ring and a new carbon-centred radical. The latter then recombines with an alkyl radical generated by the concomitant decarboxylation of the co-acid, leading to the final functionalised phthalide. A direct oxidation of the aromatic carboxylate at the electrode surface has been ruled out, given that, in the absence of co-acid, the benzoic acid was fully recovered at the end of the electrolysis. A possible oxidation of the olefin into a radical cation followed by its capture from the nucleophilic carboxylate has also been excluded since no additional product was formed upon the electrolysis of ethyl cinnamate with acetic acid.

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids

Scheme 2.5

35

Electrochemical lactonisation of hemioxalate salts. NMR yields are reported due to the volatility of the compounds.

However, due to their lower oxidation potentials, hemioxalates get oxidised faster than the aliphatic co-acid. Therefore, it was necessary to add compound 1 in six portions over one hour. This helped maintain a steady concentration of 1 throughout the electrolysis, thus avoiding its consumption before coupling with the co-acid. Following this method, a range of g-substituted butyrolactones was promptly synthesised (see Scheme 2.5). Different co-acids were tested to prove their suitability as alkylating partners. Unsurprisingly, primary carboxylic acids gave good results, with no decrease in yield upon increasing their chain length or steric hindrance. The coupling product was also obtained using cyclic carboxylic acids, although with lower yields. Ester-terminated and halogen-terminated alkyl chains

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Chapter 2

Scheme 2.8

Diastereoselective electrochemical lactonisation.

Scheme 2.9 Competing pathway for the anodic synthesis of lactams.

2.4.1.1

Limitations of the Method

Several attempts were made at applying the Kolbe electrolysis to the synthesis of g-substituted lactams. Unfortunately, under standard Kolbe conditions, the overoxidation of the oxamic radical into the more stable carbamoyl cation could not be avoided. The latter then reacts with the solvent to afford the corresponding carbamate (Scheme 2.9). A possible explanation for that behaviour lies in the nitrogen lone pair’s stabilising effect on the carbocation. It was then hypothesised that introducing electron-withdrawing groups (e.g. tosylate, triflate) on the nitrogen centre would prevent the lone pair donation and, therefore, disfavour the carbocation formation. However, no trace of the cyclised product was observed after electrolysis, which instead afforded cleanly and solely the corresponding carbamate.

2.4.2

Hofer–Moest Synthesis of Isocyanates

In the previous paragraph, it has been shown how oxamic acids react differently from their oxalic equivalents when anodically oxidised. In this section, this limitation of the Kolbe method has been turned into an advantage to access carbonyl derivatives. Ureas, carbamates, and thiocarbamates are valuable synthons that have wide applications across fields, ranging from medicinal chemistry to material chemistry.57–59 They are conventionally synthesised via isocyanates, which are usually prepared by treating a primary amine with phosgene. Due to the high toxicity

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Chapter 2

Scheme 2.11

Mechanism of the anodic oxidation of oxamic acids.

gave the best results in terms of yield. Finally, with optimal flow conditions in hand, the poorly conductive oxamic acids were tested for the electrolysis in flow (see Scheme 2.14). As a result, aryl substrates, inaccessible in batch, were successfully synthesised in flow, although with modest yields. Nevertheless, the reaction time for the electrolysis was reduced to only 6 min, with no requirement for purification once the isocyanate was allowed to react with the nucleophile. This approach represents a practical and oxidant-free approach to synthesising functionalised unsymmetrical ureas, carbamates, and thiocarbamates. Furthermore, the method showed a broad functional group tolerance, while avoiding the handling of toxic intermediates. It can therefore be used to access pharmaceutical targets on both laboratory and industrial scales.

2.4.3

Hofer–Moest Synthesis of Orthoesters

Continuing our exploration of the electrolysis of carboxylic acids, another exciting application of the Hofer–Moest reaction involves the generation of orthoesters. These highly reactive compounds are known for their use as diol protecting groups,68 acylating agents,69 Claisen rearrangement reagents,70 or coupling partners in heterocyclic chemistry.71 Despite their vast applications, common methods to prepare them remain nontrivial and usually

42

Scope of the electrochemical synthesis of carbamates and thiocarbamates.

Chapter 2

Scheme 2.13

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require harsh conditions, thus significantly limiting the tolerance of common functional groups.41,72 Consequently, only a few orthoesters are commercially available. To solve this issue, a mild and straightforward electrochemical synthesis of orthoesters has been developed.41 The synthetic strategy uses a dithiane carboxylic acid (DTCOOH) as a nucleophilic orthoester/ester equivalent (see Scheme 2.15). It was hypothesised that a Hofer–Moest decarboxylation of the dithiane moiety, followed by nucleophilic attack from the MeOH used as a solvent, would lead to the formation of the orthoester. Cyclic voltammetry experiments were crucial in this study. Cyclic voltammetry allowed to measure the oxidation potential of the dithiane acid (Epa ¼ 1.38 V vs. Fc/Fc1, a value comparable to the one preceding described for the oxidation of dithianes).73 However, it also revealed that the oxidation of these dithianes led to high passivation of the electrodes. This observation was in line with what has already been reported in the literature.74 The addition of a base not only lowered the oxidation potential (Epa ¼ 1.0 V vs. Fc/Fc1) but also completely suppressed the electrode fouling. The newly discovered reaction was then applied to a series of functionalised dithiane acid derivatives (see Scheme 2.16). As shown in Scheme 2.16, both aliphatic and aromatic orthoesters were successfully electrosynthetised, along with orthoesters bearing alkenes, alkynes, silyl, halides, fluorine, nitriles, ethers, and amide groups. Cyclic ethers and amides were also tolerated, although in this case, the anodic oxidation was carried for only 15 F mol1 instead of the usual 30 F mol1 to prevent the formation of Shono-type oxidation products (alpha-methoxylated ether or amide).75 Only a slight decrease in yield was observed upon scaling up the process up to 1.81 mmol. Interestingly, when performing the electrolysis in different solvents, new orthoesters can be formed. This was the case of tri(ethyl) and tri(trifluoroethyl) orthoesters, which have been scarcely studied in the literature so far. Regarding the reaction mechanism, it is believed to start with the anodic oxidation of one of the two sulfur centres of the dithiane since its oxidation potential is lower than that of the carboxylate (see Scheme 2.17).35 Consequently, the radical cation of the dithiane A forms the thionium B. The nucleophilic attack from the solvent then generates a mixed S,O-acetal C. The latter rapidly decomposes into the oxonium D by releasing a dithiol, which is then oxidised into the corresponding dithiolane. Alternatively, C’s sulfur moiety can get oxidised, giving the oxonium D and 1,3-propandithiyl radical, which further cyclises. Finally, the carboxylic acetal E is formed upon adding

Scheme 2.15

A new orthoester/ester synthon.

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids

Scheme 2.16

45

Scope of the anodic oxidation of dithiane acids.

another molecule of solvent onto the oxonium D. At this stage, E undergoes a classical Hofer–Moest decarboxylation, followed by a nucleophilic attack from the solvent to generate the final orthoester. Given that the reaction’s only byproduct is 1,2-dithiolane, which is volatile enough to be removed under reduced pressure, orthoesters can be utilised in further synthetic applications without the need for any purification. Among the possible subsequent reactions involving orthoesters, they can serve as condensation partners in a metal-free and Lewis acid-free one-pot preparation of benzimidazole and its thio- and oxo-analogues, which are

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Scheme 2.17

Plausible mechanism of the anodic oxidation of dithiane acids.

known for their antituberculosis, anticancer, antimalarial, antihistamine, and antimicrobial properties (see Scheme 2.18).76,77 Alternatively, orthoesters can be directly involved in Pd-catalysed cross-couplings (see Scheme 2.18).

2.4.4

Electrochemical Methoxylation

Functional group chemistry is an essential part of organic synthesis. Among the various protecting groups for alcohols, methoxymethyl (MOM) ethers are popular due to their high tolerance towards a wide range of reaction conditions.78–81 However, the main limitation to their use lies in the severe carcinogenicity of the derivatising agent chloromethyl ether (MOMCl).82–85 Other alternatives for the transformation, such as using formaldehyde dimethyl acetal (FDMA) in combination with various catalysts (e.g. P2O5, p-toluenesulfonic acid, Nafion-H, trimethylsilyl iodide, molybdenum(VI) acetylacetonate, BF3), are still expensive and time consuming.86 The issue can be circumvented by performing the etherification electrochemically. For this purpose, a-alkoxy carboxylic acids were subjected to standard Hofer–Moest conditions. As a consequence, high yields (up to 91%) of the

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids

Scheme 2.18

Further applications of orthoesters.

Scheme 2.19

Mechanism of the electrochemical methoxylation.

47

MOM-type ether were obtained.34,87 As previously, the reaction begins with the oxidation of the carboxylate to form a carboxyl radical, which rapidly undergoes decarboxylation and further oxidation into the corresponding oxonium cation. Subsequent trapping of the oxonium by the solvent affords the final MOM ether (see Scheme 2.19). Unlike conventional chemical protecting methods, the electrochemical methoxylation proceeds at room temperature, under mild conditions, without the need for drying or degassing the solvent prior to the electrolysis. Changing the current density or the nature of the base used to deprotonate the starting material had little impact on the success of the electrolysis, as did scaling up the process to 5 g. However, some substrates remained challenging to synthesise. For instance, compounds bearing easily oxidisable moieties, such as benzylic groups, have been shown to get overoxidised to the corresponding dimethoxylated species during the electrolysis. Once again, this issue can be solved by using flow electrosynthesis. Indeed, the smaller interelectrode gap and the low residence time associated with flow

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Chapter 2 88

setups avoid the product’s overoxidation. Running the reaction in flow helps also minimise the electrolyte resistance, significantly diminishing the need for supporting electrolytes. A space- and cost-efficient flow system was hence designed with the help of 3D printing.89 The resulting polypropylene flow cell equipped with carbon graphite electrodes can be attached and easily removed from commercially available electrosynthesis equipment (see Figure 2.1). The use of a potassium salt avoided the need for any additional wasteful supporting electrolytes and helped monitor the reagents’ passage within the flow cell. When only the pure solvent, methanol in this case, flowed through the cell, a large voltage (30 V) and a low current (1–3 mA) were observed. However, a sharp increase of current (up to 100 mA) and a voltage decrease to about 5 V were detected when the starting material passed through the cell.89 The endpoint of the transformation was indicated by the current/ voltage couple returning to their initial values. Compared to the batch

Figure 2.1

3D-printed electrochemical flow setup. Reproduced from ref. 89 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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2.5 Recent Advances in the Electrolysis of Carboxylic Acid Derivatives Although the electrolysis of carboxylic acids intends to be the main focus of this chapter, the electrochemical processes involving their derivatives have also been shown interesting applications. Therefore, they will be briefly presented below.

2.5.1

Electrochemical Deprotection of Aromatic Esters

As discussed before, organic electrochemistry can be used as a practical method for protecting alcohols under MOM-type ethers. Interestingly, electrochemistry can also be used to perform deprotection reactions. Due to their enhanced stability compared to their nonaromatic analogues, aromatic esters demonstrated to be challenging to hydrolyse under chemical conditions.102 However, they can be easily deprotected electrochemically by using a divided H-cell fitted with carbon graphite electrodes and NMP/iPrOH as a solvent mixture (see Scheme 2.24).103 Unlike the other reactions described herein, degassing the solvent was fundamental as oxygen inhibits the transformation. In order to facilitate the product formation, it was also necessary to increase the reaction temperature to 90 1C. Interestingly, ester, amide, free alcohol, and silyl ether

Scheme 2.24

Electrochemical deprotection of aromatic esters.

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Scheme 2.25

Chemoselective deprotection of 7 affording 8, 9, or 10 according to the applied potential.

functions were tolerated by the method. Moreover, electroanalytical studies confirmed that aromatic esters could be chemoselectively deprotected depending on their reduction potential. This can be modulated according to the nature and number of substituents on the aromatic ring. For example, the voltammogram of substrate 7 indicates three resolved reductions at 1.51 V, 1.80 V, and 2.5 V vs. Ag/AgCl.104 Single, double, and total deprotection can therefore be achieved by carefully tuning the electrode potential (see Scheme 2.25). This exquisite control of selectivity, which is impossible to achieve using chemical methodologies, represents one of the unique and most important features of electrosynthesis.

2.5.2

Electrochemical Deoxygenation of Diphenylphosphinates

The Barton–McCombie reaction is a common choice to achieve the deoxygenation of alcohols to form an alkane. However, the reliance on toxic tin reducing agents, expensive silicon hydrides, or the exploitation of lightsensitive intermediates complicates its application.105,106 The electrochemical deoxygenation of alcohols could be performed with ease when using diphenylphosphinic esters as starting materials.107 When electrolysing the phosphorus ester on graphite electrodes using DMF as solvent, a reduction occurs and forms the corresponding diphenylphosphinyl radical anion (see Scheme 2.26).

Recent Advances in the Kolbe and Non-Kolbe Electrolysis of Carboxylic Acids

Scheme 2.26 Table 2.1

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Electrochemical deoxygenation of diphenylphosphinates.

Decomposition rates for the phosphinate-derived radical anion.

Entry

R

k (s1)

1 2 3 4

Ethyl Cyclohexyl 1-Adamantyl Allyl

0.19 0.33 0.70 Too fast to be measured

Subsequently, the radical anion decomposes into diphenylphosphonic acid and an alkyl radical R . The decomposition rate for the radical anion can be measured using cyclic voltammetry and is correlated to the stability of the alkyl group R (see Table 2.1).107 Increasing the reaction temperature slightly to 60 1C improved the yields. Nevertheless, temperatures above that had a detrimental impact on the final products’ yield, as did performing the electrolysis at current densities higher than 100 mA cm2. More noticeably, by applying this method, primary, secondary, and tertiary phosphinates can be converted into their corresponding saturated products with excellent yields while retaining high functional group compatibility (see Scheme 2.27).

2.6 Future Perspectives This chapter shows that carboxylic acids are among the most attractive electrochemical precursors for C–C, C–X, and C–H bond formation. Their low cost and marked versatility have unlocked access to a wide range of value-added chemicals. Despite these evident merits, new and exciting reactions using these scaffolds are yet to be discovered. Focusing on paired

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Scheme 2.27

Chapter 2

Scope of electrochemical deoxygenation of diphenylphosphinates.

electrolyses would be desirable as both electrodes would yield valuable compounds.108,109 Another possible route would be the development of enantioselective electrochemical synthesis.110,111 Developing new asymmetric electrosynthetic strategies is crucial given the chiral nature of most pharmaceutical drugs.112 Lastly, the combination of electrochemistry with other disciplines such as flow chemistry and photochemistry represents an exciting prospect for organic electrosynthesis. For instance, the use of flow technology would enable rapid screening and scale-up of electrochemical processes.7 Moreover, as demonstrated by the flow electrochemical projects discussed in this chapter, excellent control of the selectivity and improved current efficiencies without the need for wasteful supporting electrolytes can be achieved. Merging electrochemistry with photochemistry would also be desirable as it improves the redox chemoselectivity and broadens the accessible ‘‘redox window’’ for single-electron transfer (SET),10,113 opening new opportunities to develop sustainable radical reactions.

2.7 Conclusion In summary, the latest advances in the electrolysis of carboxylic acids using Kolbe and non-Kolbe methods have been presented. A wide variety of reactions involving these precursors, such as oxidative decarboxylations, intramolecular cyclisations, and intermolecular radical cross-couplings, have been discussed. Most of them can be achieved under unprecedently mild reaction conditions, avoiding the use of toxic redox reagents or metal catalysts and showing a broad functional group tolerance. After a glorious past, a bright future is awaiting this exciting research area.

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Abbreviations ACN DBTDL DMAP DMF Fc NMP SCE SET TMSI

Acetonitrile Dibutyltin dilaurate 4-Dimethylaminopyridine Dimethylformamide Ferrocene N-Methyl-2-pyrrolidone Saturated calomel electrode Single-electron transfer Trimethylsilyl iodide

Acknowledgements We are grateful to the Engineering and Physical Sciences Research Council (Grant EP/S017097/1 to K.L.), the Leverhulme Trust (Grant RPG-2021-146 to K.L.) for their financial support, and the University of Greenwich (Vice Chancellor’s PhD Scholarship to A.P.).

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CHAPTER 3

Novel Electrolytic Processes KOICHI MITSUDO Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan Email: [email protected]

3.1 Introduction In the 1990s, combinatorial chemistry, which enables the creation of libraries containing numerous novel compounds, became a major field in chemical research. This breakthrough technology has been used for highthroughput screening in drug discovery1–13 as well as the discovery of innovative materials.14–17 From a technological point of view, the advances in combinatorial chemistry are related to miniaturisation and automation, which have made dramatic progress in the past few decades. This method has been used in combination with automated parallel synthesisers to construct chemical libraries. In the field of electrochemistry, a number of unique topics have been reported over the last two decades, including combinatorial electrosynthesis, site-selective synthesis, synthesis using solid-supported acids and bases, and device-assisted synthesis (microreactors, small bipolar electrodes, PEM reactors, etc.). However, electrochemical reactions require the examination of many parameters such as the electrode materials, supporting electrolytes, solvents, potentials, and currents, which makes determining the optimal reaction conditions time consuming. If combinatorial screening of the reaction parameters is possible, then the optimal conditions may be identified in a short period of time. Combinatorial synthesis can also be performed in Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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flow systems because electricity can act as an oxidant and reductant, with no chemical residues being generated. Thus, a combination of electrochemical and combinatorial approaches should be advantageous. Organic electrolysis is a synthetic method that strongly depends on the equipment used, unlike conventional organic synthesis, which uses glassware such as flasks, where the artifacts associated with the equipment hinder the efficient production of the desired product in the reaction system. Therefore, the development of electrochemical processes using novel electrochemical devices should significantly improve the optimisation of an electrochemical process when compared to conventional devices. In this chapter, the electrochemical processes used in organic synthesis have been described.

3.2 Parallel Batch Systems Used for Electroorganic Synthesis 3.2.1

Parallel Batch Systems Using the Cation Pool Method

One of the most efficient ways to build molecules is through the use of parallel processes. One example has been reported by Yoshida and Suga. They developed the ‘‘cation pool’’ method, which enables the generation and accumulation of cationic species as a ‘‘cation pool’’ using low-temperature electrolysis (Figure 3.1).18–35 The generated cations can also react with nucleophiles, which are readily oxidised under oxidative conditions (Scheme 3.1).18 Yoshida and Suga applied the ‘‘cation pool’’ method in both conventional organic synthesis and combinatorial parallel synthesis in the solution phase using a robotic synthesis system (Figure 3.2). After the substrate was electrochemically oxidised at low temperature to generate the ‘‘cation pool’’, it was divided into several portions and each portion was reacted with a different nucleophile (Nu1–5) to obtain the desired product in each vessel of the synthetic system. A variety of products were obtained using a range of different nucleophiles.

3.2.2

Parallel Batch Processes for Electrosynthesis

To efficiently perform an electrochemical reaction, it is important to construct a parallel electrochemical batch process. The first study on this concept was reported by Yudin in 2000.36,37 Yudin developed a parallel electrochemical system known as a ‘‘spatially addressable electrolysis platform (SAEP),’’ which was composed of 16 cells equipped with a graphite rod

Figure 3.1

The concept of the ‘‘cation pool method’’.

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Scheme 3.1

Figure 3.2

Chapter 3

Generation and accumulation of iminium cation pools and their reactions with nucleophiles.

Parallel batch system using the ‘‘cation pool’’ method.

anode and a stainless steel cathode connected to a DC supply (Figure 3.3). Constant-current electrolysis can be simultaneously carried out in all of the cells. Yudin described the electrochemical a-alkoxylation reactions of carbamates and sulphonamides (Scheme 3.2). This system enabled the simultaneous screening of the conditions and substrate scope used in the reaction. In 2001, Yudin also applied this method toward the electroreductive dimerisation of imines for the synthesis of 1,2-diamines.38 A simple parallel system was reported by Tajima et al. in 2009.39 They described the parallel anodic methoxylation of carbamates using silicasupported piperidine (Scheme 3.3). The electrolysis was carried out in five connected undivided cells at a constant current (Figure 3.4). Five substrates were methoxylated in each cell simultaneously. The methoxylated products were obtained using filtration and their subsequent concentration. Recently, Waldvogel reported a highly evolved parallel electrochemical synthesis system,40 which uses eight undivided cells (5 mL) equipped with two electrode holders placed on a steel block with eight cavities (Figure 3.5). All of the electrodes were connected to a multichannel DC power supply. Waldvogel developed a similar system using six divided cells (6 mL). The temperature of the stainless steel block was controlled and a galvanostat (DC power supply) equipped with a coulomb meter was used to adjust the current and voltage for each channel. Waldvogel used this system for a variety of electrochemical transformations, including electrochemical oxidative cross-coupling,41–43 dehalogenation,44 deoxygenation,45–47 and domino redox reactions48 (Scheme 3.4).

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Figure 3.3

63

Experimental setup for SAEP: (a) Before and (b, c) after assembly. Reproduced from ref. 36 with permission from American Chemical Society, Copyright 2010.

3.3 Combinatorial Flow System for Electroorganic Chemistry 3.3.1

Flow Electrochemistry

Microflow systems have become an important technology in the field of organic synthesis because of their ability to provide precise temperature control and efficient mass transport.49–51 The integration of flow chemistry and electrochemistry is certainly an attractive strategy, especially since the intermediates generated via electrochemical reactions are highly reactive

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Figure 3.5

65

(a) Electrochemical setup used for the parallel electrochemical synthesis developed by Waldvogel. (b) Cross-section of the undivided and divided cells. (c) Cross-sections of the carousels. Reproduced from ref. 40 with permission from American Chemical Society, Copyright 2015.

and unstable. In such cases, microflow systems are much better suited than batch systems.52–56 For instance, Yoshida and Suga developed a ‘‘cation flow method’’ in 2001 using a new electrochemical microflow system.53 The flow cell was composed of diflone and stainless steel, which was equipped with an anode (carbon felt) and a cathode (Pt) (Figure 3.6). The two compartments of the electrochemical reactor were separated using a PTFE membrane filter. Electrooxidation performed at low temperature afforded a cationic intermediate, which immediately reacted with nucleophiles to produce the final coupling products (Scheme 3.5). This ‘‘cation flow’’ system facilitated a sequential and continuous combinatorial approach toward the synthesis of multiple products by simply changing the flow path (Figure 3.7). Another type of microflow reactor used for electrochemical reactions has been reported by Atobe et al. They developed a thin-layer microflow cell used for electrosynthesis (Figure 3.8).57–68 In 2005, Atobe reported the use of the electrochemical thin-layer microflow cell for the synthesis of 2,5-dihydrofuran (Scheme 3.6).57 The advantage of this reaction system was that the

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reaction proceeded without the need for an electrolyte. In 2007, Atobe designed a laminar flow system using an ionic liquid as the reaction medium (Figure 3.9), which was used for electrochemical allylation (Scheme 3.7).60 Injecting two different solutions via two inlets formed a liquid–liquid phase, (i) Electro-oxidative cross-coupling reactions

(ii) Electrochemical dehalogenation reactions

Scheme 3.4

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67

(iii) Electrochemical deoxygenation reactions

(iv) Electrochemical domino oxidation reduction reactions

Scheme 3.4

Electrochemical reactions using the parallel electrolysis system developed by Waldvogel.

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Figure 3.10

71

Chemoselective reduction using a parallel laminar flow electrochemical system. Reproduced from ref. 63 with permission from the Royal Society of Chemistry.

metal-supported catalysts such as Pt/C, Rh/C, Ru/C, and PtRu/C as cathodes and achieved the conversion of toluene with high current efficiency (490%) under sufficiently mild conditions. In particular, the electrochemical hydrogenation of toluene using PtRu/C gave methylcyclohexane with a current efficiency of 94%. In the hydrogenation reactions using Pt/C, Rh/C, and Ru/C, the current efficiency appeared to decrease at high current densities, but the hydrogenation reaction using PtRu/C proceeded with excellent efficiency even at high current densities. In 2019, Atobe reported the chemo- and regioselective hydrogenation of alkynes using a PEM reactor (Scheme 3.10).80 They found that the selectivity of the hydrogenation could be controlled by the catalyst and cathode potential. In particular, Z-alkenes were obtained selectively when the Pd catalyst was used as the cathode catalyst. The control of cathode potential was also important. For instance, cis-stilbene was obtained selectively from diphenylacetylene at an appropriate potential (þ53 to 90 V). PEM reactors can also be used for asymmetric reactions. In 2020, Atobe reported enantioselective hydrogenation of a,b-unsaturated acids using a PEM reactor (Scheme 3.11).81 Reasonable enantioselectivity and excellent current efficiency were obtained during the asymmetric hydrogenation of a-phenylcinnamic acid in the presence of cinchonidine (CD).

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Figure 3.11

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Schematic setup of (a) a PEM reactor and (b) MEA. Reproduced from ref. 80 with permission from American Chemical Society, Copyright 2019.

Electrochemical hydrogenation proceeded under mild conditions without the addition of a supporting electrolyte. The current density was proven to be very important for obtaining these results.

3.4 Bipolar Electrochemical System Bipolar electrodes are isolated conductive materials in solution placed in an electric field, which serve as both the anode and cathode, and make it possible to investigate chemical reactions on an isolated electrode.82–101 In 2010, Inagi and Fuchigami used a U-shaped cell system containing bipolar electrodes to pattern conductive polymers (Figure 3.12(a)).87,88 This method can provide conductive polymer films with a gradient of functionality

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Scheme 3.8

Electroreduction of furanic compounds using a PEM reactor.

Scheme 3.9

Electroreduction of toluene to methylcyclohexane using a PEM reactor.

Scheme 3.10

Electrochemical hydrogenation of diphenylacetylene using a PEM reactor.

(Figure 3.12(b)). In this bipolar method, there is no need to attach the polymer to the circuit. Inagi and Fuchigami also used bipolar electrodes as a patterning technique for conductive polymer films.91 Local electrochemical doping was achieved by applying a local anodic potential to a polythiophene film on the bipolar electrode and complex patterns could be drawn on the polymer ‘‘canvas’’ in a site-controlled manner.

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Scheme 3.11

Figure 3.12

Chapter 3

Enantioselective hydrogenation of a,b-unsaturated acids using a PEM reactor.

(a) Schematic illustration of a U-type cell with a bipolar electrode (BPE) covered with a conducting polymer and (b) photographs of gradually doped films on the BPE. Reproduced from ref. 88 with permission from American Chemical Society, Copyright 2019.

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Figure 3.13

75

AC bipolar electropolymerization of EDOT. Reproduced from ref. 95, https://doi.org/10.1038/ncomms10404, under the terms of the CC BY 4.0 license, http://creativecommons.org/licenses/by/4.0/.

Recently, Inagi reported several applications of bipolar electrodes in organic synthesis.92–101 For example, they developed a click-type reaction using bipolar electrodes.92 They also reported an electrochemically mediated ATRP using bipolar electrolysis to produce patterned gradient polymer brushes,93 as well as the construction of microfiber networks consisting of poly(3,4ethylenedioxythiophene) (PEDOT) through electrolysis using a bipolar strategy and AC power supply (Figure 3.13).95

3.5 Conclusion In this chapter, the recent advances in organic electrochemical processes have been described. Several approaches, both in batch and flow systems, have been proposed to improve reaction efficiency. Because electrochemical reactions have many more parameters than classical organic reactions, it is important to further improve the equipment and processes to produce efficient organic electrochemical reactions. In the future, it is expected to be further developed by integrating electrochemical organic reactions with

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various fields, such as mechanical and electronic engineering and machine learning. This fascinating technology will be further applied to various research fields in life science and material science.

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CHAPTER 4

A Sugar Machiney HIROFUMI ENDO,a MD AZADUR RAHMANb AND TOSHIKI NOKAMI*a,b,c a

Department of Engineering, Graduate School of Sustainability Science, Tottori University, 4-101 Koyamachominami, Tottori city, 680-8552 Tottori, Japan; b Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, 4-101 Koyamachominami, Tottori city, 680-8552 Tottori, Japan; c Center for Research on Green Sustainable Chemistry, Faculty of Engineering, Tottori University, 4-101 Koyamachominami, Tottori city, 680-8552 Tottori, Japan *Email: [email protected]

4.1 Introduction The recent development of methodologies for the synthesis of oligosaccharides enables access to natural and unnatural ones more easily. Not only chemical methods based on synthetic organic chemistry but also enzymatic methods using glycosyl transferases and glycosyl hydrases have been used to synthesize oligosaccharides. Moreover, the chemoenzymatic method, which is a combination of chemical and enzymatic methods, has been developed to prepare complex oligosaccharides.1 Automated synthesis of oligosaccharides has also been reported based on both chemical and enzymatic methods.2 There are several types of chemical methods, such as solid-phase synthesis, solution-phase synthesis, and fluorous-phase synthesis. To synthesize the desired oligosaccharides on a preparative scale, solution-phase synthesis is preferable because it does not require additional y

This chapter is a translation of an article previously published in Japanese (ref. 29).

Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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steps for immobilization and removal. Of course, solid-phase synthesis and fluorous-phase synthesis have advantages in the purification of products. Therefore, these methods are practical in the preparation of various oligosaccharides on a small scale. To develop a chemical method of automated synthesis for oligosaccharides, the choice of the building block with the anomeric leaving group is important. Conventional carbohydrate building blocks frequently used in the synthesis of oligosaccharides are thioglycosides 1, glycosyl halides 2, and glycosyl imidates 3 (Figure 4.1). The glycosyl imidate is one of the most reactive building blocks; however, thioglycosides are stable and storable building blocks. Indeed, thioglycosides are the most popular building blocks for the chemical synthesis of oligosaccharides. Thus, the authors decided to develop a method for the automated synthesis of oligosaccharides based on electrochemical solution-phase synthesis by using thioglycosides as building blocks. In this chapter, automated electrochemical assembly is introduced as a powerful tool for the synthesis of oligosaccharides. Before mentioning the authors’ own works, the advantages of organic synthesis based on electrochemical methods and the pioneering works of electrochemical glycosylation shall be introduced. Electrochemical reactions under constant current conditions are carried out using a Galvanostatic method and both anodic oxidation and cathodic reduction occur at a constant rate. The amount of electricity can be controlled precisely because reaction time can be started and stopped within a second. If the electrochemical reaction requires 1 h (3600 s) for the consumption of 1.0 equiv. of electricity, 1 s is equal to 0.00028 equiv. of electricity. This level of accuracy is not necessary for organic synthesis. Moreover, redox reactions can be performed without adding reagents, and these are the benefits for the development of synthetic methodologies for automation based on electrochemical methods. Electrochemical glycosylation is a reaction initiated by electrochemical oxidative activation of carbohydrate building blocks and the thus-generated intermediates form glycosidic linkages by reactions with alcohols. The pioneering study was reported by Noyori and Kurimoto in 1986 utilizing aryl glycoside 4 (Scheme 4.1).3 Sinay and Lubineau independently reported electrochemical glycosylation of thioglycosides 1, which have lower oxidation potentials than aryl glycosides 4 (Scheme 4.2a and b).4,5 The number of reports regarding electrochemical glycosylation was around 10, including the 3 mentioned above, when the authors started their project in 2005. At

Figure 4.1

Conventional carbohydrate building blocks. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

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Scheme 4.1

Electrochemical glycosylation of phenyl glycoside. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

(a)

(b)

Scheme 4.2 Electrochemical glycosylation of thioglycosides. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

that time, the longest oligosaccharide synthesized by electrochemical glycosylation was trisaccharide.6

4.2 Electrochemical Generation of Glycosylation Intermediates 4.2.1

Generation of Glycosyl Triflate Intermediates

The reaction mechanism of electrochemical glycosylation proposed by Noyori in their first paper is shown in Figure 4.2.4 The reaction is initiated by single-electron oxidation of aryl glycoside 4, which is oxidized to the corresponding radical cation 8. The subsequent bond cleavage between the anomeric carbon atom and the oxygen atom of the aryloxy group affords the aryloxy radical species and glycosyl cation intermediate 9. The reaction is completed by the reaction of intermediate 9 with methanol as a nucleophile to

A Sugar Machine

Figure 4.2

83

Proposed reaction mechanism of electrochemical glycosylation. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

afford methyl glycoside 5 as a glycosylation product. In the case of electrochemical glycosylation using thioglycoside, the eliminated aryl thiol radical forms disulfide by dimerization. Glycosyl cations have been proposed as reactive intermediates of glycosylation including electrochemical glycosylation; however, they have not been detected spectroscopically yet. Glycosyl cations are so reactive that anions coordinate with the anomeric carbon. Indeed, glycosyl fluorides have been obtained in the presence of an electrolyte bearing BF4 anion. Based on the results of pioneering works and the authors’ studies, electrochemical oxidation of thioglycosides, including 1c, was performed using tetrabutylammonium triflate (Bu4NOTf) as an electrolyte in the absence of nucleophiles.7 As a result, the generation and accumulation of glycosyl triflates, which were covalent chemical species between the glycosyl cation and the triflate anion, were confirmed (Figure 4.3). For example, electrochemically generated glycosyl triflate 10 in CD2Cl2 was stable enough to be detected by NMR at low-temperature conditions by transferring the anodic solution to the NMR tube stored in the dry ice cooling bath. The NMR spectrum contains fewer peaks derived from reagents and/or by-products. Therefore, this method is advantageous from the viewpoints of synthesis and spectroscopic analysis. Of course, peaks of tetrabutylammonium cation appear at a higher magnetic field above 3.5 ppm in 1H NMR; however, these peaks do not overlap with peaks of the pyran ring of carbohydrates. Anodic oxidation is powerful enough to convert thioglycoside quantitatively. Therefore, protecting groups of hydroxyl groups that are labile under oxidative conditions are not tolerant when using this method. The normal 4,6-O-benzylidene acetal protection must be modified by introducing a chlorine atom at the para-position of the benzene ring to raise the oxidation potential of the protecting group. Thus-generated glycosyl triflates are as reactive as chemically generated glycosyl triflates. Hence, the electrochemical method is an alternative to chemical methods using reagents such as N-iodosuccinimide (NIS)/TfOH.

84

Chapter 4

Figure 4.3

4.2.2

NMR spectrum of electrochemically generated glycosyl triflate (a) anodic oxidation under constant current condition. (b) Chemical activation using diphenyl sulfoxide, triflic anhydride, and tri-tert-butylpyrimidine. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

Generation of Glycosyl Sulfonium Ion Intermediates

Glycosyl sulfonium ions are observable glycosylation intermediates that have a positively charged sulfur atom connecting to the anomeric carbon (Figure 4.4).8 The simplest method for the preparation of glycosyl sulfonium ions is the alkylation of the sulfur atom of thioglycoside; however, this method is applicable to limited types of thioglycosides (Figure 4.4a). The most common method for the generation of glycosyl sulfonium ions is intermolecular (Figure 4.4b and c) and intramolecular glycosylation (Figure 4.4d) with sulfides. Activation of glycosyl sulfoxide is an alternative to glycosylation (Figure 4.4e). Based on these results, it was envisioned that electrochemically generated glycosyl triflates might work as precursors of glycosyl sulfonium ions. Glycosyl triflates 17 and 18 generated from thioglycoside 1e and 1f are electrophilic enough to give glycosyl sulfonium ions 19 and 20 by reaction

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85

(a)

(b)

(c)

(d)

(e)

Figure 4.4

Preparation and structure of glycosyl sulfonium ions. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

Figure 4.5

Preparation and structure of glycosyl sulfonium ions. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

88 Table 4.1

Chapter 4 Oxidation potentials of thioglycosides. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

Thioglycoside

Ar group

Oxidation potential [V vs. SCE]

1f 1h 22a 1g 1i 22b

4-MeC6H4 4-MeC6H4 4-MeC6H4 4-FC6H4 4-FC6H4 4-FC6H4

1.65 1.54 1.64 1.73 1.67 1.67

already been generated and accumulated through low-temperature anodic oxidation of thioglycoside. To prove the authors’ concept, electrochemical iterative one-pot glycosylation was demonstrated (Figure 4.8).15 Anodic oxidation at low temperature converted thioglycoside 1f into glycosyl triflate 18 and the subsequent addition of CH2Cl2 solution of thioglycoside 1h bearing a protecting-group-free hydroxyl group at C-6 (6-OH) afforded the corresponding disaccharide of thioglycoside 22a with 84% yield together with 1,6-anhydrosugar 23 as a byproduct with 13% yield. The formation of 1,6-anhydrosugar 23 indicated the occurrence of the side reaction between glycosyl triflate 18 and the anomeric sulfur atom of thioglycoside 1h. To prevent this side reaction, the aryl substituent on the anomeric sulfur atom was changed from the 4-methylphenyl group to the 4-fluorophenyl group, and the desired disaccharide 22b was obtained with increasing yield without the formation of 1,6-anhydrosugar 23. The oxidation potential of disaccharide 22b (Eox ¼ 1.67 V vs. saturated calomel electrode [SCE]) was found to be lower than that of thioglycoside 1g (Eox ¼ 1.73 V vs. SCE), which was the precursor of glycosyl triflate 18. Thus, the conversion of 22b into the corresponding glycosyl triflate under the same electrochemical conditions was expected (Table 4.1). Thioglycosides 1h and 1i bearing protecting-group-free 6-OH have lower oxidation potentials than fully protected thioglycosides 1f and 1g. This fact suggested that electrochemical oxidation of 1f and 1g in the presence of 1h and 1i did not give desired disaccharides 22a and 22b selectively.

4.3.2

Demonstration of Automated Electrochemical Assembly of Oligosaccharides

Oligosaccharides with the desired chain length can be prepared by repeating the cycle of chain elongation the same number of times as the chain length. A single cycle of automated electrochemical assembly is equal to the one-pot iterative glycosylation, which consists of the conversion of thioglycoside to glycosyl triflate and coupling of the glycosyl triflate with an alcohol. To carry out this process in an automated manner, the authors have developed the electrochemical synthesizer by assembling personal computer, DC power supply, syringe pump, magnetic stirrer, and modified UC Reactors, which was

A Sugar Machine

Figure 4.9

89

First-generation automated electrochemical synthesizer. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

a cooling machine for variable temperature (Figure 4.9). Demonstration of the synthesis of oligoglucosamine with b-1,6-glycosidic linkages was performed by using the synthesizer (Figure 4.10). Under the same reaction procedure as disaccharide synthesis, automated electrochemical assembly of hexasaccharide 26 was accomplished. Unfortunately, the longer the target oligosaccharides became, the fewer the products obtained; however, the corresponding tetrasaccharide was obtained with 52% yield, which means an average of 81% yield per cycle. Thus, it is postulated that automated electrochemical assembly must be useful to prepare other oligosaccharides with different types of glycosidic linkages.

4.4 Synthesis of Biologically Active Oligosaccharides 4.4.1

Synthesis of TMG-chitotriomycin

There are various types of biologically active oligosaccharides with the same repeating unit as the corresponding polysaccharides or some modifications from the original structures. Here, two biologically active oligosaccharides, which are derivatives of chitooligosaccharides, are introduced to demonstrate practical applications of automated electrochemical assembly. Enzymatic hydrolysis of chitin is a crucial process for its metabolism. The process consists of two enzymatic reactions catalyzed by chitinase and N-acetyl glucosaminidase. Chitin is converted to oligo-N-acetyl glucosamine by chitinase, and oligo-N-acetyl glucosamine is converted to N-acetyl glucosamine by N-acetyl glucosaminidase. Trimethylammonium glycosidechitotriomycin (TMG-chitotriomycin) (27), which is a derivative of chitotetraose, was found to be an inhibitor of N-acetyl glucosaminidase in Streptomyces anulatus NBRC 13369 (Figure 4.11).16 It consists of chitotriose and TMG at the nonreducing end of the pseudo tetrasaccharide and all

A Sugar Machine

Figure 4.12

91

Total synthesis of TMG-chitotriomycin. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

A Sugar Machine

Figure 4.14

93

Stereoselective synthesis of disaccharides under the mixed-electrolyte conditions. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

0.025 M of Bu4NOTf and 93% b-selectivity was observed; however, the cell voltage became more than 100 V at a low concentration of Bu4NOTf. This was not an ideal reaction condition, even if we could improve selectivity. There were two roles for Bu4NOTf: first, that of an electrolyte, and second, that of a reservoir of triflate anion (TfO). Thus, another tetrabutylammonium salt was added as the second electrolyte to exempt Bu4NOTf from its role as an electrolyte. At first, the authors hypothesized that the counter anion of the second electrolyte should not be more reactive than TfO; however, Bu4NBF4, which had more nucleophilic BF4 anion, also improved b-selectivity. After optimization of the second electrolyte, b-selectivity of disaccharide 28 increased up to 95% in the presence of Bu4NOTf and Bu4NNTf2 (1 : 3 ratio). The relationship between yield and selectivity is a trade-off in this combined electrolyte system. Therefore, 1 : 1 to 3 : 1 ratios of Bu4NOTf and Bu4NNTf2 are recommended to obtain glycosylation products with reasonable yield and selectivity. Disaccharide building block 32 with a b-glycosidic bond and two monosaccharide building blocks 1k and 33 were used for automated electrochemical assembly to prepare tetrasaccharide precursor 34 (Figure 4.15). Subsequent deprotection of phthalimide group and N-acetylation were employed to convert 34 to N-acetyl product. Further reduction of the azido group and the introduction of the palmitoyl group afforded intermediate 35. Deprotection of the tert-butyldiphenylsilyl (TBDPS) group of 6-OH, the introduction of the SO3Na group to 6-OH, the removal of acetyl groups at 3-OH, and the final deprotection of benzyl groups by hydrogenation were carried out to complete total synthesis of Myc-IV(C16:0, S) (36).23

4.5 Synthesis of 1,2-trans Glycosidic Linkages of Hexoses via Automated Electrochemical Assembly Glucosamines are not the only examples of building blocks that can be applied to automated electrochemical assembly. Hexoses such as

94

Figure 4.15

Chapter 4

Total synthesis of Myc-IV (C16:0, S). Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

mannosides and glucosides bearing stereocontrolling group at 2-OH were also applicable to the method. For example, the glycosylphosphatidylinositol (GPI)-anchor core trisaccharide containing a-1,6- and a-1,4-mannosidic linkages was synthesized by using automated electrochemical assembly (Figure 4.16).24 Although the introduction of an acyl

96

Chapter 4

4.6 Synthesis of Cyclic Oligosaccharides via Automated Electrochemical Assembly Cyclodextrins, which are the most famous cyclic oligosaccharides, have been used as functional materials because of their unique properties as molecular cages. Chemical synthesis of natural and unnatural cyclic oligosaccharides has also been reported by many groups.26 The authors were interested in the synthesis of cyclic oligosaccharides because linear oligosaccharides, which are precursors of cyclic oligosaccharides, could be prepared via automated electrochemical assembly (Figure 4.18). Moreover, intramolecular glycosylation under the electrochemical conditions had never been investigated at that time. Automated electrochemical assembly to prepare linear oligosaccharides 39 with a protecting-group-free 6-OH was initiated from building block 1u bearing 6-OH protected by the 9-fluorenylmethyloxycarbonyl (Fmoc) group. The Fmoc group was removed in the same pot by adding triethylamine after the electrolysis. Purified linear oligosaccharides 39 was converted to the corresponding cyclic oligosaccharides 40 by intramolecular electrochemical glycosylation.27 It might be thought that electrochemical glycosylation was an alternative to the conventional chemical glycosylation; however, the same intramolecular glycosylation was sluggish with NIS/TfOH. Moreover, the

Figure 4.18

Electrochemical synthesis of cyclic oligosaccharides. Reproduced from ref. 29 with permission from Society of Synthetic Organic Chemistry, Japan, Copyright 2021.

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97

stereoselectivity of intramolecular glycosylation was moderate even in the presence of the N-phthalimide group as the stereocontrolling group.28 In intramolecular electrochemical glycosylation, stereoselectivity depended on the electrolyte. Further study to reveal the effects of the electrochemical method for intramolecular glycosylation is in progress.

4.7 Conclusion In this chapter, the authors’ electrochemical methods and a sugar machine were introduced.29 They believe that electrochemical glycosylation for oligosaccharide synthesis is one of the examples of chemical processes that can be performed by using automated electrochemical assembly. Electrochemical synthesis using an automated synthesizer has a bright future because it can be easily connected to artificial intelligence and machine learning. Now, the authors spent most of their time on the design of the reactions and the optimization of reaction conditions; however, the focus will be more on products and their utilities in the near future.

Acknowledgements The authors highly appreciate late Professor Jun-ichi Yoshida and Professor Emeritus Toshiyuki Itoh for their continuous support and encouragement. Financial support from Grants-in-Aid for scientific research from MEXT and JSPS and private foundations are deeply acknowledged.

References 1. W. Li, J. B. McArthur and X. Chen, Carbohydr. Res., 2019, 472, 86. 2. M. Panza, S. G. Postorio, K. J. Stine and A. V. Demchenko, Chem. Rev., 2018, 118, 8105. 3. R. Noyori and I. Kurimoto, J. Org. Chem., 1986, 51, 4320. 4. C. Amatore, A. Jutand, J.-M. Mallet, G. Meyer and P. Sinay¨, J. Chem. Soc., Chem. Commun., 1990, 718. 5. G. Balavoine, A. Gref, J.-C. Fischer and A. Lubineau, Tetrahedron Lett., 1990, 31, 5761. 6. R. R. France, R. G. Compton, B. G. Davis, A. J. Fairbanks, N. V. Rees and J. D. Wadhawan, Org. Biomol. Chem., 2004, 2, 2195. 7. T. Nokami, A. Shibuya, H. Tsuyama, S. Suga, A. A. Bowers, D. Crich and J. Yoshida, J. Am. Chem. Soc., 2007, 129, 10922. 8. T. Nokami, Trends Glycosci. Glycotechnol., 2012, 24, 203. 9. T. Nokami, A. Shibuya, S. Manabe, Y. Ito and J. Yoshida, Chem. – Eur. J., 2009, 15, 2252. 10. T. Nokami, Y. Nozaki, Y. Saigusa, A. Shibuya, S. Manabe, Y. Ito and J. Yoshida, Org. Lett., 2011, 13, 1544. 11. Z. Zhang, I. R. Ollmann, X.-S. Ye, R. Wischnat, T. Baasov and C.-H. Wong, J. Am. Chem. Soc., 1999, 121, 734.

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12. H. Tanaka, M. Adachi, H. Tsukamoto, T. Ikeda, H. Yamada and T. Takahashi, Org. Lett., 2002, 4, 4213. 13. S. Yamago, T. Yamada, O. Hara, Y. Ito and J. Yoshida, Org. Lett., 2001, 3, 3867. 14. S. Yamago, T. Yamada, T. Maruyama and J. Yoshida, J. Angew. Chem., Int. Ed., 2004, 43, 2145. 15. T. Nokami, R. Hayashi, Y. Saigusa, A. Shimizu, C.-Y. Liu, K.-K. T. Mong and J. Yoshida, Org. Lett., 2013, 15, 4520. 16. H. Usuki, T. Nitoda, M. Ichikawa, N. Yamaji, T. Iwashita, H. Komura and H. Kanzaki, J. Am. Chem. Soc., 2008, 130, 4146. 17. Y. Yang, Y. Li and B. Yu, J. Am. Chem. Soc., 2009, 131, 12076. 18. T. Nokami, Y. Isoda, N. Sasaki, A. Takaiso, S. Hayase, T. Itoh, A. Shimizu, R. Hayashi and J. Yoshida, Org. Lett., 2015, 17, 1525. 19. Y. Isoda, N. Sasaki, K. Kitamura, S. Takahashi, S. Manmode, N. Takeda-Okuda, J. Tamura, T. Nokami and T. Itoh, Beilstein J. Org. Chem., 2017, 13, 919. 20. Y. Morimoto, S. Takahashi, Y. Isoda, T. Nokami, T. Fukamizo, W. Suginta and T. Ohnuma, Carbohydr. Res., 2021, 499, 108201. ´, V. Puech-Page `s, A. Haouy, M. Gueunier, 21. F. Maillet, V. Poinsot, O. Andre L. Cromer, D. Giraudet, D. Formey, A. Niebel, E. A. Martinez, H. Driguez, ´card and J. De ´narie ´, Nature, 2011, 469, 58. G. Be 22. Y. Isoda, K. Kitamura, S. Takahashi, T. Nokami and T. Itoh, ChemElectroChem, 2019, 6, 4149. 23. K. Yano, T. Itoh and T. Nokami, Carbohydr. Res., 2020, 492, 108018. 24. S. Manmode, T. Sato, N. Sasaki, I. Notsu, S. Hayase, T. Nokami and T. Itoh, Carbohydr. Res., 2017, 450, 44. 25. S. Manmode, M. Kato, T. Ichiyanagi, T. Nokami and T. Itoh, Asian J. Org. Chem., 2018, 7, 1802. 26. G. Gattuso, S. A. Nepogodiev and J. F. Stoddart, Chem. Rev., 1998, 98, 1919. 27. S. Manmode, S. Tanabe, T. Yamamoto, N. Sasaki, T. Nokami and T. Itoh, ChemistryOpen, 2019, 8, 869. 28. D. V. Titov, M. L. Gening, A. G. Gerbst, A. O. Chizhov, Y. E. Tsvetkov and N. E. Nifantiev, Carbohydr. Res., 2013, 381, 161. 29. K. Yano, N. Sasaki, T. Itoh and T. Nokami, J. Syn. Org. Chem. Jpn, 2021, 79, 839.

Part 2 Sustainable Redox Catalysis

CHAPTER 5

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species TORU AMAYA Graduate School of Science, Nagoya City University, Nagoya, Japan Email: [email protected]

5.1 Introduction The development of intermolecular oxidative cross-coupling of enolate species has attracted the interest of chemists because this reaction offers a direct route to the formation of unsymmetrical 1,4-dicarbonyl compounds, which are common substructures found in naturally occurring products and medicinal compounds, and their synthetic intermediates as well as building blocks that can be used for the construction of heterocyclic compounds (Scheme 5.1).1,2 The first intermolecular oxidative cross-coupling, which was reported by Saegusa and coworkers in 1975 (Scheme 5.2a), involves crosscoupling between silyl enolates induced by Ag2O.3 Around the same time, the intermolecular oxidative cross-coupling of lithium enolates from ester by electrochemical oxidation was reported by Tokuda et al. (Scheme 5.2b).4 In these reports, the use of an excess of one enolate (3 equiv.) is considered to be a key to achieving selective cross-coupling. Such a tactic utilizing the stoichiometric advantage of one coupling partner (2- to 10-fold) has been often used in the intermolecular oxidative cross-coupling of enolate species.5–14 The difference in reactivity between enolate species is also a powerful way to control selectivity in such cross-coupling reactions. Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

101

102

Scheme 5.1

Chapter 5

(a) Intermolecular oxidative cross-coupling of enolate species. (b) Examples for naturally occurring products, medicinal compounds, and synthetic intermediates for heterocyclic compounds.

For example, the higher reactivity (a lower oxidation potential) of an enamine compared to a silyl enol ether that allows selective cross-coupling to occur was first reported by Narasaka et al. (Scheme 5.2c).8 The difference in reactivity between a more reactive a-stannyl carbonyl derivative and silyl enolate was also utilized for this type of selective cross-coupling (Scheme 5.2d).10 The use of a combination of imide enolates with a chiral auxiliary and a ketone or ester enolate as well as that between amide and ketone enolates was also reported by Baran and coworkers as a form of equimolar cross-coupling (Scheme 5.2e).1,15 As a unique strategy for selective cross-coupling, utilizing the selective heteroaggregation of lithium enolates by a steric effect was reported by Flowers (Scheme 5.2f).16 Another strategy involves the use of two different enolates tethered by silicon being crosscoupled intramolecularly by treatment with an oxidant (Scheme 5.2g).2,17–19 Most of the examples cited in the above paragraph require a stoichiometric amount of a metallic oxidant. Therefore, the development of catalytic systems that avoid the use of stoichiometric metal oxidants would be desirable. However, examples of such catalytic systems are very limited. Combinations of a catalyst and a terminal oxidant in previous reports are described below: [Ru(bpy)3]21 (bpy ¼ 2,2 0 -bipyridine)/duroquinone under visible light (Scheme 5.3a),20 FeCl3/(t-BuO)2 (Scheme 5.3b),21 cyclometalated (BzIm ¼ benzimidazole)–graphite anode [Rh(2-PhBzIm)2(MeCN)2]1 (Scheme 5.3c),22 and Mn(OAc)3/Pt-anode23 (Scheme 5.3d). In these examples, specific substrates were used to control the cross-coupling, and this is an

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

Scheme 5.2

103

Previous examples of the oxidative cross-coupling of enolate species.

104

Scheme 5.3

Chapter 5

Previous examples of the catalytic oxidative cross-coupling of enolate species.

issue that needs to be solved.20–23 The next challenge would be the use of O2 as a terminal oxidant, which is inexpensive, readily available, and desirable from the viewpoint of sustainable chemistry. However, this has not yet been achieved. As related research, aerobic oxidative cross-coupling of ethyl cyclopentanone-2-carboxylate derivatives and enol acetate catalyzed by Ce(III) has been reported, although Baeyer–Villiger-type rearrangement of the peroxide intermediate occurred to result in g-ketoester, not 1,4-diketone.24 The homocoupling of a zinc enolate catalyzed by Cu(II) with O2 as a terminal oxidant has been reported, but no reports for cross-coupling.25

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

Scheme 5.4

105

Oxidative cross-coupling of enolate species induced by stoichiometric and catalytic amounts of vanadium(V).

Against these background studies, we independently developed a selective oxovanadium(V)-induced oxidative cross-coupling strategy based on the use of a combination of boron and silyl enolates as the key substrates (Scheme 5.4).26,27 We also developed a catalytic system that involves an oxovanadium oxidant with O2 as a terminal oxidant.28 In this chapter, the results of our research efforts concerning the oxovanadium(V)-induced oxidative cross-coupling of enolate species are summarized.26–29

5.2 Oxovanadium(V)-induced Intermolecular Selective Oxidative Cross-coupling between Boron and Silyl Enolates26 Our working hypothesis for the oxidative cross-coupling of enolate species is described as follows. There are two key steps: (1) the selective one-electron oxidation of one enolate by a transition metal oxidant M00 n1 to generate an electrophilic carbonyl a-radical species activated by a Lewis acid30 and (2) its radical addition with a silyl enolate, which functions as a radical acceptor8,10,12 (Scheme 5.5). As a coupling partner for silyl enolates, boron enolates were selected because the reactivity of a boron enolate is higher than that of a silyl enolate.26 The oxovanadium(V) oxidant was chosen based on the knowledge regarding oxovanadium oxidants that has accumulated by Hirao and coworkers over the years.29,31,32 For example, an oxovanadium(V) species such as VO(OR)xCl3x has been shown to be a Lewis acid with a one-electron oxidation capability.31 VO(OEt)Cl2 enables the one-electron oxidation of electron-rich silyl enolates for oxidative cross-coupling (Scheme 5.6a).9 Imidovanadium(V) can be used in this reaction as well (Scheme 5.6a).33 We also reported the VO(OPr-i)2Cl-induced diastereoselective oxidative homocoupling of boron enolates (Scheme 5.6b).34 The oxidative cross-coupling of the boron enolate 1a and the silyl enolate 2a involved treating 1.25 equiv. (based on the sum of 1a and 2a) of VO(OEt)Cl2 under a nitrogen atmosphere at room temperature (Scheme 5.7). The yield of the cross-coupling product 3aa was 96% and the selectivity (mole ratio) of 3aa to homocoupling products 4a and 5a was 99 : 1 : 0. It should be noted that a high yield and selectivity are achieved, although no excess of one enolate was needed in this reaction. This reaction can be scaled up to a gram-scale reaction without severe loss of yield and selectivity. Instead of the boron enolate 1a, the use of the lithium enolate resulted in a

106

Chapter 5

Scheme 5.5

Proposed working hypothesis for oxidative cross-coupling of enolate species.

Scheme 5.6

V(V)-induced oxidative (a) cross-coupling of silyl enolates and (b) homocoupling of a boron enolate.

Scheme 5.7

V(V)-induced oxidative cross-coupling of 1a and 2a.

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

107

significant decrease in the yield for 3aa (trace), although the homocoupling product 4a was obtained in 31% yield. This result may be explained by aggregation of the lithium enolate, as exemplified by the results reported in the literature.16 Instead of VO(OEt)Cl2, the use of other metallic oxidants such as VO(OPr-i )2Cl, [Ce(NO2)6](NH4)2, FeCl3, and CuCl2 was not effective for this reaction. The scope of both boron and silyl enolates (1 and 2, respectively) in this reaction are summarized in Table 5.1. In the case of a combination of a more highly substituted boron enolate and a less-substituted silyl enolate, the reactions tend to proceed in high yields with a high degree of selectivity. Aliphatic enolates can be employed for this reaction (entries 11–13 and 17), and quaternary carbons can be constructed as well (entry 15). Cross-coupling between monosubstituted ketone enolates at 2-position (1f and 2d) was also demonstrated (entry 16). A radical clock reaction was investigated using the boron enolate 1j with a cyclopropyl group at the 2-position (Scheme 5.8). In this case, the ringopened product 6 was obtained as the main product (83% yield), suggesting the generation of a carbonyl a-radical species.

5.3 Oxidative Cross-coupling between Various Boron and Silyl Enolates27 In the previous paragraph, selective oxidative cross-coupling between ketone enolates was described. However, other carbonyl-containing compounds in addition to ketones, including aldehydes, carboxylates, esters, and amides, are also candidates as enolate precursors. The use of a wide spectrum of combinations of such derivatives in oxidative cross-coupling makes the reaction quite beneficial and versatile in organic synthesis. Although the homocoupling of enolates derived from carboxylic acid derivatives has been reported since 1935,35–38 their cross-coupling has not been extensively studied. A combination of an ester and a ketone in the oxidative crosscoupling of lithium enolates using electrochemical oxidation was reported by Tokuda et al.4 and CuCl2 by Saegusa et al.5 Hirao and coworkers also reported on a few examples of the cross-coupling of silyl enolates.9,33 Narasaka et al. reported some selective cross-coupling reactions using stannyl enolates of esters or amides and silyl enolates of ketones using ceric ammonium nitrate as an oxidant.10 The Fe(III)- and Cu(II)-induced cross-coupling of lithium enolates for imide–ketone and imide–ester combinations was investigated by Baran et al., which was applied to the total synthesis of natural products.1,15 In this context, the combinations of enolates such as ketone–ester, ester–ketone, ester–ester, amide–ketone, and amide–ester were investigated in the oxovanadium(V)-induced intermolecular oxidative cross-coupling of boron and silyl enolates.27 In the case of boron enolates from ketones and silyl enolates from esters, the optimized conditions for the oxidant and temperature were VO(OPr-i )2Cl

3ac: 71i

83 : 17 : 0

2a

3ha: 57

98 : 2 : 0

2a

3ia: 85

99 : 1 : 0

3fd: 75(dr ¼ 71 : 29)

98 : 2 : 0k

3ae: 85i(dr ¼ 56 : 44)

94 : 6 : 0

1a

2c

14j 1h 15 1i 16

1f

17g,h

1a

a

2d

2e

Molar ratio for 1/2 is 1.0 unless otherwise mentioned. Isolated yield. Yield of 3 ¼ mole of 3/mole of 1 (or 2)100. Determwined by 1H NMR of the crude materials. d Reaction time was 17 h. e A little excess amount of 1 was employed (1 : 2 ¼ B1.15 : 1). f Reaction time was 18 h. g A little excess amount of 2 was employed (1 : 2 ¼ 1 : 1.1). h Reaction temperature was 35 1C. i NMR yield. j 1.1 equiv. of VO(OEt)Cl2 were used to each enolate. Reaction time was 0.5 h, and THF (33 equiv. to 1h) was added. k The ratio was determined by isolation of 3fd and 4f. b c

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

13g,h

109

110

Scheme 5.8

Chapter 5

Radical clock reaction.

and 35 1C, respectively, where the yield and selectivity as a molar ratio of 3af : 4a : 5f were 91% and 97 : 3 : 0, respectively (Scheme 5.9a). The conditions were optimized in order to restrain the homocoupling of the reactive silyl enolate that is derived from the ester. This reaction can be scaled up to a 2 mmol scale. The optimized conditions for the combination of a boron enolate from an ester and a silyl enolate from a ketone were the same as that for the reaction of a combination of ketone–ketone enolates (oxidant: VO(OEt)Cl2, temperature: room temperature). The yield and selectivity as a molar ratio of 3ka : 4k : 5a were 71% and 96 : B4 : 0, respectively (Scheme 5.9b). The coupling of ester–ester enolates was carried out with VO(OEt)Cl2 at 70 1C to give the product 3 kg in 67% yield (Scheme 5.9c). In the case of a boron enolate 1l derived from an amide, a severe side reaction occurred, with the formation of an a,b-unsaturated amide 7. After a series of investigations, the use of an excess (more than 5 equiv.) of the silyl enolate coupling partner was found to be effective. The conditions for the oxidant and temperature were VO(OEt)Cl2 and 70 1C, respectively. The yield of the amide–ester coupling product 3lf was increased to up to 39% (Scheme 5.9d), and 7 was still formed in 14%. The amide–ketone enolate cross-coupling was performed using 1l and 2a under similar conditions to give the product 3la in 30% yield (Scheme 5.9e).

5.4 Oxovanadium(V)-catalyzed Oxidative Crosscoupling between Boron and Silyl Enolates under O2 as a Terminal Oxidant28 As described in the introduction, the development of a catalytic system that would permit stoichiometric amounts of the metallic oxidants used to be reduced would be desirable. Molecular oxygen is an ideal terminal oxidant, but side reactions are a concern. More specifically, the enolate species can react with O2 and this makes this reaction more complicated.24 In the previous report on the oxidative ligand coupling of tetraarylborate, O2 was used as a terminal oxidant for a high-valent oxovanadium(V) catalyst.39

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

Scheme 5.9

111

V(V)-induced oxidative cross-coupling of boron and silyl enolates: combination for (a) ketone–ester, (b) ester–ketone, (c) ester–ester, (d) amide–ester, and (e) amide–ketone.

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In this context, the oxovanadium(V)-catalyzed oxidative cross-coupling of boron and silyl enolates in the presence of O2 was performed (Scheme 5.10).28 Simple ketone enolates were used in the cross-coupling reaction. To minimize side reactions with O2 as much as possible, the boron and silyl enolates 1a and 2a were first treated with 22 mol% VO(OEt)Cl2 under an atmosphere of N2 and O2 was allowed to slowly flow (10 mL min1) through the flask. It should be noted that the catalyst needs to be reoxidized at least nine times for the reaction to reach completion under these conditions because this reaction is a two-electron oxidation reaction and the vanadium oxidant is a one-electron oxidant. The desired 1,4-diketone 3aa was obtained in 62% yield (Table 5.2, entry 1) and a trace amount of the homocoupling product 4a of the boron enolate 1a was also produced. A side reaction product with O2, an a-hydroxycarbonyl compound from 1a, was detected by gas chromatography–mass spectrometry (GC–MS) but, fortunately, the amount was very small. Increasing the amount of catalyst used up to 40 mol% improved the yield to 87% (entry 2). This reaction can be carried out on a gram-scale reaction without notable losses in yield (entry 3). In comparisons with other metallic catalysts such as FeCl3, CuCl2, Cu(acac)2, CeCl3, and Mn(acac)3, VO(OEt)Cl2 was clearly found to be the superior catalyst (entries 4–10). The effect of the solvents used in the reaction was also investigated. Relatively less-polar solvents such as 1,2-dichloroethane, toluene, benzene, and 1,2-dichlorobenzene tended to provide a better yield than the polar tetrahydrofuran (THF) and MeCN solvents. The substrate scope for the oxovanadium-catalyzed oxidative cross-coupling reaction under O2 is summarized in Table 5.3 and the characteristic points of the table are described below. Aryl halide moieties such as chloride, bromide, and fluoride groups were tolerated, as reported in the previous reports, and such substrates gave good yields. The aliphatic silyl enol ethers 2g and 2e and the ketene silyl acetal 2f gave relatively low yields (entries 2–4). The use of the aliphatic boron enolate 1g also resulted in a relatively low yield (entry 11). Boron enolates 1k and 1l derived from esters and amides were not applicable for this catalytic reaction (entries 12 and 13). The allyl silane 2h can be used for this cross-coupling reaction instead of silyl enolate (entry 14). The reaction pathway was investigated by 51V NMR and electron spin resonance (ESR) spectroscopy (Figure 5.1). Figure 5.1a shows the 51V NMR spectrum for VO(OEt)Cl2 in CDCl3. The peak for VO(OEt)Cl2 appears at 295 ppm. When the enolates 1a and 2a were added to the VO(OEt)Cl2 solution, a paramagnetic species was produced. The ESR spectrum for this solution showed octet peaks, which is typical for oxovanadium(IV) (I ¼ 7/2) (Figure 5.1b). The introduction of O2 gas into the NMR tube resulted in the regeneration of VO(OEt)Cl2, as shown in the 51V NMR spectrum (Figure 5.1c). Some small peaks were also observed in the region of V(V) having dialkoxy and monochloride ligands (500 ppm). The V(IV) species that was formed was not only oxidized to V(V) but the chloride ligand is retained in the V(V) as well. The oxidative ability of V(V) depends on the number of chloride ligands. The more chloride ligands it contains, the

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

Scheme 5.10

Table 5.2

113

Catalytic system based on V(V)/O2 for the oxidative cross-coupling of enolate species.

Optimization of reaction conditions for the catalytic oxidative crosscoupling.

Entry

Catalysts

Mol%

Yield(%)a,b,c

1 2 3d 4 5 6 7 8 9 10

VO(OEt)Cl2 VO(OEt)Cl2 VO(OEt)Cl2 VO(OPr-i)2Cl None FeCl3 CuCl2 Cu(acac)2 CeCl3 Mn(acac)2

22 40 40 40 — 40 40 40 40 40

62 87 (72) 77 (72)e 13 Trace Trace 19 Trace Trace 30

a

Standard conditions. Amounts of starting materials used [1a: 1.0 mmol, 2a: 2.0 mmol, VO(OEt)Cl2: 0.4 mmol, CH2Cl2: 10 mL]. Flask: 50 mL round-bottomed flask. NMR yield. c Isolated yield in parentheses. d Gram-scale reaction. e A trace amount of an impurity is included, which can be removed by further purification by recycling preparative gel permeation chromatography (GPC). b

higher is the reactivity of the catalyst.40 Thus, the recovery of the chloride ligand is crucial for maintaining the reactivity of V(V) species in the catalytic cycle. The following experiments suggest that B-Cl-9-BBN (9-borabicyclo[3.3.1]nonane) is generated in the reaction and that it plays a critical role in the recovery of a chloride ligand to form the active V(V) species rather than Me3SiCl. The use of oxovanadium(V), having dialkoxy and monochloride ligands, was examined in the presence of B-Cl-9-BBN or Me3SiCl, which is a possible source of the chloride generated in this reaction (Schemes 5.11a and b). As a result, the monochloride VO(OPr-i )2Cl was completely converted into the dichloride VO(OPr-i )Cl2 with B-Cl-9BBN, but, with Me3SiCl, there was no reaction.

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Table 5.3

Substrate scope of the catalytic oxidative cross-coupling .

Entry

Substrates

1

1a

2b

76 (65)

2

1a

2g

49 (46)

3

1a

2e

37 (major/minor isomers: 1.4/1)

4

1a

2f

22

5d

1m

2a

92 (89)

6 7d 8

1n 1o 1p

2a 2a 2a

86 (44)e 68 (39)e 56 (20)e

9

1b

2a

68 (30)e

10

1c

2a

54 (45)e

11

1g

2a

37

12

1k

2a

13

13 14

1l 1a

2af 2h

Not detected 50 (24)e

a

Product

Yield(%)bc

Standard condition. Amount of starting materials used [1: 1.0 mmol, 2: 2.0 mmol, VO(OEt)Cl2: 0.4 mmol, CH2Cl2: 10 mL]. Flask: 50 mL round-bottomed flask. b NMR yield. c Isolated yield in parentheses. d 1,2-Dichloroethane was used instead of CH2Cl2 as a solvent. e Purified with recycling preparative GPC to remove a small amount of impurity after column chromatography. f 4 equiv. of 2a were used.

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

Figure 5.1

115

(a) 51V NMR spectrum before reaction. (b) ESR spectrum after addition of 1a and 2a. (c) 51V NMR spectrum after the introduction of O2.

Scheme 5.11

Ligand exchange reactions from monochloride to dichloride V(V) species with (a) B-Cl-9-BBN or (b) Me3SiCl.

A plausible catalytic cycle is shown in Scheme 5.12. The boron enolate 1 is oxidized by a V(V) species such as VO(OEt)Cl2 to generate the corresponding a-carbonyl radical species 8 and B-Cl-9-BBN. The radical reacts with the silyl enol ether 2 to form 9, which undergoes a one-electron oxidation by V(V) to

116

Scheme 5.12

Chapter 5

Plausible reaction pathway.

yield the product 3 by way of the oxonium cation 10. As another possibility, O2 might oxidize 9 to form 10. The V(V) species is then regenerated from the V(IV) species in the presence of B-Cl-9-BBN and O2.

5.5 Conclusion The oxovanadium(V)-induced intermolecular oxidative cross-coupling of enolates is reviewed. The key point of the proposed strategy is the use of a combination of boron and silicon as the metallic part of the enolate, which permits two enolates to have different roles in the reaction. Oxovanadium(V) is an effective oxidant because it has sufficient oxidative capability and Lewis acidity to permit the a-radical species of the carbonyl group to be activated, thus leading to a high cross-selectivity. It is noteworthy that this strategy can be extended to various combinations of enolates, as exemplified by ketone– ketone, ketone–ester, ester–ketone, ester–ester, amide–ester, and amide– ketone enolates. In addition, the V(V/IV)/O2 catalytic system was constructed for use in the selective enolate cross-coupling of simple ketone enolates. Among a series of metallic catalysts that were examined, VO(OEt)Cl2 was revealed to be the best for this catalytic reaction. Further study of this V(V/IV)/O2 catalytic system is expected to not only lead to further applications but also approaches to the construction of other catalytic oxidation systems.

Abbreviations Ac acac BzIm Bn

acetyl acetylacetonate benzimidazole benzyl

Vanadium(V)-induced Oxidative Cross-coupling of Enolate Species

Bu 9-BBN bpy CAN Cy DMAP Et ESR GC–MS i-Pr Me NMR Ph THF

117

butyl 9-borabicyclo[3.3.1]nonane 2,2 0 -bipyridine ceric ammonium nitrate cyclohexyl 4-dimethylaminopyridine ethyl electron spin resonance gas chromatography–mass spectrometry isopropyl methyl Nuclear magnetic resonance phenyl tetrahydrofuran

Acknowledgements The author wishes to express his gratitude to Prof. Toshikazu Hirao for his continuous guidance and encouragement over the years. The author also sincerely thanks the continuous efforts of his students, Takaya Masuda, Yusuke Maegawa, Yuma Osafune, and Yuqing Jin, who directly contributed to this work. The author appreciates all members who supported him in accomplishing this work. This work was supported by Grants-in-Aid for Scientific Research (C) from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 17K05782 and 26410046.

References 1. M. P. DeMartino, K. Chen and P. S. Baran, J. Am. Chem. Soc., 2008, 130, 11546, and references therein. 2. F. Guo, M. D. Clift and R. J. Thomson, Eur. J. Org. Chem., 2012, 4881, and references therein. 3. Y. Ito, T. Konoike and T. Saegusa, J. Am. Chem. Soc., 1975, 97, 649. 4. M. Tokuda, T. Shigei and M. Itoh, Chem. Lett., 1975, 4, 621. 5. Y. Ito, T. Konoike and T. Saegusa, J. Am. Chem. Soc., 1975, 97, 2912. 6. Y. Ito, T. Konoike, T. Harada and T. Saegusa, J. Am. Chem. Soc., 1977, 99, 1487. 7. E. Baciocchi, A. Casu and R. Ruzziconi, Tetrahedron Lett., 1989, 30, 3707. 8. K. Narasaka, T. Okauchi, K. Tanaka and M. Murakami, Chem. Lett., 1992, 21, 2099. 9. T. Fujii, T. Hirao and Y. Ohshiro, Tetrahedron Lett., 1992, 33, 5823. 10. Y. Kohno and K. Narasaka, Bull. Chem. Soc. Jpn., 1995, 68, 322. 11. K. Ryter and T. Livinghouse, J. Am. Chem. Soc., 1998, 120, 2658. 12. H.-Y. Jang, J.-B. Hong and D. W. C. MacMillan, J. Am. Chem. Soc., 2007, 129, 7004.

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´ and R. ˇ 13. P. Tisovsky´, M. Mecˇiarova Sebesta, Org. Biomol. Chem., 2014, 12, 9446. 14. A. Mambrini, D. Gori, R. Guillot, C. Kouklovsky and V. Alezra, Chem. Commun., 2018, 54, 12742. 15. P. S. Baran and M. P. DeMartino, Angew. Chem., Int. Ed., 2006, 45, 7083. 16. B. M. Casey and R. A. Flowers, II, J. Am. Chem. Soc., 2011, 133, 11492. 17. M. Schmittel and A. Haeuseler, J. Organomet. Chem., 2002, 661, 169. 18. M. D. Clift, C. N. Taylor and R. J. Thomson, Org. Lett., 2007, 9, 4667. 19. C. T. Avetta, L. C. Konkol, C. N. Taylor, K. C. Dugan, C. L. Stern and R. J. Thomson, Org. Lett., 2008, 10, 5621. 20. Y. Yasu, T. Koike and M. Akita, Chem. Commun., 2012, 48, 5355. 21. T. Tanaka, T. Tanaka, T. Tsuji, R. Yazaki and T. Ohshima, Org. Lett., 2018, 20, 3541. 22. X. Huang, Q. Zhang, J. Lin, K. Harms and E. Meggers, Nat. Catal., 2019, 2, 34. 23. J. Strehl and G. Hilt, Org. Lett., 2019, 21, 5259. 24. I. Geibel, A. Dierks, M. Schmidtmann and J. Christoffers, J. Org. Chem., 2016, 81, 7790. 25. H.-Q. Do, H. Tran-Vu and O. Daugulis, Organometallics, 2012, 31, 7816. 26. T. Amaya, Y. Maegawa, T. Masuda, Y. Osafune and T. Hirao, J. Am. Chem. Soc., 2015, 137, 10072. 27. T. Amaya, Y. Osafune, Y. Maegawa and T. Hirao, Chem. – Asian J., 2017, 12, 1301. 28. Y. Osafune, Y. Jin, T. Hirao, M. Tobisu and T. Amaya, Chem. Commun., 2020, 56, 11697. 29. T. Amaya and T. Hirao, in Vanadium Catalysis, ed. M. Sutradhar, A. J. L. Pombeiro, J. Armando and L. da Silva, Royal Society of Chemistry, 2021, ch. 19, pp. 464–482. 30. D. Yang, Q. Gao and O.-Y. Lee, Org. Lett., 2002, 4, 1239. 31. T. Hirao, Chem. Rev., 1997, 97, 2707. 32. Synthetic Methods for Redox Reactions Using Phosphorus, Vanadium and Samarium Compounds, in Functionalized Redox Systems: Synthetic Reactions and Design of p- and Bio-Conjugates, ed. T. Hirao, T. Hirao, T. Moriuchi, T. Amaya, A. Ogawa and A. Nomoto, Springer, Tokyo, 2015, pp. 5–50. 33. M. Nishina, T. Moriuchi and T. Hirao, Dalton Trans., 2010, 39, 9936. 34. T. Amaya, T. Masuda, Y. Maegawa and T. Hirao, Chem. Commun., 2014, 50, 2279. 35. D. Ivanoff and A. Spassoff, Bull. Soc. Chim. Fr., 1935, 2, 76. 36. M. W. Rathke and A. Lindert, J. Am. Chem. Soc., 1971, 93, 4605. 37. N. Kise, K. Tokioka, Y. Aoyama and Y. Matsumura, J. Org. Chem., 1995, 60, 1100. ´ky¨ and J. Plumet, Chem. Soc. Rev., 2001, 30, 313. 38. A. G. Csa 39. H. Mizuno, H. Sakurai, T. Amaya and T. Hirao, Chem. Commun., 2006, 5042. 40. T. Ishikawa, A. Ogawa and T. Hirao, Organometallics, 1998, 17, 5713.

CHAPTER 6

Mediated Electron Transfer in Electrosynthesis: Concepts, Applications, and Recent Influences from Photoredox Catalysis ´JEKc ROBERT FRANCKE*a,b AND MICHAL MA a

Leibniz Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany; b Institute of Chemistry, Rostock University, Albert-Einstein-Str. 3a, 18059 Rostock, Germany; c Department of Organic Chemistry, ´ Dolina, Ilkovicˇova 6, 84215 Bratislava, Comenius University, Mlynska Slovakia *Email: [email protected]

6.1 Introduction In comparison to conventional protocols for the oxidation and reduction of organic compounds, electrosynthesis provides several practical, economic, and ecological advantages.1–3 First, replacing redox reagents with electricity reduces waste generation and can thereby enhance cost efficiency and sustainability. Second, the working electrode potential as the driving force of the reaction can be precisely adjusted to the redox potential of a substrate, either by using a reference electrode (‘‘potentiostatic mode’’) or by choice of the appropriate current density (‘‘galvanostatic mode’’).4 Third, electrosynthesis Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Chapter 6

can be considered inherently safe, as it allows for controlled in situ generation of highly reactive intermediates or reagents. Fourth, electrochemical reactions are usually carried out under mild conditions, i.e. at room temperature (or at slightly elevated temperatures) and under atmospheric pressure. Fifth, electrochemical reactions inevitably undergo different mechanisms than classical redox reactions, often resulting in different chemoselectivity and enabling access to reactive intermediates that are not readily available by classical means. These attractive features, together with the possibility for direct utilization of electricity from renewable sources, have led to a tremendous intensification of research in this field over the past five years.5–12 Despite all the abovementioned advantages, electrosynthesis is of course not the method of choice for all redox reactions. The most obvious and trivial reason is that the technology and expertise are not available in many laboratories or companies. A true technical challenge, however, is the required supporting electrolyte, which must be separated after completed electrolysis. If the salt additives cannot be recycled in a resource-saving manner, other environmental benefits of electrochemistry may be negated. Another problem is the frequent kinetic inhibition of electron exchange between the electrode and the reactant (‘‘kinetic overpotential’’), which leads to an increase in energy consumption and often impairs the selectivity of the electrode process. While approaches for mitigating the supporting electrolyte issue exist and are described elsewhere,13–18 electrocatalysis can be used to lower kinetic inhibitions. Here, the electron transfer (ET) is catalyzed by a chemical interaction either between substrate and electrode (‘‘heterogeneous electrocatalysis’’) or by a socalled mediator, which is interposed between the electrode and the starting material (‘‘mediated ET’’ or ‘‘homogeneous electrocatalysis’’). Such mediators can not only ensure a lowering of the overpotential but are often also capable of steering the product electivity in a completely different direction. Not rarely, it is the use of certain mediators that makes electroorganic reactions possible in the first place. Another interesting aspect that has only recently come into focus is that, in principle, mediators are capable of absorbing energy from both the electrode and incident light. This fact allows mediated electrochemistry to be combined with photocatalysis, thereby harnessing advantages from both fields. A broad range of well-studied mediators has become available for numerous conversions. The long list includes organometallic compounds, halide salts, triarylamines, iodoarenes, and N-oxyl radicals. For a more detailed treatment of the plethora of reported mediators and applications, the reader is referred to ref. 19–27. Instead, the present chapter focuses on conceptual aspects, strategies for recycling, and the recently emerging combination of electro- and photocatalysis using a few classic examples and some recent developments for illustration.

6.2 Concepts and Applications The focus of this section is on general mechanisms and concepts associated with mediated ET in electrosynthesis. For readers with a particular interest

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the indirect oxidation A-P from Scheme 6.1 as an example. Initially, CV can be employed to investigate the behavior of a mediator in the absence of substrate, thus allowing one to test whether a candidate is oxidized or reduced within the desired potential regime and at which electrode potential the mediated reaction can be set up. Voltammetry experiments should therefore first be conducted in the absence of substrate to determine the mediator potential E0(Mox/Mred) and the peak current iP in the forward scan (curve 1). Figure 6.1 also shows the appropriate range for the mediator potential E0(Mox/Mred). At more positive potentials, this range is confined by the onset potential (Eon) for direct oxidation of A to B (Figure 6.1, curve 3). Toward less positive potentials, the window is limited by the thermodynamic (equilibrium) potential E0 of the A/P couple, since mediators with E0(Mox/Mred)oE0(A/P) lack the required driving force. A general trend is that increasingly positive mediator potentials (corresponding to a higher overpotential Zcat with respect to the A/P couple) lead to higher catalytic rates, although at the expense of energy efficiency. A reversible response of the Mox/Mred couple, as shown in the example, may be beneficial, as it indicates that the active species Mox is stable toward degradation (at least on the voltammetry time scale). After characterizing the mediator under noncatalytic conditions, a test for catalytic turnover can be performed by the addition of A and repetition of the scan. When a catalytic process takes place, the reversible shape of the CV (curve 1) frequently changes to the irreversible form indicated by

Figure 6.1

Typical voltammetric responses of a mediator in the absence (1) and presence (2) of substrate A according to the catalytic mechanism in Scheme 6.1. Exemplary cyclic voltammograms of a blank electrolyte (3) and A (4) are inserted for comparison. Reproduced from ref. 36 with permission from American Chemical Society, Copyright 2018.

Mediated Electron Transfer in Electrosynthesis

123

y

curve 2. Here, no reduction peak is observed in the reverse scan, since Mox is reduced by catalytic turnover of A. As under these conditions, a single mediator unit can be electrooxidized multiple times during the forward scan, the anodic current increases. The ratio between the resulting catalytic current (icat) and iP thus reflects the rate of the homogeneous catalytic process.

6.2.3

Redox Catalysis and Chemical Catalysis

According to the type of interaction between the active mediator species and the substrate, the catalytic processes can be assigned to one of the following two categories: ‘‘redox catalysis’’ and ‘‘chemical catalysis’’ (Scheme 6.2).32 In the former type, electron exchange occurs via an outer-sphere process without bonding interactions. In such processes, merely small potential gradients nE between M and the A/B couple can be overcome (typically a few hundred mV).19,38 The catalytic effect of such outer-sphere mediators may be caused by two phenomena: First, the availability of electrons or positive charges is enhanced due to a dispersion over the same 3D space as the substrate. Second, the ET between M and A may be kinetically favorable compared to the electron exchange with the electrode when, for instance, the redox-active site on A is sterically shielded. Either way, rapid and thermodynamically favorable chemical steps have to follow to pull the unfavorable ET equilibrium in the desired direction. In contrast to redox catalysis, chemical catalysis is based on an intimate binding interaction between A and the active form of M (‘‘inner-sphere ET’’). Such a scenario can be considered a classical homogeneous catalytic process that involves the regeneration of the active species by the electrode instead of a stoichiometric reagent. More precisely, two cases are distinguishable: The first one includes a binding interaction in the transition state of the electron exchange that decreases the energy barrier compared to an outer-sphere ET of the same driving force (‘‘one-step chemical catalysis’’). In the second case, the binding interaction is stronger and thus leads to the generation of an adduct between the active species and A (‘‘two-step chemical catalysis’’). Compared to direct and redox-catalyzed electrosynthesis, chemical catalysis can proceed via entirely different and often more favorable pathways. Thus, the potential gradients nE vs. the A/B couple of direct conversion are often much larger than with redox catalysis, sometimes even higher than 2.0 V. For an illustration of the two catalytic concepts, the electrochemical oxidation of alcohols constitutes an excellent example. Generally, a direct conversion at catalytically inactive (outer-sphere) anodes such as y

It should be noted that depending on the experimental parameters (concentrations, scan rate v, rate of the catalytic process, etc.) and possible side effects, the shape of a catalytic CV can vary significantly from curve 2. For a correct interpretation, it is useful to be familiar with the characteristic shapes and the parameters influencing them. A detailed treatment of this matter can be found in ref. 28, 32, and 37.

Mediated Electron Transfer in Electrosynthesis

127

oxidations, several advantages arise from the mechanistic differences: First, the potential gradient nE between mediator and direct substrate oxidation is not a limiting factor and can therefore be 2 V or higher. Second, groups that stabilize radical (ionic) intermediates are not necessary, broadening the substrate scope to allylic and aliphatic alcohols. Third, the omission of high energy intermediates 2–4 increases the product selectivity. Therefore, TEMPO-mediated alcohol oxidations are excellent examples for illustrating chemical catalysis in the sense of the general mechanism depicted in Scheme 6.2. The benefits of TEMPO-mediated oxidations are demonstrated by the original procedure of Semmelhack and coworkers depicted in Scheme 6.7.49 The protocol is similar to the triarylimidazole-mediated reaction in Scheme 6.5 (potentiostatic electrolysis, aprotic electrolyte solution, divided cell, 2,6-lutidine as proton scavenger). However, the TEMPO-based protocol is also applicable to aliphatic (1a–1c) and allylic alcohols (1d). Furthermore, a comparison between the conversions 6a-7a mediated by TEMPO and 12 shows that the former renders a higher yield (83% vs. 65%) at decreased mediator loading (5 mol% for TEMPO vs. 10 mol% for 12) and a lower applied potential (0.30 V vs. 0.97 V). Since the publication of the seminal work by Semmelhack et al., numerous advances in terms of practicality, sustainability, and scope have been reported. For a detailed overview of the electrochemistry of N-oxyl species, interested readers are referred to ref. 25. Some milestones of these developments include protocols for aqueous electrolyte systems that render carboxylic acids as the products,50 double-mediated oxidations in biphasic electrolyte solutions for spatial separation between alcohol oxidation and electrochemical steps,51 and the use of chiral N-oxyl mediators for kinetic electrochemical racemate resolution of sec-alcohols.52,53 Furthermore, there has been major progress in unraveling the mechanistic background of TEMPO-catalyzed electrooxidations.25 Key findings for the optimization of

Scheme 6.7

Anodic oxidation of primary alcohols 1 to carbonyl compounds 5 mediated by TEMPO.

130

Scheme 6.11

Chapter 6

Anodic partial fluorination of alkenes mediated by 4-iodotoluene (18a).

(‘‘electroauxiliary’’). The scope was recently extended to vicinal difluorination of nonactivated alkenes 20a by Lennox et al. (Scheme 6.11).69 The choice of an appropriate mediator (4-iodotoluene, 18a) and electrolyte composition (NR35.6 HF in a 7 : 3 mixture of CH2Cl2 and hexafluoroisopropanol [HFIP]) turned out to be essential for successful difluorination. The reaction is carried out under galvanostatic conditions in an undivided cell using one equivalent of 18a, the latter being oxidized to the corresponding difluoro-l3iodane fluorination agent 19a along with H2 as the by-product. The choice between the in-cell and ex-cell methods is determined by the oxidizability of the starting material. While the conversion of electron-deficient alkenes (Eox41.8 V) is carried out in-cell, electron-rich substrates (Eoxo1.8 V) have to be added after the completed formation of the fluorination agent. It is noteworthy that despite being an in-cell process, the fluorination of nonactivated alkenes still requires stoichiometric quantities of 18a. This is likely due to the rate of the alkene fluorination being too low for establishing an efficient catalytic cycle. A related method was reported by Waldvogel et al. for converting N-allyland N-propargylamides (20b) into monofluorinated oxazoles and 2-oxazolines 21b (Scheme 6.12).70,71 Since the tested starting materials are difficult to oxidize, the in-cell method was applied in most cases. Using a similar setup and reaction medium, the conversions are mediated by 18a (1–2 equivalents) at 50 mA cm2 passing 3.0 charge equivalents per mole 20b. Since under these conditions, fluorocyclization is not completed after switching off the electrolysis, an extension of the reaction time of several hours is necessary. Noteworthily, products 21a and 21b can also be obtained via nonelectrochemical

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Chapter 6

Scheme 6.14

Bromanes 25 as examples for electrogenerated reagents.

well as to the challenging preparation from the corrosive and highly toxic BrF3 precursor.75 In this context, the electrochemical approach toward the synthesis of chelation-stabilized l3-bromanes 25 from parent bromoarenes 24 represents a straightforward and scalable alternative (galvanostatic electrolysis in an undivided cell). Several para-substituted derivatives with remarkably high redox potentials spanning a range from 1.86 V to 2.60 V vs. Ag/ 0.01 M AgNO3 were prepared using the electrochemical method. Developing an in-cell mediated process under these conditions is not feasible, as previously discussed in the case of anodic fluorinations of electron-rich olefins shown in Scheme 6.11. Not surprisingly, the synthetic applications (homocoupling of electron-rich arenes, Csp3–H aminations, intramolecular cyclization of 22) were exclusively performed with stoichiometric amounts of pregenerated 25. Since the test reactions in the HFIP electrolyte solution gave unsatisfactory results, 25 was first isolated and then used in dichloromethane or acetonitrile. Furthermore, in the case of arene coupling, the intrinsic reactivity of the bench-stable bromine(III) species has to be unlocked by the addition of a Lewis or a Brønsted acid. Under these circumstances, 25 can no longer be referred to as an ex-cell mediator; the term ‘‘electrogenerated reagent’’ is clearly more appropriate here.

6.3 Approaches Toward Facilitating Mediator Recycling 6.3.1

Ionically Tagged Mediators

Apart from the positive aspects discussed above, the use of mediators also has disadvantages such as additional costs, more complex separation of the product mixture, and the generation of additional waste (provided that no recycling is performed). With this in mind, it can be beneficial to equip the mediator with an ionic group, which can greatly simplify the separation from less polar products. Several ionically tagged iodoarenes were studied by Francke et al. for application in indirect electrolysis.76–78 In a representative example, cyclization of 22 was achieved in HFIP by using 4-iodoaryl sulfonate (18c), the latter acting both as an ex-cell mediator and a supporting electrolyte (Scheme 6.15). Advantages of the method comprise rapid and

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Figure 6.3

6.3.3

Voltammetric response of 4-AcO-TEMPO, HP-1, and TEMPO in the absence (dotted lines) and presence of 4-methoxybenzyl alcohol and base (solid lines). Reproduced from ref. 83 with permission from John Wiley & Sons, Copyright r 2021 Wiley-VCH GmbH.

Mediator-modified Electrodes

Attaching mediators to the electrode surface appears quite promising for several reasons but has only rarely been exercised in synthetic organic electrochemistry thus far. The most interesting features of the method are (i) that the mediator does not have to be separated from the reaction mixture after completed electrolysis, (ii) that discharge of the active species at the counter electrode is inhibited (which may enable the use of an undivided cell), and (iii) that the turnover number may be enhanced through stabilization of the mediator units. The latter was observed, for example, in the case of a pyrene-tethered TEMPO derivative that was noncovalently attached to a carbon cloth anode and used for indirect alcohol oxidation.84 Further electroorganic applications involving TEMPO-85–88 and phenanthroimidazolemodified89 electrodes have been reported. For conversions of small molecules (e.g. CO2, H2O, O2) using organometallic catalysts, the electrode immobilization strategy is much more widespread (for comprehensive treatments of the topic, see ref. 90–92).

6.4 Mediators in Photoelectrochemical Synthesis Similar to the renewed interest that has been shown by the organic synthetic community toward the field of electrochemistry, organic photochemistry has received enormous attention throughout the last decade. The main driving force behind this is the recognition of the potential of photoredox catalysis

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to open avenues toward new reactions and improved selectivity. New technological advances also greatly enhanced the appeal of photochemical methods to the community. Most importantly, the availability of powerful LED irradiation sources permits any chemist to irradiate reaction mixtures under well-defined conditions (wavelength, power). LEDs are superior to the traditional arc lamp or fluorescence bulbs, not only by being inherently more efficient with regard to power but also due to the ease of operation and the suppression of side reactions stemming from broad band irradiation.98,99 Another important innovation was the development of photochemical flow reactors, allowing for uniform irradiation of reaction mixtures, increasing the robustness of the design, and thus opening the field of photochemistry to industrial applications.100,101 When looking at photochemistry through the prism of reaction mechanisms, the parallels to electrochemistry are immediately recognizable. Photoand electrochemically driven reactions are usually based on single-electron transfer processes and, as such, involve free organic radicals. In the same way that the applied potential can be utilized to influence the selectivity of electroorganic reactions, tuning the wavelength of the incident light can be used with similar effects in photochemistry. While electrochemically inactive species can be involved in electrochemistry by mediators, in photochemistry, the photocatalysts play the same role toward the compounds that do not absorb the applied photons. A parallel between mediated electrosynthesis and photocatalysis is the upper limit of the energy input: While in electrosynthesis, this limit is represented by the onset potential of (the undesired) direct substrate conversion, it is the long-wave edge of the absorption spectrum of the substrate in photocatalysis. One of the main differences between mediated electrochemistry and homogeneous photocatalysis is the lifetime of the active species. Excited states exhibit inherently low lifetimes that often prevent them from engaging in productive processes, while their open-shell electronic structure triggers single electron exchanges with the substrate molecules, which are generally very fast (often at diffusion limit). According to the electrochemist’s nomenclature introduced in Section 2.3, this behavior would be referred to as redox catalysis (compare Scheme 6.2b). In this context, it should be pointed out once again that in mediated electrosynthesis, the electrogenerated active species may also be a closed-shell system that induces chemical catalysis (Scheme 6.2c) via polar mechanisms (see, for example, the catalytic cycle of TEMPO in Scheme 6.6). Even though one can draw so many parallels between photo- and electrochemistry on a conceptual level, in reality, there were very few attempts to merge photochemistry with electrochemistry in organic synthesis until a few years ago.102–105 The reason for this is mainly the necessity to obtain specific equipment and several practical skills to run a photo- or electrochemical reaction in the laboratory, thus creating a significant barrier for electrochemists to enter the field of photochemistry (and vice versa).

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Transformations at Photoelectrodes

Arguably the simplest way to merge photo- and electrochemistry for driving chemical reactions is the use of photoactive electrodes in an electrochemical cell – photoelectrodes. Photoelectrochemical cells are usually designed as a typical electrochemical three-electrode configuration in a divided cell, with the difference from a traditional electrolysis cell being the material of the electrodes. At least one of the electrodes is made of a conductor coated with a photoactive material, usually a semiconductor, which is used to harvest light. Among the commonly used semiconductors for this purpose are hematite, BiVO4, or TiO2, the latter often functionalized with a material photoactive in the visible part of the spectrum.106–108 The obvious appeal of photoelectrochemistry stems from the combination of two energy sources to obtain sufficient energy for running the desired reaction. This means that lower potentials are required in comparison with traditional electrolysis and, on the other hand, that less energetic photons can be used in comparison with methods that would solely rely on photochemistry. Such milder conditions translate to the decreased likelihood of unwanted side reactions. To illustrate the effects of photoelectrocatalysis on the reaction energetics, a comparison between electrooxidation on a conventional anode and on a photoanode is useful (see Figure 6.4, a similar figure can also be drawn for a photocathode).109 The potential applied to a conventional anode is used to directly oxidize substrate A (with redox potential E0), the excess voltage between the applied potential and the reaction reduction potential translates to overpotential Z – the driving force for the transformation, and a quantity that has an effect on the selectivity of the transformation (Figure 6.4a). Changing the applied potential Eappl will lead to a shift of the Fermi level (EF) of the electrode and will have a direct influence on Z. When comparing this with the photoanode (Figure 6.4b), the obvious difference is the much lower applied potential. This decrease is due to the fact that a part of the energy is now injected into the system by exciting electrons from the valence (a)

Figure 6.4

(b)

Comparison between oxidations at a conventional anode (a) and photoanode (b). EF ¼ Fermi level, CB ¼ conduction band, VB ¼ valence band, E0 ¼ redox potential of the A/A 1 couple, Eappl ¼ potential applied to electrode, Z ¼ overpotential.

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Scheme 6.19

Photoelectrocatalytic valorization of 5-hydroxymethylfurfural.

the corresponding cation radicals 31, which react with pyrazoles 32 to give the aminated coupling products 34. High yields, as well as high regioselectivity, could be obtained by using this approach. Hematite was used as the electrode material in a potentiostatic electrolysis at a relatively low potential (1.13 V vs. saturated calomel electrode [SCE]) and irradiation with blue LED lights. This is in contrast with the results from conventional electrolysis, where much higher potentials must be used to obtain the conversion of the starting material (1.93 V vs. SCE), leading to decreased yields and deterioration of regioselectivity of the C–H amination. Photoelectrocatalytic cells have been successfully coupled with homogenous in-cell mediators – examples include TEMPO and N-hydroxysuccinimide,114 as well as halides.115 The development of such systems usually stems from attempts to replace the water oxidation reaction in photocells for CO2 reduction with a reaction that would offer more valuable products at lower potentials. An example for such an endeavor was the work of Choi et al., where a TEMPO-mediated oxidation of 5-hydroxymethylfurfural was performed on a BiVO4 photoanode, under irradiation with simulated sunlight (Scheme 6.19).109 Under these conditions, oxidation went smoothly to the 2,5-furandicarboxylic acid (an important industrial feedstock) with a high Faradaic efficiency (493%). The use of a photocathode allowed for the application of a less positive potential (a difference of over 0.6 V compared to reaction in the dark). Mediation with TEMPO was instrumental in this case, as oxidation in the absence of TEMPO proceeded at very high potentials and gave only traces of product.

6.4.2

Sequential Activation of Substrates by Electro- and Photochemistry

For the development of a new synthetic method, strategic placement of sequential photochemical and electrochemical steps can be used in a beneficial manner, even when these steps are not intertwined within the same catalytic cycle. This approach is often referred to as decoupled photoelectrochemistry. When designing such reaction systems, the compatibility of the photo- and electrochemical steps must be considered. The photoactive components are often electroactive (and vice versa), so unwanted degradation processes have to be prevented by performing the reactions with controlled potential, selective irradiation, and optimizing the concentrations of the active species. A promising approach is to generate the photoactive

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applications in organic electrochemistry. This can be mainly ascribed to the fact that mediated reactions frequently afford higher and/or different selectivity than direct conversions at the electrode. Another interesting aspect is the electrocatalytic effect that allows one to operate below the potential of direct substrate conversion and thereby save energy. A distinction must be made between outer-sphere and inner-sphere ET mediators. While the former ‘‘merely’’ shuttle electrons or holes from the electrode to the substrate (‘‘redox catalysis’’), the latter rather resemble chemical catalysts. The binding interaction between mediator and substrate often provides greater selectivity compared to redox catalysis and allows a stronger decrease of the overpotential with respect to the substrate–product couple. Ex-cell/in-cell mediation represents another important concept for the classification of indirect electrosyntheses. Thus, pregeneration of the active species must occur in stoichiometric amounts if the potential of the mediator is more positive (negative) than that of the substrate to be oxidized (reduced). This approach can be useful if the desired selectivity cannot be achieved in an in-cell process. Iodoarenes represent a prominent mediator system that is frequently employed in ex-cell procedures in which hypervalent iodine(III) species are anodically generated for subsequent use in various transformations. A broad range of well-characterized mediator systems is available from all the abovementioned mediator classes, only some of which have been discussed in detail in this chapter. At this point, it should be noted that most of the reported mediator systems are designed for anodic oxidation, whereas progress in indirect electroreduction was not able to keep pace. This means, of course, that there are a lot of opportunities in the area of mediated electroreductions. Although mediators display all the initially mentioned advantages, it should not be ignored that their use also entails disadvantages such as additional costs and a more complex separation of the product mixture. To address these issues, several concepts have been developed, three of which have been discussed in this chapter: bifunctional mediator-supporting electrolyte systems, polymediators, and mediator-modified electrodes. These approaches feature the facilitation of mediator recovery by changing the polarity, tuning the molecular size, and immobilization, respectively. Surely, new interesting developments can be expected in this area in the near future. Even though the overlap between the electrochemical and photochemical research community has not been large so far, it is likely that a new vibrant field on the interface between these two disciplines will emerge. So far, photoredox catalysts have fulfilled a role in photochemistry similar to that of the mediators in electrochemistry. However, as recent examples show, the use of a combination of the energy of both photons and electrons to drive a single transformation will likely bear fruit in the discovery of more selective transformations under milder conditions. In the known examples, the proposed reaction mechanisms are based on highly reactive species with extremely short lifetimes. Therefore, a critical evaluation of the proposed

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mechanisms will be required in the future to probe alternative mechanisms – a complicated task, as the experimental study of such short-lived intermediates is extremely challenging. Both regarding applications and mechanistic understanding, there is no doubt that further important and exciting developments will follow in the future.

Acknowledgements R.F. is grateful for funding by the DFG Heisenberg Program (FR 3848/4-1). M.M. acknowledges the support from the European Union’s Horizon 2020 research and innovation program (Marie Sk"odowska-Curie Grant Agreement No. 892479).

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125. H. Huang and T. H. Lambert, Angew. Chem., Int. Ed., 2020, 59, 658–662. ¨nig, Chem. Eur. J., 2008, 14, 1854– 126. J. Svoboda, H. Schmaderer and B. Ko 1865. 127. W. Zhang, K. L. Carpenter and S. Lin, Angew. Chem., Int. Ed., 2020, 59, 409–417. ´jek, U. Faltermeier, B. Dick, R. Perez-Ruiz and A. Jacobi von 128. M. Ma Wangelin, Chem. – Eur. J., 2015, 21, 15496–15501. ¨nig, Science, 2014, 346, 129. I. Ghosh, T. Ghosh, J. I. Bardagi and B. Ko 725–728. ¨nig, Angew. Chem., Int. Ed., 2016, 55, 7676–7679. 130. I. Ghosh and B. Ko ´jek, V. A. de la Pen ˜ a O’Shea, 131. M. Neumeier, D. Sampedro, M. Ma A. Jacobi von Wangelin and R. Perez-Ruiz, Chem. – Eur. J., 2018, 24, 105–108. 132. N. G. W. Cooper, C. P. Chernowsky, O. P. Williams and Z. K. Wickens, J. Am. Chem. Soc., 2020, 142, 2093–2099. 133. H. Huang, Z. M. Strater, M. Rauch, J. Shee, T. J. Sisto, C. Nuckolls and T. H. Lambert, Angew. Chem., Int. Ed., 2019, 58, 13318–13322. 134. H. Huang, Z. M. Strater and T. H. Lambert, J. Am. Chem. Soc., 2020, 142(4), 1698–1703. 135. T. Shen and T. S. Lambert, Science, 2021, 371, 620–626. 136. H. Kim, H. Kim, T. H. Lambert and S. Lin, J. Am. Chem. Soc., 2020, 142, 2087–2092. 137. C. P. Chernowsky, A. F. Chmiel and Z. K. Wickens, Angew. Chem., Int. Ed., 2021, 60, 21418–21425. 138. N. G. W. Cowper, C. P. Chernowsky, O. P. Williams and Z. K. Wickens, J. Am. Chem. Soc., 2020, 142(5), 2093–2099. 139. A. J. Rieth, M. I. Gonzalez, B. Kudisch, M. Nava and D. G. Nocera, J. Am. Chem. Soc., 2021, 143, 14352–14359.

CHAPTER 7

Synergy of Electrochemistry and Asymmetric Catalysis YI-MIN JIANG,a YI YU,a ZHAOJIANG SHI,a YI-LUN LI,a HONG YANa AND KE-YIN YE*a,b a

Key Laboratory of Molecule Synthesis and Function Discovery (Fujian Province University), Institute of Pharmaceutical Science and Technology, College of Chemistry, Fuzhou University, Fuzhou 350108, China; b State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, P.R. China *Email: [email protected]

7.1 Introduction Organic electrosynthesis, featuring the use of electrons as the green oxidants or reductants, is a highly sustainable synthetic platform for novel redox reactions.1–10 The constructive interplay of electrochemistry with state-of-theart catalysis further profoundly enhances its capability to contribute to the extension of chemical space, such as C–H activation,11–15 difunctionalization of alkenes,16–18 biselectrophile cross-coupling,19–21 electrocarboxylation,22–25 and so on.26–32 Particularly, the asymmetric synthesis powered by electrochemistry has also experienced a dynamic renaissance.33–37 Though electrochemical asymmetric synthesis could be achieved with the use of chiral solvents, chiral supporting electrolytes, chiral substrates/ auxiliaries, and chiral electrodes, the electrochemical asymmetric catalysis appears to be the most practical and promising strategy.38–41 In addition, the efficiency and stereoselectivities of the electrochemical asymmetric catalysis can be further fine-tuned via the proper design of the chiral catalysts, based Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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between catalysts and substrates forming the catalyst–substrate complexes, which then turns on the electrochemical reactivity of asymmetric catalysis (Scheme 7.1C). Since there already exist many elegant reviews on electrochemical asymmetric synthesis, this chapter only focuses on asymmetric catalysis under electrochemical conditions. In line with this theme, literature reviews are usually organized based on the types of catalysts used, catalytic transformations realized, or the redox nature (oxidation or reduction) of the catalytic asymmetric reactions. Instead, we would like to highlight recent advances in the synergy of electrochemistry and asymmetric catalysis, paying particular attention to the organic entities (substrates and/or catalysts) participating in redox events,42 as shown above. Therefore, other asymmetric electrochemical reactions using chiral substrates/auxiliaries, chiral electrodes, chiral electrolytes, and chiral solvents are beyond the scope of this chapter.

7.2 Substrates as the Redox Entities in Electrochemical Asymmetric Catalysis By tuning the polarity of functional groups via selective addition or removal of electrons, electrochemistry could be used in the challenging bondforming reactions, which otherwise are difficult to achieve. The synthesis of highly enantioenriched meta-alkylated anilines, an impossible transformation by the conventional ortho- and para-selective Friedel–Crafts reaction, was realized by Jørgensen and coworkers by using an anodic oxidation/ organocatalytic protocol (Scheme 7.2).43 Galvanostatic electrolysis (applied current: 25 mA, current density: 10 mA cm2) employing Hayashi–Jørgensen catalyst 3 (10 mol%) with electron-rich aromatic aniline 1 and alkyl aldehydes 2 led to meta-substituted dihydrobenzofurans 4 (and their downstream diols 5) with good yields (65–83%) and high enantiomeric excess (89–96% ee). The high stability of the Hayashi–Jørgensen catalyst under these electrolysis conditions further allowed the reaction to run at low catalytic loading (5 mol%) or on a gram-scale reaction while maintaining the same level of high enantioselectivity. Two combined sequences, namely the anodic oxidation of aniline and asymmetric organocatalysis, were anticipated to take place in this electrochemical meta-selective alkylation of electron-rich anilines. The electrochemically generated umpolung electrophilic intermediate 8 (from the nucleophilic hydroxylaniline 1), which could also be prepared in situ chemically, reacted with the electron-rich enamine 7, giving rise to the intermediate 9. The use of water was proposed to assist both the hydrolysis of intermediate 9 to regenerate the catalyst and the subsequent protontransfer step to form the product. This hypothesis was supported by the control experiment, where a dramatically decreased conversion was observed when this reaction was run in CH3CN in the presence of only 5 equiv. of H2O.

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cross-coupling reaction between 2-acyl imidazole 94 and tertbutyldimethylsilyl (TBS)-protected catechol 95 (Scheme 7.21).65 This chiral a,a-diaryl carbonyl 96 was then employed as the key intermediate in the formal total synthesis of isopavine alkaloid, (þ)-amurensinine 98. Extensive CV studies suggested that this reaction proceeded via a radical–radical crosscoupling mechanism between the chiral nickel-bound a-carbonyl radical and para-phenoxyl radical.

7.5 Conclusion In conclusion, recent advances in the synergy of electrochemistry and asymmetric catalysis have been highlighted in this chapter. The introduction of electrochemistry into the state-of-the-art asymmetric catalysis indeed brings forth more chances for the discovery of novel reactivity, most notably in green and sustainable manners. Future directions of the investigations and understanding of the behaviors of chiral catalysts under electrochemical conditions are of great significance,66 as they would guide the design of novel chiral catalysts and catalytic asymmetric reactions. The authors envisage that the synergy of electrochemistry and asymmetric catalysis should find broad applications in the preparation of diverse chiral molecules in organic synthesis.

Acknowledgements Financial support from the National Natural Science Foundation of China (No. 21901041), State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (No. 202008), Hundred-Talent Project of Fujian (No. 50012742), and Fuzhou University (No. 510841) is gratefully acknowledged.

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CHAPTER 8

Alternative Approaches for Scalable Artificial Photosynthesis via Sustainable Redox Processes HAN SEN SOO Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore Email: [email protected]

8.1 Introduction The world faces multiple impending environmental crises now. Around 80% of the global energy needs are still fulfilled by using fossil fuels, which are limited and result in the release of large amounts of greenhouse gases that are recognized as the leading causes of anthropogenic global climate change.1 Moreover, the proliferation of synthetic plastics in consumer products has led to the indiscriminate disposal of over 6300 megatons (Mt) of plastics waste, of which around 79% are accumulating in landfills and marine ecosystems.2 Depressingly, in 2015, for example, around 302 Mt (74%) of the annual plastics production had become ‘‘waste’’, with 14% mechanically recycled, 14% combusted, and the remainder irresponsibly discarded.3 The situation is so dire that plastics waste has been observed in the depths of the Mariana Trench, almost 11 km below sea level, and has been proposed to disrupt the global biogeochemical nitrification and denitrification cycles.4–9 Furthermore, marine plastics pollution is projected to increase to 62 Mt in 2030 and 80 Mt in 2040 from around 19–23 Mt in 2016.10,11 Ultimately, the plastics pollution Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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problem also converges with global climate change since an estimated 1800 Mt of CO2-equivalents of greenhouse gases are released annually over a typical life cycle of plastics, with the amounts expected to grow to 6500 Mt (15% of the global carbon budget) by 2050.12 Patently, we need sustainable solutions to overcome all these problems. In this context, an approach that has received enduring attention over multiple decades from scientists is artificial photosynthesis.13–15 According to its etymology, photosynthesis is fundamentally any process that harvests light (photo, a derivative of ‘‘light’’ in Greek) to produce (synthesis, or put together in Greek) fuels and chemicals.16 In nature, the most common form of photosynthesis involves two independent half-reactions, namely water oxidation to O2 by photosystem II and water reduction to produce hydride equivalents for converting NADP1 to NADPH by photosystem I.1,13 These two half-reactions that combine to form natural photosynthesis are thus essentially water splitting, which is the quintessential sustainable and functional redox process since it produces NADPH as the fuel to reduce the respiratory waste CO2 and also the O2 needed for aerobic metabolism. Inspired by nature, scientists have sought to create simpler manmade chemical or hybrid water-splitting systems so that artificial photosynthesis is often synonymous with water splitting.13–15 Nonetheless, in this chapter, we will revert to the original meaning of artificial photosynthesis and consider all processes that harness light to synthesize chemicals. To distinguish between photocatalysis and photosynthesis, the former involves light mainly to overcome kinetic barriers in exothermic reactions, while the latter stores energy overall via two redox half-reactions.1 Lately, a number of research teams, including Meyer,17,18 Wang,19,20 Sun,21 Reisner,22,23 and the author’s team1,16,24 have been exploring and advocating for artificial photosynthetic systems beyond water splitting or CO2 reduction that produce more value-added products. A technoeconomic analysis of artificial photosynthesis solely by water splitting suggests that it will not be economically viable in the near future because among the two products, H2 and O2, only the former can serve as a fuel, whereas O2 already constitutes 21% of the atmospheric gaseous contents and has little economic value. Instead, if we frame chemical processes in terms of scale and process value (see Figure 8.1a), many industrialized chemical processes fall within the top left and bottom right quadrants. For example, in the top left quadrant, the petrochemical industry thrives on the relative abundance of and access to fossil fuels to be very scalable, but the primary products are fuels and platform chemicals of only modest commercial value. In contrast, the pharmaceutical industry would fall in the bottom right quadrant since expensive reagents and catalysts are employed to produce even higher-value products on small scales. Currently, artificial photosynthetic reactions focused on water splitting or CO2 reduction only will fall into the top left quadrant since water is abundant and H2 from water splitting is not cost competitive with H2 derived from fossil fuels.24,25 However, if artificial photosynthesis can move beyond water to other abundant feedstocks, some of which are treated as ‘‘waste’’ like nonfood biomass and plastics, we may

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(a) Comparisons of the scalabilities versus the product values of chemical processes. For artificial photosynthesis (AP) to become commercially viable, the process must be scalable and the products should also be of high value, as represented by the position of the yellow star. (b) A model photoelectrochemical cell for simultaneous biomass valorization or plastics upcycling by oxidative catalysis at the (photo)anode coupled with the production of fuels by reductive catalysis at the (photo)cathode. Reproduced from ref. 24 with permission from Elsevier, Copyright 2020.

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be able to valorize them into more economically viable products, providing new and more realistic paths to commercialization. This chapter will examine the state-of-the-art in alternative approaches to achieve scalable and valuable artificial photosynthesis through more sustainable redox reactions. Since artificial photosynthesis necessarily consists of two redox half-reactions, it is ideal if both halves are sustainable and produce value-added products (see Figure 8.1b). In Section 8.2, the examples in which nonfood biomass or feedstocks derived from nonfood biomass are oxidized photocatalytically, electrocatalytically, or with both methods combined will be discussed. Subsequently, Section 8.3 will include some of the latest results in heterogeneous and homogeneous oxidation of synthetic polymers, which opens up the possibility of ‘‘mining plastics waste’’ to alleviate the problems of both environmental pollution and global climate change.3,16,24,26–36 Next, the application of a reemerging class of materials, metal halide perovskites, for photodriven redox processes will be examined in Section 8.4. Section 8.5 is a summary of and outlook on the future prospects of sustainable redox processes in artificial photosynthesis.

8.2 Nonfood Biomass Oxidation Other than fossil fuels, the most abundant source of carbon that we can potentially harvest is biomass, the majority of which can be classified as nonfood biomass lignocellulose. Biomass mainly consists of cellulose (40–50%), hemicellulose (25–35%), and lignin (15–20%), the last of which is the most abundant source of aromatic compounds in the world.37 To avoid competition with food production, nonfood lignocellulose is an appealing alternative, which will be the focus of this section. Thus, the controlled and selective oxidation of lignocellulose or feedstocks derived from lignocellulose into small molecules can be a sustainable way to obtain chemicals that are not derived from fossil fuels. In fact, the thermocatalytic conversion of lignocellulose has been comprehensively studied and reviewed, including a themed collection on ‘‘catalytic advances for biomass conversion and upgrading’’ by Chemical Society Reviews.37–41 However, thermocatalytic or pyrolytic biomass oxidation processes ultimately consume a substantial portion of their feedstocks and generally lead to intractable mixtures that are typically combusted as low-grade fuels. To emphasize sustainability, only catalytic oxidation processes driven by light, electricity, or both will be reviewed in this chapter, based on the premise that the light or electricity can be generated from renewable resources such as the sun, wind, tidal waves, or geothermal energy.

8.2.1

Photocatalytic Nonfood Biomass Oxidation

The exploitation of light for the photocatalytic oxidation of lignocellulose has been studied since the 1980s, although previous efforts have largely focused on the use of TiO2 with various cocatalysts.42 Nonetheless, one of the

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major challenges in the reactivity of lignocellulose is its poor solubility in water and other common organic solvents. Consequently, a number of research teams have developed models43–47 to study complex biopolymers like lignin to facilitate the development of catalysts that target vulnerable links such as the b-O-4 group (see Figure 8.2a). The model compounds are soluble in common organic solvents and can, in principle, be used to provide insights for the development of catalysts that target the weakest link in lignin. Building on the early work from Hanson and Toste on using molecular vanadium complexes to target C–C and C–O bond cleavage of lignin model compounds,43–47 the author’s team explored the application of molecular vanadium photocatalysts to effect bond scission in lignin model compounds as well.48,49 Homogeneous catalysts have the advantage that each molecule should be active and can possibly intercalate within the lignocellulose composite to cause bond cleavage, even if the biomass has not been dissolved. The photocatalysts can oxidatively cleave the C–C bonds of the b-O-4 linkage into vanillin and guaiacol derivatives and also be simultaneously coupled with H2 evolution catalysts to give a biomass photoreforming system (see Figure 8.2b).48 The author’s team discovered that a VV complex supported by a conjugated hydrazone benzohydroxamate ligand is able to absorb visible light via ligand-to-metal charge transfer (LMCT) to form a highly reactive ligand radical cation, which is capable of oxidatively cleaving an alcoholic b C–C bond of a coordinated lignin model compound.49 The products are the ethyl ester of vanillin and the formate ester of guaiacol (see Figure 8.2b).49 To understand the origins of this remarkable ambient aerobic C–C bond cleavage, the author’s team conducted studies involving 2H and 13C isotopically labeled lignin model compounds, cyclic voltammetry (CV), UV–Vis spectroscopy, radical trapping, and density functional theory (DFT) calculations.49 The mechanism that is consistent with all the aggregated data is that the benzylic alcohol of the lignin model compound replaces one of the coordinated methanol molecules, after which visible light absorption results in an LMCT to form a transient ligand radical cation (see Figure 8.2c).49 In this excited state, the b C–C bond of the bound lignin model is the weakest link that cleaves to form a coordinated carbonyl product and an alkyl radical.49 Since the reaction is performed with air, the radical is trapped by O2, which eventually goes on to produce the guaiacol formate.49 Subsequently, the author’s group designed a set of five new VV complexes bearing electron-deficient substituents at the two aryl rings of the photocatalyst (see Figure 8.2d) and conducted electrochemistry and kinetic measurements, as well as additional DFT calculations.48 It was discovered that the reaction was first order in photocatalyst and substrate, but zeroth order in O2, suggesting that the rate-determining step is likely the C–C bond cleavage after LMCT (See Figure 8.2c).48 This work demonstrates a proof of concept that nonfood biomass lignin can be selectively and controllably oxidized as part of an artificial photosynthetic system. Nonetheless, one of the practical shortcomings is that the homogeneous VV photocatalysts are difficult to separate and recover from the reaction products.

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To address the need for convenient catalyst recovery, a number of research groups have moved beyond TiO2 systems42 to other visible light absorbing photocatalysts, such as Wang’s team.19,20,50–57 Likewise, they started with lignin model compounds and examined a number of visible light-absorbing materials, including ZnIn2S4, Pd/ZnIn2S4 with TiO2, Ru-doped ZnIn2S4, CuOx, and mesoporous graphitic CNx.19,20,50–55,57 In a typical example with ZnIn2S4, the lignin model compound is mixed with the photocatalyst at room temperature to 42 1C and irradiated with blue light-emitting diodes (LEDs) under an Ar atmosphere (see Figure 8.3a).54 The C–O bond of the b-O-4 linkage cleaves to give a ketone and phenolic products.54 They tested several lignin model compounds and examined the effects of exogenous H2 on the reaction.54 In the proposed mechanism, the ZnIn2S4 absorbs visible light to generate e/h1 pairs and the h1, which has an estimated þ1.44 V potential, will initiate the C–O bond cleavage process by first oxidizing the benzylic alcohol to a ketone (see Figure 8.3b).54 At the same time, the e in the conduction band of the photoexcited ZnIn2S4 will reduce the released protons from benzylic alcohol oxidation to produce surface hydrogen atoms, which will hydrogenolyze the intermediate to effect C–O bond cleavage (see Figure 8.3b).54 They further demonstrated that ZnIn2S4 is capable of fragmenting a synthetic polymer composed of b-O-4 linkages and dioxanesolv poplar lignin, the latter of which gave up to 17% of p-hydroxyl acetophenone derivatives.54 Wang’s team has thus shown that heterogeneous semiconductor photocatalysts are suitable for oxidative redox processes that can valorize nonfood biomass in artificial photosynthetic systems to produce aromatic chemical feedstocks. In a further step toward practical implementation, Reisner and coworkers first employed CdOx-coated CdS quantum dot (QD) photocatalysts for the photoreforming of lignocellulose into carboxylates and carbonates while producing H2 as a fuel.58 They reported that 4.8 nm CdS QDs decreased in size by about 0.6 nm and became passivated with CdOx after being suspended in 10 M KOH (aq.) solutions. The CdS/CdOx QDs were then employed

Figure 8.2

(a) Representative structure of nonfood biomass lignin and an example of a commonly used model compound that contains the vulnerable b-O-4 linkage. Reproduced from ref. 49 with permission from the Royal Society of Chemistry. (b) Idealized artificial photosynthetic or photoreforming system composed of the photocatalytic oxidative deconstruction of lignin coupled with the simultaneous production of H2. (c) Correlation of the kinetics of photocatalytic C–C cleavage with the computed electronic structure of several molecular vanadium complexes. Reproduced from ref. 48 with permission from the American Chemical Society, Copyright 2017. (d) Mechanism for oxidative C–C bond cleavage initiated by a ligand-to-metal charge transfer (LMCT) from the redox noninnocent ligand to VV, followed by the trapping of the aliphatic radical with O2. This mechanism has been proposed based on DFT calculations and is consistent with a series of studies including isotope labeling, kinetic measurements, and spectroscopic analyses.

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(a) Greener photocatalytic benzylic alcohol oxidation and tandem C–O cleavage targeting the b-O-4 linkage of nonfood biomass lignin model compounds to phenols and aryl ketones driven by visible light. (b) Proposed two-step mechanism for benzylic alcohol oxidation by the photogenerated holes in ZnIn2S4, followed by transfer hydrogenolysis to cleave the C–O bond. Reproduced from ref. 54 with permission from the American Chemical Society, Copyright 2017.

for the visible light photoreforming of a-cellulose, hemicellulose, lignin, and eventually lignocellulosic materials such as printer paper, cardboard, wooden branches, bagasse, sawdust, and grass.58 After adding Co(BF4)2 as a cocatalyst, H2 evolved from all of these materials, with an external quantum yield of 1.2% for a-cellulose when 430 nm monochromatic radiation was used.

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As a more sustainable extension of their earlier pioneering work, Reisner’s group reported that cyanamide-functionalized carbon nitride (NCNCNx) is also an effective catalyst for the photoreforming of lignocellulose in aqueous solutions between pH 2 and pH 15.59 The NCNCNx was first activated by ultrasonication, after which it was mixed with different cocatalysts (e.g. molecular Ni bis-(diphosphine), Pt, or MoS2) for photoreforming the components of lignocellulose (see Figure 8.4).59 Remarkably, the photoreforming systems containing NCNCNx mixed with Pt or MoS2 could steadily evolve H2 over 6 days, with higher yields of H2 from more alkaline solutions.59 Up to 22% conversion was observed with a-cellulose as the substrate after 6 days of illumination in 10 M KOH (aq.) solutions, which is over twice as much as that in their previously reported study with CdS/CdOx QDs (9.7%).58,59 Mechanistically, they ruled out the operation of OH radicals since they did not detect fluorescence from 2-hydroxyterephthalic acid when terephthalic acid was used as a radical trap. Instead, they proposed that the lignocellulosic substrates were directly oxidized via h1 transfer from the photocatalysts owing to the strong intermolecular interactions (electrostatic, hydrogen bonding, p–p stacking) with the substrates, which resulted in C–C bond cleavage and the production of formate and carbonate.58,59 Under the strongly alkaline 10 M KOH (aq.) solutions, partial hydrolysis of the lignocellulose into dissolved fragments may also help to accelerate the photoreforming process.58,59 Consequently, in their latest work, they examined the use of metal salt hydrate solutions to completely dissolve lignocellulose in aqueous media, although the reported activities were lower than their previous studies.60

Figure 8.4

(a) Depiction of the use of cyanamide-functionalized carbon nitride (NCNCNx) as a photoreforming catalyst for the oxidation of nonfood lignocellulose biomass, with simultaneous production of H2 fuel in the presence of other cocatalysts. (b) Quantification of the H2 generated from the different components of lignocellulose during photocatalysis in the presence of NCN CNx and a molecular Ni bis-(diphosphine) cocatalyst in a phosphate buffer solution under simulated AM 1.5G solar radiation at 25 1C. Reproduced from ref. 59 with permission from the American Chemical Society, Copyright 2018.

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The works examined in this section have laid the foundations for the sustainable oxidation of nonfood biomass using light as the energy source. However, the processes are all batch-scale and photocatalytic reactors are notoriously difficult to scale up owing to the limitations of light penetration according to the Beer–Lambert law. In the next section, electrocatalytic and photoelectrocatalytic processes are reviewed since more scalable flow reactors can be developed for them.

8.2.2

Electrocatalytic and Photoelectrocatalytic Nonfood Biomass Oxidation

Another approach to oxidizing nonfood biomass with minimal reliance on fossil fuels is the development of electrocatalytic redox processes to selectively transform the substrates. Although the majority of the electricity produced worldwide is still generated from fossil fuel combustion, the basis of this chapter is that the studies discussed here are laying the groundwork to exploit renewable electricity, which will become available in the near future. Similar to the previous section, the research involves mainly actual and derived feedstocks from nonfood biomass. Owing to the low solubility of lignocellulose in water and organic solvents, most of the reported research works thus far have involved simpler smallmolecule substrates originating from carbohydrates. For instance, electricity and light have been combined in photoelectrochemical cells where TiO2 and WO3 photoanodes were employed to oxidize substrates, including urea, pig urine, oxalic acid, alcohols, polyols, and other sugars.61–63 However, the fate of the biomass is invariably mineralization to CO2, leaving H2 as the main source of value recovery, which is not economically viable as explained previously. Lately, Choi and Sun have independently worked on the oxidation of 5-hydroxymethylfurfural (HMF), which is derived from C6 monosaccharides from cellulose depolymerization, to 2,5-furandicarboxylic acid (FDCA).64–69 Choi first deployed BiVO4 photoanodes together with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) as a redox mediator for a visible lightpromoted photoelectrochemical system to oxidize HMF at 1.04 V vs. reversible hydrogen electrode (RHE) with the simultaneous generation of H2 at the cathode from pH 9.2 borate buffer solutions.64 Sun and coworkers then achieved even higher current densities for the same 6e oxidation of HMF to FDCA (see Figure 8.5a) by using electrodeposited cobalt phosphide on copper foam (Co–P/CF) for both electrodes.66 Unlike BiVO4, which is unstable in strongly alkaline or acidic pH conditions, the Co–P/CF electrodes were utilized for HMF oxidation at 1.42 V vs. RHE in 1.0 M KOH (aq) solutions and operated for 6 h.66 Full conversion of the HMF was observed with FDCA as the predominant product (B90% yield) and only transient amounts of the intermediates were detected (see Figure 8.5b).66 Besides the additional value of FDCA being a terephthalic acid replacement for the preparation of bioderived polyesters, this system is advantageous over water splitting since

Alternative Approaches for Scalable Artificial Photosynthesis

Figure 8.5

185

(a) Two pathways for the oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a monomer for polyesters. (b) The concentration of the HMF substrate and the oxidation product yields during chronoamperometry experiments performed in 1.0 M KOH at 1.42 V vs. the reversible hydrogen electrode (RHE). (c) Linear sweep voltammograms of the Co–P catalyst conducted with (red) and without (black) HMF present as the substrate. Reproduced from ref. 66 with permission from the American Chemical Society, Copyright 2016. (d) A novel electrolyzer design for the decoupled oxidation of HMF with the hydrogen evolution reaction (HER) in the left chamber, connected to the [Fe(CN)6]3/[Fe(CN)6]4 redox couple in the right chamber. Reproduced from ref. 67 with permission from Elsevier, Copyright 2018.

HMF oxidation requires a lower onset potential and can be operated at only 1.44 V vs. RHE (150 mV lower than water splitting) to reach a current density of 20 mA cm2 (see Figure 8.5c).66

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Sun’s group has gone on to explore the use of other electrode materials for the concurrent oxidation of HMF coupled with H2 evolution, including nickel phosphide nanoparticle arrays on nickel foam (Ni2P NPA/NF), Ni2P/Ni/NF, and hierarchically porous nickel sulfide on nickel foam (Ni3S2/NF).65,68,69 New electrolyzer designs were also created to facilitate the decoupling of water splitting with the biomass oxidation reactions.67,69 Typically, the electrocatalytic oxidation and reduction half-reaction rates are not matched and conventional water-splitting electrolyzers require high-voltage inputs due to the substantial potential (minimum of 1.23 V) needed. By using an electrochemically stable redox couple of judiciously selected potentials like [Fe(CN)6]3/[Fe(CN)6]4 as redox mediators in a separate compartment, they were able to temporally decouple the HMF oxidation process from the H2 evolution reaction (see Figure 8.5d).67 This electrolyzer design permits the application of lower applied potentials for each redox half-reaction when Co–P was used as the working electrode in 1.0 M NaOH (aq.).67 Furthermore, like the light and dark reactions of natural photosynthesis, a photovoltaic can be used to drive H2 evolution with solar irradiation while charging the [Fe(CN)6]3/[Fe(CN)6]4 reservoir, and a modest voltage (0.2 V proposed by the authors) can be applied together with the discharge of the e reservoir in the absence of sunlight for the oxidation of HMF to FDCA.67 Thus, the electrocatalytic oxidation of cellulose-derived HMF offers a sustainable redox reaction for a scalable artificial photosynthetic system. Nonetheless, despite the promise offered by the selective oxidation of HMF to FDCA, HMF is still derived after the dehydration of hexose sugars, which adds additional processing costs. Moreover, the hexose sugars are typically obtained from agricultural food crops such as sugar cane or corn, which will ultimately compete with food production. Ideally, the origin of nonfood biomass feedstocks should be minimally processed lignocellulose. In this context, Stephenson and coworkers have reported a breakthrough in combining both electrocatalysis with photocatalysis to oxidatively functionalize pine native lignin.70 To facilitate a systematic and mechanistic understanding of the process, they also started with lignin model compounds containing the b-O-4 linkage (see Figure 8.6a).70 N-hydroxyphthalimide (NHPI) was then used as a homogeneous redox mediator in the presence of 2,6-lutidine as a base.70 As anticipated, the redox wave of NHPI shifted cathodically by 580 mV upon deprotonation by 2,6-lutidine and the behavior also became more reversible, as suggested by the small potential difference between the anodic and cathodic peaks (82 mV from 155 mV without 2,6-lutidine), approaching the ideal 59 mV (see Figure 8.6b).70 Subsequently, CV measurements were conducted on 1-(3,4dimethoxyphenyl)ethanol and ethanol to compare their reactivities, which ascertained the selectivity for benzylic alcohol oxidation over primary alcohol oxidation (see Figure 8.6c).70 With these results in hand, they experimented with the electrocatalytic oxidation process by varying the electrolyte, the base, and the duration of electrolysis, with the optimized conditions shown in Figure 8.6a (condition B).70

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Figure 8.6

187

(a) Sequential electrocatalytic benzylic oxidation followed by photoredox C–O bond cleavage for the valorization of nonfood biomass lignin models and actual lignin. (b) Cyclic voltammograms showing the cathodic shift and increased reversibility of the N-hydroxyphthalimide (NHPI) redox couple in the presence of the 2,6-lutidine base. (c) Cyclic voltammograms exhibiting increased catalytic currents that support the selectivity for benzylic alcohol over primary alcohol oxidation. (d) Proposed propagation and termination reactions for oxidizing the benzylic alcohol to a ketone, which occur during the electrocatalytic oxidation process in the absence (bottom) and the presence (top) of O2. Reproduced from ref. 70 with permission from the American Chemical Society, Copyright 2017.

After establishing the conditions for the electrocatalytic oxidation of the benzylic alcohols to ketones, Stephenson’s team then proceeded to evaluate the compatibility of these conditions with a photoredox C–O bond cleavage.70 They investigated if a one-pot reaction by adding [Ir(ppy)2(dtbbpy)](PF6) (ppy ¼ 2-phenylpyridine, dtbbpy ¼ 4,4 0 -di-tert-butyl-2,2 0 -dipyridyl), diisopropylethylamine, and formic acid could tolerate all the other reagents present

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during the electrochemical processes, such as the NHPI, 2,6-lutidine, and electrolyte.70 Furthermore, a flow system was utilized for the reaction to become scalable. Moderate to high conversions and yields of several lignin model compounds were observed under these conditions (see Figure 8.6a).70 To target pine native lignin, they discovered that a cosolvent of 98 : 2 acetone/ DMSO could be used to give a homogeneous solution, after which the optimized one-pot, electrocatalytic/photoredox two-step conditions were successfully applied.70 Although only modest yields (1.1–1.3%) of monoaromatic products were obtained,70 this study represents a critical proof of concept that macromolecular nonfood biomass can be oxidatively cleaved into valuable small molecules. Kinetic isotope effect experiments were conducted to support the mechanism shown in Figure 8.6d, where phthalimide N-oxyl (PINO) radicals generated in situ by electrocatalytic oxidation will first abstract the benzylic protons, after which a second oxidation can take place at the anode or in the presence of air aided by proton-coupled electron transfer (PCET).70 The intermediate ketone will then undergo the photoredox C–O bond cleavage to produce the monoaromatic products, which are derivatives of vanillin, for example. Currently, these are some of the latest proofs of concept that nonfood biomass, several of which do not need much preprocessing, can be oxidized to more valuable products with potentially renewable energy sources through photocatalysis, electrocatalysis, or a combination of both methods. Nonfood biomass constitute the world’s most abundant natural polymers and will undoubtedly be a sustainable source of chemical feedstocks. In the next section, the chemistry of the oxidation of synthetic polymers that we have been producing since the middle of the 20th century, which are now considered recalcitrant pollutants, but which can in fact become another sustainable source of carbon-based chemical feedstocks, will be examined.

8.3 Synthetic Polymer Oxidation As mentioned in the Introduction section, the proliferation of synthetic plastics and their irresponsible disposal has led to another global environmental crisis. However, the carbon-rich and low-oxidation state nature of the polyolefins and other nonperfluorinated plastics offer an opportunity for the oxidative transformation into partially oxidized platform chemicals, which could offer sustainable approaches to ‘‘mine’’ the synthetic polymers.32 Although plastics can theoretically be oxidized electrocatalytically, the dearth of reports suggests that their low solubility in commonly used organic solvents or water has proven to be a formidable barrier to the development of such processes. The sole report on the electrochemical degradation of polyethylene terephthalate (PET) was essentially just the electrocatalytic reduction of protic methanol as an in situ method to generate alkoxide ions, which led to alkaline conditions for the partial hydrolysis of the PET.71 Given this scarcity of research on the electrocatalytic oxidation of synthetic polymers, the following subsections will focus mainly on the photooxidative

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deconstruction of plastics, especially the use of heterogeneous semiconductor catalysts and homogeneous photoredox catalysts.

8.3.1

Heterogeneous Photocatalytic Oxidation of Synthetic Polymers

The heterogeneous photocatalytic oxidation of synthetic polymers has had a long history since the 1980s and 1990s, dominated by the use of wide bandgap metal oxides such as TiO2 and ZnO.72,73 Depending on the particle size and morphology, the bandgaps for TiO2 are typically estimated at 3.2–3.4 eV, with the valence band at around þ2.7 V vs. normal hydrogen electrode (NHE). Since the valence band of TiO2 and most metal oxides is localized as holes on the oxygen atoms, other wide bandgap metal oxides like ZnO share similar valence band levels, giving them strong oxidizing potentials to react with synthetic polymers. Since the valence band potentials are so high, the polymers are typically mineralized to CO2 or carbonates. Such photooxidative processes have been applied to multiple types of plastics, including polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), polyvinyl borate, and PET, with a recent example on the complete mineralization of PE.72–80 However, wide bandgap semiconductor materials require UV radiation, which constitutes only around 5% of the solar spectrum. For the photocatalytic oxidation of synthetic polymers to be part of a sustainable artificial photosynthetic system, extending the absorption range to the visible region can offer the potential to harvest another 43% of the solar spectrum. Consequently, this has prompted research in the introduction of dopants to reduce the bandgap of TiO2,81 the attachment of visible light-absorbing molecules or materials to improve light harvesting,74,75,82–84 and the exploration of smaller bandgap materials,76,85 including some from the author’s team.86–89 Besides expanding the spectral range of light absorption, another pathway toward sustainability is to obtain more valuable products from the oxidation of plastics. The mineralization of plastics to CO2 is ultimately no greener than waste-to-energy schemes and merely transforms plastics pollution into global climate change by producing more greenhouse gases.12 Lately, Reisner and coworkers have extended their oxidative photoreforming catalytic systems from lignocellulosic biomass to plastics as well. Instead of only mineralizing the plastics into carbonates, they have developed heterogeneous photocatalysts that absorb visible light so that the h1 will oxidize plastics, while the e can react with water in the presence of cocatalysts to give H2 (see Figure 8.7a).90 They first demonstrated this concept by using the same CdS/CdOx QDs that were successfully applied for biomass substrates.90 In typical experiments, the synthetic polymers were pretreated by dispersion as powders in 10 M NaOH (aq) for 24 h at 40 1C first and then irradiated with AM 1.5G solar irradiation in the presence of the CdS/CdOx QDs under N2 at 25 1C.90 A number of synthetic polymers were examined in their study, including polyvinyl pyrrolidone (PVP), polyethylene glycol (PEG), PE, PVC,

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Figure 8.7

Chapter 8

(a) Representative reactions for the photoreforming of plastics with H2 production driven by a semiconductor photocatalyst. In this study, CdS/CdOx quantum dots were used, although the same diagram is also applicable when more eco-friendly photocatalysts such as CNx are used. (b) H2 production from the photoreforming of polylactic acid (PLA), polyethylene terephthalate (PET), and polyurethane by using 1 nmol of CdS/CdOx quantum dots under simulated solar radiation at 25 1C before and after treatment in 10 M NaOH (aq). Reproduced from ref. 90 with permission from the Royal Society of Chemistry. (c) Photograph of a batch photoreactor for the photoreforming of plastics using CNx as the photocatalyst. (d) H2 production from the photoreforming of polyester microfibers, a PET bottle, and a PET bottle coated with soybean oil, using CNx with nickel phosphide (Ni2P) as the catalysts in 1 M KOH under simulated sunlight over the course of five days. Adapted from ref. 91 with permission from the American Chemical Society, Copyright 2019.

PS, polymethyl methacrylate (PMMA), and polycarbonate (PC).90 However, their alkaline reaction conditions mainly favor condensation polymers that can be partially hydrolyzed to shorter chain oligomers or monomers, so the highest yields of H2 from photoreforming were observed for PLA, PET, and polyurethane (PUR) (see Figure 8.7b).90 NMR spectroscopic analyses of the supernatant before and after the photoreforming suggested that the organic products remaining in solution were mainly carboxylates like formate, acetate, lactate, and isophthalate, depending on the identity of the plastic

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precursor. However, the conversions remained low at 38.8% for PLA, 16.6% for PET, and 22.5% for PUR, although they were able to demonstrate the generation of H2 in a photoreforming process over 6 days.90 Nonetheless, Cd is highly toxic and Cd-based QDs will unlikely be widely deployed for environmental applications. Fortunately, the concept illustrated in Figure 8.7a is general and the heterogeneous photocatalyst can be replaced with more environmentally benign alternatives. Reisner’s team subsequently showed that CNx can be combined with nickel phosphide (Ni2P) to form a composite CNx |Ni2P, which could similarly photoreform plastics to produce H2.91 CNx prepared from melamine was subjected to hydrothermal conditions with NiCl26H2O and NaH2PO2H2O at 200 1C to form Ni2P nanocrystals deposited on the CNx.91 The same alkaline pretreatment protocol was employed with synthetic polymers such as PE, PET, PLA, polypropylene (PP), PS, PUR, and PS-block-polybutadiene, but the highest H2 yields were observed for the condensation polymers PLA and PET, as expected.91 Batch reactions were conducted in up to 120 mL of solvent (see Figure 8.7c) and ‘‘real-world’’ plastic samples were subjected to the reaction conditions over 5 days.91 As shown in Figure 8.7d, 104, 22.0, and 11.4 mmolH2 gsub1 of H2 were produced from polyester microfibers, a PET bottle, and a PET bottle contaminated by soybean oil, respectively.91 Interestingly, the rate of H2 produced accelerated for the polyester microfibers, which the authors attributed to an increase in the exposed surface area as the microfibers disintegrated.91 They also compared the photocatalytic activity of CNx |Ni2P with CNx |Pt, TiO2 |Pt, TiO2 |Ni2P, and CdS/CdOx and found that although CdS/CdOx resulted in the highest H2 yield, CNx |Ni2P offered the best compromise in long-term stability, utilization of visible light, price, and ecofriendliness.91 However, conversions remained low at around 50% for polyethylene glycol after 18 days, 24.5% for PET, and 6.7% for PLA, with the same mixture of different carboxylates.91 Although CNx |Ni2P is promising for the oxidative degradation of some synthetic polymers and produces H2 as a fuel, the low conversions, limited scope of plastics, alkaline pretreatment, and the mixture of products hinder further developments in this technology. Recently, Sun, Xie, and coworkers reported that Nb2O5 nanosheets could be used as wide bandgap heterogeneous photocatalysts for the oxidative degradation of several polyolefins.92 Niobic acid was used to prepare single unit cell-thick Nb2O5 layers, which have estimated bandgaps of 3.4 eV and valence band levels of around þ2.5 V vs. NHE.92 A number of polyolefins were tested, including PE, PP, and PVC, by irradiation with a 300 W Xe lamp under an AM 1.5G filter in the presence of Nb2O5 nanosheets dispersed in pure water.92 Remarkably, PE, PP, and PVC were fully photooxidized within 40, 60, and 90 h, respectively.92 However, instead of complete mineralization as observed previously for most wide bandgap semiconductor materials, they observed the formation of acetic acid by 1H NMR spectroscopy, although the yields were very low on the order of 0.2%.92 Their proposed mechanism

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is summarized in the following equations for PE as an example. PE is first oxidatively degraded completely to CO2. H2O þ h1- OH þ H1  OH;O

2

(8.1)

ðCH2  CH2 Þn ! 2nCO2

(8:2)

O2 þ e-O2 

(8.3)

O2  þ e þ 2H1-H2O2

(8.4)

H2O2 þ 2e þ 2H -2H2O

(8.5)



1

In the subsequent step, CO2 was reduced to acetic acid by the conduction band e. CO2 þ e þ H1- COOH

(8.6)

2 COOH-HOOC–COOH

(8.7)

HOOC–COOH þ 6e þ 6H -CH3COOH þ 2H2O

(8.8)



1

To confirm their proposed mechanism, the authors conducted isotope labeling experiments with H218O and D2O and observed the formation of C16O18O, as well as CDH2COO, CD2HCOO, and CD3COO by in situ Fourier transform infrared (FTIR) and mass spectrometry experiments.92 These results are consistent with the fact that water oxidation intermediates are involved in the mineralization of the PE, after which the acetic acid forms from the reduction of the CO2 in the presence of D2O as the hydrogen atom source. Despite the tangible progress in the development of heterogeneous photocatalytic oxidation processes for plastics, the low conversions, low yields, and poor selectivity for desired products leave much room for improvements. Part of the problem likely arises from the low interfacial kinetic rates between the insoluble plastics and the heterogeneous catalysts. In the next section, homogeneous photocatalysts that can, in principle, be uniformly dispersed throughout the plastics so that every molecule is active will be discussed.

8.3.2

Homogeneous Photocatalytic Oxidation of Synthetic Polymers

Similar to the trajectory with heterogeneous photocatalysts, early efforts in the development of homogeneous variants mainly led to mineralization. This included the use of UV, FeCl3, and H2O2 in photoassisted Fenton processes that led to indiscriminate oxidation of PS to CO2 by reactive oxygen species.93 In contrast, the author’s team focused on developing ways to add value to the oxidized plastics. Building on the mechanistic insights on the LMCT-driven photocatalytic C–C bond cleavage mediated by VV complexes (see Figure 8.2c), the author’s team sought to expand the substrate scope

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beyond lignin model compounds and also beyond lignin-based nonfood biomass. Having identified the fastest VV photocatalyst in the previous study conducted by the author’s team on lignin model compound substrates (see Figure 8.2d),48 they embarked on evaluating the functional group tolerance of the photocatalytic C–C bond cleavage reaction.25 Gratifyingly, electrondonating methyl and methoxy groups and electron-withdrawing boronate, nitrile, nitro, ester, and halide groups were all compatible with the reaction conditions.25 Furthermore, it was found that nonbenzylic alcohols could undergo the same C–C bond cleavage.25 Notably, products from secondary C–C bond cleavage in aliphatic alcohols like 1-butanol were detected, suggesting that the initial product after b-scission can continue to behave as a substrate.25 After reanalyzing the mechanism in Figure 8.2c, the author’s team realized that the peroxide intermediate from radical trapping by O2 will eventually convert into an aliphatic alcohol, which is a substrate for C–C bond cleavage again. This prompted them to consider the potential applications of oxidative cascade C–C bond cleavage in macromolecules. In the context of abundant macromolecules that can be valorized for artificial photosynthesis by cascade C–C bond cleavage, other than nonfood lignocellulosic biomass, synthetic polymers come to mind. Accordingly, they started by examining the photooxidative C–C cleavage of the biodegradable, alcohol-terminated PEG and found that it could be quantitatively converted to formic acid with up to 75% yields within 2.5 days.25 They progressively examined more challenging substrates such as the copolymer of PEG and polycaprolactone, PE–PEG, and finally the nonbiodegradable PE as well.25 Remarkably, they observed the quantitative cascade C–C bond oxidative cleavage of PE to produce mainly formic acid within 6 days, instead of the estimated centuries in the natural environment.25 Thus, this study represents a benchmark in the oxidation of synthetic polymers for artificial photosynthesis. Formic acid has numerous applications as a preservative, an antibacterial agent, and as a fuel, either directly in fuel cells or as a liquid organic hydrogen carrier (see Figure 8.8a).25 They then conducted isotope labeling studies with deuterated methanol CD3OD and 1H, 2H, and 13C NMR spectroscopy to ascertain that the formic acid originated from the synthetic polymers and not the solvent.25 Based on the previously established mechanism for small molecular substrates, the cascade C–C bond cleavage is proposed to start by the coordination of an alcohol-terminated polymer, followed by a visible light-triggered LMCT and C–C bond cleavage (see Figure 8.8b).25 The aliphatic alkyl radical is trapped as a peroxide, which undergoes further transformation to become an alcohol-terminated polymer that is shorter by one carbon.25 This initial product can then reenter the catalytic cycle until the entire polymer chain is sequentially cleaved so that each carbon is at least partially oxidized to formic acid (see Figure 8.8b).25 An alternative approach to valorizing plastics by photooxidative catalysis, other than polymer chain deconstruction, is the increased functionalization to synthetic polymers, especially those containing aromatic groups, as adopted by Leibfarth and coworkers.94–96 Synthetic polymers containing aromatic rings

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Figure 8.8

Chapter 8

(a) Schematic showing the oxidative depolymerization of nonbiodegradable polyethylene (PE) by cascade C–C bond cleavage using a molecular vanadium photocatalyst with atmospheric pressure air as the oxidant. The main products are formates, which can be directly used as chemical feedstocks or in fuel cells and can also be considered liquid forms for storing H2. (b) An adaptation of the mechanism in Figure 8.2d for small molecules, with the C–C bond in PE sequentially broken by the photocatalyst in a cascade C–C bond cleavage process. Each time the C–C bond is cleaved, the alkyl radical is trapped by O2 to eventually turn into another shorter-chain alcohol, which becomes a substrate again to continue the C–C bond cleavage process. Thus, the vanadium photocatalyst is akin to a zipper that unravels nonbiodegradable polyolefins like PE.

like PS can be further oxidized by C–H activation to become new functional materials. Building on the seminal procedure for radical trifluoromethylation of small molecules by Stephenson and coworkers,97 Leibfarth’s team adapted the conditions for the fluoroalkylation of PS.94 They first used Ru(bpy)3Cl2 (bpy ¼ 2,2 0 -bipyridine) as the photocatalyst and a mixture of trifluoroacetic anhydride and pyridine-N-oxide to oxidatively introduce CF3 groups to the benzene rings in PS.94 By controlling the number of equivalents of trifluoroacetic anhydride with pyridine-N-oxide between 0.5 and 5, they were able to introduce 19 to 108 CF3 groups per 100 repeat units, meaning that

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more than one CF3 group could be introduced for each benzene ring. The reaction could be successfully applied to other synthetic polymers, including poly(4-methylstyrene), poly(4-tert-butylstyrene), poly(bisphenol A carbonate), PET, and Eastman’s Tritans.94 The oxidative C–H activation concept is also general for the functionalization of PS and can be extended from substitution by CF3 groups to C2F5, C3F7, CClF2, CBrF2, and C7F15.94 These multifluorinated PS derivatives became more hydrophobic with static contact angles rising from 941 in PS to 1111 for the material functionalized by C7F15.94 Moreover, the new material containing CBrF2 groups was employed as a precursor for additional chemical diversification with atom transfer radical polymerization by using CuBr and heat in the presence of methacrylate esters to produce amphiphilic poly(styrene-graft-acrylic acid) copolymers.94 They have thus demonstrated that synthetic polymers containing aromatic rings can be upcycled by oxidative C–H activation by homogeneous photocatalysis. After establishing the concept of synthetic polymer upcycling by photocatalysis, Leibfarth’s team enhanced the sustainability of the process by replacing the heavy metal Ru(bpy)3Cl2 photosensitizer with the organic photocatalyst 5,10-di(2-naphthyl)-5,10-dihydrophenazine.95 They retained the use of fluorinated acyl anhydrides and pyridine-N-oxide as the source of fluorinated alkyl radicals and screened a family of phenoxazines, phenothiazines, and phenazines to evaluate their efficacy in adding CF3 groups to PS.95 Interestingly, although all the tested organic photocatalysts were able to introduce different amounts of CF3 groups, the majority of them also changed the molecular weight distribution of the synthetic polymers, which was attributed to chain coupling reactions due to defluorination by the highly reducing photosensitizers.95 Compound 5,10-di(2-naphthyl)-5,10dihydrophenazine was identified as the optimal photocatalyst that achieved among the highest trifluoromethylation yields with minimal change to the molecular weight distribution.95 With the organic photocatalyst in hand, they discovered that 1 mol% was the most cost-effective concentration and also extended the reaction beyond substitution with CF3 to C2F5, C3F7, and CClF2, and also demonstrated the applicability on postindustrial and postconsumer PS waste, as well as poly(bisphenol A carbonate).95 The use of organic photocatalysts to achieve oxidative C–H functionalization and upcycling of plastics is clearly a more sustainable approach than the original method using expensive and toxic Ru(bpy)3Cl2. Despite the immense promise of homogeneous photocatalysis for the oxidative valorization of synthetic polymers, a number of challenges remain. For the C–H substitution on aromatic rings along the polymer chain, there are currently no known uses for the new fluorinated materials, which limits the function of the protocol. A more general problem afflicting homogeneous reactions is the difficult reuse of the catalysts and the lack of scalable reactors to manage the constraints of the exponential drop in light penetration because of the Beer–Lambert law. Additional efforts to develop microreactors and continuous flow systems will undoubtedly be necessary for future progress in the applications of homogeneous photocatalysis for

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the oxidative functionalization of synthetic polymers to become integrated into artificial photosynthetic systems.

8.4 Photosynthetic and Photocatalytic Reduction by Metal Halide Perovskites The previous sections of this chapter have focused on catalytic oxidation processes that can be scalable alternatives to water oxidation in an integrated artificial photosynthetic system. However, the reductive half-reaction is typically the part where fuel is created through energy storage. In this section, the focus will be on the emergence of metal halide perovskites as outstanding and promising light harvesters for photosynthesis and photocatalysis. Organic ammonium lead halide perovskites had experienced a renaissance since 2009 when Miyasaka and coworkers first reported the use of CH3NH3PbI3 and CH3NH3PbBr3 as light harvesters for dye-sensitized solar cells with modest but respectable solar conversion efficiencies of up to 3.8%.98,99 Since then, there has been intensive interest by scientists worldwide and a meteoric rise in solar conversion efficiencies to the current record of 29.15% for a perovskite/silicon tandem solar cell in 2020.100 The impressive photon collection properties of metal halide perovskites have stimulated research interest in their exploitation for artificial photosynthetic purposes,101–103 with notable successes in solar-to-H2 energy conversion efficiencies of 12.3% by water photolysis using perovskite solar cells in 2014.104 However, the most effective lead halide perovskites are notoriously unstable when exposed to air, water, irradiation, or some combination thereof, which has been attributed to the lability of the halide and hydrophilic cations and their low thermodynamic stability, among other reasons.16,105–107 This spurred the development of strategies to encase the perovskite with more robust materials such as Ni to protect it from direct exposure during water-splitting photoelectrocatalysis.108 Interestingly, it was discovered that CH3NH3PbI3 can be directly used for photocatalytic H2 generation from HI when [I]r[H1] and both are at such high concentrations that the solution is saturated with CH3NH3PbI3.109 After the publication of this seminal report, other lead halide perovskites were found to be suitable for photocatalytic CO2 reduction when suitably modified. Kuang and coworkers revealed that a composite of CsPbBr3 QDs with graphene oxide (GO) could be employed for the photocatalytic reduction of CO2 to give a mixture of CO, CH4, and H2 (see Figure 8.9a).110 The authors prepared the CsPbBr3 QD/GO composites by precipitation at room temperature and characterized the material by absorption and photoluminescence spectroscopy, powder X-ray diffraction (XRD), transmission electron microscopy, and energy-dispersive X-ray spectroscopy.110 As illustrated in Figure 8.9b, the CsPbBr3 QD/GO samples consisted of B6 nm QDs randomly distributed on the GO nanosheets.110 The X-ray photoelectron spectra were consistent with the perovskite phase composition of CsPbBr3 and thermogravimetric analysis showed that the GO composition was around 8.73%, while the majority is

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110

CsPbBr3. The CsPbBr3 QD/GO composite was then employed for photocatalytic CO2 reduction with ethyl acetate as the solvent and no other protic source or sacrificial electron donor.110 After 12 h of photoreaction, the CsPbBr3 QD/GO composite produced a mixture of CO and CH4 with little H2, as shown in Figure 8.9c.110 By combining all the identified products, the average e consumption was 29.8 mmol (g h)1 for the composite, which was higher than the 23.7 29.8 mmol (g h)1 for CsPbBr3 QDs alone.110 However, CO and CO2 were also generated when the reactions were conducted under N2, which was attributed to the partial oxidation of the ethyl acetate solvent, suggesting that the solvent is both the source of e and hydrogen atoms, and no 13C isotope labeling experiments were used to verify that CO2 reduction was the direct source of the reduced products.110 Thus, it is inconclusive whether the CsPbBr3 perovskite directly reduced CO2 as a photocatalyst. In the same vein, Tan, Wu, and coworkers reported that CsPbX3 (X ¼ Cl, Br, I) perovskite nanocrystals could be used for thiol coupling and dehydrogenative C–P cross-coupling reactions.111 The generic structure of CsPbX3 perovskites is illustrated in Figure 8.10a, with a three-dimensional (3D) lattice created by PbX6 octahedra bridged by corner-shared halide ions and Cs occupying the interstitial holes.111 Since the band structure of these CsPbX3 perovskites is such that the valence band comprises mostly contributions from the halide p orbitals, while the conduction band is a combination of halide

Figure 8.9

(a) Schematic of a composite containing CsPbBr3 quantum dots (QDs) deposited on graphene oxide (GO) as a photocatalyst for the reduction of CO2 to a mixture of CO, CH4, and H2. (b) Comparisons of the product yields of CO2 reduction using CsPbBr3 QDs only as the photocatalysts with experiments where the composite of CsPbBr3 QDs/GO are used after 12 h of irradiation. (c) TEM image of the CsPbBr3 QDs/GO used as the photocatalysts, showing the QDs dispersed randomly on the GO nanosheet. Reproduced from ref. 110 with permission from the American Chemical Society, Copyright 2017.

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p orbitals with the Pb 6s and 6p orbitals, the periodic trends are that the bandgap, lattice energy, and thermodynamic stability increase from CsPbI3 to CsPbCl3 (see Figure 8.10a). The authors first evaluated all three perovskites as well as those with mixed halide compositions (e.g. CsPbBr1.5Cl1.5) for the photocatalytic coupling of thiophenol to diphenyl disulfide in air and discovered that CsPbBr3 offered the best compromise in white light-harvesting properties and stability in the dichloromethane solvent.111 They subsequently demonstrated that the CsPbBr3 nanocrystals were effective photocatalysts for the coupling of 12 aryl and alkyl thiols to disulfides, as well as 14 examples of dehydrogenative C–P cross-coupling between tertiary amines and phosphite esters (see Figure 8.10b).111 These studies have thus established the feasibility of metal halide perovskites to mediate photoredox processes in aerobic conditions and organic solvents, although it was unclear if they could be reused, which limits the advantages of their nature as heterogeneous photocatalysts. To overcome the instability issues of the most common 3D metal halide perovskites, the author’s team chose to focus on multidimensional, especially two-dimensional (2D) variants with hydrophobic hexadecylammonium (HDA) cations.106,112 The HDA chains would be water repellent and the substantial van der Waal’s forces between the chains could offer increased thermodynamic stability and lower the lability of the ions. Accordingly, we synthesized both (HDA)2PbI4 and (HDA)2SnI4 and characterized them spectroscopically and structurally.112 As anticipated, the single-crystal X-ray crystallographic structure of both materials showed the layered 2D perovskite structure composed of the Pb–I and Sn–I nanosheets intercalated by the orderly corrugated C16 alkyl chains of the HDA cations (see Figure 8.10c).112 More critically, (HDA)2PbI4 is not only water repellent but is in fact stable enough to be suspended in water for at least 30 min, with no changes to the powder XRD pattern (see Figure 8.10d) and the photoluminescent properties.112 We proceeded to exploit the structural stability of (HDA)2PbI4 and (HDA)2SnI4 for the photocatalytic decarboxylation of several indoline-2-carboxylic acids under white LED illumination at room temperature with dichloromethane as the solvent (see Figure 8.10e).112 In the absence of air, indolines were the products of redox-neutral reactions, whereas indoles formed in the presence of air due to further oxidation and aromatization.112 We then conducted experiments with radical scavengers including ferrocene (for h1), AgBF4 (for e), and the commercially sourced Tiron (for O2) and found that the rate-determining step is likely the decarboxylation by the h1 in the photoexcited perovskite and O2 is only required for hydrogen atom abstraction to produce indole.112 Furthermore, we could recover the (HDA)2PbI4 by centrifugation after a photocatalytic reaction and reuse it since the powder XRD and X-ray photoelectron spectroscopic data indicated that the composition remained unchanged, thus reinforcing the strategy of enhancing perovskite stability with hydrophobic organic ammonium cations.112 Nonetheless, one of the biggest barriers to the widespread commercialization of metal halide perovskites in devices and for photosynthesis or

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(a) Representation of the unit cell structure of the CsPbX3 (X ¼ Cl, Br, I) perovskites and the estimated valence and conduction band levels. (b) Examples of the photoredox catalytic reactions facilitated by CsPbBr3 nanocrystals, including disulfide formation from thiols (top) and C–P cross-dehydrogenative coupling between tertiary amines and phosphite esters. Reproduced from ref. 111 with permission from the Royal Society of Chemistry. (c) Single-crystal X-ray diffraction (XRD) structures of hydrophobic and water-stable hexadecylammonium (HDA) lead and tin iodides ((HDA)2PbI4 and (HDA)2SnI4) showing the close-packed corrugated structures of the HDA chains. (d) Powder XRD patterns of freshly prepared (HDA)2PbI4 and after the perovskite has been suspended in water for at least 30 min, demonstrating the structural stability even when directly exposed to water. (e) Examples of the photoredox catalytic reactions facilitated by (HDA)2MI4 (M ¼ Pb, Sn) such as the decarboxylation of indoline carboxylic acids to indoline in the absence of air or indole in the presence of air at room temperature with white LEDs as the energy source. Reproduced from ref. 112 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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photocatalysis is the toxicity of Pb in the most effective visible light harvesters. Consequently, several groups, including the author’s,113 have sought to develop Pb-free, potentially less toxic metal halide perovskites for light harvesting and other applications.114–119 Karunadasa’s team has been especially prolific at expanding the inventory of 3D perovskites by synthesizing new double perovskites Bi and Tl halides that are more hydrolytically and oxidatively stable and yet can still absorb substantial portions of the visible spectrum.114–118 Although their applications in optoelectronic devices have not taken off so far owing to the lower solar conversion efficiencies, a Cs2AgBiBr6 double perovskite was reported by Kuang and coworkers for the photocatalytic reduction of CO2 to CO and CH4 in ethyl acetate as the solvent again.119 Similar to their previous report, ethyl acetate is proposed to be the sacrificial electron donor with no 13CO2 isotope labeling to verify the origins of the reduced products.119 Currently, the application of metal halide perovskites as heterogeneous catalysts for photosynthesis and photoreduction chemistry is still in embryonic stages, partly due to their previous perceived instabilities in the presence of solvents. However, as new metal halide perovskites and double perovskites are discovered with different optoelectronic and photosensitizing properties, it is only a matter of time before they may become the new benchmarks in photocatalysis, similar to how TiO2 has dominated research activities in artificial photosynthesis since Fujishima’s and Honda’s groundbreaking study.14

8.5 Conclusions and Outlook Global climate change and plastics pollution are two interrelated environmental crises that warrant the attention of scientists and world leaders urgently. One of the potential solutions that can concurrently alleviate both problems is developing artificial photosynthetic systems that can store solar energy in chemical fuels. The majority of the research efforts in artificial photosynthesis are directed toward water splitting and CO2 reduction, which may never be practical and cost competitive in the foreseeable future, especially in light of the worldwide glut in fossil fuel supplies and consequent low prices. Nevertheless, there has been growing interest in the development of other scalable oxidation alternatives to water oxidation as a commensurate source of e for the reduction of CO2 and/or water to fuels. This includes the photocatalytic and electrocatalytic oxidation of nonfood biomass and nonbiodegradable plastics that the author’s team and others have recently been pursuing. Furthermore, there has also been a revival of interest in the photophysical and photochemical behavior of metal halide perovskites, which portends their future exploitation in artificial photosynthesis as well. Going forward, global climate change and plastics pollution are evidently unsolved problems and require extraordinary and coordinated efforts worldwide. Continuous flow systems that can be scaled up for photo- and electrocatalytic reactions will have to be improved to achieve the high

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throughput necessary for scalable production. Simultaneously, technologies that can bring price parity of renewable energy with fossil fuels will need to be developed so that the photo- and electrocatalytic processes in artificial photosynthesis can be deployed. In addition, for artificial photosynthesis to approach being economically viable, more detailed technoeconomic analyses will have to be conducted to examine the cost-effectiveness of the processes. However, scientists alone cannot overcome these crises. Governments, industries, and the public all need to cooperate to expand efforts in carbon capture and utilization as well as promote the upcycling of plastics. With all these in place, the sustainable and functional redox chemical processes discussed in this chapter may have a chance of forestalling or even reversing the environmental damage from anthropogenic activities since the Industrial Revolution.

Acknowledgements Han Sen Soo acknowledges that this contribution is supported by A*STAR under the AME IRG grants A2083c0050 and A1783c0002. He is also grateful for the Singapore Ministry of Education Academic Research Fund Tier 1 grants RG 111/18 and RT 05/19. Han Sen Soo thanks NTU for the 5th ACE Grant. In addition, acknowledgement is made to the ACS GCI Pharmaceutical Roundtable Research Grant for partial support of this research. The work discussed in this chapter is the culmination of multiple current and previous group members, all of whom have been vital to the progress his team has made in this field.

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CHAPTER 9

Bioinspired Catalyst Learned from B12-dependent Enzymes HISASHI SHIMAKOSHI Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Nishi-ku, Motooka 744, Fukuoka 819-0395, Japan Email: [email protected]

9.1 Introduction Bioinspired chemistry is expanding to various fields of chemistry such as material,1,2 polymer,3 energy conversion,4 electronic devices,5 actuators and robots,6,7 and catalysis.8–11 The sophisticated structure and notable function of biological systems prompt chemists to design an artificial system that mimics their structure and function. Inspired by a natural system, we can construct excellent molecular systems. Among the bioinspired chemistry, a bioinspired catalyst system was developed in the interdisciplinary fields of bioorganic chemistry and bioinorganic chemistry. In nature, enzymes catalyze various molecular transformations under mild conditions at room temperature and physiological pH with high efficiency and selectivity. Therefore, we can learn about many excellent systems from nature. As for the catalyst design, a metal complex inspired by the active site of a metal enzyme has been developed. Since metal enzymes catalyze a variety of molecular transformations by the action of the metal ion, they have been considered one of the desirable catalysts in organic synthesis. Among the metal enzymes, tetrapyrrole pigments, such as porphyrin, chlorophyll, and corrin, that have emerged in nature concerned with various metabolisms (see Figure 9.1) have attracted a lot of attention. Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Other classes of B12-dependent enzymes that do not appear to use an upper alkyl ligand have also been discovered. Reductive dehalogenase isolated from anaerobic bacteria contains the vitamin B12 derivative as a cofactor lacking an upper ligand for dehalorespiration by reducing 1,1,2,2tetrachloroethene (PCE) to cis-dichloroethene (DCE) via trichloroethene (TCE).16 The low valent cobalt center of cobalamin could attack the substrate organohalogen compound, leading to carbon–halogen bond cleavage. Nucleophilic attack of the Co(I) species of the vitamin B12 derivative on a substrate or electron transfer from an electron-rich Co(I) species to a substrate should be the key step of the reaction. The vitamin B12 derivatives are applicable for various molecular transformations based on the reactivity of the Co(I) species. Therefore, the reduction of the cobalt center is a key step for the catalytic reaction. In this chapter, B12-dependent enzyme-inspired catalytic reactions with photoredox systems are summarized.

9.1.2

Catalyst Design for B12-dependent Enzyme-inspired Reactions

The various cobalt complexes having an equatorial tetradentate ligand have been developed not only as vitamin B12 models but also as novel catalysts for organic syntheses by utilizing the high nucleophilicity of their monovalent cobalt species (see Figure 9.4).17 The amphiphilic vitamin B12 derivative with high solubility in an organic solvent was developed for the practical application of the vitamin B12 derivative in organic synthesis.18,19 Among the amphiphilic vitamin B12 derivatives, the simplest heptamethyl cobyrinate perchlorate with peripheral methyl ester groups instead of natural amide groups has excellent solubility in various organic solvents.20 The electronic structure and redox behavior of the heptamethyl cobyrinate perchlorate are close to those of natural B12 caused by the same equatorial tetradentate ligand.21 For example, the Co(II)/ Co(I) redox couple is observed at around 0.5 V vs. Ag/AgCl in organic solvents such as DMF and CH3CN during cyclic voltammetry (CV). This value is close to that of natural cobalamin. The combination of the vitamin B12 derivative with a suitable redox system could construct an efficient B12-inspired catalyst system.

9.2 Photo-driven Molecular Transformation 9.2.1

Heterogeneous Catalyst System

The combined use of a photocatalyst and metal complex was developed for the B12-inspired catalyst. A semiconductor is a functional inorganic material, which is activated by light irradiation corresponding to its bandgap energy. The bandgap generation of a hole (h1) and an excited electron (e) is utilized for the oxidation and the reduction process, respectively. Among the semiconductors, titanium oxide (TiO2) has been used in various fields of science

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The hybrid catalyst converted benzotrichloride to N,N-dialkylbenzamide by visible light irradiation at room temperature in air with a tertiary alkyl amine. The tertiary alkyl amine was oxidized to form a secondary amine and the produced secondary amine was used for the amide formation. Both the electrons and holes during the bandgap excitation of the photocatalyst are utilized for this photo-duet reaction.

9.2.4

B12-inspired Hydrogen Production and Alkene Reduction

The Co(I) species of the vitamin B12 derivatives have been used for other molecular transformations. For example, the B12–TiO2 hybrid catalyst catalyzes proton reduction to form hydrogen (see Figure 9.11).37 The recent growth of studies on hydrogen evolution by the cobalt complex comes from the high reactivity of the Co(I) complex toward a proton to form the cobalt– hydrogen complex (Co–H complex) as the intermediate.38 UV light irradiation of the aqueous disodium dihydrogen ethylenediaminetetraacetate (EDTA2Na) solution produced hydrogen in the presence of the B12–TiO2. The hydrogen production efficiency of the hybrid catalyst using the anatasetype TiO2 was superior to that of the rutile TiO2 since the conduction band electron is more negative for anatase TiO2 (Ered ¼ ca. 0.5 V vs. NHE in pH 7 aqueous solution) than that of the rutile (Ered ¼ ca. 0.3 V vs. NHE in pH 7 aqueous solution) for the B12 reduction. An initial turnover frequency for the hydrogen evolution was 2.1 h1 and 0.47 h1 for the anatase TiO2 and the rutile TiO2, respectively. The turnover number based on the B12 complex was estimated to be about 1 h1 in B12–TiO2 (anatase) for a 10-h reaction.

Figure 9.11

Photocatalytic H2 evolution by the B12–TiO2 from EDTA2Na aqueous solution (0.1 M) at room temperature under N2; B12–TiO2 (anatase) (red circle), B12–TiO2 (rutile) (blue square), B12 (heptamethyl cobyrinate perchlorate) þ TiO2 (anatase) (green triangle). Reproduced from ref. 37 with permission from John Wiley & Sons, Copyright r 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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and photoredox catalysts enabled various molecular transformations with environmentally friendly organic synthesis reactions and transformations of organic halides by a photochemical reaction. Combining the benefits of natural enzymes and engineering methods will allow the development of a new catalyst system with light driven reaction. This new function of bioinspired catalyst could exceed normal biological reactions with high efficiency of light energy.

Acknowledgements Parts of this study were supported by JSPS KAKENHI grant No. JP19H02735 from MEXT and the Cooperative Research Program of ‘‘Network Joint Research Center for Materials and Devices: Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials’’ grant No. 20214031.

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Part 3 Functional Redox System

CHAPTER 10

Redox-active Molecules and Their Energy Device Application AKIHIRO SHIMIZU Division of Chemistry, Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Email: [email protected]

10.1 Introduction Batteries store energy by redox reactions at cathodes and anodes. Among the many kinds of secondary batteries, lithium-ion (Li-ion) batteries and redox flow batteries especially have attracted the attention of organic chemists. Typical Li-ion batteries use solid cathodes composed of transition metal oxides such as LiCoO2 as active materials, solid anodes composed of graphite as an active material, and electrolytes to transport Li ions. The Li-ion battery using LiCoO2 as a cathode active material and graphite as an anode active material has a theoretical cathode capacity of 137 mAh g1 and a voltage of 43.5 V. In contrast, redox flow batteries use liquid catholyte and anolyte composed of inorganic active materials and electrolytes, and ion-exchange membranes to transport ions to balance the charge of catholyte and anolyte. The all-vanadium redox flow battery, which is the most promising technology, uses VO21/VO21 as a catholyte and V21/V31 as an anolyte. Transport of proton (H1) through ion-exchange membrane balances the charge of catholyte and anolyte. The all-vanadium redox flow battery has a voltage of 1.26 V, a concentration of vanadium ofo1.7 M, and an energy density ofo25 W h L1. Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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In terms of sustainability, organic materials composed of C, H, N, and O atoms are promising as active materials for Li-ion batteries and redox flow batteries. To date, various organic active materials have been developed as described in many excellent reviews of organic active materials for Li-ion batteries1–6 and redox flow batteries.7–10 This chapter mainly introduces our approach for developing organic active materials for Li-ion batteries and redox flow batteries.

10.2 Organic Active Materials for Li-ion Batteries 10.2.1

Basic Concepts

Tarascon and Poizot et al. showed the usefulness of organic active materials,11 which are fascinating in terms of sustainability12 and flexibility.13 Because the redox potential difference between the cathode and anode active materials corresponds to the voltage of batteries, materials with high redox potentials and low redox potentials are used as cathode and anode active materials, respectively. In addition, to increase the capacity of batteries, the active materials should be as lightweight as possible and can accept as many electrons as possible. The redox of active materials should be reversible to achieve the high cyclability of the batteries. To date, numerous organic cathode active materials have been developed (Figure 10.1). The organic cathode active materials can be classified into two categories based on the redox couples; (1) cation/neutral, such as 2,2,6,6tetramethylpiperidine 1-oxyl (TEMPO) and tetrathiafulvalene (TTF), and (2) neutral/anion, such as disulfides and carbonyls. Although the high redox potentials of TEMPO and TTF are promising for increasing the voltage of the battery, the use of a redox couple between cation/neutral needs an equimolar amount of supporting electrolytes and hence reduces the capacity of the battery. On the other hand, the use of a neutral/anion couple only requires the transfer of Li ions. We have developed quinone-based organic cathode active materials for Li rechargeable batteries focusing on (1) increasing capacity, (2) increasing cyclability, and (3) increasing voltage, utilizing the advantage of organic materials.

10.2.2

Capacity Increase

The carbonyl group (CQO) is a simple and the most abundant functional group in nature that can be reduced to generate a radical anion. Its theoretical capacity is 957 mAh g1 (1 electron per molecular weight of 28), which is larger than that of the theoretical capacity of LiCoO2 (137 mAh g1, 0.5 electron). Although the radical anion of CQO is reactive because of the unpaired electron mainly located on the carbon atom, the dianions of 1,2-diketones and 1,4-diketones are stable because no unpaired electron exists. In addition, dianions of quinones are stabilized by aromaticity. Therefore, quinones, such as benzoquinone (BQ),14 naphthoquinone (NQ),15 anthraquinone (AQ),16

Redox-active Molecules and Their Energy Device Application

Figure 10.1

Representative examples of organic cathode active materials for Li rechargeable batteries.

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18

pentacene-5,7,12,14-tetraone (PT), phenanthrenequinone (PQ), and pyrene4,5,9,10-tetraone (PYT)19 are promising molecules as redox-active core structures (Figure 10.2). Notably, these quinones can store two or four electrons per molecule, and their theoretical capacities are more than 250 mAh g1. In particular, ortho-quinones such as PQ and PYT are promising because the dianions of ortho-quinones can coordinate to Li ions at the 1,2-diketone moiety, which stabilizes the dianions and increases redox potentials of ortho-quinones, hence increasing the voltage of Li rechargeable batteries. Nokami and Yoshida et al. examined the ring size effect of cyclic 1,2diketone structure theoretically and experimentally by a comparison of benzocyclobutenedione (BBD), acenaphthenequinone (ANQ), and pyrene4,5-dione (PYD), which are representative four-, five-, and six-membered ring 1,2-diketones, respectively (Figure 10.3).19 Density functional theory (DFT) calculations show that the lowest-unoccupied molecular orbital (LUMO) energy level of PYD (2.95 eV) is lower than those of BBD (2.54 eV) and ANQ (2.60 eV), indicating that the two-electron reduction of PYD is energetically more favorable than that of BBD and ANQ. DFT calculations also show that the dianion of BBD is destabilized by antiaromaticity, while the dianion of PYD is stabilized by aromaticity. Cyclic voltammograms show that redox potentials of PYD are higher than those of BBD and ANQ. These theoretical and experimental results suggest that six-membered ring 1,2-diketones, that is, ortho-quinones, are promising as a core structure of organic active materials. Because of its high capacity and 1,2-diketone structure, Nokami and Yoshida et al. chose PYT (Figure 10.3), which is synthesized by the oxidation of pyrene,20 as a cathode active material for Li rechargeable batteries.19 The theoretical capacity of PYT is 409 mAh g1, and the mean voltage of PYT as calculated by the DFT method is 3.0 V. The energy levels of LUMO and LUMO þ 1 of PYT are 3.56 and 3.46 eV, respectively. PYT exhibited reduction waves at 3.0, 2.8, 2.3, and 2.2 V vs. Li/Li1 in the cyclic voltammetry measurement to generate tetraanion. The Li rechargeable battery using PYT as a cathode active material showed a discharge capacity of ca. 320 mAh g1. However, the capacity rapidly decreased to ca. 120 mAh g1 after 20 cycles, probably because of the dissolution of PYT to electrolytes.

10.2.3

Cyclability Increase

To avoid the dissolution of PYT to electrolytes, Nokami and Yoshida et al. synthesized polymer-bound PYT (PPYT), choosing polymethacrylate as polymer support considering its physical flexibility and Li-ion affinity (Figure 10.4).19 The theoretical capacity of PPYT is 262 mAh g1. The Li rechargeable battery using PPYT as a cathode active material showed a discharge voltage of 2.5 V and a discharge capacity of 231 mAh g1, which is 88% of the theoretical capacity. PPYT exhibited high rechargeability; the capacity retention after 500 cycles (193 mAh g1) was 83% of the initial capacity.

Redox-active Molecules and Their Energy Device Application

Figure 10.2

Structures, redox, and theoretical capacities of quinones.

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Figure 10.3

Structures, redox, and theoretical capacities of cyclic 1,2-diketones.

(a)

(a) Structure and theoretical capacity of polymer-bound PYT (PPYT) and (b) charge–discharge cycling of Li rechargeable batteries using PYT and PPYT as cathode active materials. Reproduced from ref. 19 with permission from American Chemical Society, Copyright 2012.

Chapter 10

Figure 10.4

(b)

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235

To decrease the dissolution to electrolytes by increasing the molecular weight of redox-active materials, Zhang et al. synthesized a linear polymer of poly(pyrene-4,5,9,10-tetraone) (PPTO), in which PYT was directly connected, and poly(2,7-ethynylpyrene-4,5,9,10-tetraone) (PEPTO), in which PYT was connected by ethynyl moieties (Figure 10.5).21 They used minimum redox-inactive units to minimize the decrease of theoretical capacity (PPTO: 412 mAh g1, PEPTO: 377 mAh g1). The discharge voltages and discharge capacities of Li rechargeable batteries using PPTO as a cathode active material were 2.44 V and 234 mAh g1, and those using PEPTO were 2.47 V and 244 mAh g1, respectively. Interestingly, PEPTO showed better rate stability than PPTO. The discharge capacity of Li rechargeable battery using PPTO as a cathode active material was 161 mAh g1 at current densities of 100 mA g1, but it dropped to the infinitesimal value at 800 mA g1. On the other hand, the discharge capacities of Li rechargeable batteries using PEPTO as a cathode active material were 189 and 152 mAh g1 at current densities of 100 and 800 mA g1, respectively. Capacity retentions of PPTO at 100 mA g1 and PEPTO at 800 mA g1 after 1000 cycles were 74% and 79%, respectively. Because the dihedral angles between adjacent PYT units are ca. 341 for PPTO and 01 for PEPTO based on the DFT calculations, they concluded that the enhanced conjugation and planarity improved the rate and long-term stabilities. Zhang et al. also synthesized a two-dimensional (2D) covalent organic framework using boroxine flamework (poly(pyrene-4,5,9,10-tetraone-2,7diboroxine), PPTODB), which has a theoretical capacity of 342 mAh g1

Figure 10.5

Structures and theoretical capacities of PYT-based cathode active materials.

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(Figure 10.5). The Li rechargeable battery using PPTODB as a cathode active material showed a discharge voltage of ca. 2.5 V and a discharge capacity of 198 mAh g1. The capacity retention after 150 cycles was 68% of the initial capacity. To decrease the dissolution of monomeric organic active materials to electrolytes, intermolecular interaction is an alternative approach. Yao et al. investigated the effect of the chain length of alkoxy quinone derivatives (Figure 10.6).23 The Li rechargeable battery using 2,5-di-decyloxy-1,4-benzoquinone (DDBQ) as a cathode active material showed a relatively good cycle performance (60% of initial capacity after 20 cycles), indicating the importance of the higher hydrophobicity of the long decyloxy groups than the short alkoxy groups. Morita et al. investigated the effect of heavy atoms. They synthesized trioxotriangulenes (TOTs) having three tert-butyl groups ((t-Bu)3TOT) and three bromo groups ((Br)3TOT).24 The theoretical capacities of (t-Bu)3TOT and (Br)3TOT are 219 and 192 mAh g1, respectively. The discharge capacities of (t-Bu)3TOT and (Br)3TOT after 100 cycles were 22 and 177 mAh g1, which were 17% and 85% of the initial capacity, respectively. The high cycle performance of (Br)3TOT indicates that strong intermolecular interactions through heavy atoms prevent the dissolution to electrolytes. The intermolecular interaction of salts suppresses the dissolution of organic active materials to electrolytes. Sun et al. reported the use of Li2C6H2O4, a quinone-based coordination polymer, as a cathode active material (Figure 10.7).25 The X-ray single-crystal analysis indicates that Li2C6H2O4 forms Li–O bridged coordination polymer framework. The uncoordinated oxygen atoms of CQO groups could enable Li ions to be inserted and deinserted reversibly when the carbonyl groups are reduced and oxidized, respectively. The discharge capacity of the Li rechargeable battery using Li2C6H2O4 as a cathode active material was 137 mAh g1 after 10 cycles, which is 78% of the initial capacity (176 mAh g1), indicating that the Li–O bridge suppresses the dissolution. A drawback of the introduction of a lithiumoxy (–OLi) group to quinones is a decrease in voltages due to its electron-donating nature. The discharge voltage of the Li rechargeable battery using Li2C6H2O4 as a cathode active material was ca. 1.9 V. Poizot et al. reported the use of Li2C6O4Cl2, which has a higher discharge voltage (2.3 V) than Li2C6H2O4 because of the electronwithdrawing Cl atoms.26 The capacity retention after 25 cycles was 70% of the initial capacity (130 mAh g1). Li et al. reported the use of anthracene-based salt.27 The Li rechargeable battery using the anthracene salt as a cathode active material showed a discharge voltage of ca. 1.8 V and a discharge capacity of 127 mAh g1. The capacity retention after 50 cycles was 94% of the initial capacity (120 mAh g1). Poizot et al. reported that Li4C6O6 works as a cathode and an anode active material because Li4C6O6 is oxidized to Li2C6O6 and reduced to Li6C6O6 (Figure 10.7).28 The batteries using Li4C6O6 as cathode and anode active materials showed a discharge voltage of ca. 0.5 V, a discharge capacity of ca. 200 mAh g1, and a capacity retention of ca. 65% of the initial capacity after 50 cycles.

Structures and theoretical capacities of benzoquinones having alkoxy groups and trioxotriangulenes (TOTs).

Redox-active Molecules and Their Energy Device Application

Figure 10.6

237

238

Figure 10.7

Chapter 10

Structures, redox, and theoretical capacities of quinones having –OLi groups.

Tarascon et al. showed that dilithium trans-trans-muconate (Li2C6H4O4) and dilithium terephthalate (Li2C8H4O4) work as a two-electron redox system (Figure 10.8).29 The discharge voltages of the Li rechargeable batteries using Li2C6H4O4 and Li2C8H4O4 as cathode active materials were 1.4 and 0.8 V, respectively. Although the discharge voltages are low, these materials could be used as anode active materials. The capacity retention of the Li rechargeable battery using Li2C6H4O4 as a cathode active material after 50 cycles was 78% of the initial capacity and that using Li2C8H4O4 after 80 cycles was 74% of the initial capacity. The low solubility of Li2C6H4O4 and Li2C8H4O4 to electrolytes suggests that the lithiumoxy carbonyl (–CO2Li) group is a promising substituent group. Tao and Chen et al. reported a para-benzoquinone having two –CO2Li groups ( p-Li4C8H2O6) (Figure 10.9).30 The Li rechargeable battery using p-Li4C8H2O6 as a cathode active material showed a discharge voltage of ca. 2.4 V and a discharge capacity of 223 mAh g1. The capacity retention after 50 cycles was 95% of the initial capacity. Poizot et al. studied orthoand para-benzoquinones having two –CO2Li groups, Li2-o-DHT and Li2-p-DHT.31,32 The Li rechargeable battery using Li2-o-DHT as a cathode active material showed a higher discharge voltage (2.85 V) than that using Li2-p-DHT (2.55 V). The discharge capacity of the battery did not decrease significantly. Shimizu and Yoshida et al. developed quinones (AQ, PQ, PYT) having two –CO2Li (LC) groups (Figure 10.9).33 Notably, the introduction of two LC groups only slightly increases the LUMO energy levels (AQ: 2.79 eV, LCAQ: 2.64 eV, PQ: 2.98 eV, LCPQ: 2.77 eV, PYT: 3.56 eV, LCPYT: 3.33 eV) and does not significantly affect the discharge voltages of Li rechargeable batteries

Figure 10.9

Structures and theoretical capacities of quinone dicarboxylates and a disulfonate.

239

Structures, redox, and theoretical capacities of dilithium carboxylates.

Redox-active Molecules and Their Energy Device Application

Figure 10.8

240

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using these quinones as cathode active materials (AQ: 1.92 eV, LCAQ: 1.79 eV, PQ: 2.55 eV, LCPQ: 2.11 V, PYT: 2.32 V, LCPYT: 2.39 V). On the other hand, the capacity retention of Li rechargeable batteries using LCAQ, LCPQ, and LCPYT as cathode active materials was significantly higher than those using AQ, PQ, PYT, and MCPYT, probably because of the intermolecular interaction through –CO2Li groups (Figure 10.10). Sodium sulfonate (–SO3Na) group is also used to prevent the dissolution of organic active materials to electrolytes. Zhou et al. reported an AQ derivative having two –SO3Na groups (Figure 10.9).34 The Li rechargeable battery using the AQ derivative as a cathode active material showed a higher discharge voltage of 2.4 V than that using AQ and no obvious capacity decrease after 100 cycles (120 mAh g1). However, the high molecular weight of the –SO3Na group significantly decreases the theoretical capacity of organic active materials.

Figure 10.10

Discharge curves and cyclings of (a) AQ (dotted line, open circle) and LCAQ (solid line, filled circle), (b) PQ (dotted line, open circle) and LCPQ (solid line, filled circle), and (c) PYT (dotted line, open circle), LCPYT (solid line, filled circle), and MCPYT (dashed line, open triangle). Reproduced from ref. 33 with permission from Elsevier, Copyright 2014.

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10.2.4

241

Voltage Increase

The most common and effective way to increase the voltage of Li batteries using organic cathode active materials is to introduce electron-withdrawing substituent groups, which lowers the LUMO energy levels and increases redox potentials of organic cathode active materials. For example, Matsubara et al. reported benzoquinones having electron-withdrawing substituent groups (Figure 10.11).35 The discharge voltage of the Li rechargeable battery using CF3-BQ (3.0 V) as a cathode active material was higher than that using CH3-BQ (2.6 V). However, this approach inevitably decreases the theoretical capacity of the batteries because of the large molecular weight of the electron-withdrawing substituent groups. Theoretical capacities of BQ, CH3-BQ, and CF3-BQ are 496, 394, and 220 mAh g1, respectively. Another approach is to use heteroatoms. Chen et al. reported heteroatomcontaining quinones, benzofuro[5,6-b]furan-4,8-dione (BFFD), benzo[1,2-b: 4,5-b0 ]dithiophene-4,8-dione (BDTD), and pyrido[3,4-g]isoquinoline-5,10-dione (PID), which are isoelectronic with AQ (Figure 10.12).36 The discharge voltages of Li rechargeable batteries using BFFD (2.61 V), BDTD (2.51 V), and PID (2.71 V) as cathode active materials were higher than that using AQ (2.27 V), indicating that heteroatoms increase the voltage. Notably, the theoretical capacities of these heteroatom-containing quinones (BFFD: 285 mAh g1, BDTD: 243 mAh g1, and PID: 255 mAh g1) are comparable to that of AQ (257 mAh g1). Shimizu and Yoshida et al. succeeded at increasing the redox potentials of quinones by replacing its C–H moiety with a nitrogen atom (Figure 10.12).37 They reported 1,4,5,8-tetraaza-9,10-anthraquinone (TAAQ) as AQ analogous. The LUMO energy level of TAAQ (3.43 eV) is lower than that of AQ (2.79 eV), probably because of the higher electronegativity of nitrogen than carbon. Notably, the theoretical capacities of TAAQ and PQ are 253 and 257 mAh g1, respectively, indicating that the molecular design only slightly decreases the theoretical capacity because the molecular weights of C–H (13) and N (14) are similar. This molecular design is also applicable to PQ derivatives. The LUMO energy levels of 4,5-diaza-9,10-phenanthrenequinone (4,5-DAPQ) (3.32 eV) and 1,8-diaza-9,10-phenanthrenequinone (1,8-DAPQ) (3.24 eV) are lower than that of PQ (2.98 eV). The theoretical capacities of 4,5-DAPQ (255 mAh g1) and 1,8-DAPQ (255 mAh g1) are close to that of PQ (257 mAh g1). The Li rechargeable battery using TAAQ as a cathode active

Figure 10.11

Structures and theoretical capacities of benzoquinones having electron-withdrawing substituent groups.

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Figure 10.12

Structures, redox, and theoretical capacities of quinones containing heteroatoms.

material showed a discharge voltage of 2.75 V, which is higher than that of AQ (2.13 V) (Figure 10.13). The coordination effect was elucidated in comparison between 4,5-DAPQ and 1,8-DAPQ. The discharge voltage of 1,8-DAPQ is 2.94 V, which is higher than those of 4,5-DAPQ (2.73 V) and PQ (2.52 V). DFT calculations show that 1.8-DAPQ coordinates to Li ions at N–C–C–O unit, while 4,5-DAPQ coordinates at O–C–C–O and N–C–C–N units. These results show that nitrogen atom b to the carbonyl group of ketones can form chelate coordination to Li ions.

10.3 Organic Active Materials for Redox Flow Batteries 10.3.1

Aqueous Electrolyte

Xu et al. used 4,5-dihydroxy-1,3-benzenedisulfonate or 2,5-dihydroxybenzenedisulfonate in 3 M H2SO4 as a catholyte (Figure 10.14).38 This study shows that organic molecules could replace vanadium ions. Aziz et al. reported 9,10-anthraquinone-2,7-disulfonic acid (2,7-AQDS), which realized a high concentration of 1 M in 1 M H2SO4.39 The flow battery using 2,7-AQDS as an anolyte coupled with a bromine (Br2/Br3) catholyte has a voltage of

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Figure 10.13

Discharge curves of Li rechargeable batteries using (a) AQ and TAAQ and (b) PQ, 4,5-DAPQ, and 1,8-DAPQ as cathode active materials. Reproduced from ref. 37 with permission from John Wiley & Sons, Copyright r 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 10.14

Structures of quinones used in acidic aqueous electrolytes.

0.6 V with the galvanic discharge capacity retention above 99%. The cell potential would be increased by 11% by the introduction of two hydroxy groups at 1 and 8 positions (DHAQDS). Yang et al. reported an all-organic aqueous flow battery using 0.2 M 1,2-dihydrobenzoquinone-3,5-disulfonic acid as a catholyte and 0.2 M anthraquinone-2-sulfonic acid (AQS) or 0.2 M anthraquinone-2,6-disulfonic acid (2,6-AQDS) as an anolyte.40 The battery has a voltage of ca. 0.4 V.

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Figure 10.15

Structures of quinones used in alkaline aqueous electrolytes.

Aziz et al. also reported quinone flow batteries using alkaline solution (Figure 10.15).41 In total, 0.5 M of 2,6-dihydroxyanthraquinone (2,6-DHAQ) in 1 M KOH was used as an anolyte, and 0.4 M of ferrocyanide (FeCy) was used as a catholyte. The 2,6-DHAQ/FeCy battery showed a voltage of ca. 1.2 V. The capacity retention after 100 cycles was 90% of the initial capacity. The voltage of the battery can be increased using 2,3,6,7-tetrahydroxyanthraquione (1.33 V) and 1,5-dimethyl-2,6-dihydroxyanthraquione (1.34 V) (Figure 10.15).

10.3.2

Nonaqueous Electrolyte

The advantage of nonaqueous electrolytes is their wider potential windows than aqueous electrolytes, which would increase the voltage of batteries. However, the solubility of organic active materials in nonaqueous electrolytes is often low, which eventually decreases the capacity and energy density of the batteries. To solve the problem, nonaqueous redox flow batteries using highly soluble organic active materials based on TEMPO,42,43 dialkoxybenzene,44,45 phenothiazine,46 boron-dipyrromethene,47 fluorenone,48 and N-methylpyridinium ion49 have recently been developed to achieve high capacity (Figure 10.16). However, a new approach for nonaqueous redox flow batteries is required. Takeuchi et al. proposed the idea to maximize the solubility of redox compounds (Figure 10.17).50 Because the minimum requirements for the catholyte are stoichiometric couples of a redox species and a supporting electrolyte, they selected 4-methoxy-2,2,6,6-tetramethylpiperidine 1-oxyl (MT) as a redox species and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, LT) as a supporting electrolyte. Interestingly, a 1 : 1 molar mixture of MT and LT is liquid. The flow battery using MTLT (1/1) þ H2O (17 wt%) as a catholyte and Li metal anode showed a voltage of 3.6 V and a capacity of 55.5 A h L1 and an energy density of 200 W h L1. As quinone-based active materials, Wang et al. reported 1,5-bis(2-(2-(2methoxyethoxy)ethoxy)ethoxy)anthracene-9,10-dione (15D3GAQ), which is liquid at room temperature (Figure 10.17).51 The concentration of 15D3GAQ was 0.25 M in 1.0 M LiPF6/propylene carbonate (PC). The Li/15D5GAQ flow battery exhibited a voltage of ca. 2.3 V and an energy density close to 25 W h L1. Shimizu and Yoshida et al. reported the liquid quinone as a cathode active material for nonaqueous redox flow batteries (Figure 10.17).52 To achieve

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245

Figure 10.16

Representative examples of organic active materials used in nonaqueous electrolytes.

Figure 10.17

Structures of liquid organic active materials.

high capacity, they chose p-benzoquinone and 1,4-naphthoquinone as core structures. To lower the melting points, ethylene glycol monomethyl ether or diethylene glycol monomethyl ether groups were introduced to quinones. Although benzoquinone and naphthoquinone derivatives having ethylene glycol monomethyl ether groups are solid at room temperature, those having diethylene glycol monomethyl ether groups (DEGBQ and DEGNQ) are liquid at room temperature. The reduction peak potential of DEGBQ (0.64 V vs. Fc/Fc1, two-electron reduction) in LiBF4/PC is higher than those of DEGNQ (0.85 and 0.98 V), which is consistent with the LUMO energy levels of DEGBQ (2.87 eV) and DEGNQ (2.84 eV) and the energy difference between the neutral and reduced forms of DEGBQ (8.33 eV) and DEGNQ (7.47 eV) estimated using the DFT calculations. DEGNQ works as a catholyte with and without solvent. The flow battery using 1 mM DEGNQ in LiBF4/PC as a catholyte and Li metal anolyte shows a voltage of 2.8 V. The concentration of DEGNQ can be increased up to 600 mM, and an energy density of

246

Figure 10.18

Chapter 10

Charge and discharge curves of the battery using DEGNQ þ LiTFSI as catholyte with the current of 0.5, 1.0, 2.0, 3.0, and 5.0 mA. Reproduced from ref. 52 with permission from John Wiley & Sons, Copyright r 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

57.6 W h L1 was achieved. A catholyte composed of DEGNQ:LiTFSI at a molar ratio of 1:0.2 with a concentration of 2.69 M also works as a catholyte. A static battery using the catholyte and a Li-metal anolyte shows a voltage of 2.57 V and an energy density of 264 W h L1 (Figure 10.18). This result suggests the high potential of nonaqueous redox flow batteries based on liquid redox-active materials.

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CHAPTER 11

Redox-active Polymeric Materials NAOKI SHIDAa,b AND SHINSUKE INAGI*a,c a

Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan; b Department of Chemistry and Life Science, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama, Kanagawa 240-8501, Japan; c PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan *Email: [email protected]

11.1 Introduction Redox-active polymers are deeply involved everywhere in our daily lives. Organic polymers are generally lighter and more flexible than inorganic materials, and redox-active polymers are applied to a variety of electronic materials that take advantage of these characteristics. There are two main types of redox-active polymers: conjugated polymers, which have a main chain structure with overlapping p-orbitals of the constituent elements, and nonconjugated polymers with redox-active sites. Following doping, conjugated polymers exhibit electrical conductivity comparable to that of metals, and so they are also called conducting polymers. In conjugated polymers, the molecular orbitals of the repeating units overlap and the polymers have a band structure, so redox reactions in one unit affect the electronic structure of the entire polymer. On the other hand, in nonconjugated redox-active polymers, the redox-active sites are isolated from each other, and a redox reaction at one active site has little effect on the electronic state of other active sites. However, if the pendant groups are sufficiently close together, electronic conduction can be achieved by electron hopping. Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Figure 11.12

11.3.5

(a) Design of dendrimer containing ferrocenes at the periphery. (b) Reversible swelling behavior in response to the redox state of ferrocene moiety.69

Redox-active Inorganic Polymers

Inorganic polymers with inorganic elements in the main chain have also been reported to be redox active. Manners and coworkers conducted a systematic study on polyferrocenylsilanes (PFS) and reported their redox activity.70 In general, polymers with ferrocene in the side chain exhibit single wave oxidation behavior in CV measurements, but PFS shows two independent oxidation waves with comparable current values.71 This is because when one ferrocene group in the main chain of the polymer is oxidized, the adjacent ferrocene moieties are affected electronically and become difficult to oxidize (Figure 11.13). As a result, the second oxidation wave appears to be about 250 mV more positive than the first oxidation wave. The neutral PFS is a nonconductor (conductance s ¼ 1014 O1 cm1), but the conductivity increases from 108 to 104 with oxide doping, and thus it can be classified as a p-type semiconductor.72 Vinyl pyridine complexes can be reductively polymerized at the cathode.73 Zhong and coworkers expanded the ligand design and application of polypyridine complexes and reported a variety of polymeric materials.74 The monomers are composed of transition metals (Ru, Os, Fe, Co, Cr, Ir) and ligands with vinyl groups (pyridine, bipyridine, terpyridine). As a representative example, the electropolymerization of the ruthenium complex [Ru(dvbpy)(bpy)2]21 (bpy ¼ 2,2 0 -bipyridine, dvbpy ¼ 5,5 0 -divinyl-2,2 0 -bipyridine) is described (Scheme 11.14).75 In CV measurements of this monomer, three reduction peaks originating from the ligand are observed, and polymerization proceeds at the potential of the second reduction wave. Measurement of the CV curve for a film obtained in the monomer-free electrolyte showed that the redox couple Ru(II)/(III) appeared at almost the same potential as that for the monomer, indicating that the metal center was not damaged by electropolymerization. On the other hand, the reduction potential for the ligand shifted to a negative direction after polymerization because the vinyl moiety changed to a

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utilizing the redox reactions in the solid state of this polymer. The device functions as four information storage units with independent interconversion and shows unique memristive switching characteristics.

11.5 Conclusion In this chapter, redox-active polymeric materials, namely conjugated, nonconjugated, and combined materials, are reviewed, focusing on their redox behavior. Redox-active polymers are highly practical for use as materials because they can easily form films. In addition, they have unique redox properties that are not found in small molecules due to their polymer characteristics, and there are many interesting examples of them in basic scientific research. As redox chemistry of small molecules is undergoing a renaissance and new methods and knowledge are being accumulated, new developments in the synthesis, reaction, and properties of redox-active polymers based on these methods are expected.

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CHAPTER 12

Chiral Metal Electrodes for Enantioselective Analysis, Synthesis, and Separation CHULARAT WATTANAKIT*a AND ALEXANDER KUHN*a,b a

School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand; b University of Bordeaux, CNRS UMR 5255, Bordeaux INP, Site ENSCBP, 16 avenue Pey Berland, Pessac 33607, France *Emails: [email protected]; [email protected]

12.1 Background Chirality is a very important feature, especially in chemical research, because it plays a key role in various fields, ranging from materials engineering and surface science to analytical chemistry, pharmaceutics, and catalysis.1–22 Up to date, there are various approaches that have been proposed to obtain enantiomerically pure compounds (EPCs), including the use of the chiral pool,23–25 enantiospecific crystallization,3,26–30 enantioselective synthesis,31–38 and chiral separation.39–43 All these concepts require the presence of chiral environments. For example, enantioselective synthesis, being one of the most promising strategies to control the production of EPCs, can be achieved using catalysts such as chiral coordination complexes,44–47 chiral-imprinted polymers,48–52 and chiral biocatalysts.53–57 Nevertheless, they all have also some limitations, such as the (too) flexible molecular structure of soft materials, complicated preparation processes, low thermal and chemical stability of some materials, and the difficulty to retain chiral information on solid surfaces.58,59 Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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Successful strategies have also been developed based on homogeneous catalytic systems, but the separation of the catalyst from the enantiomeric products is the main problem for applications at an industrial scale. In order to overcome the limitations of homogeneous catalytic systems, heterogeneous reactions are an interesting alternative, particularly due to a more straightforward reusability and catalyst regeneration.60–62 Over the past decades, heterogeneous catalysts have played an increasingly important role in tailoring the selectivity and kinetics of reactions. For example, surface grafting or immobilization on solid surfaces, such as silica and titania, of chiral molecules or chiral coordination complexes63 can lead to the formation of chiral self-assembled monolayers (SAM).64 In addition, electrodes with chiral surfaces have been employed as heterogeneous catalysts in the frame of chiral technologies. Indeed, electrodes with tailored chiral surfaces have been prepared by following various approaches. They include the generation of intrinsically chiral properties on crystal surfaces, the adsorption of chiral or achiral molecules on electrode surfaces, and the elaboration of chiral polymers by surface grafting and molecular imprinting.65–74 Although chiral solid surfaces have been successfully obtained by following different strategies, one of the most interesting concepts is based on the molecular imprinting of solids with the help of chiral templates.75–77 Typically, this generates chiral features with specific recognition properties, which can be easily controlled by the template structure.76,78–80 In this context, molecularly imprinted polymers (MIPs) with chiral surface properties have been widely studied. However, template removal is rather difficult and constitutes a drawback. Also, low binding constants, slow binding kinetics, and a too flexible structure induce limitations for the development of such chiral imprinting materials. Some of these limitations have been overcome, for example, with chiral mesoporous polymers, polymer–inorganic composite materials, and chiral carbon.81–83 In this context, the molecular imprinting of metals has also been considered, even though it seems to be at first sight a rather unconventional idea. Initial work, dealing with the elaboration of metal structures bearing chiral features, was essentially based on the following approaches: (i) adsorption of chiral/achiral molecules on the metal surface;66,70,84–104 (ii) binding of chiral ligands to metal surfaces;105–118 (iii) cutting a bulk metal along specific crystal planes to break the highly symmetric metal structure;4,85,119–134 and (iv) chiral molecular imprinting.100,135–139 The latter one will be the main focus of this chapter. As metals are excellent electronic conductors, it is at the same time quite natural to explore particularly their use for electrochemical applications.

12.2 Elaboration of Chiral Metal Electrodes Although the introduction of chiral features on and in metal structures has been less explored compared to other solid materials, various approaches have been proposed to generate a chiral metal interface, as will be illustrated in more detail in the following.140–143

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Adsorption of Chiral/Achiral Molecules on Metal Surfaces

In the frame of a molecular adsorption approach, the generation of chiral metal interfaces can be based on different concepts, including the adsorption of achiral molecules on achiral surfaces, the adsorption of chiral molecules on achiral surfaces, and the adsorption of chiral molecules on chiral surfaces.66,70,84–103,133 Although the isolated adsorbate molecule and the metal might be achiral structures, they are able to generate a chiral interface when they are combined, because the mirror plane of the isolated system is changed by a decrease of symmetry compared to the isolated species. For example, an achiral molecule, in this case 2,5,8,11,14,17-hexatert-butylhexabenzo[bc,ef,hi,kl,no,qr]coronene (HtB-HBC), can generate a chiral surface when adsorbed on Cu(110), due to the presence of van der Waals (vdW) interactions, forcing the molecules to simultaneously adjust to the atomic template of the substrate geometry and self-assemble in a closepacked geometry.65 In a more general way, chiral interfaces can be obtained simply by the adsorption of chiral compounds either on achiral or chiral metal surfaces.144–147 A well-known example is based on the adsorption of amino acids, such as the enantiomers of lysine, on achiral surfaces148 of Cu(001).149 The adsorption of lysine on Cu, combined with annealing, leads to a surface arrangement with higher index planes of Cu, namely Cu(3,1,17)R. Interestingly, Cu(3,1,17)R can be considered as a chiral structure due to a lack of mirror symmetry, as shown in Figure 12.1.149 Other amino acids such as phenylalanine, tryptophan, asparagine, aspartic acid, and cysteine have also been used to modify metals and lead to chiral surface features.146,150,151 Although chirality can be introduced rather easily with this adsorption approach, leaching of the adsorbed species has to be considered, as it eventually causes a gradual loss of chirality.152

12.2.2

Binding of Chiral Ligands to Metal Surfaces

An alternative approach, which allows avoiding the loss of chiral information due to desorption, is surface grafting or surface modification with chiral ligands. In this case, the chiral interface is obtained by modifying metal nanoparticles via a strong interaction with molecular ligands.108–118 This has been illustrated with metal–ligand complexes such as Au2X2(BINAP), where BINAP is the bidentate phosphine ligand 2,2 0 -bis(diphenylphosphino)-1,1 0 binaphthyl. The reduction of this metal–ligand complex is able to generate metal nanoparticles, which interact with and are stabilized by the chiral ligand to generate a chiral metal nanoparticles surface.106 Apart from this, other organic ligands or macromolecules, interacting with metal surfaces, can be used.117,118,153,154 For example, thiolate-stabilized and protected gold nanoparticles have been successfully elaborated. Interestingly, ultrasmall gold nanoparticles with an average diameter ranging from 0.57 to 1.75 nm, protected by a chiral ligand, such as D-penicillamine and L-penicillamine,

Chiral Metal Electrodes for Enantioselective Analysis, Synthesis, and Separation

Figure 12.1

277

Scanning tunneling microscope (STM) image of a Cu(001) surface modified with L-lysine and annealed at 430 K for 20 min. Reproduced from ref. 149 with permission from American Chemical Society, Copyright 2000.

exhibit opposite behavior in terms of their circular dichroism (CD) spectra.107,108 Apart from thiolate-stabilized metals, chiral phosphines can also be used as modifiers. Theoretically, the bis-phosphine ligands alter the chiroptical properties of metal clusters with respect to the isolated system.107,154,155 Another interesting strategy to create a chiral metal surface is based on the binding of some biological compounds such as DNA.107,156–161

12.2.3

Controlled Cutting of Bulk Metals

Bulk metals typically have a high symmetry, such as Cu, Ni, and Pt with their face-centered cubic (fcc) structures. However, breaking such a symmetric structure via cutting the bulk metal along low symmetry planes allows lowering the symmetry and generates high-Miller-index surfaces. Up to date, there are various high-Miller-index surfaces that have been successfully obtained, such as the enantiomorphic structures of Cu and Pt (e.g. Cu[531], Pt[643]).124,128,162 This strategy leads to specific chiral adsorption sites, mostly located on kinks, vacancies, and adatoms.163 The two enantiomorphic surface structures exhibit enantioselective recognition abilities, because the two enantiomers adsorb or desorb differently on such surfaces. For instance, the two enantiomers of D- and L-glucose are oxidized with different efficiencies on Pt{643}R and Pt{643}S, which are enantiomorph interfaces.121,122 Apart from the cutting of bulk metal structures, the epitaxial deposition of ¯¯ Au films on commercial Si(643) and Si(6 4¯ 3) wafers by electrochemical methods is also able to generate chiral information. It was found that two ¯¯ enantiomorphic surfaces, Au(643) and Au(6 4¯ 3), were obtained during the electrodeposition on the different sides of the Si wafer.164 In addition, several

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Figure 12.2

Illustration of the proposed models of chiral Au surfaces: (a) Au(643); ¯¯ ¯ 14 ¯ 17 ¯). Reproduced from (b) Au(6 4¯ 3); (c) Au(8 14 17); and (d) Au(8 ref. 164 with permission from American Chemical Society, Copyright 2018.

metals including Pt, Ni, Cu, and Ag can be deposited on top of the epitaxial Au layers, thus leading to various types of chiral metal surfaces.164 To confirm the epitaxial growth of these metal films on the Au-coated Si wafers (Figure 12.2), their behavior with respect to the electrochemical oxidation of ¯¯ glucose isomers was studied for Ag/Au/Si(643) and Ag/Au/Si(6 4¯ 3).164 A complementary approach to introducing chirality on metal surfaces relies on the distortion of metal structures.109 In this case, gold clusters have been distorted by the passivation with thiol molecules. For an intermediate cluster size, it has been suggested that the adsorption of thiol monolayers induces forces that are strong enough to distort the gold surface structure.109 These findings were also confirmed by theoretical calculations, demonstrating that the most stable configurations with the lowest structural energies are chiral structures.109

12.2.4

Chiral Molecular Imprinting

An alternative way to encode chiral information in a metal is based on the molecular imprinting approach. According to this concept, the metal matrix

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can be generated around the chiral templates, leading to specific chiral cavities depending on the shape and structure of the chiral templates. This has been exemplified with several metals such as Pd, Au, Pt, Ag, and Cu.135,136 Such materials can be successfully prepared by chemical reduction of a metal precursor in the simultaneous presence of chiral compounds, which get entrapped inside the metal matrix.137,138,165–173 To date, various types of metal nanoparticles with chiral features, for example, Ag, Au, and Pd clusters imprinted with organic compounds, have been successfully obtained.112,135,174 Avnir and Rothenberg et al. have generated such chiral interfaces on metal clusters by following the imprinting approach. They reported that not only Ag and Au but also Pd can be successfully modified using a variety of organic molecules as chiral templates, such as the cinchona alkaloid family, to produce chiral-imprinted metalloorganic hybrid materials.112,135,174 They also showed that after the removal of the chiral dopant, the chiral features are preserved, indicating the presence of chiral footprints in the matrix.135 In order to further confirm the chirality of these metal surfaces, photoelectron emission spectroscopy with clockwise circularly polarized light (cw-CPL) or counterclockwise circularly polarized light (ccw-CPL) has been used to study the metal films.135,174 It clearly showed that even after extraction of the chiral template, a different photoelectron emission yield is observed for ccw-CPL and cw-CPL, indicating the presence of enantiomorphic structures.135,168,174 With an analog philosophy, the entrapment of chiral metal complexes inside the metal matrix has also been tested. For example, a [Co]@Ag composite with chiral interfaces has been successfully prepared by the reduction of AgNO3 in the presence of a chiral cobalt complex.169 The designed [Co]@Ag composite exhibits good stability and can be easily reused for various catalytic cycles. The introduction of chirality is not limited to metal nanoparticles but can also be achieved on more extended surfaces. Using electrochemical approaches, chiral information could be stored on a variety of metal and metal oxides surfaces.175–177 For example, tartrate enantiomers are able to generate chiral surface features on Au(001) (Figure 12.3)175,176 and Cu(111).176,178,179 The chiral structure of the imprinted materials can be retained even after the removal of the tartrate enantiomers. Such surfaces behave like perfect antipodes with inverse configuration. For example, a CuO film, generated by using L-tartaric acid as a chiral molecular template, ¯¯ is composed to 95% by domains with a (11 1) orientation, whereas the one ¯11) orientation.176 Apart imprinted with D-tartaric acid exhibits 93% of (1 from tartaric acid, various other chiral molecules, such as malic acid, have also been successfully employed to introduce chiral information on CuO surfaces.180 Chiral metal interfaces can be obtained not only by electrochemical deposition but also by a chiral etching process.181,182 In this case, an achiral CuO film was etched in a solution containing enantiomers, such as L-(þ)-tartaric acid and D-()-tartaric acid. The chiral information generated at the etched CuO surface was verified by the different reactivity of L- and

280

Figure 12.3

D-tartaric

Chapter 12

Illustration of the pole figures for CuO films on Au(001) imprinted with: (a) L-tartaric acid; (b) D-tartaric acid, and (c) DL-tartaric acid. The films obtained from L-tartaric acid and D-tartaric acid have an opposite ¯¯ ¯11) orientation, respectively, whereas configuration with (11 1) and (1 the one deposited from the DL-tartaric acid solution contains equivalent ¯¯ ¯11) orientations. Reproduced from ref. 176 with portions of (11 1) and (1 permission from American Chemical Society, Copyright 2004.

acid during electrooxidation. A preferential electrooxidation of L-enantiomer is observed when using a film that has been etched in 182 L-(þ)-tartaric acid and vice versa. The chiral recognition properties obviously depend on the number of recognition sites, which in turn depends on the available active surface area. Thus, it is of interest to examine the added value of artificially increasing the active surface area by using porous materials in the frame of the molecular imprinting approach.183 Mesoporous metals might be a good first choice in this context, because they can be generated by both chemical and electrochemical means. They can be prepared by following two main strategies: (i) a hard template approach, in which rigid materials such as silica and carbon are used as solid templates; (ii) a soft template approach, in which selfassembled surfactants or polymers are involved,184–186 combined with a reducing agent, allowing the control of the deposition rate of metal around the template. In addition to chemical synthesis, electrodeposition of mesoporous metals is also a well-known strategy to precisely control their porous structure. In this case, the deposition rate around the template can be easily controlled by the potential, leading to well-defined mesoporous structures. Attard et al. first reported the synthesis of highly ordered mesoporous

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platinum by using self-assembled lyotropic liquid crystals (LLC) as a soft template. The synthesis of chiral-imprinted metals with mesoporous features has been recently proposed by using both chemical and electrochemical synthesis routes.136 In the case of electrochemical deposition, chiral-encoded mesoporous platinum (Pt) films have been prepared in the simultaneous presence of the metal salt and a self-assembled lyotropic liquid crystal (LLC) using nonionic surfactant as mesoporogen,136 as shown in Scheme 12.1. As a first step, an LLC gel, mixed with platinum precursor and the chiral template molecule, has been prepared. It is placed on a gold-coated glass slide, adapting a self-assembled hexagonal columnar surfactant structure (H1-phase) with the hydrophilic parts forming the outer surface of the columns. This hydrophilic part directly interacts with the chiral molecules. By applying a suitable reduction potential, metallic Pt is generated around the template structure. Finally, after the removal of the templates, chiral-imprinted mesoporous Pt is obtained.136 This concept has been first tested with 3,4-dihydroxyphenylalanine (DOPA) as a chiral template.136 In order to extend the concept, several other chiral molecules such as mandelic acid (MA),188,189 phenyl ethanol (PE),190 and tryptophan191 have also been studied. Although chiral mesoporous Pt has been successfully tested, it suffers from the drawback of being an expensive noble metal that prevents the use of such surfaces for practical applications. Therefore, an analog strategy has been developed for earth-abundant first-row transition metals. In this context, nickel seems to be a good first choice and chiral-imprinted mesoporous nickel has been successfully prepared by the electrodeposition from an LLC containing nickel salts and the chiral compounds, as illustrated in Scheme 12.2. After template dissolution, the chiral-imprinted mesoporous nickel has been studied with respect to its enantioselective properties.192 Monometallic chiral films have been successfully synthesized, as mentioned above; however, their chemical, electrochemical, and mechanical stability is often limited. In order to develop more stable chiral-imprinted mesoporous metal electrodes, the electrodeposition of alloys might be a promising strategy. In the case of Pt, alloys can be formed with various metals, including ruthenium (Ru), rhodium (Rh), iridium (Ir), and gold (Au), to improve its mechanical properties.193–195 As a first example of this approach, mesoporous platinum–iridium (Pt–Ir) alloys with chiral features have been elaborated by electrodeposition, following the abovementioned procedure.196 By fine-tuning the deposition potential, the thickness of the chiral films, and the relative content of Pt and Ir, the chiral recognition properties can be optimized. Chiral-imprinted mesoporous metals can be prepared not only by electrodeposition but also by following a chemical route. For example, mesoporous bimetallic Pd@Pt nanoparticles with chiral features have been obtained using Pluronic F127 as a mesoporogen to control the structure of the mesoporous network. In a typical procedure, Pd and Pt salts are mixed in

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Scheme 12.1

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Illustration of the synthesis of chiral-imprinted mesoporous Pt electrodes by electrodeposition in the simultaneous presence of metal salt and 3,4dihydroxyphenylalanine (DOPA) as a chiral template: (a) Self-assembled LLC and chiral molecules on a gold-coated glass slide; (b) Growth of metallic Pt around the template structure; (c) the extraction of templates to form the mesoporous structure with chiral features. Reproduced from ref. 136 with permission from Springer Nature, Copyright r 2014, The Author(s).

the presence of Pluronic F127 and ascorbic acid (AA). In order to generate chiral features in the Pt shell, DOPA enantiomers were added to the mixture, leading to chiral-imprinted mesoporous metals with a core–shell structure.197

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Scheme 12.2

283

Schematic illustration of the synthesis of chiral-imprinted mesoporous nickel films by the electrodeposition of Ni in the simultaneous presence of a lyotropic liquid crystal (LLC) also containing the enantiomers of phenylethanol (PE): (a) molecular structure of R- and S-enantiomers of phenylethanol (PE); (b) the columnar structure of nonionic surfactants (H1-phase) interacting with Ni salt and chiral templates; (c) electrodeposition of Ni around the self-assembled surfactant and chiral molecules; (d) template dissolution to obtain the chiralimprinted mesoporous nickel films. Reproduced from ref. 192 with permission from American Chemical Society, Copyright 2019.

As stated above, to date, a large diversity of chiral-imprinted metal structures has been successfully developed. They exhibit some outstanding properties, such as high active surface area, good thermal and mechanical stability, and, in some cases, a very pronounced enantioselectivity. The latter allows envisioning some exciting applications, as illustrated in the next section.

12.3 Applications of Chiral Metal Electrodes Over the past decade, several types of chiral metal electrodes have been developed, as stated above. In order to illustrate their benefits, they have been studied for a variety of applications, ranging from enantioselective analysis198–206 and asymmetric synthesis188,190,192,207–212 to chiral separation.20,191

12.3.1

Enantioselective Analysis

Materials with chiral surfaces are well known and are crucial ingredients for enantioselective analysis and chiral recognition. In particular, chiral metals can be used advantageously in conjunction with concepts of electrochemistry,6 providing a rapid and easy readout of the corresponding analytical information. One popular strategy is based on the presence of chiral or achiral molecules adsorbed on metal surfaces. Various chiral compounds, such as tartaric acid, glucose, amino acids (e.g. leucine, alanine, DOPA, homocysteine [Hcy]), 1,1 0 -binaphthalene-2,2 0 -dithiol (BNSH), amino esters, and amines

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have been studied in terms of chiral discrimination. For example, self-assembled monolayers (SAMs) of a dithiol, such as BNSH on gold single-crystal surfaces, provide an asymmetric surface structure with mirror symmetry when using S-BNSH or R-BNSH. The resulting interface allowed the chiral discrimination of organic molecules such as phenylalanine.216 Beyond the normal SAM approach, chiral ligand exchange (CLE) has also been used and exemplified by the deposition of Cu complexes on the SAM surface of L-homocysteine on gold surfaces.215,217 It allows efficient chiral discrimination of various amino acids.215,217–219 In addition to enantioselective analysis based on the SAM approach, chiral discrimination can also be achieved with metal surfaces having an encoded chiral feature. The first example is a chiral encoded CuO film, obtained by electrodeposition on Au(001) surfaces in the presence of chiral molecules including tartaric acid, alanine, and valine enantiomers, which can form a complex with Cu(II).175,176,178 For example, a CuO electrode generated electrochemically in the presence of L-tartaric acid preferentially oxidizes L-tartaric acid and vice versa. In a control experiment, using racemic DL-tartaric acid, no chiral discrimination could be observed using cyclic voltammetry (CV) studies. In addition, the enantioselectivity of chiral metal films can be greatly enhanced by etching with solutions containing chiral molecules such as tartaric acid enantiomers under either acidic or basic conditions due to the amphoteric properties of CuO. Remarkable chiral discrimination could be observed when using the etched CuO films as electrodes. Only weak enantioselectivity could be obtained for unetched chiral CuO films, when employing them for the electrooxidation of tartrate enantiomers. However, when using a chiral CuO film after etching with L-(þ)-tartaric acid, the chiral discrimination was significantly improved.181 Apart from imprinted CuO films, metalloorganic hybrid materials have been generated by the entrapment of various types of alkaloids in metals, exhibiting chirality even after alkaloid extraction.135,137,138,165,168,220–222 The designed materials exhibit chiral recognition for (S,S)- and (R,R)-tartrate when examined using linear sweep voltammetry (LSV). For example, an alkaloid@Ag composite, obtained by entrapping cinchonine (CN), shows a higher activity for the electrooxidation of (S,S)-tartrate compared to the R-enantiomer, even after extraction of the alkaloid. In contrast, using the pseudoenantiomer of cinchonine, cinchonidine, an opposite trend was observed (Figure 12.4). Similar results were obtained when employing quinine (QN) or quinidine (QD) as dopants.220 As mentioned above, the epitaxial growth of metal films on Si wafers can also generate chiral surfaces. The presence of chirality for Ag electrodeposited on gold-coated Si(643) has been confirmed by the preferential electrooxidation of D-glucose, whereas the opposite behavior is observed for ¯¯ Ag electrodeposited on a gold-coated Si(6 4¯ 3) surface.164 A synergy between mesoporosity and chiral metal surface properties has been achieved with both chemical and electrochemical approaches.

Chiral Metal Electrodes for Enantioselective Analysis, Synthesis, and Separation

Figure 12.4

285

Illustration of chiral recognition using linear sweep voltammetry with a sweep rate of 10 mV s1 with a: (A) pure Ag electrode; (B) CN@Ag after template extraction; (C) cinchonidine@Ag after template extraction; (a, d, and g are obtained in the pure supporting electrolyte; c, f, and h in the presence of 20 mM (S,S)-tartrate; b, e, and i in the presence of 20 mM (R,R)-tartrate). Reproduced from ref. 220 with permission from the Royal Society of Chemistry.

For example, the chemically synthesized Pd@Pt core–shell structures with mesoporous features and imprinted with DOPA enantiomers show chiral discrimination when using them as electrodes for the detection by differential pulse voltammetry (DPV).197 The concept of using chiral-imprinted mesoporous metals as working electrodes for the electrochemical detection of chiral compounds has already been reported previously.136 The chiral discrimination of DOPA enantiomers was investigated with different configurations of chiral-imprinted mesoporous Pt. When employing mesoporous Pt imprinted with L-DOPA, followed by the removal of the chiral template, the electrochemical oxidation of L-DOPA is significantly higher than the one with D-DOPA and vice versa.136 Nonimprinted platinum obviously did not allow chiral discrimination. The presence of chiral surface features was further confirmed by destroying on purpose the chiral surface properties at high positive potentials. As a result, no more enantiodiscrimination was possible with these electrodes. Not only DOPA enantiomers but also various other compounds, including mandelic acid (MA),188,189 phenyl ethanol (PE),190,192 and tryptophan

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enantiomers, have been used as chiral organic template molecules. All of these imprinted mesoporous platinum electrodes showed an unprecedented efficiency for chiral discrimination. Furthermore, this concept is not limited to the design of chiral-imprinted mesoporous noble metals such as Pt, but other metals, in particular nonnoble metals such as nickel, which are earth abundant and cheap, have also been successfully synthesized by combining mesoporosity with chiral features.192 In order to improve the electrochemical stability of the chiral-imprinted metals, especially at positive potentials due to surface oxidation, alloy structures have been proposed recently as an alternative. Simultaneous electrodeposition from Pt and Ir precursors in the presence of LCC and chiral templates leads to mesoporous chiral Pt–Ir alloys, as illustrated in Figure 12.5. Interestingly, compared to monometallic Pt, improved chiral recognition properties have been observed. In addition, the Pt–Ir alloy

Figure 12.5

Chiral recognition of L-DOPA (black) and D-DOPA (red) electrooxidation illustrated by differential pulse voltammetry (DPVs) carried out with different Pt–Ir electrodes: Electrochemical signals recorded (a, b) for fresh L-DOPA- and D-DOPA-imprinted mesoporous Pt–Ir alloys, respectively, (c) after 10 cycles in the range from 0.20 to 1.20 V vs. Ag/AgCl and (d) after 40 cycles from 0.20 to 1.80 V vs. Ag/AgCl, leading to a gradual loss of chiral information. All experiments were performed in 4 mM L- or D-DOPA in 50 mM HCl solution as the supporting electrolyte (pH ¼ 1.4) with electrodes synthesized with a deposition charge density of 6 C cm2. Reproduced from ref. 196, https://doi.org/10.1038/s41467-021-21603-8, under the terms of the CC BY 4.0 license, http://creativecommons.org/ licenses/by/4.0/.

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structure is very robust even after oxidizing it at high positive potentials for several cycles. Not only chiral metals alone but also hybrid materials, obtained by combining chiral metals and conducting polymers, can be used for enantioselective analysis.223,224 In this case, a hybrid bilayer, composed of a free-standing conducting polymer film, polypyrrole, and electrodeposited chiral-imprinted mesoporous platinum, has been successfully tested for wireless enantioselective actuation. Using bipolar electrochemistry, selective electrooxidation of chiral molecules has been observed at one extremity of the object, coupled with the reduction of the polymer at the other extremity. The latter leads to a volume change and induces significant bending as an electromechanical readout of chiral information.223

12.3.2

Asymmetric Synthesis

Another important application of chiral metal structures is the selective synthesis of chiral compounds from prochiral molecules as starting reagents. One of the strategies for designing heterogeneous catalysts for asymmetric synthesis is based on the adsorption of chiral catalysts on metal surfaces.225 For example, chiral nickel surfaces, obtained by the adsorption of optically active tartaric acid enantiomers on achiral nickel structures, have been applied for asymmetric hydrogenation.207,208 Other examples are based on the entrapment of molecules, belonging to the family of alkaloids, in bulk metal structures. The generated chiral information allowed the use of these materials for asymmetric synthesis,135 such as the hydrogenation of isophorone, and acetophenone. Pd imprinted with cinchonidine leads to an ee of approximately 16% of (R)-dihydroisophorone. In contrast, an ee of 7% of (S)-dihydroisophorone can be observed when using cinchonine-imprinted Pd.135,226 Similar results can be obtained for the hydrogenation of acetophenone, where cinchonine-imprinted Pd preferentially produces (R)-1-phenylethanol, while the S enantiomer is the major product in the case of cinchonidine imprinted Pd. These alkaloid@metal systems are not only useful in the frame of chemical synthesis of chiral compounds, but can also be applied as metal electrodes for electrochemical synthesis. For example, silver particles with entrapped alkaloid species have been used as a cathode for the asymmetric synthesis of a chiral compound, methyl benzoylformate, via an electrochemical route. Electrohydrogenation presents various benefits compared to a chemical process, because it can be operated at a much lower hydrogen pressure and reaction temperature.220,227–229 The galvanostatic electrohydrogenation of methyl benzoylformate using tetraethylammonium iodide as a supporting electrolyte and cinchonine@silver as a working electrode results in a high ee of S-methyl mandelate (460%). In contrast, a preferential production of R-methyl mandelate with 58% ee could be observed in the case of the cinchonidine@silver system. In addition, the alkaloid@silver catalyst can be easily reused and recycled for several catalytic cycles without a significant change in catalytic performance.220

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Recently, an alternative strategy has been proposed for using chiralimprinted mesoporous metals as working electrodes for the selective electroconversion of prochiral molecules to chiral compounds.8,188,190,230 Chiral-imprinted platinum with mesoporous features has been employed for the electrotransformation of a prochiral molecule, phenylglyoxylic acid (PGA), into mandelic acid enantiomers,188 using conventional constant potential electrosynthesis. Interestingly, even after the removal of the chiral template, platinum imprinted with (R)-mandelic acid produced a significantly higher amount of (R)-mandelic acid compared to the S-enantiomer. On the other hand, a higher selectivity for (S)-mandelic acid could be achieved when using platinum imprinted with (S)-mandelic acid. The ee has been approximately 20%, which is an acceptable value for the asymmetric synthesis of chiral compounds on heterogeneous catalytic systems.188 In addition, this work demonstrated the possibility of optimizing the ee by increasing the amount of imprinted chiral molecules during the elaboration of the metal surfaces. However, the loading with chiral molecules has an upper limit, because otherwise the ordered mesoporous structures of platinum can be disturbed, resulting in a decrease of ee. However, even for rather high rates of imprinting, the selectivity of chiral-imprinted platinum is limited by unselective reactions at nonimprinted sites, eventually producing racemic mixtures.190 One possibility to prevent such undesired reactions at unspecific sites, located at the outermost surface of the electrode, is to protect these regions by the adsorption of self-assembled monolayers. In this case, the ee can be considerably increased, because only the inner volume of the imprinted mesoporous channels can contribute to the reaction. Another elegant option to decrease the impact of unspecific reactions at the outer electrode surface is the concept of pulsed enantioselective electroconversion.190 In this case, the reduction potential is switched off for a certain period of time, before switching it on again. The electroreduction of acetophenone to phenylethanol enantiomers has been chosen as a model reaction using chiral-imprinted mesoporous platinum as a working electrode. During pulse electrosynthesis, the prochiral reactant, acetophenone, is first adsorbed in the chiral cavities of the electrode. Subsequently, the adsorbed species are converted selectively into chiral compounds when applying the reduction potential. When the potential is switched off, the product can diffuse out from the chiral cavities, liberating the selective reaction spots for the adsorption of further prochiral molecules. Repeating this sequence for many cycles, a highly selective production of chiral compounds with an ee above 90% has been obtained.190 This concept has also been employed with other metals such as chiral-imprinted mesoporous nickel for the electroreduction of acetophenone to phenylethanol enantiomers.192 Interestingly, when using pulsed electrosynthesis, again, a high ee of up to 80% is generated, despite the lower chemical and mechanical stability of nickel compared to noble metals.192 Although a highly selective production of chiral compounds via the electroreduction of prochiral molecules could be successfully demonstrated with chiral-imprinted mesoporous metals, they often suffer from low

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electrochemical stability, even noble metals such as Pt. Indeed, when the electrode is immersed in an acidic solution at a high positive potential, Pt is easily oxidized to form surface oxides, and therefore, the chiral information is lost due to a rearrangement of the atoms. This results in a decrease of enantioselectivity when using them for several catalytic cycles.196 To overcome these problems, chiral-imprinted mesoporous alloys have been proposed.196 For example, a Pt–Ir alloy shows a remarkably enhanced electrocatalytic performance for the electroreduction of acetophenone to phenylethanol with high ee (490%), even when using it for several electrosynthesis cycles. These observations illustrate that bimetallic mesoporous alloy systems with chiral features constitute promising materials for the highly selective electrosynthesis of chiral compounds.

12.3.3

Electrochemical Separation

The separation of enantiomers is also very important in the frame of chiral technologies. Therefore, a straightforward extension of the application spectrum of chiral metal structures is their use as selective stationary phases, analogous to what has been practiced already with conventional methods such as high-performance liquid chromatography (HPLC). In this context, chiral-imprinted mesoporous platinum has been successfully applied as a stationary phase in a microfluidic device for the electroseparation of chiral compounds. In this case, the chiral encoded platinum is electrochemically deposited in the presence of LLC and chiral templates, such as tryptophan enantiomers, to generate specific chiral interfaces in microfluidic devices, as illustrated in Scheme 12.3.191 By optimizing the potential

Scheme 12.3

Illustration of the experimental setup of a microfluidic electrochromatography device using a chiral-encoded mesoporous metal film as a stationary phase. Reproduced from ref. 191 with permission from John Wiley & Sons, Copyright r 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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applied to the surface of the stationary metal phase, the interaction between the imprinted sites and chiral compounds can be fine-tuned, eventually enhancing the capacity of chiral separation. Without applying a potential, overlapping peaks of the two enantiomers have been observed at the channel outlet by fluorescence detection. However, when injecting the racemate into the microfluidic channel and simultaneously applying a positive potential in the range of 200 mV vs. an external Ag electrode, complete baseline separation of the two enantiomers could be achieved. Under these conditions, the negatively charged chiral molecules interact more efficiently with the stationary phase due to its positive charge. This first example opens up interesting perspectives for the application of chiral metal electrodes as stationary phases for the separation of chiral compounds.191

12.4 Conclusion and Perspectives In this chapter, the development of chiral metal electrodes has been summarized and illustrated. Various approaches have been studied so far, including the adsorption of chiral molecules on metal surfaces, binding of chiral ligands to metal surfaces, cutting bulk metals along certain crystal planes, and chiral molecular imprinting. Some of these concepts allow generating intrinsically chiral interfaces, even without adsorbed chiral species or chiral template molecules. The presence of chirality in these structures can be verified with different techniques, such as photoelectron emission spectroscopy, scanning tunneling microscopy, or electrochemical measurements (e.g. DPV). To illustrate the interesting features of chiral metal systems, various applications have been considered, including enantioselective analysis, asymmetric synthesis, and chiral separation. Encouraging results, such as a highly selective recognition, efficient synthesis with high ee and baseline resolution in electrochromatography constitute an interesting basis for the further development of high-performance strategies. Despite these very promising first achievements, several issues, such as long-term stability and synthetic time–space yield, have to be further investigated in order to allow the practical use of such materials, for example, in the frame of large-scale industrial synthesis. Furthermore, in addition to the reusability of chiral metal surfaces, the synthesis of real pharmaceutical compounds should be demonstrated and is one of the main perspectives for the presently mostly academic studies, together with the elaboration of such surfaces based on cheap and earth-abundant metals. We therefore anticipate that this topic will still make considerable progress in the next few years, addressing the different challenges that have been described in this chapter.

Acknowledgements We would like to thank the Vidyasirimedhi Institute of Science and Technology (VISTEC) and the French Embassy in Thailand. In addition, the project has been funded by the European Research Council (ERC) under the European

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Union’s Horizon 2020 research and innovation program (grant agreement no. 741251, ERC Advanced Grant ELECTRA) and has also been supported by the bilateral PICS program of CNRS. C. W. thanks the Mid-Career Research Grant 2020 from the National Research Council of Thailand (NRCT5-RSA63025-03). This research has received funding support from the NSRF via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation (B05F640207).

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CHAPTER 13

Fluorescent Sensors for Water YOUSUKE OOYAMA Graduate School of Advanced Science and Engineering, Hiroshima University, Higashi-Hiroshima, Japan Email: [email protected]

13.1 Introduction The detection and quantification of water in solutions, solids, and gases or water on materials surface are doubtlessly significant in not only organic and analytical chemistry but also environmental, biomedical, and quality control monitoring systems and industrial applications, including food inspection and manufacturing of pharmaceutical, electronic, and petroleum products.1,2 In synthetic chemistry, the presence of water in organic solvents causes serious problems, such as the generation of by-products, quenching of reactions, lowering of the product yields, and furthermore, catastrophic dangers of fire and explosion. Particularly, in a large-scale industrial process, careful attention should be paid to this impurity to avoid worst-case scenarios. Therefore, various analytical approaches and techniques have been developed to detect and quantitate water content in solids and gases as well as organic solvents. As a common and classical method for the determination of water content, the Karl Fischer titration method, which utilizes coulometric or volumetric titration to determine the amount of water (0.001–100 wt%) in a sample such as a solution or solid, is widely used in the laboratory and industry. The coulometric titration is suitable for the determination of a low amount of water below 0.1 wt% and it is based on the Karl Fischer reaction I2 þ SO2 þ 3Base þ ROH þ H2O-2BaseHI þ BaseHSO4R Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

300

(13.1)

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301

and 2I – 2e-I2,

(13.2)

that is, 1 mole of water will react with 1 M of iodine generated electrolytically by eqn (13.2) so that the electricity for 1 mg of water is equivalent to 10.71 coulombs. Therefore, the Karl Fischer titration method has sufficient accuracy, but it is a batch (ex situ) analysis process, which leads to timeconsuming measurements as well as the inability of real-time monitoring and flow (in situ) analysis of the water content. On the other hand, the optical sensing method utilizing organic colorimetric and fluorescent sensors for the determination of water content has become of considerable scientific and practical concern in recent years, because it allows the visualization as well as the detection and quantification of water content in samples and products using a highly sensitive and quick flow analysis based on the changes in wavelength, intensity, and lifetime of photoabsorption and photoluminescence depending on the water content.2 Consequently, fluorescent sensors for water are one of the most promising functional materials contributing to the achievement of the 2030 agenda for Sustainable Development Goals (SDGs), which has been adopted by all United Nations Member States in 2015 and provides a shared blueprint for peace and prosperity for people and the planet now and in the future. In fact, to date, some kinds of organic fluorescent sensors and polymers for the determination of water content based on intramolecular charge transfer (ICT), photoinduced electron ¨rster resonance energy transfer (FRET), excited-state intratransfer (PET), Fo molecular proton transfer (ESIP), solvatochromism (SC), or solvatofluorochromism (SFC) have been designed and synthesized.2 The optical sensing properties of these fluorescent sensors for the detection and quantification of water content were investigated from the viewpoints of the relationship between ICT, PET, FRET, or ESIP characteristics and the intermolecular interaction of the sensor with water molecules. As a result, it was found that most of the previous fluorescent sensors for water content determination, including conjugated polymers3 and organic fluorescent dyes with ICT4–6 and ESIP characteristics,7 are based on a fluorescence quenching (turn-off) system, that is, the fluorescence intensity of the sensor decreases as a function of water content in organic solvents. However, this fluorescence quenching system makes it difficult to detect a trace amount of water. In contrast, a fluorescence enhancement (turn-on) system exhibiting a fluorescence response with an increase in water content in organic solvents is useful for the visualization, detection, and quantification of a trace amount of water in organic solvents. For example, chemodosimeters based on watertriggered reactions such as Schiff base hydrolysis and spiro-ring opening reactions,8 and PET-based fluorescent sensors9–16 based on the suppression of PET (from the electron donor part to the photoexcited fluorophore) due to the intermolecular interactions between the fluorescent sensor and water molecules belong to the fluorescence enhancement systems for the determination of water. In particular, the use of PET-based fluorescent sensors is useful for

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the detection and quantification of a trace amount of water in organic solvents because the fluorescence intensity of the sensor increases as a function of water content in organic solvents. In addition, AIEE (aggregation-induced emission enhancement) is the photophysical property of organic fluorophores in the aggregation state, which is attributed to the emission enhancement induced by the aggregate formation of organic fluorophores upon the addition of large amounts of water (over 40 wt% in almost every case) into the solution and has been reported as a fluorescence enhancement system for high water content.17 On the other hand, colorimetric and ratiometric fluorescent sensors based on FRET and ICT characteristics are preferable because the ratio of photoabsorption or fluorescence intensities at two wavelengths is in fact independent of the total concentration of the sensor, photobleaching, fluctuations in light source intensity, and the sensitivity of the instrument and leads to an effective avoidance of self-quenching and fluorescence detection errors.18 Based on the above mentioned research background, the authors designed and synthesized PET-based9–16 or the PET/FRET-based19,20 fluorescent sensors for the detection of a trace amount of water in solvents, PET/AIEE-based fluorescent sensors21,22 for the detection of water in the low- and high-watercontent regions in solvents, SFC/AIEE-based fluorescent sensor23 for the detection of water over a wide range from low-water-content to high-watercontent regions in solvents, and the ICT-based colorimetric and ratiometric fluorescent sensors24–28 for the detection of water over a wide range from low-water-content to high-water-content regions in solvents. Therefore, this chapter provides a direction in molecular design toward creating highly efficient fluorescent sensors for the determination of water in solvents, as well as fluorescence analysis for the detection, quantification, and visualization of water based on PET, FRET, ICT, or AIEE characteristics of newly developed colorimetric and fluorescent dyes and their optical sensing mechanism for the detection and quantification of water content in solvents. Moreover, the authors propose that polymer films doped with fluorescent sensors for water are one of the most promising and convenient functional materials for visualizing moisture and water droplets.29

13.2 PET-based Fluorescent Sensors PET-based fluorescent sensors have been commonly developed for detecting cations such as H1, Na1, K1, Ca21, and Mg21, anions such as monohydrogen phosphate and pyrophosphate ions, and neutral organic species such as saccharides in biochemical analyses.18,30 They are generally composed of a fluorophore–spacer–receptor structure, that is, a fluorophore skeleton linked to a cation binding site such as an amino moiety (electron donor) via a methylene spacer. Thus, for the PET-based fluorescent sensors, the PET takes place from the nitrogen atom of the amino moiety to the photoexcited fluorophore skeleton, leading to fluorescence quenching of the fluorophore. When the nitrogen atom of the amino moiety is protonated or strongly

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interacts with a cation, a drastic enhancement in fluorescence is observed because of the retardation of PET. Therefore, as shown in Figure 13.1a, upon photoexcitation of fluorophore, an electron is excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) of the fluorophore, which enables PET from the HONO of the electron donor to that of the fluorophore, resulting in fluorescence quenching of the fluorophore. Upon protonation or interaction of the donor with cation species, the corresponding HOMO of the donor becomes lower in energy than that of the fluorophore. As a result, PET is no longer possible, leading to the appearance of fluorescence emission (Figure 13.1b). Thus, the authors devised and developed an anthracene–amino acid structure, OS-1, as PET-based fluorescent sensors for water for the first time, where anthracene is used as the fluorophore skeleton, a methylene unit as the spacer, and an amino group as the binding site for the H3O1 (Figure 13.2a).9 It is worth mentioning here that a carboxyl group is incorporated into the structure as the first recognition site for water molecules. Actually, it was found that the photoabsorption spectra of OS-1 in 1,4-dioxane, tetrahydrofuran (THF), acetonitrile, or ethanol did not undergo appreciable changes in terms of intensity and shape upon the addition of water (Figure 13.3a). In contrast, the corresponding fluorescence spectra of OS-1 exhibited significant changes in intensity with a negligible change in their spectral shapes (Figure 13.3b). For OS-1, in the

Figure 13.1

Mechanisms of PET-based fluorescent sensors for the detection of cations: (a) PET active state (non-fluorescence) and (b) PET inactive state (fluorescence).

(a)

(b)

(c)

304

(d)

(e)

(f)

Proposed mechanisms of PET-based fluorescent sensors: (a) OS-1 and OS-2; (b) OM-1, OM-2, OF-1, and OF-2; (c) MH-1 and MH-2; (d) OU-1; (e) OU-2; and (f) OA-1 and OA-2 for the detection of water in solvent.

Chapter 13

Figure 13.2

Fluorescent Sensors for Water

Figure 13.3

305

(a) Photoabsorption and (b) fluorescence spectra (lex ¼ 366 nm) of OS-1 (c ¼ 2.1105 M) in acetonitrile containing water (0.045–100 wt%). Fluorescence peak intensity at ca. 417 nm (lex ¼ 366 nm) of OS-1 as a function of water content (c) in the region of 0.045–100 wt% in acetonitrile and (d) below 1.2 wt% in 1,4-dioxane, THF, acetonitrile, and ethanol. Reproduced from ref. 9 with permission from the Royal Society of Chemistry.

low-water-content region below 1.0 wt%, the fluorescence intensities increased almost linearly with the increase in water content for all four solvents (Figure 13.3c,d). On the basis of this result, it was proposed that the addition of water to organic solvents containing OS-1 promotes the dissociation of the carboxyl proton, followed by the formation of fluorescent zwitterionic structure OS-1a by the protonation of the amino group, leading to fluorescence enhancement due to the suppression of PET (Figure 13.2a). The detection limit (DL) of OS-1 for water was determined from the plot (Figure 13.3d) of the fluorescence intensity at around 417 nm versus the water fraction in the low-watercontent region below 1.0 wt% (DL ¼ 3.3s/ms, where s is the standard deviation of the blank sample and ms is the slope of the calibration curve). The pots revealed that the slopes for 1,4-dioxane and THF as less polar solvents are smaller than those for acetonitrile and ethanol as polar solvents. As a result, the DL values of OS-1 are 0.1 wt% for both acetonitrile and ethanol (Table 13.1). On the other

306 Table 13.1

Chapter 13 DL values of fluorescent sensors for water determination in various organic solvents.

Sensor

Solvent

ms

DL

Mechanism

Ref.

OS-1a

1,4-Dioxane THF Acetonitrile Ethanol 1,4-Dioxane THF Acetonitrile Ethanol 1,4-Dioxane THF Acetonitrile Ethanol 1,4-Dioxane THF Acetonitrile Ethanol 1,4-Dioxane THF Acetonitrile Ethanol Acetonitrile 1,4-Dioxane THF Acetonitrile 1,4-Dioxane THF Acetonitrile Acetone 1,4-Dioxane THF Acetonitrile Acetone THF Acetonitrile Acetonitrile THF THF

o5 o5 31 72 35 8.2 31 35 14 19 67 106 12 6.7 55 86 334 390 382 362 13 8.9 7.4 31 9.2 6.7 8.9 7.2 3.1 5.9 7.3 0.7 1.0 13 56 — —

41.0 wt% 41.0 wt% 0.1 wt% 0.1 wt% 0.1 wt% 0.4 wt% 0.1 wt% 0.1 wt% 0.2 wt% 0.2 wt% 0.04 wt% 0.04 wt% 0.3 wt% 0.5 wt% 0.06 wt% 0.04 wt% 0.01 wt% 0.008 wt% 0.009 wt% 0.009 wt% 0.25 wt% 0.37 wt% 0.44 wt% 0.11 wt% 0.36 wt% 0.49 wt% 0.37 wt% 0.46 wt% 41.0 wt% 0.56 wt% 0.45 wt% 41.0 wt% 41.0 wt% 0.25 wt% 0.06 wt% 0.002 vol% 0.02 vol%

PET

9

PET

10

PET

11

PET

14

PET

14

PET/FRET PET/AIEE

19 21

PET/AIEE

22

PET/AIEE

22

SFC/AIEE ICT ICT ICT ICT

23 25 24 4 5

OS-2a

OM-1a

OF-1a

OF-2a

DJ-1a RN-1a RS-1a

RS-2a

TPE-(An-CHO)4a 9-MP-BF3a YNI-2-BF3a KD-F0021 3c a

DL ¼ 3.3s/ms, where s is the standard deviation of blank sample and ms is the slope of the calibration curve in the region of the low water content below ca. 1.0–3.0 wt%.

hand, the DL value of OS-1 for water in 1,4-dioxane and THF is over 1.0 wt%, which is inferior to those for acetonitrile and ethanol. Thus, in order to further improve the fluorescence-sensing abilities of PET-type fluorescent sensor for water based on anthracene–amino acid structure, an anthracene–amino acid OS-2 having two carboxyl groups was developed (Figure 13.2a) in the anticipation that the addition of water to organic solvents containing OS-2 causes an efficient formation of fluorescent zwitterionic compounds OS-2a, because of a more acidic nature of OS-2 than OS-1.10 As a result, the DL values of OS-1 for water are

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307

0.1 wt% for 1,4-dioxane, 0.4 wt% for THF, 0.1 wt% for acetonitrile, and 0.1 wt% for ethanol, which are superior to those of OS-1. However, the DL values of PET-type fluorescent sensors for water based on anthracene–amino acid structure in less polar organic solvents are inferior to those in polar organic solvents due, most likely, to poor dissociation of the carboxyl proton in the former solvents. Meanwhile, an anthracene–(aminomethyl)phenylboronic acid system, which was developed by Shinkai et al., has been used as a PET-based fluorescent sensor for saccharides.31 Therefore, in order to overcome the drawback of anthracene–amino acid structure, they inspired the authors to conceive an anthracene–(aminomethyl)phenylboronic acid structure as a new PET-based fluorescent sensor for water. Actually, anthracene–(aminomethyl)phenylboronic acid pinacol esters (AminoMePhenylBPin) OM-1, OF-1, and OF-2 (Figure 13.2b) were designed and synthesized, where OF-1 and OF-2 have methoxy group as an electron-donating substituent and a cyano group as an electron-withdrawing substituent, respectively, at the para-position on PhenylBPin.11,14 It is expected that PhenylBPin enhances the Lewis acidity of a boron atom and the solubility of the sensor in organic solvents. Thus, the addition of water to organic solvents containing OM-1, OF-1, or OF-2 causes a drastic enhancement of fluorescence due to the suppression of PET, which is attributed to the formation of PET inactive species (fluorescent ionic structure) such as OM-1a, OF-1a, or OF-2a by interactions with a water molecule. In fact, as in the cases of OS-2 and OS-1, upon addition of water into the solution, the photoabsorption spectra did not undergo appreciable changes, but the corresponding fluorescence spectra exhibited a drastic enhancement of fluorescence intensity. For all the three sensors, in the low-water-content region below 1.0 wt%, the fluorescence intensities increased almost linearly with the increase in water content for all four solvents. These results indicate that the fluorescence enhancement of OM-1, OF-1, or OF-2 with the increase in the water content can be attributed to suppression of PET due to the formation of fluorescent ionic structure OM-1a, OF-1a, or OF-2a (Figure 13.2b). Thus, the DL (¼ 3.3s/ms) value for water was determined from the plot of the fluorescence intensity at around 415 nm versus the water fraction in the low-water-content region below 1.0 wt% (Figure 13.4). The plots indicate a big difference in the ms values among OM-1, OF-1, and OF2 (Table 13.1). The ms value for OF-1 becomes steeper in the following order: THF (ms ¼ 6.7)o1,4-dioxane (ms ¼ 12)oacetonitrile (ms ¼ 55)oethanol (ms ¼ 86). That is, the ms values in less polar organic solvents (1,4-dioxane and THF) are much smaller than those in polar organic solvents (acetonitrile and ethanol), as in the case of OM-1 (ms ¼ 14 for 1,4-dioxane, 19 for THF, 67 for acetonitrile, and 106 for ethanol). However, the ms values for OF-1 in all four solvents are smaller than those of OM-1. On the other hand, it is worth mentioning that for OF-2, there is a little difference in the ms value among the four solvents, and the ms values for OF-2 (ca. 330–390) in all four solvents are much larger than those of OM-1 and OF-1. The DL values of OF-1 for water are 0.3 wt% for 1,4-dioxane, 0.5 wt% for THF, 0.06 wt% for acetonitrile, and 0.04 wt% for ethanol, which are inferior to those of OM-1 (DL ¼ 0.2 wt% for

308

Figure 13.4

Chapter 13

Fluorescence peak intensity (lex ¼ 367 nm) at ca. 415 nm of (a) OM-1, (b) OF-1, and (c) OF-2 in 1,4-dioxane, THF, acetonitrile, and ethanol in a low-water-content region below 1.2 wt%. Reproduced from ref. 14 with permission from the Royal Society of Chemistry.

1,4-dioxane, 0.2 wt% for THF, 0.04 wt% for acetonitrile, and 0.04 wt% for ethanol). Thus, for both OM-1 and OF-1, the DL values for water in polar organic solvents (acetonitrile and ethanol) were inferior to those in less polar organic solvents (1,4-dioxane and THF). On the other hand, the DL values of OF-2 for water are 0.01 wt% for 1,4-dioxane, 0.008 wt% for THF, 0.009 wt% for acetonitrile, and 0.009 wt% for ethanol, which are superior to those of OM-1 and OF-1 and are equivalent to or superior to those of the fluorescence quenching system (KD-F0021 and 3c in Table 13.1) based on the reported ICTtype fluorescent sensors.4,5 This result can be attributed to the fact that for OF2, the electron-withdrawing cyano group at the para-position on PhenylBPin enhances the Lewis acidity of the boron atom due to its electron-withdrawing substituent, leading to the facilitation of the formation of fluorescent ionic structure OF-2a by interaction with water molecules. In contrast, for OF-1, the electron-donating methoxy group at the para-position on PhenylBPin diminishes the Lewis acidity of the boron atom, leading to the retardation of the formation of fluorescent ionic structure OF-1a by the addition of water molecules. In addition to PET-type fluorescent sensors for water based on anthracene fluorophores such as OM-2,12 OU-1, OU-2,13 OA-1, and OA-2,15 it was found that BODIPY fluorophore–AminoMePhenylBPin (MH-1 and MH-2)16,20 can act as a fluorescent sensor for trace amounts of water based on the PET method with fluorescence enhancement and attenuation systems (Figure 13.2c–f). Consequently, on the basis of the authors’ continuing work to gain insight into the optical sensing mechanism of PET-based fluorescent sensors for water, it is postulated that the fluorescence enhancement system based on the PET-type fluorescent sensors for water is useful for the detection and quantification of a trace amount of water in organic solvents.

13.3 PET/FRET-based Fluorescent Sensors As mentioned in Section 13.2, the PET method makes it possible to visualize, detect, and determine a trace amount of water in organic solvents. However,

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309

the PET-based fluorescent sensor usually has the disadvantage of a very small Stokes shift (SS), causing serious self-quenching and fluorescence detection errors due to photoexcitation and scattering light from the excitation source; SS is the difference in wavelength or frequency units between the maxima of the first photoabsorption band and the fluorescence band. On the other hand, the FRET-based sensor is useful for applications in biochemistry and environmental research such as nucleic acid and ion analysis, signal transduction, and light harvesting, as well as for designing ratiometric fluorescent sensors.18 FRET is well described as an energy transfer process between an excited-state donor fluorophore and a groundstate acceptor fluorophore linked together by a nonconjugated spacer, and, as a result, a fluorescence spectrum from the acceptor fluorophore is observed. In order to achieve an effective FRET, a strong spectral overlap between the donor fluorescence and the acceptor photoabsorption is required. Consequently, the pseudo-SS between the maxima of the donor photoabsorption band and acceptor fluorescence band of the FRET-based sensor is larger than the SS of either the donor or acceptor fluorophore, leading to an effective avoidance of self-quenching and fluorescence detection errors. Thus, in order to develop a fluorescent sensor possessing large SS for a trace amount of water in solvents, the authors have designed and developed PET/FRET-based fluorescent sensors DJ-1 and DJ-2 (Figure 13.5).19,20 DJ-1 is composed of an anthracene–AminoMePhenylBPin skeleton as the PET-type

Figure 13.5

Proposed mechanisms of PET/FRET-based fluorescent sensors: (a) DJ-1 and (b) DJ-2 for the detection of water in solvent. Reproduced from ref. 20 with permission from the Royal Society of Chemistry.

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donor fluorophore and a BODIPY skeleton as the acceptor fluorophore in the FRET process. In contrast, DJ-2 is composed of an anthracene skeleton as the donor fluorophore and a BODIPY–AminoMePhenylBPin skeleton as the PET-type acceptor fluorophore in the FRET process. That is, as shown in Figure 13.5a, it is expected that for DJ-1 in absolute solvents, the PET takes place from the amino moiety to the photoexcited fluorophore (anthracene skeleton), leading to fluorescence quenching. On the other hand, the addition of water to solvents containing DJ-1 causes both the suppression of PET in the anthracene–AminoMePhenylBPin and energy transfer from the photoexcitedstate anthracene to the ground-state acceptor fluorophore (BODIPY skeleton) through a FRET process, thus resulting in the appearance of the fluorescence band originating from the BODIPY skeleton. For DJ-2 in absolute solvents, however, FRET takes place from the photoexcited-state donor fluorophore (anthracene skeleton) to the ground-state acceptor fluorophore (BODIPY skeleton), but fluorescence emission originating from the BODIPY skeleton is not observed due to the occurrence of PET in the BODIPY– AminoMePhenylBPin (Figure 13.5b). As in the case of DJ-1, the addition of water to organic solvents containing DJ-2 causes both the suppression of PET and energy transfer from the photoexcited-state anthracene to the ground-state BODIPY skeleton through the FRET process, thus resulting in the appearance of the fluorescence band originating from the BODIPY skeleton. In fact, DJ-1 and DJ-2 show two photoabsorption bands in the ranges of 300–400 nm and 420–540 nm, which are assigned to the anthracene skeleton and the BODIPY skeleton, respectively (Figure 13.6a,c). It was found that DJ-1 in absolute acetonitrile exhibits only one fluorescence band with the fluorescence maximum (l2em) at 508 nm in the range of 480–600 nm originating from the BODIPY skeleton by photoexcitation (l1ex ¼ 367 nm) of the anthracene skeleton, as well as photoexcitation (l2ex ¼ 472 nm) of the BODIPY skeleton (Figure 13.6b). However, in contrast to the case of DJ-1, DJ-2 in absolute acetonitrile exhibits two fluorescence bands with the l1em at 407 nm and the l2em at 520 nm originating from the anthracene skeleton and the BODIPY skeleton, respectively, by photoexcitation (l1ex ¼ 367 nm) of the anthracene skeleton (Figure 13.6b). On the other hand, the addition of water to acetonitrile solution containing DJ-1 or DJ-2 caused both PET suppression and energy transfer from the donor fluorophore to the acceptor fluorophore through the FRET process, thus resulting in an enhancement of the fluorescence band originating from the BODIPY skeleton. In fact, DJ-1 exhibits an enhancement of fluorescence band (l2em) at 508 nm originating from the BODIPY skeleton by the photoexcitation (l1ex ¼ 367 nm) of the anthracene skeleton upon the addition of water to the acetonitrile solution. This result indicates that the FRET efficiency of DJ-1 in acetonitrile solution containing water is quantitative due to the fact that no fluorescence band originating from the anthracene skeleton is observed. On the other hand, upon the addition of water to the acetonitrile solution, DJ-2 exhibits an enhancement and redshift (ca. 15 nm) of the fluorescence band (l2em) at 520 nm originating from the BODIPY skeleton by the photoexcitation (l1ex ¼ 367 nm) of the anthracene skeleton, but the

Fluorescent Sensors for Water

Figure 13.6

311

(a) Photoabsorption and (b) fluorescence spectra (lex ¼ 367 nm) of DJ-1 (c ¼ 4.0106 M) in acetonitrile containing water (0.0070–39 wt%). (c) Photoabsorption and (d) fluorescence spectra (lex ¼ 367 nm) of DJ-2 (c ¼ 4.0106 M) in acetonitrile containing water (0.0033–40 wt%). Fluorescence peak intensity (lex ¼ 367 nm) at (e) 508 nm of DJ-1 and (f) 520–535 nm of DJ-2 as a function of water content below 40 wt% in acetonitrile. Inset in (e): Fluorescence peak intensity (lex ¼ 367 nm) at 508 nm of DJ-1 as a function of water content below 1.0 wt% in acetonitrile. Reproduced from ref. 20 with permission from the Royal Society of Chemistry.

312

Chapter 13

(l1em)

fluorescence band at around 410 nm originating from the anthracene skeleton does not undergo appreciable changes, which might be attributed to the low FRET efficiency. Actually, it was found that the FRET efficiency for DJ-1 is quantitative, but that for DJ-2 was estimated to be ca. 50% based on timeresolved fluorescence lifetime measurements. In addition, the pseudo-SS values of DJ-1 and DJ-2 between the photoabsorption maximum of the anthracene fluorophore and the fluorescence maximum of the BODIPY fluorophore are 7563 cm1 (141 nm) and 8017 cm1 (153 nm), respectively, which are significantly higher than those of typical PET-based fluorescent sensors OM-1 (395 cm1) and MH-2 (3775 cm1). The ms and DL values of DJ-1 for the acetonitrile solution in the low-water-content region below 1.0 wt% are 13 and 0.25 wt%, respectively (Table 13.1), which are inferior to those of the PET-based fluorescent sensor OM-1 (ms ¼ 67, DL ¼ 0.04 wt%). The ms and DL values of the PET/FRET-based fluorescent sensor may depend on the nonconjugated spacer between the donor fluorophore and the acceptor fluorophore, that is, the substituent on the PhenylBPin. In fact, the ms value (55) and DL value (0.06 wt%) of OF-1 having an electron-donating methoxy group are inferior to those of OM-1, but the ms (382) and DL (0.009 wt%) values of OF-2 having an electron-withdrawing cyano group are superior to those of OM-1 and OF-1. On the other hand, the DL values (over 10 wt%) of MH-2 (ms ¼ 0.16) and DJ-2 (ms ¼ 0.24) are much inferior to those of OM-1 and DJ-1. The inferior DL values of MH-2 and DJ-2 might be attributed to the highly active PET characteristics of the BODIPY-AminoMePhenylBPin skeleton, compared to the anthracene–AminoMePhenylBPin skeleton in OM-1 and DJ-1. These results suggest that the ms and DL values of a PET/FRETbased fluorescent sensor for water can be improved not only by modifying the nonconjugated spacer between the donor fluorophore and the acceptor fluorophore but also by selecting a PET-type fluorophore. Consequently, based on the fluorescence-sensing mechanisms of DJ-1 and DJ-2 for water, it is proposed that a combination of a PET-type donor fluorophore and an acceptor fluorophore in the FRET process is one of the most promising molecular designs to create an efficient PET/FRET-based fluorescent sensor possessing large SS for the detection of water in organic solvents.

13.4 PET/AIEE-based Fluorescent Sensors As mentioned in Sections 13.2 and 13.3, PET- or FET/FRET-based fluorescent sensors belong to fluorescence enhancement systems for the detection of water in solvents containing low water content. On the other hand, the AIEE of organic fluorophores in the aggregation state has been reported as a fluorescence enhancement system for high water content.17 That is, an AIEE dye such as tetraphenylethene (TPE) and diphenyldibenzofulvene (DPDBF) and their derivatives exhibit emission enhancement due to the restricted intramolecular rotation (RIR) in the molecular structures induced by aggregate formation upon the addition of large amounts of water (over 40 wt% in almost every case) into the solution. Thus, the RIR by the aggregation of

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313

fluorophore results in the elimination of radiationless (nonemissive) relaxation of the photoexcited fluorophore. Actually, TPE- or DPDBF-based compounds in a dilute solution exhibit almost no emission due to the radiationless relaxation of the excitons by dynamic rotation of the phenyl groups. Upon aggregate formation due to the addition of large amounts of water into the solution, on the other hand, TPE- or DPDBF-based compounds exhibit AIEE characteristics due to the RIR of the phenyl groups. Consequently, AIEE-based fluorescent sensors are suitable for the detection of water in solvents containing high water content. Thus, in order to gain insight into a direction in molecular design toward creating fluorescent sensors for the detection of water over a wide range from low water content to high water content in organic solvents, the authors have designed and developed a anthracene–AminoMePhenylBPin–TPE structure RN-1 possessing PET and AIEE characteristics, where the BPin group is attached to the para-position of the TPE unit (Figure 13.7a).21 The main advantage of the PET/AIEE-based fluorescent sensor is that it acts as a dual-fluorescence emission sensor, which makes it possible to detect water in the low- and high-water-content regions because fluorescence emission due to the suppression of PET occurs at a different wavelength from that of AIEE, compared to simple PET-based and AIEE-based fluorescent sensors. Actually, for RN-1, it was found that the appearance of the fluorescence band at ca. 420 nm for the THF, 1,4-dioxane, or acetonitrile solution in the low-water-content region below 1.0 wt% is attributed to the fluorescence emission originating from the anthracene skeleton due to the formation of PET inactive species RN-1(H2O) upon the addition of water molecules (Figure 13.7b,c for acetonitrile solution). Moreover, in the water content region above ca. 70 wt%, a new and broad fluorescence band at ca. 500 nm gradually appears with the simultaneous decrease in the fluorescence band at around 420 nm. Evidently, the appearance of fluorescence band at ca. 500 nm is attributed to the AIEE characteristics due to the RIR of the TPE unit associated with the aggregate formation (RN-1(H2O)-A) (Figure 13.7a). For THF, 1,4-dioxane, and acetonitrile solutions, in a water content region below 1.0 wt%, the DL values of RN-1 are 0.44 wt%, 0.37 wt%, and 0.11 wt% (Table 13.1). The DL values of RN-1 are inferior to those (0.2 wt% for both THF and 1,4-dioxane and 0.04 wt% for acetonitrile) of OM-1, which may be attributed to the dynamic rotation of the TPE unit, leading to the inhibition of the formation of PET inactive species RN-1(H2O) by interaction with water molecules. However, this work proposes that the PET/AIEE hybrid fluorescent sensor can act as a dual-fluorescence emission sensor for the detection of water in the low-water-content and high-water-content region in solvents, although RN-1 showed nonresponse in the moderate water content range moving from the PET inactive to the AIEE active. Furthermore, a TPE–anthracene–AminoMeCNPhenylBPin structure RS-1 and a DPDBF–anthracene–AminoMeCNPhenylBPin structure RS-2 as the PET/AIEEbased fluorescent sensor (Figure 13.8a,b) have been developed, where the TPE or DPDBF unit is directly attached to the 10-position on the anthracene skeleton, in

314

Figure 13.7

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(a) Proposed mechanisms of PET/AIEE-based fluorescent sensor RN-1 for the detection of water in solvent. (b) Fluorescence spectra (lex ¼ 367 nm) of RN-1 (c ¼ 2.0105 M) in acetonitrile containing water (0.006–90 wt%). (c) Fluorescence peak intensity (lex ¼ 367 nm) at ca. 420 nm and ca. 500 nm of RN-1 as a function of water content (0.006–90 wt%) in acetonitrile. Reproduced from ref. 21 with permission from John Wiley & Sons, Copyright r 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

contrast to RN-1.22 For both RS-1 and RS-2 in 1,4-dioxane, THF, acetonitrile, and acetone, the enhancement of fluorescence band at around 430 nm in the lowwater-content region below 1.0 wt% is attributed to the fluorescence emission originating from the anthracene skeleton due to the formation of PET inactive species RS-1(H2O) and RS-2(H2O) upon the addition of water molecules (Figure 13.8c–f for acetonitrile solution). Furthermore, the appearance of new and broad fluorescence bands at around 450 nm for RS-1 and 530 nm for RS-2 in the high-water-content region of over 60–70 wt% is attributed to the AIEE characteristics due to the RIR of the TPE or DPDBF unit associated with aggregate formation (RS-1(H2O)-A and RS-2(H2O)-A). It is worth noting that for RS-2, the separation of fluorescence peak wavelengths originating from the anthracene skeleton (atB430 nm) and the AIEE characteristics (atB530 nm) due to the RIR of the DPDBF unit with the aggregate formation is clear (Figure 13.8d), but for RS-1, the separation of fluorescence peak wavelengths is completely indistinct (Figure 13.8c). Consequently, RS-2 having the DPDBF unit on the anthracene skeleton can act as a PET/AIEE-based dual-fluorescent sensor

Fluorescent Sensors for Water

Figure 13.8

315

Proposed mechanisms of PET/AIEE-based fluorescent sensors (a) RS-1 and (b) RS-2 for the detection of water in solvent. (c) Fluorescence spectra (lex ¼ 375 nm) of RS-1 (c ¼ 2.0105 M) in acetonitrile containing water (0.0053–80 wt%). (d) Fluorescence spectra (lex ¼ 375 nm) of RS-2 (c ¼ 2.0105 M) in acetonitrile containing water (0.01–80 wt%). (e) Fluorescence peak intensities (lex ¼ 300 nm and 375 nm) at 430–450 nm of RS-1 as a function of water content (0.0053–80 wt%) in acetonitrile. (f) Fluorescence peak intensities (lex ¼ 375 nm) at ca. 430 nm and ca. 530 nm of RS-2 as a function of water content (0.01–80 wt%) in acetonitrile. Reproduced from ref. 22 with permission from the Royal Society of Chemistry.

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for the detection of water in the low- and high-water-content regions in solvents. For 1,4-dioxane, THF, acetonitrile, and acetone solutions in a water content region below 1.0 wt%, the DL values of RS-1 are 0.36 wt%, 0.49 wt%, 0.37 wt%, and 0.46 wt% (Table 13.1). On the other hand, the DL values of RS-2 are 0.56 wt% for THF, 0.45 wt% for acetonitrile, and over 1.0 wt% for both 1,4-dioxane and acetone. The DL values of RS-1 and RS-2 are inferior to those (0.01 wt% for 1,4-dioxane, 0.008 wt% for THF, and 0.009 wt% for acetonitrile) of OF-2 and those (0.37 wt% for 1,4-dioxane, 0.44 wt% for THF, and 0.11 wt% for acetonitrile) of RN-1. The inferior DLs of RS-1 and RS-2 are attributed to the dynamic rotation of the TPE unit or the DPDBF unit, which is directly attached to the anthracene skeleton, leading to the promotion of nonradiative decay in the photoexcited anthracene skeleton. Consequently, this work demonstrates that the main advantage of the PET/AIEE-based fluorescent sensors is to make it possible to detect water in the low- and high-water-content regions, compared to PET-based fluorescent sensors for the detection of water in the low-water-content region and AIEEbased fluorescent sensors for the detection of water in high-water-content regions. However, developing a more efficient PET/AIEE hybrid fluorescent sensor that can accelerate the appearance of AIEE characteristics is necessary to overcome the nonresponse range (the moderate water content region) for the detection of water in solvents.

13.5 SFC/AIEE-based Fluorescent Sensors As mentioned in Section 13.4, PET/AIEE-based fluorescent sensors for water show nonresponse in the moderate water content range, moving from the PET inactive species to the AIEE active. Thus, in order to overcome this drawback, the focus was shifted to the SFC of fluorescent dyes. SFC is also a photophysical property of organic fluorophores, that is, a redshift (positive SFC) or a blueshift (negative SFC) of their fluorescence bands with an increase in solvent polarity.18 Donor–p–acceptor (D–p–A) compounds are typical SFC dyes based on the ICT characteristics, which are widely used in polarity sensors and polarity-sensitive fluorescence probes to detect a slight polarity change by the shift of their fluorescence bands. Previously, solvatochromic AIEE luminogens based on TPE derivatives32 having electron-donating or electron-accepting groups as well as D–p–A fluorophores33 have been reported as fluorescent sensors for water. However, to the best of the authors’ knowledge, there are only a few reports on the SFC/ AIEE compounds composed of different fluorophores,34 although such an SFC/AIEE-based sensor allows the colorimetric or ratiometric fluorescence measurements, which are preferable because the ratio of the photoabsorption or fluorescence intensities at the two wavelengths is in fact independent of the total concentration of the sensor, photobleaching, fluctuations in light source intensity, sensitivity of the instrument, etc. Moreover, the disadvantage of these SFC/AIEE-based fluorescent sensors for water developed so far is that they give no response in the low-water-content region; that is, the

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fluorescence intensity as a function of water content does not increase linearly with the increase in the water content over a wide range from a low-water-content to a high-water-content region in organic solvents. In fact, a linear change in fluorescence intensity as a function of water content is one of the factors required for the practical use of the fluorescent sensor for water, as in the case of PET-based fluorescent sensors for water.9–16 Therefore, to verify the superiority and extensibility of SFC/AIEE-based fluorescent sensors for water over a wide range from a low-water-content to a high-water-content region in organic solvents, a donor–acceptor-type fluorescent sensor TPE-(An-CHO)4 composed of an electron-donating TPE core and four electron-accepting anthraldehydes as peripheral units possessing both solvatofluorochromic properties and AIEE characteristics for the detection of water has been designed and synthesized (Figure 13.9a).23 Actually, TPE-(An-CHO)4 in absolute THF showed two photoabsorption bands in the range of 300–350 nm and 350–450 nm, which are assigned to TPE and the anthraldehyde units, respectively (Figure 13.9b). In the water content regions below 30 wt%, the photoabsorption spectra of TPE-(An-CHO)4 show unnoticeable changes with an increase in the water content. On the other hand, in the water content regions greater than 40 wt%, the photoabsorption bands in the range from 350 to 400 nm were redshifted with an appearance of a level-off tail, indicating the presence of nanosized or micrometer-sized particles. In fact, the aggregate formation of TPE-(An-CHO)4 in THF with 40–90 wt% water content was determined by Tyndall scattering and scanning electron microscopy (SEM). For the corresponding fluorescence spectra, TPE-(An-CHO)4 in absolute THF showed a feeble fluorescence band at around 500 nm originating from the anthraldehyde units (Figure 13.9c). The addition of water to the THF solution containing TPE-(An-CHO)4 caused redshifts of the fluorescence band and changes in the fluorescence intensity. In the low-water-content region below 30 wt%, TPE-(An-CHO)4 exhibited a linear increase in the fluorescence intensity with a redshift of the fluorescence band with the increase in the water content, that is, the positive SFC based on ICT characteristics due to increasing the solvent polarity. A broad fluorescence band at around 540 nm originating from AIEE characteristics due to the RIR of the TPE core associated with the aggregate formation was enhanced and dominated the fluorescence spectra from the relatively low water content of 40 wt%. The AIEE characteristics of TPE-(An-CHO)4 observed in the relatively low-water-content region are attributed to the fact that the four anthraldehyde units lowered the solubility in organic solvents and water. Indeed, the plots of the fluorescence intensity versus the water fraction in the THF solutions for TPE-(An-CHO)4 indicated that the fluorescence intensity gradually and almost linearly increased with the increase in the water content over a wide range from 0 wt% to 90 wt% (Figure 13.9d). Furthermore, the plots of the fluorescence maximum wavelength versus the water fraction for TPE-(An-CHO)4 revealed that the fluorescence maximum wavelength significantly redshifted as a function of water content in the region below 10 wt% and then gradually redshifted in

318 (a) Proposed mechanisms of the SFC/AIEE-based fluorescent sensor TPE-(An-CHO)4 for the detection of water in solvent. (b) Photoabsorption and (c) fluorescence spectra (lex ¼ 408 nm) of TPE-(An-CHO)4 (c ¼ 1.0105 M) in THF containing water (0.3–84 wt%). (d) Fluorescence peak intensity at around 497–541 nm and (e) the fluorescence maximum wavelength of TPE-(An-CHO)4 (lex ¼ 408 nm) as a function of water content (0.3–84 wt%) in THF. Reproduced from ref. 23 with permission from the Royal Society of Chemistry.

Chapter 13

Figure 13.9

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the region above 20 wt% (Figure 13.9e). This result indicates that the fluorescence behavior of TPE-(An-CHO)4 for the detection of water by fluorescence enhancement system based on the SFC and AIEE exhibited a good linear response in water over a wide range from the low-water-content to high-water-content region in solvents, although the DL of TPE-(An-CHO)4 is estimated to be over 1.0 wt% from a water content region below 1.0 wt%, as shown in Figure 13.9d (Table 13.1), which is inferior to those of the reported SFC/AIEE-based fluorescent sensors (DL ¼ 0.005–0.01 wt%).32 Consequently, this work proposed that donor–acceptor-type TPE-(anthracene-acceptor)4 structures can act as SFC/AIEE-based fluorescent sensors for the detection of water over a wide range from low-water-content to highwater-content regions in organic solvents.

13.6 ICT-based Fluorescent Sensors D–p–A dyes bearing both electron-donating (D) and -accepting (A) groups linked by p-conjugated bridges have a large dipole moment and exhibit strong photoabsorption properties associated with the ICT excitation from the donor to the acceptor moiety in the D–p–A structures.18 Moreover, there are also D–p–A dyes possessing fluorescence properties and thus, it is wellknown that D–p–A fluorescent dyes exhibit SFC due to the large dipole moment. As a noteworthy structural feature of D–p–A dyes, the HOMOs are delocalized over the p-conjugated systems, in many cases, in configurations centering on the D moiety, whereas the LUMOs are delocalized over the A moiety. Indeed, for the ICT-type optical sensors with a D–p–A structure, the dipole moment and electronic structure change due to the intermolecular interaction (electrostatic interaction) between the D or the A moiety of the sensor and the species accompanying the detection (recognition) of the analytes such as cations, anions, and neutral organic species, resulting in changes in photoabsorption, fluorescence (intensity and wavelength), and electrochemical properties (oxidation and reduction potentials). Therefore, the ICT-type optical sensor allows the colorimetric and ratiometric fluorescence measurements, which are preferable because the ratio of the photoabsorption or fluorescence intensities at the two wavelengths is in fact independent of the total concentration of the sensor, photobleaching, fluctuations in light source intensity, sensitivity of the instrument, and so on. For this reason, the focus was on b-carboline (9H-pyrido[3,4-b]indole) with a D–A structure, which has a pyridine ring as a hydrogen bond acceptor, and it is one of the most promising colorimetric and ratiometric fluorescent sensor skeletons for hydrogen bond donor species such as Brønsted acids, water, alcohols, and nucleotides.35–37 For example, 9-methyl pyrido[3,4-b]indole (9-MP) in aprotic solvents exhibited a vibronic-structured fluorescence band at around 370 nm (Figure 13.10a).35–37 Interestingly, 9-MP in alcohols or solvents containing water showed a new fluorescence band at 450 nm arising from the photoexcited state of the cationic exciplex (CL) and it is attributed to the formation of the hydrogen-bonded proton transfer complex (PTC) between the

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0.4

BF 3 -OEt 2

Absorbance

0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 250

Figure 13.10

350 450 Wavelength/ nm

(c) 0 0.1 eq 0.2 eq 0.3 eq 0.4 eq 0.5 eq 0.6 eq 0.7 eq 0.8 eq 0.9 eq 1.0 eq 1.5 eq 2.0 eq

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0 0.1 eq 0.2 eq 0.3 eq 0.4 eq 0.5 eq 0.6 eq 0.7 eq 0.8 eq 0.9 eq 1.0 eq 1.5 eq 2.0 eq

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(a) Proposed mechanisms for the formation of the Py(N)–B complex of 9-MP with BF3. (b) Photoabsorption and (c) fluorescence (lex ¼ 362 nm) spectral change of 9-MP (c ¼ 1.0105 M) upon the addition of BF3-OEt2 (0–2 eq.) in acetonitrile; inset: color and fluorescence color images of 9-MP with and without the addition of BF3-OEt2. Reproduced from ref. 38 with permission from the Royal Society of Chemistry.

pyridinic nitrogen atom of 9-MP and the hydroxyl group of the alcohol or water as a proton donor, which has been already reported. Furthermore, it was found that 9-MP can form a Py(N)–B complex (BF3-9-MP) with boron trifluoride (BF3) as a Lewis acid in the solution, which exhibits photoabsorption and fluorescence bands at a longer wavelength region than pristine 9-MP (Figure 13.10b,c).38 In fact, upon the addition of boron trifluoride diethyl etherate (BF3-OEt2) to 9-MP in acetonitrile, the photoabsorption band at around 350 nm decreased with the simultaneous appearance of a new photoabsorption band at around 390 nm with an isosbestic point at 362 nm, which suggested the formation of the complex (BF3-9-MP) with BF3. Moreover, the corresponding fluorescence band at around 370 nm decreased gradually with the simultaneous appearance of a new fluorescence band at around 460 nm with an isoemissive point at 415 nm. These interesting results inspired the authors to conceive 9-methyl pyrido[3,4-b]indole-BF3 complex, 9-MP-BF3, as a colorimetric and ratiometric fluorescent sensor for the detection of water in solvents (Figure 13.11a). Indeed, 9-MP-BF3 was prepared by the reaction of 9-MP with BF3-OEt2 and its photoabsorption and fluorescence spectral measurements in acetonitrile containing various concentrations of water and the 1 H NMR spectral measurements with and without the addition of water in the solution have been performed.25 It was found that in the low-water-content

Fluorescent Sensors for Water

Figure 13.11

321

(a) Proposed mechanisms of the colorimetric and ratiometric fluorescent sensor 9-MP-BF3 for the detection of water in solvent. Photographs are color (left) and fluorescence color (right) images of the acetonitrile solutions with varying amounts of water. (b) Absorbance at 290 nm, 305 nm, 360 nm, and 390 nm of 9-MP-BF3 as a function of water content (0.043–80 wt%) in acetonitrile. (c) Fluorescence peak intensities (lex ¼ 323 nm) at 370 nm and 460 nm of 9-MP-BF3 as a function of water content (0.043–80 wt%) in acetonitrile. Reproduced from ref. 25 with permission from the Royal Society of Chemistry.

region below 2.1 wt%, the photoabsorption and fluorescence spectral changes with isosbestic and isoemissive points were observed. That is, a new photoabsorption band at around 360 nm and a fluorescence band at around 370 nm gradually appeared with a simultaneous decrease in the photoabsorption band at around 390 nm and the fluorescence band at around 460 nm originating from 9-MP-BF3, which could be attributed to the dissociation of 9-MP-BF3 into 9-MP by water molecules (Figure 13.11b,c). In the moderate-water-content region from 2.1 wt% to 40 wt%, the photoabsorption band at around 360 nm and the fluorescence band at around 370 nm gradually shifted to a longer wavelength region with an increase in the fluorescence intensity, which could be ascribed to the formation of a hydrogen-bonded complex (9-MP-H2O) with water molecules. Furthermore, in the high-water-content region from 40 wt% to 80 wt%, two photoabsorption bands at around 305 nm and 390 nm and one fluorescence band at around 460 nm gradually reappeared with a simultaneous decrease in the photoabsorption band at around 290 nm and the fluorescence band at around 370 nm, which was attributed to the formation of a hydrogen-bonded proton transfer complex (9-MP-H1) with water

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molecules. The DL value of 9-MP-BF3 for the acetonitrile solution in the lowwater-content region below 2.1 wt% was 0.25 wt%, which was inferior to those of the reported ICT-based4–6 and PET-based fluorescent sensors (Table 13.1).9–16 However, this work clearly demonstrated that the pyrido[3,4-b]indole–BF3 complex can act as a colorimetric and ratiometric fluorescent sensor for the detection of water over a wide range from low water content to high water content in solvents. Furthermore, a julolidine-structured pyrido[3,4-b]indole–BF3 complex ET-1-BF3 having a strong electron-donating julolidine (quinolizidine) moiety as an ICT-based colorimetric and fluorescent sensor for water was developed (Figure 13.12).26 It was found that in acetonitrile, ET-1-BF3 can respond to water differently depending on the content, accompanying gradual color and fluorescence changes. In the range of low water content below ca. 10 wt%, ET-1-BF3 releases BF3 to generate ET-1, and ET-1 forms the hydrogen-bonded complex with one water molecule (ET-1-H2O). At higher water contents over ca. 10 wt%, the hydrogen-bonded PTC (ET-1-H1) is gradually generated. Compared with 9-MP-BF3, for ET-1-BF3, the sensitivity and spectral response to water are significantly improved over a wide concentration range, which is attributed to the fact that the electron-donating julolidine moiety enhances the basicity of the pyridinic nitrogen atom and the ICT characteristic. Indeed, the yellow color of the ET-1-BF3 solution faded away with increasing water content due to the release of BF3 and the subsequent formation of ET-1-H2O, followed by the restoration of the yellow color as a result of the ET-1-H1 formation. The fluorescence color also changed in the order of light blue, blue, and green, together with the abovementioned color changes. Meanwhile, the focus was on D–p–A-type pyridine–BF3 complexes as ICT-based colorimetric and fluorescent sensors for water, and thus a D–(p–A)2-type pyridine–BF3 complex YNI-2-BF3 composed of a carbazole skeleton as the donor moiety and two pyridine–BF3 units as the acceptor moiety was developed (Figure 13.13a).24 As a result, the addition of water to the acetonitrile solution of YNI-2-BF3 causes the blueshift of photoabsorption from 450 nm to 360 nm and the enhancement of fluorescence intensity at around 485 nm in the low-water-content region, which is attributed to the change in the ICT characteristics due to the dissociation of YNI-2-BF3 into YNI-2 (Figure 13.13b,c). Furthermore, a redshift of fluorescence bands by ca. 10 nm with the decrease in fluorescence intensity in the high-water-content region was observed, and it is attributed to the formation of the hydrogenbonded PTC (YNI-2-H2O) with water molecules as well as the solvatofluorochromic properties of YNI-2 (Figure 13.13d). Indeed, the color of YNI-2-BF3 in acetonitrile is yellow. Upon the addition of water, the color changed from yellow to colorless due to the dissociation of YNI-2-BF3 into YNI-2 (Figure 13.13a). The ms and DL values of YNI-2-BF3 for the acetonitrile solution in the water content region below 3.0 wt% were 56 and 0.06 wt%, respectively, which are equivalent to those of the reported PET-based4–6 and ICT-based9–16 fluorescent sensors (Table 13.1). Moreover, the plot of the fluorescence intensity at ca. 485–495 nm versus the water fraction in the

Fluorescent Sensors for Water

Figure 13.12

Proposed mechanisms of the colorimetric and ratiometric fluorescent sensor ET-1-BF3 for the detection of water in solvent. Photographs are color (left) and fluorescence (right) images of the acetonitrile solutions with varying amounts of water. Reproduced from ref. 26 with permission from the Royal Society of Chemistry.

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3-, 5-, and 8-positions and the BODIPY core. ST-3-BF3 exhibited a characteristic fluorescence band originating from the BODIPY skeleton at around 730 nm. It was found that by the addition of a trace amount of water to the acetonitrile solution of ST-3-BF3, the photoabsorption band at around 415 nm and the fluorescence band at around 730 nm increased linearly as a function of water content below only 0.2 wt%, which could be ascribed to the change in the ICT characteristics due to the dissociation of ST-3-BF3 into ST-3 by water molecules (Figure 13.14b,c). Consequently, the presented works confirm that D–A or D–p–A-type pyridine–BF3 complex is one of the most promising ICT-based colorimetric and ratiometric fluorescent sensors for the detection of water in the low-, moderate-, and high-water-content regions in solvents.

13.7 Fluorescent Sensor-doped Polymer Films From 2020 to the present, a novel coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that causes the Coronavirus Disease 2019 (COVID-19), dramatically changed the world to give people a sense of fear of death. Infectious viruses are generally released into the atmosphere through droplet spread from coughing and sneezing by an infected person. Thus, the infection route from an infected person to an uninfected person is predominantly via the droplet. Actually, face shields made of polyester or polycarbonate films and partitions made of acrylic resin are commercially available for reducing the risk of droplet infection. Therefore, if we can visually confirm the droplet on the face shields and partitions, this allows us to accurately remove the viruses by wiping away the droplet. However, because the virus-containing droplet is generally 5 mm or more, it is practically difficult for us to visually confirm the droplet. Meanwhile, over 90% of the droplet is composed of water, and thus techniques and methods capable of visualizing water are undoubtedly useful for detecting the viruscontaining droplet. Thus, in order to develop fluorescent polymeric materials for the visualization and detection of water, the authors have achieved the preparation of various types of polymer films (polystyrene [PS], poly(4-vinylphenol) [PVP], polyvinyl alcohol [PVA], and polyethylene glycol [PEG]), which were doped with PET-type fluorescent sensor OF-2 at 50 wt%, and investigated the optical sensing properties of the OF-2-doped polymer films before and after exposure to moisture.29 For example, as shown in Figure 13.15a,b, the as-prepared OF-2-doped PS film (in dry process) shows a vibronically structured photoabsorption band in the range of 300–400 nm and a feeble and broad fluorescence band in the range of 400–600 nm attributable to the excimer emission originating from the anthracene skeleton in the PET active state. When the OF-2-doped PS films were exposed to moisture (in wet process), the photoabsorption spectral shape did not undergo appreciable changes, whereas the fluorescence spectra underwent a change in the spectral shape to the vibronically structured monomer emission

Fluorescent Sensors for Water

Figure 13.15

327

(a) Photoabsorption and (b) fluorescence spectra (lex ¼ 366 nm) of spincoated PS film with 50 wt% OF-2 before (in dry process) and after (in wet process) exposure to moisture. Photographs (under 254 nm irradiation) of 50 wt% OF-2-doped PS film before and after (c) exposure to moisture and (d) water droplet. Inset in (b) shows reversible switching of fluorescence intensity at around 470 nm in dry process and at 415 nm in wet process of 50 wt% OF-2-doped PS film. Reproduced from ref. 29 with permission from the Royal Society of Chemistry.

at 415 nm arising from the PET inactive state. In fact, one can see that an as-prepared OF-2-doped PS film initially exhibits the green excimer emission in the PET active state, but the blue monomer emission in the PET inactive state upon exposure to moisture or by water droplet (Figure 13.15c,d). For the OF-2-doped PS, PVP, PVA, and PEG films during the wet–dry repeated cycles, the photoabsorption spectral shape did not undergo appreciable changes, and the corresponding fluorescence spectra show a change in spectral shape from the excimer emission to the monomer emission before and after exposure to moisture. Actually, the dry–wet cycles of the OF-2-doped PS films show that a reversible switching in fluorescent intensity between the excimer and monomer emissions was still observed in the third dry–wet process (Figure 13.15b inset). Consequently, it was found that PET-type fluorescent sensor-doped polymer films exhibit a reversible switching in fluorescent color between the excimer emission in the PET active state under a drying process and the monomer emission in the PET inactive state upon exposure to moisture or by water droplet. It is proposed that polymer films doped with fluorescent sensors for water are convenient functional materials for visualizing the virus-containing droplet.

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13.8 Conclusion It was demonstrated that the fluorescence analysis for the determination of water based on PET, FRET, ICT, AIEE, or SFC characteristics of organic dyes is one of the most promising fluorescence enhancement and the colorimetric and ratiometric fluorescence systems for the detection and quantification of water in the low-, moderate-, and high-water-content regions in solvents. However, the DL value for water by using the Karl Fischer coulometric titration method is a few ppm (o0.001 wt%). Thus, much effort is necessary to develop highly sensitive fluorescent sensors for water to further improve the fluorescence analysis. As is well known, on the other hand, organic dyes, which have the potential to be functional materials, have promise for application in optical sensors and probes for environmental, biomedical, and quality control monitoring systems, as well as photosensitizers and emitters for optoelectronic devices. Actually, it was mentioned that polymer films doped with PET-type fluorescent sensors for water based on a fluorescence enhancement (turn-on) system are one of the most promising and convenient functional materials for visualizing moisture and water droplets. Thus, as a future perspective in the study of organic fluorescent sensors for water, developing functional dye materials such as fluorescent sensor–doped polymer films and fluorescent polymer sensors for water would be required for practical use in the detection and quantification of water in products as well as solutions, solids, and gases or water on material surfaces.

Acknowledgements This work was supported by Grants-in-Aid for Scientific Research (B) from the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 19H02754 and by the JST Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP) Grant Number JPMJTM20RB.

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34. N. Zhao, Z. Yang, J. W. Y. Lam, H. H. Y. Sung, N. Xie, S. Chen, H. Su, M. Gao, I. D. Williams, K. S. Wong and B. Z. Tang, Chem. Commun., 2012, 48, 8637. ´n, A. S. Coronilla and M. A. Mun ˜oz, J. Phys. Chem. A, 35. C. Carmona, M. Balo 2004, 108, 1910. ˜oz and M. Balo ´n, J. Fluoresc., 36. A. S. Coronilla, C. Carmona, M. A. Mun 2009, 19, 1025. ´nchez-Coronilla, M. Balo ´n, M. A. Mun ˜ oz and 37. J. Hidalgo, A. Sa C. Carmona, Photochem. Photobiol. Sci., 2009, 8, 414. 38. T. Enoki, K. Matsuo, J. Ohshita and Y. Ooyama, Phys. Chem. Chem. Phys., 2017, 19, 3565.

CHAPTER 14

Photoredox Chemistries of Cyclometalated Ir(III) Complexes YOUNGMIN YOU Division of Chemical Engineering and Materials Science, and Graduate Program in System Health Science and Engineering, Ewha Womans University, 03760 Seoul, Republic of Korea Email: [email protected]

14.1 Photoinduced Electron Transfer of Cyclometalated Ir(III) Complex Cyclometalated complexes of Ir(III) exhibit electrochemical silence in their ground states. The inertness is due in part to their large bandgap energies, which necessarily locate oxidation and reduction potentials of the complexes incapable of facilitating ground-state electron transfer. In contrast, photoexcitation provides a thermodynamic allowance for electron transfer. The excited-state electron transfer of cyclometalated complexes is valuable, as its thermicity and kinetics are widely tunable by synthetic means. In addition, the strong cyclometalation in the complex effectively suppresses the disruption of the metal–ligand bond during the course of cyclic electron transfer. These favorable features have stimulated the development of a variety of sensors and photoredox catalysts based on cycloiridated complexes.1–6 This chapter introduces the excited-state redox processes of cyclometalated Ir(III) complexes. The complexes are chosen due to their ability to mediate a variety of photoredox chemistries. The first part of this chapter provides general knowledge about the photoinduced electron transfer of molecules. Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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In contrast, photoexcitation enables electron transfer. Photoexcitation of D promotes one electron transition from HOMO to LUMO of D, generating two singly occupied molecular orbitals (SOMOs). The upper and the lower orbitals are denoted as the higher SOMO (HSOMO) and the lower SOMO (LSOMO), respectively. The HSOMO energy corresponds to the excited-state oxidation potential (E*ox), whereas the LSOMO energy is the excited-state reduction potential (E*red). In Figure 14.1, the electron in HSOMO of D* can be transferred to the LUMO of A. It is worth noting that photoexcitation reverses the redox roles of the molecular orbitals of D. The E*ox and E*red can be expressed as eqn (14.2) and (14.3), respectively: E*ox ¼ Eox  DEg

(14.2)

E*red ¼ Ered þ DEg

(14.3)

In eqn (14.2) and (14.3), DEg is the bandgap energy of D. By combining eqn (14.1)–(14.3), one can obtain –DGeT for photoinduced electron transfer involving D* and A* (eqn (14.4) and (14.5)). DGeT ¼ e  [E*ox(D)  Ered(A)] ¼ e  [Eox(D)  DEg(D)  Ered(A)]

(14.4)

DGeT ¼ e  [Eox(D)  E*red(A)] ¼ e  [Eox(D)  Ered(A) þ DEg(A)]

(14.5)

It should be noted that the geminate radical ion pair (i.e., {D 1  A }) is stabilized by Coulomb attraction (the C term in Figure 14.1) as much as e2/4per, where e and r are the dielectric constant of media and the distance between D and A. Therefore, DGeT can be expressed as eqn (14.6) and (14.7):   e2 DGeT ¼ e  Eox ðDÞ  DEg ðDÞ  Ered ðAÞ  4per   e2 DGeT ¼ e  Eox ðDÞ  Ered ðAÞ þ DEg ðAÞ  4per

(14:6) (14:7)

These relationships enable one to predict an occurrence of photoinduced electron transfer: photoinduced electron transfer is allowed when DGeT40 and is forbidden when DGeT o0. The knowledge of DGeT also permits an estimation of the kinetics of electron transfer. Marcus developed the classical electron transfer theory that relates DGeT and the rate constant of electron transfer (k0eT) (eqn (14.8)):10–13   kB T ðDGeT þ lÞ2 0 (14:8) keT ¼ exp  4lkB T h In eqn (14.8), kB is the Boltzmann constant, T is the absolute temperature, h is the Plank constant, and l is the reorganization energy for electron transfer. The two thermodynamic parameters, DGeT and l, determine the rate of electron transfer. An interesting feature of eqn (14.8) is the exponential dependence of k0eT on the square of DGeT. In addition, l appears in both the numerator and the denominator of the exponential term. If l4|DGeT|, k0eT will increase with |DGeT|. On the other hand, k0eT decreases with |DGeT| when

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lo|DGeT|. The former case is more broadly found; therefore, the electron transfer becomes faster at a large driving force for electron transfer.14,15 l involves nuclear reorganization of molecules, which is the energy difference before and after the electron transfer between the D and A pair. The internal reorganization energy is usually estimated by using quantum chemical calculations. Reorganization of solvent molecules around the D and A pair also contributes to l. This solvent reorganization energy (lS) can be expressed as eqn (14.9):     ðDeÞ2 1 1 1 1 1   (14:9) þ   lS ¼ eop eS 4pe0 2rD 2rA d In eqn (14.9), De is the amount of transferred charge (i.e., De ¼ e), e0 is the dielectric constant in a vacuum, rD and rA are the van der Waals radii of electron donor and acceptor molecules, d is the contact distance, eop is the optical dielectric constant, and eS is the dielectric constant of the solvent. The above-described electron-transfer theory does not account for the electronic structure of a molecule. Several semiclassical methods have been established to overcome the limitation of the classical Marcus theory. Jortner established a modified model of electron transfer.16 The Jortner theory considers internal reorganization which proceeds through the highfrequency vibrational modes of a molecule. This semiquantum chemical approach avoids overprediction of the Marcus-inverted region of electron transfer where k0eT decreases with |DGeT| (eqn (14.10)): 0 keT ¼

2p J2  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h 4plout kB T "  n    # 1 X lin lin 1 ðDGeT þ lin þ nhoÞ2  exp     exp  ho  ho 4lout kB T n!  n¼0

(14:10)

In eqn (14.10), J2 is taken as 5.0105 eV, and h  o is approximately 0.21 eV, which is typical for CQC stretching in aromatic hydrocarbons. lout is equal to lS (eqn (14.9)). In fluid solution, heterobimolecular electron transfer is diffusion limited: experimental electron transfer cannot be greater than a diffusion rate of solvent molecules. The rate constant for diffusion (kdiff) can be expressed as in eqn (14.11): kdiff B kdiff ¼

8kB NA T 3Z

(14:11)

In eqn (14.9), NA and Z are the Avogadro’s number and viscosity of the solvent, respectively. The observed electron-transfer rate levels off, as shown in eqn (14.12):   kB T ðDGeT þ lÞ2 kdiff  exp  0 kdiff  keT 4lkB T h ¼ (14:12) keT ¼   0 kdiff þ keT kB T ðDGeT þ lÞ2 kdiff þ exp  4lkB T h

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the original D species. If diffusion is faster than charge recombination, the radical ion species will be released into the bulk. These radical species are strong oxidants (D 1) and reductants (A ), so they initiate radical reactions of substrates. This irreversible process can be employed as a useful principle for photoredox catalysis.19–31

14.2 Electronic Structures of Cyclometalated Complexes of Ir(III) Cyclometalated Ir(III) complexes are appealing platforms for the utilization of photoinduced electron transfer. To fully exploit the potential of Ir(III) complexes, it is essential to understand their electronic structures. Figure 14.2a depicts the chemical structure of fac-[Ir(ppy)3] (ppy ¼ 2-phenylpyridinato), an archetype of cyclometalated Ir(III) complexes. fac-[Ir(ppy)3] possesses the trivalent Ir core and three monoanionic bidentate ligands, and thus, it is overall charge-neutral. The HOMO of fac-[Ir(ppy)3] is delocalized over the t2g orbital of Ir and the p orbital of the phenylide moiety of ppy. The LUMO is localized exclusively within the p* orbital of the pyridine moiety of ppy. This spatial disparity in the frontier orbital topologies leads to a multitude of electronic transitions. As shown in Figure 14.2b, four electronic transitions are possible. The metalcentered (MC) transition and the ligand-to-metal charge-transfer (LMCT) transition are usually inaccessible; therefore, they are neglected in the

Figure 14.2

(a) Chemical structure of fac-[Ir(ppy)3], a representative triscyclometalated Ir(III) complex, and its frontier molecular orbitals. (b) Electronic transitions of fac-[Ir(ppy)3]. Note that [IrIIIL3] denotes the general structure of fac-[Ir(ppy)3]. (c) Excited states of fac-[Ir(ppy)3]. The general structures show the valence change upon the formation of the state. Refer to the main text for the symbols.

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photophysical processes of cyclometalated Ir(III) complexes. Photoexcitation typically generates the ligand-centered (LC) transition state and the metal-toligand charge-transfer (MLCT) transition state. Large spin–orbit coupling provided by the central Ir atom facilitates ultrafast intersystem crossing from the singlet MLCT (1MLCT) transition state to the triplet MLCT (3MLCT) transition state. There are three substates of 3MLCT (GI, GII, and GIII), with different electronic energies. The GI substate is mixed strongly with the closely located singlet LC (1LC) transition state and gains substantial intensities for luminescent relaxation. This intensity borrowing actually corresponds to a perturbation between the 3MLCT and 1LC states; therefore, the luminescent state is best described as a linear combination of the 3MLCT and 1LC states (i.e., C ¼ aC(1LC) þ bC(3MLCT)).32–35 It should be noted that the four electronic transition states bear different oxidation states of Ir and ligands. The MLCT transition state of fac-[Ir(ppy)3] can be approximated as [IrIV(ppy) (ppy)2]* having the highly oxidizing Ir(IV) species and the highly reducing radical anionic species of the ppy ligand. In contrast, localized states, such as LC, do not experience changes in the formal oxidation states of the constitutional components. Nevertheless, the redox silence does not necessarily imply the inability of photoinduced electron transfer, because the HSOMO (p* orbital) and LSOMO (p orbital) localized within the ppy ligand can facilitate electron transfer with vicinal molecules. The delocalization of HOMO and the mixed nature of the lowest triplet state of an Ir(III) complex permits effective control over excited-state electrochemical potentials. Figure 14.3 summarizes the E*ox and E*red values of several cyclometalated Ir(III) complexes. E*red values of Ir complexes vary in the range of 0.61–1.47 V vs. saturated calomel electrode (SCE). E*ox values are located in the range of 0.99 to 1.85 V vs. SCE. Note that the electrochemical window of a cyclometalated Ir(III) complex, which is the difference between E*ox and E*red, is greater than that of [Ru(bpy)3]21. The wide electrochemical window is beneficial for achieving a large driving force for electron transfer. It is also found that the electrochemical potential depends sensitively on the chemical structures of ligands. For example, E*ox shifts anodically upon introducing electron-withdrawing groups at the phenylide moiety, whereas electron-donating groups exert an opposite effect. Ir(III) complexes having high-field N-heterocyclocarbenic ligands exhibit very negative E*ox despite the large stabilization of the t2g orbital.

14.3 Sensory Applications of Intramolecular Photoinduced Electron Transfer of Ir(III) Complexes Intramolecular photoinduced electron transfer serves successfully as the key principle of creating luminescent sensors. The molecular construct of the sensors is usually based on a dyad that consists of a luminophore and a receptor. The receptor functions as an electron donor or an electron

Photoredox Chemistries of Cyclometalated Ir(III) Complexes

Figure 14.3

Excited-state electrochemical potentials of Ir(III) complexes and [Ru(bpy)3]21.

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acceptor. In the absence of a target analyte, photoinduced electron transfer occurs between the luminophore and the receptor to quench the emission of the former. Analyte binding at the receptor suppresses the photoinduced electron transfer to restore the inherent emission of the luminophore. The overall response is, thus, luminescence turn-on. The strong room-temperature phosphorescence of cyclometalated Ir(III) complexes has stimulated the development of turn-on phosphorescence sensors of biological analytes.5,36–42 The first phosphorescence zinc probe is based on a heteroleptic biscyclometalated Ir(III) complex having a di(2-picolyl)amino (DPA) zinc receptor at the 1,10-phenanthroline ligand (Figure 14.4).43 The zinc probe displays negligible phosphorescence emission in the absence of zinc, but the addition of zinc promptly turns on the phosphorescence emission. The formal oxidation state of the central Ir in the MLCT excited state is þ4, which is susceptible to reduction by electron

Figure 14.4

Mechanism of phosphorescence turn-on sensors of zinc. Top image: Reproduced from ref. 43 with permission from American Chemical Society, Copyright 2011. Bottom image: Reproduced from ref. 44 with permission from American Chemical Society, Copyright 2013.

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transfer from the tertiary amino unit in DPA. The electron transfer produces an intramolecular charge-separated species that is nonphosphorescent. Fast charge recombination through back electron transfer from the one-electronreduced ligand to the one-electron-oxidized DPA restores the original sensor. Zinc binding at the DPA receptor abrogates the photoinduced electron transfer. This abrogation enhances the MLCT phosphorescence emission to produce turn-on responses to zinc. Systematic studies revealed that the occurrence of photoinduced electron transfer is governed by the thermodynamic condition, DGeT40.44 As shown in Figure 14.5, E*red of heteroleptic biscyclometalated Ir(III) complexes increases when the electron density of the cyclometalating ligand decreases. Since Eox of DPA is 1.28 V vs. SCE, the positive driving force for photoinduced electron transfer (i.e., DGeT40) can be obtained when E*red of the Ir complex is greater than 1.28 V vs. SCE. Actually, among the tested Ir(III) complexes shown in Figure 14.5, only three complexes having E*red41.28 V vs. SCE show turn-on responses to zinc. This understanding facilitates the further development of phosphorescent zinc probes. An optimized complex having the greatest DGeT could visualize intracellular zinc ion. One unique benefit of phosphorescence zinc sensors over fluorescence sensors is their ability to perform time-gated imaging. The long-lived phosphorescence emission can easily be discriminated from background noises that contain autofluorescence. Since the autofluorescence is relatively short lived, allowing time delays for image acquisition after photoexcitation can ultimately eliminate the background noises. Figure 14.6 demonstrates the lifetime imaging ability of a phosphorescence zinc probe. The total images are substantially contaminated by short-lived autofluorescence from HeLa cells (t2 and t3 images). Long-lived t1 images represent the true zinc signals. The phosphorescence turn-on mechanism can be further employed to create redox-active metal ions. Chromium(III) ion is a soft metal, so it binds to a receptor that contains soft thioether units. A phosphorescent probe having the Cr(III) receptor shows weak green emission in the absence of chromium ion (Figure 14.7).45 The Cr(III) binding produces turn-on phosphorescence emission, with an emission color change from green to yellow. The turn-on response can be explained on the basis of chromium binding-induced abrogation of photoinduced electron transfer from the receptor to the phosphorescent Ir complex unit. Very interestingly, the phosphorescence emission returns slowly to green when the solution is exposed to air. The second ratiometric phosphorescence response results from O2 activation of the bound chromium ion. Presumably, Cr(III) is activated to highly oxidizing Cr(IV)QO, which oxidatively cleaves the benzyl linker between the receptor and the Ir(III) complex. The dualstage ratiometric phosphorescence responses are selective to chromium ion, which enables high-fidelity identification of chromium. Intramolecular photoinduced electron transfer is suppressed when E*red of Ir complexoEox of a metal ion receptor. This is actually the case when cyclometalating ligands are electron rich. The heteroleptic triscyclometalated

342 Thermodynamic allowance for the intramolecular photoinduced electron transfer from the DPA zinc acceptor to the excitedstate Ir complex. Reproduced from ref. 44 with permission from American Chemical Society, Copyright 2013.

Chapter 14

Figure 14.5

Photoredox Chemistries of Cyclometalated Ir(III) Complexes

Figure 14.6

(a) Photoluminescence lifetime imaging micrographs of HeLa cells pretreated with the phosphorescence zinc probe before (bottom) and after (top) zinc enrichment. t1 panels (b) correspond to phosphorescence signals. t2 (c) and t3 (d) signals are due to autofluorescence. (e) Amplitude plots of x-scan for gray regions in image (b). Reproduced from ref. 43 with permission from American Chemical Society, Copyright 2011.

343

344 (a) Dual-responsive phosphorescence detection of chromium ion. Transient photoluminescence signals of the phosphorescence probe in the absence (b) and presence (c) of chromium(III) ion, and after subsequent incubation under an aerobic condition (d). Reproduced from ref. 45 with permission from John Wiley & Sons, Copyright r 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Chapter 14

Figure 14.7

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345

Ir(III) complex shown in Figure 14.8 does not suffer from phosphorescence quenching even in the absence of metal ions because photoinduced electron transfer from DPA is inherently forbidden.46 The inactivation of electron transfer is ascribed to the presence of three electron-rich monoanionic cyclometalating ligands. The two different chromophoric ligands, 2-phenylpyridinate and 2-(2-benzothienyl)pyridinate, produce strong green and red emissions. Copper(II) binding at DPA preferentially quenches the red

Figure 14.8

Top, phosphorescence ratiometric visualization of intracellular copper ions. Middle, photoluminescence wavelength–photoexcitation wavelength contour plots of the phosphorescence probe in the absence (left) and presence (right) of CuCl2. Bottom, photoluminescence ratiometric images of HeLa cells pretreated with the phosphorescence Cu(II) probe before (top) and after (bottom) exogenous supply of CuCl2. Adapted from ref. 46 with permission from American Chemical Society, Copyright 2011.

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phosphorescence of 2-(2-benzothienyl)pyridinate, while the green emission of 2-phenylpyridinate remains relatively unperturbed. The selective phosphorescence quenching by Cu(II) ion is presumably due to electron transfer to the oxidizing Cu(II) center from the vicinal 2-(2-benzothienyl)pyridinate. The ratiometric phosphorescence response to copper ions is unique and is applicable to estimations of intracellular copper levels. There is a challenge in the creation of luminescence probes based on the photoinduced electron transfer mechanism: the driving force for electron transfer decreases as the bandgap energy of a luminophore decreases, according to eqn (14.7) (in this case, A is the luminophore). As schematically compared in Figure 14.9, a blue-emissive Ir(III) complex possesses E*red more positive than Eox of a receptor, which facilitates photoinduced electron transfer. In contrast, E*red of a red-emissive complex is frequently located above Eox of a receptor due to its small bandgap energy. This electrochemical disposition makes DGeT negative. The decrease in DGeT becomes more problematic for phosphorescent molecules; the phosphorescent state is in triplet, so it is lower in energy than the singlet excited state by an exchange energy (Figure 14.9c). The loss by the exchange energy further decreases E*red, suppressing photoinduced electron transfer. One viable approach to circumventing this problem is the use of a receptor having a small (i.e., cathodically shifted) Eox value. Synthetic approaches to achieving this goal would involve providing a distance between a receptor and the positively charged chromophoric ligand. The zinc probe shown in Figure 14.10 consists of a biscyclometalated Ir(III) complex and a DPA receptor.47 DPA is tethered to the 1-pyridylpyrazine ligand. An amide linker is present between the two units, which alleviates the through-space electronwithdrawal effect. This electronic control enables intramolecular photoinduced electron transfer at a small bandgap energy. The phosphorescence probe successfully shows turn-on responses in the orange-to-red emission regions. This part has outlined the sensing utility of intramolecular photoinduced electron transfer of Ir complexes. Analyte binding suppresses or activates electron transfer, resulting in changes in phosphorescence intensities. The mechanism is highly valuable, as it is fully reversible and operates at high rates. These properties are essential for real-time monitoring of fluctuations of analyte concentrations. It is anticipated that electrochemical control will expand the available library of luminescence probes.

14.4 Photoredox Catalysis Based on Intermolecular Photoinduced Electron Transfer of Ir(III) Complexes Photoinduced electron transfer between two different molecules produces a geminate radical ion pair. This ion pair can dissociate in polar solvents, facilitating radical reactions. The photoredox catalysis has gathered

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(a)

(b)

(c)

Figure 14.9

Challenges in the design of metal ion sensors displaying turn-on phosphorescence responses in the red emission regions. (a) Cyclic mechanism for the turn-on detection of metal ions by inhibition of photoinduced electron transfer. (b) Comparison of the driving forces for photoinduced electron transfer for blue- (left) and red-emissive (right) phosphorescence sensors. (c) Comparison of the driving forces for photoinduced electron transfer for fluorescence (left) and phosphorescence (right) sensors. Reproduced from ref. 47 with permission from American Chemical Society, Copyright 2015.

348

Figure 14.10

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Detection of zinc ions using a red-phosphorescent probe (ZIrdap). Reproduced from ref. 47 with permission from American Chemical Society, Copyright 2011.

enormous research interest because the excited-state electron transfer exhibits strong redox power than that of conventional ground-state redox catalysis. Contrary to conventional single-electron redox agents, photoredox catalysts combined with sacrificial electron sources or traps can be easily handled. In addition, photoredox catalytic reactions do not require high temperatures and multiple uses of auxiliary catalysts and supporting ligands. Cyclometalated Ir(III) complexes have found great utility for photoredox catalytic applications. This utility is attributed to the broad electrochemical window of excited-state Ir complexes. For example, fac-[Ir(ppy)3] has the excited-state electrochemical window of 2.23 eV, which is greater than that (1.58 eV) of [Ru(bpy)3]21 (Figure 14.3). Note that fac-[Ir(ppy)3] can still be excited under photoirradiation with visible lights. The wide electrochemical window of fac-[Ir(ppy)3] is due to the efficient stabilization and destabilization of the Ir-centered t2g and eg orbitals, respectively, by cyclometalating ligands. Cyclometalated Ir(III) complexes are also highly stable, which enables one to employ organic synthesis techniques, such as silica gel column chromatography. These favorable properties stimulate evaluations of the photoredox catalysis performances of cyclometalated Ir(III) complexes. This part showcases several examples of photoredox catalysis of cyclometalated Ir(III) complexes. The first two examples illustrate the archetypical photoredox catalytic generation of alkyl radicals. Then, the merged catalysis consisting of a cyclometalated Ir(III) complex catalyst and a redox-active Pd(II) or Co(II) catalysts are introduced. The last example shows a unique catalysis system that recycles both photons and electrons.

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Figure 14.11a shows the photoredox catalytic difluoroalkylation of arenes mediated by fac-[Ir(ppy)3].48 Visible-light photoirradiation of fac-[Ir(ppy)3] promotes the MLCT transition, which produces the radical anion of the ppy ligand. This radical anionic moiety is capable of one-electron transfer to BrCF2CO2Et to reductively cleave the C–Br bond. The resulting  CF2CO2Et radical species can be added to an arene substrate. Radical–polar conversion of the difluoroalkylated radical adduct occurs through electron transfer to the one-electron-oxidized fac-[Ir(ppy)3] catalyst. Finally, the positively charged difluoroalkylated arene can be neutralized through deprotonation by KOt-Bu. The overall photoredox cycle involves photoinduced electron transfer to BrCF2CO2Et and ground-state neutralization by the aryl radical. The Brønsted–Lowry base, KOt-Bu, is essential for product formation. The key step in the photoredox catalysis cycle is the oxidative quenching of the MLCT state of fac-[Ir(ppy)3] by BrCF2CO2Et. This oxidative electron transfer competes with the inherent phosphorescence process of the MLCT state. The rate of the electron transfer can, thus, be determined by monitoring the phosphorescence lifetime. As shown in Figure 14.11c, the phosphorescence lifetime decreases with the added concentration of BrCF2CO2Et. A pseudofirst-order plot yields the bimolecular rate constant for the electron transfer. This technique is also valuable in the identification of quenching species. The invariance of the phosphorescence lifetime excludes the reductive quenching of the MLCT state of fac-[Ir(ppy)3] by a single electron donor, such as N,N,N 0 ,N 0 -tetramethylethylenediamine (TMEDA) (Figure 14.11c). The photoredox catalytic radical generation has found unique utility in the dehalogenation and coupling reactions. The MLCT state of [Ir(ppy)2(dtbbpy)]1 (dtbbpy ¼ 4,4 0 -di(t-Bu)-2,2 0 -bipyridine) can be reduced by one-electron transfer from Hantzsch ester (Figure 14.12).49 The resultant one-electron-reduced complex readily donates its extra electron to benzyl bromide, which subsequently cleaves the labile C–Br bond to yield benzyl radical. Of interest is the reaction of the one-electron-oxidized Hantzsch ester. This species is deprotonated by K3PO4 to a neutral yet unstable radical species. The neutral radical species of Hantzsch ester can donate one electron to the MLCT state of [Ir(ppy)2(dtbbpy)]1 or directly to benzyl bromide. In either case, benzyl radical can be generated. The latter electron transfer is of particular interest because the overall photoredox cycle corresponds to one-photon-induced two-electron catalysis. Finally, the benzyl radical can abstract hydrogen atom to be converted to toluene. Alternatively, homocoupling of two benzyl radicals occurs to yield bibenzyl. The majority of photoredox catalytic organic transformations involve oneelectron transfer. To overcome the limitation of electron stoichiometry, research efforts have been made to combine conventional transition metal catalysts with photoredox catalysts. Transition metals that steer two-electron redox cycles have been extensively tested. One illustrative example is shown in Figure 14.13.50 Pd(II) species can catalyze C–N coupling reactions. The intramolecular C–N coupling of 2-aminobiphenyl can produce carbazole,

350

Figure 14.11

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(a) Photoredox catalytic difluoroalkylation of arenes. Adapted from ref. 48 with permission from John Wiley & Sons, Copyright r 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Photophysical mechanism involving the photoredox catalysis, fac-[Ir(ppy)3]. (c) Phosphorescence titration experiments to investigate photoinduced electron transfer interactions involving fac-[Ir(ppy)3].

Photoredox Chemistries of Cyclometalated Ir(III) Complexes Photoredox catalytic debrominative generation of benzyl radical and its conversions to debromination and homocoupled products. Adapted from ref. 49 with permission from John Wiley & Sons, Copyright r 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

351

Figure 14.12

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Figure 14.13

Chapter 14

(a) Merged photoredox catalysis for the synthesis of carbazole compounds. (b) Plausible mechanism of the merged catalysis. Adapted from ref. 50 with permission from American Chemical Society, Copyright 2015.

which serves as a valuable building block for organic electronics materials. Visible-light illumination of a N-protected 2-aminobiphenyls yields carbazole products in the presence of Pd(OAc)2 and [Ir(dFppy)2(phen)]1 (dFppy ¼ 2-(2,4-difluorophenyl)pyridinate; phen ¼ 1,10-phenanthroline). The catalytic generation of carbazole begins with the coordination of Pd(II) to the amino moiety of N-protected 2-aminobiphenyl. Deprotonation of the amino unit and subsequent C–H activation yield the palladacycle of 2-aminobiphenyl. Transient photoluminescence experiments revealed that the palladacycle is capable of transferring one electron to the MLCT state of [Ir(dFppy)2(phen)]1. The resultant Pd(III) species has two mechanistic pathways: one is reductive elimination of the carbazole product (path A in Figure 14.13b), and the other is further oxidation by the MLCT state of

Photoredox Chemistries of Cyclometalated Ir(III) Complexes 1

353

[Ir(dFppy)2(phen)] (path B in Figure 14.13b). The latter pathway furnishes a Pd(IV) species, which readily facilitates reductive elimination to Pd(II) and carbazole. In path A, the Pd(I) species can be oxidized to Pd(II) by the photoinduced electron transfer to [Ir(dFppy)2(phen)]1. Regardless of the pathways, the photoredox cycle involving reductive quenching of [Ir(dFppy)2(phen)]1 operates twice per carbazole production. Note that the double electron transfer occurs sequentially. The sequential two-electron transfer is not ideal in terms of the entropy point of view. Therefore, there is a strong need to develop a photosensitizer capable of transferring two electrons simultaneously. Another interesting merged catalysis is shown in Figure 14.14. Planar complexes of Co(II), such as vitamin B12, can be one-electron-reduced to a Co(I) species by the MLCT state of [Ir(dFppy)2(phen)]1.51 Alternatively, the MLCT state of [Ir(dFppy)2(phen)]1 is reductively quenched by a triethanolamine (TEOA) sacrificial electron donor. Subsequent electron transfer from the one-electron-reduced Ir catalyst to the Co(II) complex generates a Co(I) species. This Co(I) species is supernucleophilic, so it can substitute chloride in dichlorodiphenyltrichloroethane (DDT), an environmentally harmful insecticide. The substitution product bearing the Co(III)–C bond is photolabile; photoillumination provokes the homolysis of the bond into the original Co(II) species and the carbon-centered radical. Finally, hydrogen atom abstraction occurs to give the dechlorinated compound. The two-electron redox cycle of Co is coupled with the single photoredox catalysis cycle. The final example illustrates a rare case of catalysis that recycles both photons and electrons. The photoredox catalysis discussed so far is photon stoichiometric; photon is consumed to steer the photoinduced cycle through photoexcitation of a photocatalyst. If photon is recycled after the photocycle, one can substantially improve the quantum yield of the reaction, which is defined as the mole of product divided by the mole (flux) of photon. The photon recycle can be achieved when the driving force for the recovery of the one-electron-oxidized or -reduced catalyst is greater than the excited-state energy of the catalyst. Electrocatalysis that recycles electrons is more common than photonrecycling catalysis. One example of electrocatalysis is radical propagation, in which charge neutralization occurs by oxidizing or reducing the starting material. The oxidized and reduced starting material enters the catalytic cycle, and the charged product is finally neutralized by the starting material. Figure 14.15 shows the photoelectrocatalytic conversion of the closed form (DTEc) to the open form (DTEo) of a 1,2-dithienylethene (DTE) dye.52 Photoexcitation of the Ir complex catalyst elicits the ring-opening reaction of DTEc. This chromic change is initiated by photoinduced one-electron oxidation of DTEc by an Ir(III) complex catalyst. Spontaneous bond rearrangements proceed for the radical cation of DTEc, yielding the radical cation of DTEo. The cationic DTEo is neutralized by abstracting one electron from DTEc. This neutralization leaves the radical cation of DTEc, which again enters the electrocatalytic ring-opening cycle. The radical

354 Mechanism for merged photoredox catalysis for the dechlorination of DDT. Reproduced from ref. 51 with permission from the Royal Society of Chemistry.

Chapter 14

Figure 14.14

Photoredox Chemistries of Cyclometalated Ir(III) Complexes

Figure 14.15

355

(a) Photoelectrocatalytic cycloreversion of photochromic 1,2-dithienylethene (DTE). (b) Plausible mechanism for photoelectrocatalysis. Adapted from ref. 52 with permission from the Royal Society of Chemistry.

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propagation significantly enhances the electron economy of the reaction. Of particular interest is the fate of the one-electron-reduced Ir catalyst. This species loses its extra electron to the radical cation of DTEo. This productive step usually restores the ground-state Ir(III) catalyst, but the excitedstate catalyst can be regenerated if the driving force of this step is greater than the triplet state energy of the catalyst. The latter case is found for electron-poor DTEo having anodically shifted Eox values. Overall, the photoinduced ring-opening reaction involves two cycles: One is the electrocatalytic cycle, and the other is the photon-recycling cycle. The unique combination yields a one order of magnitude improvement in the quantum yield of the reaction over the photon-stoichiometric reactions.

14.5 Outlook The rich photoelectrochemistry of cyclometalated Ir(III) complexes has stimulated the discovery of a variety of photofunctions. This chapter overviewed the excited-state electrochemical properties of Ir(III) complexes, with a special focus on the intra- and intermolecular photoinduced electron transfer. The excited-state electron transfer and subsequent charge recombination is fully reversible and therefore promises appealing applications. Photoinduced electron transfer of cyclometalated Ir(III) complexes has provided unique principles of creating sensors and photoredox catalysts. This chapter briefly introduced the essential features of photoinduced electron transfer theories. Electronic structures of cyclometalated Ir(III) complexes have also been outlined. The basic knowledge has been exploited to explain the mechanisms of phosphorescence sensors of biological metals, including zinc, copper, and chromium ions. Finally, the same knowledge was used to understand the photoredox catalytic dehalogenative radical generation. The photoinduced redox ability can further be extended to the generation of high-valency Pd species that catalyzes intramolecular C–N coupling reactions. An excited-state Ir(III) complex can be recovered in the final step of photoredox catalysis, which substantially improves the photon economy of the catalysis. These examples demonstrate the huge potential of the photoelectrochemistry of cyclometalated Ir(III) complexes. Since the Ir(III) complexes exhibit the strongest phosphorescence emission and a wide electrochemical window among their congeners such as Pt(II) and Ru(II) complexes, future research will create highly valuable photoelectrochemical functionality other than sensing and photoredox catalysis.

Acknowledgements The author acknowledges the financial support through the Midcareer Research Program (NRF-2019R1A2C2003969) of the National Research Foundation of Korea funded by the Ministry of Science, Information, and Communication Technology and Future Planning.

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CHAPTER 15

Electrogenerated Chemiluminescence in Functional Redox Chemistry ELENA VILLANIa AND SHINSUKE INAGI*a,b a

Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan; b PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan *Email: [email protected]

15.1 Introduction Electrogenerated chemiluminescence, or electrochemiluminescence (ECL), is a complex phenomenon where the generation of photons is achieved at the electrode surface following an electrochemical stimulus, which generates molecular species that undergo high-energy electron transfer reactions resulting in light-emitting excited states.1–5 The distinctive method to achieve light emission offers many advantages for the application of such technology in analytical and bioanalytical research fields. For example, since no external light source is required for photoexcitation as is usually needed in fluorescence, very low background signals are generated, making ECL an extremely sensitive analytical technique with a high range of sensitivity and very low detection limits. Furthermore, since light generation is achieved in situ upon an electrochemical stimulus, the location and time of the lightemitting region can be easily controlled, paving the way for the application of this technology as an emerging microscopy technique.6,7 Green Chemistry Series No. 69 Sustainable and Functional Redox Chemistry Edited by Shinsuke Inagi r The Royal Society of Chemistry 2022 Published by the Royal Society of Chemistry, www.rsc.org

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15.2 Fundamentals of ECL: Mechanisms of Light Generation There are two main reaction mechanisms to achieve ECL emission: the ‘‘annihilation’’ mechanism and the ‘‘coreactant’’ mechanism. In both cases, the generation of light is obtained after an exergonic electron transfer reaction between intermediates electrochemically generated at the electrode surface.

15.2.1

Annihilation ECL

The first studies regarding the observation of light electrochemically generated after electrolysis of aromatic hydrocarbons were reported separately by Hercules and Visco in the mid-1960s,8,9 although a previous examination of the same phenomenon after electrolysis of luminol was reported at the end of the 1920s.10 In the same period, the generation of light after electrochemical reactions of 9,10-diphenylanthracene was also reported by Bard and coworkers.11 In the ‘‘annihilation’’ pathway, radical species are electrochemically generated at the electrode surface by the alternate pulsing of the electrode potential; successively, such radical species undergo an exergonic electrontransfer reaction with the formation of an emitting excited state as a result.2 The ‘‘annihilation’’ mechanism can be summarized as depicted in eqn (15.1)–(15.4), where A can be the same chemical species or different (mixed annihilation):12,13

A

A  e - A 1 (oxidation at the electrode)

(15.1)

A þ e - A  (reduction at the electrode)

(15.2)

1

þA



- A* þ A (excited state formation)

A* - A þ hn (light emission)

(15.3) (15.4)

In this approach, the potential of the working electrode is quickly changed between two different values, generating the oxidized and reduced radical species (A 1 and A  in eqn (15.1) and (15.2), respectively) of the parent molecule A. Subsequently, the two radical species react in the diffusion layer generating the excited state (A* in eqn (15.3)), which then relaxes to the ground state through the emission of a photon (hn, eqn (15.4)), regenerating the parent molecule A at the end of the process. The energy of the electron-transfer reaction (eqn (15.3)) is responsible for the type of excited state formed. If the energy involved is sufficient to populate the lowest singlet excited state, the emitting species will be a singlet (1A*); differently, if the energy involved is sufficient to populate the lowest triplet excited state, the emitting species will be a triplet (3A*).1 The energy requirement in eqn (15.3) is directly related to the enthalpy, which can be calculated from the redox potentials of the oxidation and reduction reactions (eqn (15.1) and (15.2), respectively) as defined in eqn (15.5): DHann ¼ Ep(A/A 1)  Ep(A/A )  0.16

(15.5)

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361

where DHann (in eV) is the enthalpy for the annihilation reaction, Ep is the peak potential for the electrochemical oxidation and reduction processes (in V), and 0.16 is the entropy approximation term (TDS at 298 K, corresponding to 0.10 eV) with the addition of 0.057 eV resulting from the difference between the reversible potential and the peak potential of the redox reactions (eqn (15.1) and (15.2)). It is possible to classify the annihilation process referring to the value of the enthalpy:  if DHann4Es, the lowest singlet excited state can be directly generated. In this case, the system is called an ‘‘energy-sufficient system,’’ and the reaction is said to follow the S-route: A 1 þ A  - 1A* þ A (excited singlet formation)

(15.3a)

 if DHannoEs but DHann4Et, the lowest triplet excited state can be directly generated. In this case, the system is called an ‘‘energy-deficient system,’’ and the reaction is said to follow the T-route: A 1 þ A  - 3A* þ A (excited triplet formation)

(15.3b)

A particular case of the T-route is the triplet–triplet annihilation (TTA) when the triplet state 3A* is initially formed, but it is rapidly converted to the singlet state 1A* after reaction with another triplet state 3A*: 3

A* þ 3A* - 1A* þ A (triplet–triplet annihilation)

(15.3c)

 if DHannBEs, the T-route can contribute to the formation of 1A* in addition to the S-route, leading to a mixed system called ST-route. In addition to singlet and triplet excited-state formation, ion annihilation reactions can lead to the direct excimers (excited dimers) and exciplexes (excited complexes) formation, as depicted by eqn (15.6) and (15.7). These reactions are said to follow the E-route.

3

A 1 þ A  - A2* (excimer formation)

(15.6)

A 1 þ A  - (AA) 0 * (exciplex formation)

(15.7)

A* þ 3A* - 1A2* (TTA excimer formation)

(15.8)

Excimer and exciplex species can also be formed via the TTA mechanism, as shown in eqn (15.8). The formation of these chemical species is possible when the participating molecules are able to align for a significant p-orbital overlap. The main requirements necessary to obtain efficient annihilation ECL are i) chemical precursors able to form radicals sufficiently stable to generate the excited state; ii) good photoluminescence yield of the excited state; iii) electron-transfer reactions sufficiently exergonic to populate the excited state.1,2

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The generation of ECL emission can be explained considering the Marcus inverted region.14 Marcus revealed the kinetics of heterogeneous electron transfer reaction as a function of the driving force. The rate constant augments with the driving force (Marcus normal region); at higher driving force, the rate constant decreases (Marcus inverted region). As Marcus demonstrated, the intersection of the potential energy surface of the reactants with that of the electronic ground-state products produces a large energetic barrier compared to that generated from the intersection of the same reactant energy surface with the one of an excited state. Therefore, in the Marcus inverted region, the formation rate of the ground-state products then becomes slow relative to the formation of the excited state.15

15.2.2

Coreactant ECL

It is also possible to generate ECL in a single potential step using a coreactant. A coreactant is a chemical species that, upon oxidation or reduction, produces an intermediate sufficiently stable to react with an ECL chromophore to produce its excited state. Usually, this occurs after bond cleavage of the coreactant to form strong oxidants or reductants. For instance, oxalate ion (C2O42) was the first coreactant discovered16 and it is believed to produce the strong reductant CO2  after oxidation in an aqueous solution. Its reaction with the common ECL luminophore ruthenium(II)-tris(2,2 0 -bipyridine) ion (Ru(bpy)321) can be schematized as follow (eqn (15.9)–(15.15)): Ru(bpy)321  e - Ru(bpy)331 Ru(bpy)3

31

þ C2O4

2

- Ru(bpy)3

21

þ C2O4

(15.9) 

C2O4  - CO2  þ CO2 Ru(bpy)3

31

þ CO2



- Ru(bpy)3

21

(15.11) * þ CO2

Ru(bpy)321 þ CO2  - Ru(bpy)31 þ CO2 Ru(bpy)331 þ Ru(bpy)31

-

(15.10)

Ru(bpy)321* þ Ru(bpy)321

Ru(bpy)321* - Ru(bpy)321 þ hn

(15.12) (15.13) (15.14) (15.15)

Oxalate is often referred to as an ‘‘oxidative–reductive’’ coreactant due to its ability to form a strong reducing agent upon electrochemical oxidation. Another example of an ‘‘oxidative–reductive’’ coreactant is tri-n-propylamine (TPrA) that, together with the Ru(bpy)321 luminophore, forms a commercially important ECL system.3,5 The use of TPrA as a coreactant was firstly reported in the 1990s17 and, although a comprehensive study of several ECL reaction pathways for the Ru(bpy)321/TPrA system was reported at the beginning of the 2000s,18 the mechanism of the ECL generation is rather complicated and still unclear. Generally, the ECL emission of this system consists of two waves: the first one is in correspondence with the direct oxidation of TPrA, while the second one occurs in the potential region of the

Electrogenerated Chemiluminescence in Functional Redox Chemistry 21

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19

direct Ru(bpy)3 oxidation. Both ECL waves are associated with the emission of Ru(bpy)321 and usually merge when relatively high concentrations of the chromophore are used (mM range). However, in commercial ECL systems where the concentration of the chromophore is usually very low (mM range or less), the first ECL wave is predominant. To explain such an effect, Bard and coworkers proposed four different mechanisms (Figure 15.1), depending on the chromophore concentration and potential applied, that all of them contribute to the total ECL signal.18 All mechanisms predict that the oxidation of TPrA produces the oxidant species radical cation (TPrA 1) that, upon deprotonation, leads to the formation of the strong reductant species free radical (TPrA ). Both species are necessary to achieve ECL emission. In Figure 15.1(a), both TPrA and Ru(bpy)321 undergo oxidation at the electrode surface leading to the generation of the excited state through reaction with the strong reductant agent TPrA . This mechanism is usually referred to as ‘‘homogeneous coreactant ECL’’. TPrA can also reduce Ru(bpy)321 leading to the generation of light after annihilation reaction between the chromophore radicals (scheme (b) in Figure 15.1). In scheme (c) of Figure 15.1, the so-called ‘‘catalytic route’’ is reported, where the oxidation of the coreactant is achieved from the reaction with the oxidized form of the chromophore. Generally, this mechanism prevails when high concentrations of Ru(bpy)321 are used. In scheme (d) of Figure 15.1, only the oxidation of the coreactant generates the excited state. This mechanism is generally predominant when the diffusion of the chromophore toward the electrode surface is hindered, for example, by immobilizing the chromophore on the surface of beads or nanoparticles.20–22 Such a particular type of mechanism, which is usually referred to as ‘‘heterogeneous coreactant ECL’’, is the fundamental theory behind the actual ECL immunoassay technology.23,24 Several factors influence the ECL of this system including the presence of oxygen,25 pH, the use of surfactants,26,27 electrode surface modifications28 and the type of electrode material.29,30

Figure 15.1

The four mechanisms for Ru(bpy)321/TPrA system proposed by Bard and coworkers. Reproduced from ref. 18 with permission from American Chemical Society, Copyright 2002.

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‘‘Reductive–oxidative’’ coreactants are also used to generate ECL, such as benzoyl peroxide31 or peroxydisulfate (S2O82).32–34 For example, in the case of peroxydisulfate, reduction produces the strong oxidant SO4 , which then undergoes an electron-transfer reaction with an ECL luminophore such as Ru(bpy)321 to generate light, as reported in eqn (15.16)–(15.20): S2O82 þ e - SO4  þ SO42 Ru(bpy)321 þ e

- Ru(bpy)3

(15.16)

1

(15.17)

Ru(bpy)31 þ S2O82 - Ru(bpy)321 þ SO4  þ SO42 1

Ru(bpy)3 þ SO4



- Ru(bpy)3

21

* þ SO4

Ru(bpy)321* - Ru(bpy)321 þ hn

2

(15.18) (15.19) (15.20)

The sulfate anion is unreactive from the ECL point of view. The ECL intensity of this system is a function of peroxydisulfate concentration since it can act as both a coreactant and a quencher of the Ru(bpy)321 excited state.32,35

15.3 Applications of ECL in Molecular Electrochemistry 15.3.1

Novel ECL Reaction Systems

As stated in Section 15.2.2, the coreactant is usually added to the solution together with the luminophore to achieve ECL emission after a single potential sweep or step is applied. However, Einaga and coworkers exploited the peculiar properties of boron-doped diamond (BDD) electrodes for the in situ generation of the coreactant to afford very efficient ECL emission. For example, they developed a coreactant-free ECL system where Ru(bpy)321 emission is obtained by using sulfate instead of the more common peroxydisulfate. This particular ‘‘coreactant-on-demand’’ ECL takes advantage of the unique ability of BDD to operate at very high oxidation potentials in aqueous solutions and to promote the conversion of inert SO42 into the reactive S2O82 (Figure 15.2(a) and (b)).36 This novel procedure is rather straightforward, not requiring any particular electrode geometry, and since the coreactant is only generated in situ, the interference with biological samples is minimized. Furthermore, since BDD has a large overpotential for the evolution of hydrogen in aqueous electrolyte solutions, reductive–oxidation ECL with the coreactant peroxydisulfate can also be obtained with high reproducible signals and stable emission, without interference from the hydrogen evolution reaction.37 In a similar manner, they developed another ECL system for the in situ coreactant production using BDD electrodes, where the oxidation of carbonate (CO32) into peroxydicarbonate (C2O62) is promoted, which further reacts with water to form hydrogen peroxide (H2O2), which acts as a coreactant for Ru(bpy)321 ECL (Figure 15.2(c) and (d)). Investigation of the mechanism reveals that ECL emission is triggered by the reduction of H2O2 to hydroxyl radicals (OH ), which later react with the reduced Ru(bpy)31

Electrogenerated Chemiluminescence in Functional Redox Chemistry

Figure 15.2

365

The in situ coreactant generation at diamond electrodes. (a) ECL reaction mechanism from Ru(bpy)321 on BDD electrode with sulfate ions; (b) ECL intensity transients at various Na2SO4 concentrations, 1 mM (blue), 10 mM (green), 0.1 M (red), 1 M (purple), and 0.1 M KClO4 (black). The inset shows the integrated ECL intensity as a function of Na2SO4 concentration. Reproduced from ref. 36 with permission from American Chemical Society, Copyright 2016. (c) ECL scheme of the proposed reaction mechanism for the Ru(bpy)321/CO32 system at a BDD electrode. (d) ECL intensity for 10 mM Ru(bpy)3Cl2 in 25 mM (black), 50 mM (red), 100 mM (orange), 200 mM (green), 500 mM (sky blue), and 1000 mM (blue) Na2CO3 solutions. The inset shows the integrated ECL intensity as a function of Na2CO3 concentration. Reproduced from ref. 38 with permission from American Chemical Society, Copyright 2019.

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molecules to form excited states, followed by light emission. The in situ generation of H2O2 on BDD electrodes has also been exploited to trigger the ECL emission of luminol, avoiding instability of the peroxide species or its interference with the analytes of interest.39 Even if the applications of diamond electrodes are vast,40 such as electrosynthesis, environmental remediation, and CO2 reduction,41 these results also encourage the use of BDD in the wide field of ECL.

15.3.2

ECL for Imaging Applications

In quite recent years, ECL has emerged as a sensitive microscopy technique for the micro- and nanoimaging of single objects deposited directly on the surface of an electrode. For example, Sojic and coworkers demonstrated that the imaging of cells and cell membranes at the interface with an electrode using ECL reveals details that are not resolved with classical fluorescence microscopy.42,43 This particular effect is due to the confinement of the ECL emitting region only to the immediate vicinity of the electrode surface. However, by changing the buffer capacity (Figure 15.3(a))44 or by rationally varying the concentration of luminophore and/or coreactant, the ECL emitting layer can be modulated and extended several micrometers away from the electrode surface; for example, the latter strategy has been exploited for the sequential and selective visualization of cell–matrix adhesions and cell–cell junctions in a single sample.45 In addition, biological samples cultured directly on the electrode surface can hinder the diffusion of the ECL precursors toward the electrode, resulting in a remarkable negative optical contrast of the target biological object (Figure 15.3(b)).46,47 Inagi and coworkers employed ECL imaging to map the distribution of the potential gradient established in bipolar electrochemical systems having different geometries (Figure 15.3(c) and (d)).48 The proposed approach exploits the dependency of the ECL emission of luminol, when it is oxidized in the presence of hydrogen peroxide49 at the anodic side of a bipolar electrode (BPE), on the axial location parallel to the electric field. This remarkable advantage may help clarify the potential distribution profile established in bipolar electrochemical systems with different geometries currently in use for organic electrosynthesis.50,51 The same group also reported the feasibility of luminol ECL on the surface of entirely pure conducting polymer films, such as poly(3,4ethylenedioxythiophene) (PEDOT) and polypyrrole, when used as BPEs.52 Indeed, by using ECL imaging, they were able not only to study and compare the different ECL emission behaviors on the two types of films but also to track the propulsion of such objects under the influence of the external electric field.

15.3.3

ECL of Organic Systems

Organic molecules and conjugated systems have become ideal ECL luminophores owing to their nonmetal content, definite structure, and fine-tuning of the emission wavelength.53,54 Maran et al. reported the synthesis, photophysical,

Electrogenerated Chemiluminescence in Functional Redox Chemistry

Figure 15.3

367

Various applications of ECL imaging. (a) Top-view ECL images of labeled Ru(bpy)321-beads in 0.01 M (left) and 1 M (right) phosphate buffer solutions and their respective normalized ECL profiles extracted from bead imaging analysis for 8 mm (red), 12 mm (blue), and 14 mm (black) beads (scale bar is 10 mm). Reproduced from ref. 44 with permission from the Royal Society of Chemistry. (b) Schematic illustration of imaging cell–matrix adhesions by ECL microscopy. Reproduced from ref. 47 with permission from John Wiley & Sons, Copyright r 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) and (d) Luminol ECL imaging at different potential values on an indium tin oxide (ITO) plate (1 cm1 cm) acting as a bipolar electrode in a bipolar electrochemical cylinder configuration having (c) the driving cathode as the inner electrode and the driving anode as the outer electrode; in (d) the arrangement of the driving electrodes is reversed. Adapted from ref. 48 with permission from American Chemical Society, Copyright 2021.

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Figure 15.4

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ECL derived from various organic systems. (a) Cyclic voltammogram of 1 mM spirobifluorene-derived compound (black), spirobifluorene core (blue), and tris(4-methylphenyl)amine (red) in DMF/0.1 M tetrabutylammonium perchlorate (TBAP). (b) ECL spectra of 1 mM solutions (DMF/0.1 M TBAP) of various spirobifluorene-derived compounds. Reproduced from ref. 55 with permission from American Chemical Society, Copyright 2017. (c) Molecular structure of a BODIPY derivative; (d) cyclic voltammogram of a 0.5 mM compound in (c) in benzene/ acetonitrile (2 : 1 vol.) containing 0.1 M TBAPF6 (arrows indicate the scan direction); (e) ECL spectra of the compound (c) at five different concentrations in benzene/acetonitrile (2 : 1 vol.) solution containing 100 mM of TPrA. Reproduced from ref. 58 with permission from John Wiley & Sons, Copyright r 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

and ECL properties of a series of compounds formed of two triphenylamines linked by a fluorene or spirobifluorene bridge (Figure 15.4(a) and (b)).55 The phenylamine moieties were modified at the para-position of the two external rings by electron-withdrawing or electron-donating substituents. These modifications allowed for fine-tuning of both photoluminescence and ECL emission from blue to green. The ECL properties were investigated by direct annihilation of the electrogenerated radical anion and radical cation, which results in a very efficient annihilation process generating an intense greenish-blue ECL emission easily observable by the naked eye.

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Valenti et al. reported the electrochemical and ECL properties of a family of polysulfurated dendrimers with a pyrene core, of which the redox and luminescence properties are dependent on the generation number.56 In particular, from low to higher generation, it is both easier to reduce and oxidize dendrimers and the emission efficiency increases along the family, with respect to the polysulfurated pyrene core. Ding et al. reported a straightforward two-step synthesis of a boron difluoride formazanate dye that exhibits near-infrared ECL.57 Examination of its solid-state structure suggested that the N-aryl substituents have significant quinoidal character, leading to enhanced electronic delocalization over the p-system that includes both the formazanate backbone and the N-aryl substituents. This delocalization results in a drastic redshift in both the absorption, photoluminescence, and ECL maxima compared to related BF2 formazanates without the need for elaborate structure alteration. Remarkably, the ECL properties of such a dye, examined in the presence of TPrA, exhibit the maximum intensity at 910 nm, with at least 85 nm redshifting compared to all other organic dyes. Ishimatsu et al. reported the electrochemical and ECL properties of a borondipyrromethane (BODIPY) derivative with extended conjugation, which shows ECL in the range of 650–800 nm by using TPrA as the coreactant (Figure 15.4(c)–(e)).58 Their results showed that the triplet–triplet annihilation is likely the main pathway emitting ECL through the ion annihilation of the radical anions and cations of the studied molecule, whereas the lowest excited singlet state is directly produced via coreactant oxidation. The same group also studied the electrochemistry and ECL behavior of four kinds of electron donor–acceptor molecules exhibiting thermally activated delayed fluorescence (TADF).59 TADF molecules can harvest light energy from the lowest triplet state by spin-up conversion to the lowest singlet state because of the small energy gap between these states. Intense green to red ECL can be emitted from these TADF molecules by applying a square-wave voltage with very high efficiency.

15.3.4

Aggregation and Crystallization-induced Emission in ECL

A particular research area of ECL recently garnering considerable attention concerns the development of molecular systems that exhibit aggregationinduced ECL (AIECL). The first study regarding this new phenomenon was reported by De Cola and collaborators in 2017.60 They observed AIECL from square-planar Pt(II) complexes (Figure 15.5(a)–(c)) resulting from the formation of supramolecular nanostructures and it is the first example of ECL of Pt(II) complexes in an aqueous solution having higher efficiency than the standard Ru(bpy)321. In this system, self-assembly changes the HOMO and LUMO energies, making their population accessible via ECL pathways and leading to the generation of the luminescent excited state. After this first report, many other examples were reported in the literature,61,62 including

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Figure 15.5

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(a) ECL emission recorded during cyclic voltammetry in an aqueous solution of a Pt(II) complex physically transferred onto the electrode before (blue trace) and after (red trace) a mechanical stress; photographs of the luminescence of the Pt(II) complex onto the electrode under a bench-top UV lamp and cartoons showing the principle of its mechanochromism (b) before and (c) after grinding with a pestle. Reproduced from ref. 60 with permission from American Chemical Society, Copyright 2017. (d) Photoluminescence spectra of the crystalline film of a diboron complex (blue line) and a 0.25 mM solution of the same complex in acetonitrile (black line). Inset shows the chemical structure of the diboron complex. (e) Photoluminescence images of recrystallized diboron complex (leftmost panel), after gradual mechanical grinding (middle three panels), and powder diboron complex after exposure to acetone vapors for 24 h (rightmost panel). Reproduced from ref. 66 with permission from John Wiley & Sons, Copyright r 2020 Wiley-VCH GmbH.

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matrix coordination- and crystallization-induced emission effects. For example, an AIECL system composed of tetraphenylbenzosilole derivatives in an aqueous phase system was reported for the ultrasensitive detection of hexavalent chromium.64 Ding et al. demonstrated the crystallization-induced ECL enhancement of two new benzosilole compounds that show an ECL enhancement of 24 times and 16 times, respectively, compared with their solution.65 The same group also reported the effects of crystallization-induced blueshift emission on the ECL properties of a newly synthesized diboron complex (Figure 15.5(d) and (e)).66 The 57 nm blueshift and great enhancement in the crystalline lattice relative to this compound in solution were attributed to the restriction of intramolecular rotation. Furthermore, it was also discovered that ECL at crystalline film/solution interfaces could be further enhanced by means of coreactant, which was found to give off a redshifted light emission.

15.4 Conclusions and Future Directions In this chapter, we described and summarized several applications of ECL that are currently hot topics in this field of research. In particular, we focused our attention on the use of diamond-based electrodes that, thanks to the intrinsic characteristics and surface chemistry, can be exploited for the in situ generation of coreactants. In addition, we also explored recent trends of ECL imaging analysis and the use of a large variety of organic species as efficient and tunable ECL chromophores. Lastly, we also reviewed the phenomena of aggregation and crystallization-induced emission and their effect on the ECL emission properties. Such a wealth of novel systems and applications in the ECL field benefits from the intrinsic electrochemical nature of the light emission and we believe that many other applications and systems will become the subject of new studies in the near future.

Acknowledgements We acknowledge the financial supports by Kakenhi Grant-in-Aids (JP19F19769, JP20H02796) from the Japan Society for the Promotion of Science (JSPS), PRESTO (No. JPMJPR18T3) of the Japan Science and Technology Agency (JST), and a Support for Tokyo Tech Advanced Researchers [STAR] Grant funded by the Tokyo Institute of Technology Fund (Tokyo Tech Fund). E. V. acknowledges the JSPS Fellowship (ID No. P19769).

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Subject Index AA. See ascorbic acid (AA) acenaphthenequinone (ANQ), 232 4-acetamido-TEMPO (ACT), 128 ACT. See 4-acetamido-TEMPO (ACT) aggregation-induced ECL (AIECL), 369 aggregation-induced emission enhancement (AIEE), 302 AIECL. See aggregation-induced ECL (AIECL) AIEE. See aggregation-induced emission enhancement (AIEE) AMF. See arbuscular mycorrhiza fungi (AMF) ANQ. See acenaphthenequinone (ANQ) arbuscular mycorrhiza fungi (AMF), 92 ascorbic acid (AA), 282 B12-dependent enzymes, 222–223 adenosylcobalamin and methylcobalamin, 209 amphiphilic vitamin B12, 210 axial base, 209 bioinspired chemistry, 207 methylation reaction, 209 photo-driven molecular transformation B12–BODIPY dyad system, 220–221 B12-inspired hydrogen production and alkene reduction, 216–217 cross-coupling reactions, 218–220 dehalogenation, esters and amides formation coupled with, 213–214

heterogeneous catalyst system, 210–213 homogeneous catalyst system, 217 photocatalyst, 221–222 visible light-driven catalytic system, 214–216 porphyrin, 209 structures, 207–208, 211 tetrapyrrole metal complex, characteristic feature of, 208 vitamin B12 (cobalamin), 208–209 B12 derivative–boron dipyrromethene (BODIPY) dyad, 220–221 BBD. See benzocyclobutenedione (BBD) benzocyclobutenedione (BBD), 232 bipolar electrochemical system local electrochemical doping, 73 U-shaped cell system, 72, 74 bipolar electrode (BPE), 74 BODIPY dyad. See B12 derivativeboron dipyrromethene (BODIPY) dyad BPE. See bipolar electrode (BPE) carbon-centered radicals 1,3-dicarbonyl carbon, electrochemical oxidation of, 14, 16 Mn-catalyzed electrochemical transformations, 19–20 redox catalysis, 14 sulfonyl radicals, 18

376

carboxylic acids, electrolysis of aromatic esters, electrochemical deprotection of, 51–52 diphenylphosphinates, electrochemical deoxygenation of, 52–53 electrochemical methoxylation conventional chemical protecting methods, 47 electrosynthesis equipment, 48 flow electrochemical methoxylation, scope of, 49 mechanism of, 46–47 isocyanates, Hofer–Moest synthesis of carbamates and thiocarbamates, synthesis of, 39, 42 flow electrochemistry, 39 GC–MS analyses, 39 oxamic acids, 38–40 ureas, electrochemical synthesis of, 39, 41 ureas, flow electrochemical synthesis of, 39–40, 43 Kolbe electrolysis. See Kolbe electrolysis malonic acid derivatives, electrochemical decarboxylation of, 49–50 non-Kolbe electrolysis. See non-Kolbe electrolysis orthoesters, Hofer-Moest synthesis of aliphatic and aromatic orthoesters, 44, 45 applications of, 46–47 cyclic voltammetry, 44 dithiane acids, anodic oxidation of, 44–45 orthoester/ester synthon, 44

Subject Index

cation pool method concept of, 61 parallel batch system, 61–62 chiral metal electrodes, 290 asymmetric synthesis, 287–289 bulk metals, controlled cutting of, 277–278 chiral molecular imprinting chemical synthesis, 280 LLC gel, 281 mesoporous metals, 280–281 metal nanoparticles, types of, 279 monometallic chiral films, 281 electrochemical separation, 289–290 enantioselective analysis CuO films, 284 DOPA, 285 Pt–Ir alloys, 286–287 SAM approach, 284 Si wafers, 284 EPCs, 274 heterogeneous catalysts, 275 metal–ligand complexes, 276 molecular adsorption approach, 276–277 cobalt tetrasulfophthalocyanine (CoTSP), 212 COFs. See covalent organic frameworks (COFs) concurrent reduction–substitution (CRS) method, 254 conjugated polymers, 249 electrochemical polymer reaction 1H NMR analysis, 255–256 BF3-OEt2, 257 bromination and alkoxylation, 252 chloride ion, 253 conductive polymer films, electrochemical fluorination of, 256

Subject Index

CRS method, 254–255 electron transfer, 252 fluorene-based conducting polymer, paired reactions of, 258–259 fluorene–thiophenebased copolymer, 255 ketone group, 258 P3HS-b-P3EHS, 256–257 P3MT, 253 poly(1,4-dimethoxybenzene), 254 poly-(3-hexylthiophene), 254 polyaniline, 254 poly(thiophene-alt-fluorene) (PTF), 255 rod–rod block copolymer, 256 oxidative and reductive electropolymerization, 250–252 polythiophene, doping and dedoping behavior of, 250 redox-active moieties, 268–270 two- and three-dimensional, 259–260 CoTSP. See cobalt tetrasulfophthalocyanine (CoTSP) covalent organic frameworks (COFs), 259 CRS method. See concurrent reduction-substitution (CRS) method CV. See cyclic voltammetry (CV) cyanamide-functionalized carbon nitride (NCNCNx), 183 cyclic voltammetry (CV), 121–122 cyclometalated Ir(III) complexes, photoredox chemistries of, 356 electronic structures of, 337–338 intermolecular photoinduced electron transfer arenes, photoredox catalytic difluoroalkylation of, 349–350

377

BrCF2CO2Et, 349 closed form (DTEc), 353 electrocatalysis, 353 MLCT state, 349 open form (DTEo), 353, 356 radical ion pair, 346 transient photoluminescence, 352 transition metals, 349 intramolecular photoinduced electron transfer DPA, 340–341 electron-rich monoanionic cyclometalating ligands, 345 phosphorescence emission, 341, 343 zinc probe, 340, 346, 348 photoinduced electron transfer of electron transfer mechanism, 333 heterobimolecular electron transfer, 335 heterobimolecular photoinduced electron transfer, 332 HOMO and LUMO, 333 HSOMO and LSOMO energy, 334 photoexcitation, 334, 336 radical ion pair, 336 Rehm–Weller expression, 333 semiclassical methods, 335 singlet–triplet spin conversion, 336 DCN. See 9,10-dicyanoanthracene (DCN) DDBQ. See 2,5-di-decyloxy-1,4benzoquinone (DDBQ) DDT. See dichlorodiphenyltrichloroethane (DDT)

378

density-functional theory (DFT), 158, 232 detection limit (DL) values, 305–306 DFT. See density-functional theory (DFT) di(2-picolyl)amino (DPA), 340 dibutyltin dilaurate (DBTDL), 39 dichlorodiphenyltrichloroethane (DDT), 353 9,10-dicyanoanthracene (DCN), 146 2,5-di-decyloxy-1,4-benzoquinone (DDBQ), 236 differential pulse voltammetry (DPV), 285 3,4-dihydroxyphenylalanine (DOPA), 281, 285 5,6-dimethylbenzimidazole (DMB), 209 1,2-dithienylethene (DTE) dye, 353, 355 DL values. See detection limit (DL) values DMB. See 5,6-dimethylbenzimidazole (DMB) DOPA. See 3,4-dihydroxyphenylalanine (DOPA) DPA. See di(2-picolyl)amino (DPA) DPV. See differential pulse voltammetry (DPV) DTE dye. See 1,2-dithienylethene (DTE) dye EB. See emerald base (EB) ECCR. See electrochemically generated carbon-centred radical (ECCR) ECL. See electrochemiluminescence (ECL) electrochemical asymmetric catalysis, 155 catalysts as redox entities, 166–172 chiral sBOX ligand, 163, 165 Jacobsen–Katsuki epoxidation, 162

Subject Index

nickel-catalyzed asymmetric biselectrophile cross-coupling reaction, 165–166 palladium-catalyzed atroposelective C–H alkenylation reaction, 165–166 sharpless asymmetric dihydroxylation, 161 organic electrosynthesis, 154 substrates as redox entities, 166–172 anodic oxidation of aniline, 156 asymmetric organocatalysis, 156 cyclic b-ketoester, asymmetric a-arylation of, 159 DFT, 158 enantioselective induction, 157 meta-alkylated anilines, 156–157 p-quinone, 160 electrochemical glycosylation glycosyl sulfonium ion intermediates, 84–86 glycosyl triflate intermediates, generation of, 82–84 phenyl glycoside, 81–82 thioglycosides, 81–82 electrochemically generated carboncentred radical (ECCR), 30 electrochemical methoxylation conventional chemical protecting methods, 47 electrosynthesis equipment, 48 flow electrochemical methoxylation, scope of, 49 mechanism of, 46–47 electrochemiluminescence (ECL), 371 aggregation and crystallizationinduced emission in, 369–371

Subject Index

annihilation ECL, 360 coreactant ECL catalytic route, 363 oxidative–reductive, 362 reductive–oxidative, 364 Ru(bpy)321/TPrA system, 362–363 electron-transfer reaction, 360 energy-deficient system, 361 Marcus inverted region, 362 molecular electrochemistry ECL reaction systems, 364–366 imaging applications, 366 organic systems, 366, 368–369 electrogenerated chemiluminescence. See electrochemiluminescence (ECL) electrolytic processes combinatorial synthesis, 60–61 electroorganic chemistry, combinatorial flow system for bipolar electrochemical system, 73–75 flow electrochemistry, 63, 65–68 PEM reactor, 68–72 electroorganic synthesis, parallel batch systems for carbamates, parallel anodic methoxylation of, 62, 64 cation pool method, 61–62 electrochemical reactions, 62, 66–67 SAEP, 61, 63–64 electron spin resonance (ESR) spectroscopy, 112 electron transfer (ET), 120 electrosynthesis, mediated electron transfer in advantages, 120 catalytic current, 121–123

379

direct and indirect electrosynthesis, 121, 147 ET, 120, 147 galvanostatic mode, 119 in-cell- and ex-cell-mediated transformations, 128–132, 147 ionically tagged mediators, 132–133 mediator-modified electrodes, 136 photoelectrochemical synthesis, mediators in enhancing mediator reactivity with light, 142–146 substrates, sequential activation of, 140–142 transformations at photoelectrodes, 138–140 polymediators, 133–136 potentiostatic mode, 119 redox catalysis and chemical catalysis ACT, 128 benzylic alcohols, 124 binding interaction, 123 catalytic processes, 123 high E1/2 values, 124–125 inner-sphere ET, 123 LiClO4–CH3CN electrolyte, 125 12-mediated alcohol conversion, 125 N-oxyl species 13, 126 outer-sphere ET, 123 TEMPO-mediated alcohol oxidations, 126–127 triarylimidazolemediated conversion, 125–126 emerald base (EB), 254 enantiomerically pure compounds (EPCs), 274 EPCs. See enantiomerically pure compounds (EPCs)

380

ESR spectroscopy. See electron spin resonance (ESR) spectroscopy ET. See electron transfer (ET) FDCA. See 2,5-furandicarboxylic acid (FDCA) flow electrochemistry, 63 cation flow method, 65 carbamates, electrooxidative C–C bond formations of, 65, 68 sequential and continuous combinatorial synthesis, 65, 69 microflow reactor, 65 parallel laminar flow reactor, 66, 70 thin-layer flow cell system, 65, 69 electrochemical dimethoxylation, 65, 70 electrooxidative allylation, 66, 70 Fourier transform-infrared (FT-IR), 254 FT-IR. See Fourier transforminfrared (FT-IR) 2,5-furandicarboxylic acid (FDCA), 184 gel permeation chromatography (GPC), 255 GO. See graphene oxide (GO) GPC. See gel permeation chromatography (GPC) graphene oxide (GO), 196–197 halide-mediated ionic cyclization reactions alkenes and indoles, 19, 22 hypervalent iodine species, 19 iodobenzenes, anodic oxidation of, 19, 22 ketones, iodide-mediated electrochemical dehydrogenative cyclizations of, 23

Subject Index

HDA. See hexadecylammonium (HDA) heteroatom-centered radicals aza-Wacker-type cyclization reactions, 8–9 halides, 8 KBr or KI, 10 NCRs, 4, 6–7 N–I bond, 10 P-centered radicals, 14 TEMPO, 4 heterogeneous electrocatalysis, 120 hexadecylammonium (HDA), 198 higher SOMO (HSOMO), 334 highest occupied molecular orbital (HOMO), 303, 333 HMF. See 5-hydroxymethylfurfural (HMF) Hofer–Moest decarboxylation, 31 HOMO. See highest occupied molecular orbital (HOMO) homogeneous electrocatalysis, 120 HSOMO. See higher SOMO (HSOMO) 5-hydroxymethylfurfural (HMF), 184 isocyanates, Hofer–Moest synthesis of carbamates and thiocarbamates, synthesis of, 39, 42 flow electrochemistry, 39 GC–MS analyses, 39 oxamic acids, 38–40 ureas, electrochemical synthesis of, 39, 41 ureas, flow electrochemical synthesis of, 39–40, 43 Kolbe electrolysis background of, 30–31 Kolbe intramolecular cyclisation aliphatic carboxylate, 33–34 aromatic carboxylates, 33 bioactive pyrrolidones, class of, 37

Subject Index

co-acids, 35–36 functionalised phthalides, electrosynthesis of, 33–34 lactones 2i–2p, 36 method, limitations of, 38 nonalkylated lactone, 36 pyrrolidones, electrosynthesis of, 37 Kolbe intramolecular cyclisation tandem reaction, 31 LB. See leukoemeraldine base (LB) LEDs. See light-emitting diodes (LEDs) leukoemeraldine base (LB), 254 ligand-to-metal charge transfer (LMCT), 179 light-emitting diodes (LEDs), 181 Li-ion batteries, organic active materials for, 229 basic concepts, 230–231 capacity increase, 230, 232 cyclability increase Li4C6O6, 236 PEPTO, 235 PPTO, 235 PPTODB, 235–236 PYD, 232 PYT, 232 sodium sulfonate (–SO3Na) group, 240 voltage increase, 241–242 LLC. See lyotropic liquid crystal (LLC) LMCT. See ligand-to-metal charge transfer (LMCT) lower SOMO (LSOMO), 334 lowest unoccupied molecular orbital (LUMO), 232, 303, 333 LSOMO. See lower SOMO (LSOMO) LUMO. See lowest unoccupied molecular orbital (LUMO) lyotropic liquid crystal (LLC), 281

381

MEA. See membrane electrode assembly (MEA) membrane electrode assembly (MEA), 69 metal-to-ligand charge-transfer (MLCT), 338 methoxymethyl (MOM), 46 9-methyl pyrido[3,4-b]indole (9-MP), 319 MIPs. See molecularly imprinted polymers (MIPs) MLCT. See metal-to-ligand chargetransfer (MLCT) molecularly imprinted polymers (MIPs), 275 MOM. See methoxymethyl (MOM) 9-MP. See 9-methyl pyrido[3,4-b]indole (9-MP) Myc-LCOs. See mycorrhizal lipopolysaccharides (Myc-LCOs) mycorrhizal lipopolysaccharides (Myc-LCOs), 92–93 naphthalenediimide (NDI), 268 NCRs. See Nitrogen-centered radicals (NCRs) NDI. See naphthalenediimide (NDI) NHE. See normal hydrogen electrode (NHE) Nitrogen-centered radicals (NCRs), 4, 6–7 N,N,N’,N’-tetramethylethylenediamine (TMEDA), 349 nonconjugated polymers, 249 block copolymers, 261–263 dendrimers, periphery of, 265–266 polymeric materials mimicking metalloproteins, 263–265 redox-active inorganic polymers, 266–268 side chain, 261 nonfood biomass oxidation electrocatalytic and photoelectrocatalytic, 184–188

382

nonfood biomass oxidation (continued) photocatalytic b C–C bond, 179–180 CdS/CdOx QDs, 181–182 homogeneous catalysts, 179 NCN CNx, 183 non-Kolbe electrolysis background of, 31–32 carboxylic acids, electrolysis of, 32 normal hydrogen electrode (NHE), 189, 211 oligosaccharides automated electrochemical solution-phase synthesis of one-pot iterative glycosylation, principle of, 86–88 tetrasaccharide, 89 biologically active oligosaccharides, synthesis of Myc-LCOs, 92–93 TMG-chitotriomycin, 89–92 cyclic oligosaccharides via automated electrochemical assembly, 96–97 orthoesters, Hofer-Moest synthesis of aliphatic and aromatic orthoesters, 44, 45 applications of, 46–47 cyclic voltammetry, 44 dithiane acids, anodic oxidation of, 44–45 orthoester/ester synthon, 44 oxovanadium(V)-catalyzed oxidative cross-coupling, 110, 112–116 oxovanadium(V)-induced intermolecular selective oxidative cross-coupling, 105–109 P3AS. See poly(3-alkylselenophene) (P3AS) P3AT. See poly(3-alkylthiophene) (P3AT)

Subject Index

P3MT. See poly(3-methylthiophene) (P3MT) PDHA. See poly(2,5-dihydroxyaniline) (PDHA) PEDOT. See poly(3,4-ethylenedioxythiophene) (PEDOT) PEFC. See polymer electrolyte fuel cell (PEFC) PEM reactor. See proton exchange membrane (PEM) reactor PEPTO. See poly(2,7-ethynylpyrene4,5,9,10-tetraone) (PEPTO) PET. See photoinduced electron transfer (PET) photo-driven molecular transformation B12–BODIPY dyad system, 220–221 B12-inspired hydrogen production and alkene reduction, 216–217 cross-coupling reactions, 218–220 dehalogenation, esters and amides formation coupled with, 213–214 heterogeneous catalyst system B12–TiO2 powder system, 212–213 TiO2, 210–211 TiO2-CoTSP catalyst system, 212 homogeneous catalyst system, 217 photocatalyst, 221–222 visible light-driven catalytic system, 214–216 photoelectrochemical synthesis, mediators in enhancing mediator reactivity with light doublet state, mediators photoactive in, 144–146 sacrificial electron acceptor/donor, 143–144 LEDs, 137 mediated electrosynthesis, 137

Subject Index

substrates, sequential activation of, 140–142 transformations at photoelectrodes 5-hydroxymethylfurfural, 140 photoelectrocatalytic C–H amination, 139 photoinduced electron transfer (PET), 301 polymer electrolyte fuel cell (PEFC), 68 poly(3-alkylselenophene) (P3AS), 256 poly(3-alkylthiophene) (P3AT), 256 poly(3-methylthiophene) (P3MT), 253 poly(2,5-dihydroxyaniline) (PDHA), 268 poly(3,4-ethylenedioxythiophene) (PEDOT), 75 poly(2,7-ethynylpyrene-4,5,9,10-tetraone) (PEPTO), 235 poly(pyrene-4,5,9,10-tetraone) (PPTO), 235 poly(pyrene-4,5,9,10-tetraone-2,7-diboroxine) (PPTODB), 235–236 poly(thiophene-alt-fluorene) (PTF), 255 PPTO. See poly(pyrene-4,5,9,10-tetraone) (PPTO) PPTODB. See poly(pyrene-4,5,9,10tetraone-2,7-diboroxine) (PPTODB) proton exchange membrane (PEM) reactor a,b-unsaturated acids, enantioselective hydrogenation of, 71, 74 diphenylacetylene, electrochemical hydrogenation of, 71, 73 furanic compounds, electroreduction of, 69, 73 hydrogenation reactions, 71 MEA, 69 methylcyclohexane, electroreduction of toluene, 70, 73 proton transfer complex (PTC), 319

383

PTC. See proton transfer complex (PTC) PTF. See poly(thiophene-alt-fluorene) (PTF) PYD. See pyrene-4,5-dione (PYD) pyrene-4,5,9,10-tetraone (PYT), 232 pyrene-4,5-dione (PYD), 232 PYT. See pyrene-4,5,9,10-tetraone (PYT) QD. See quantum dot (QD) quantum dot (QD), 181, 190, 197 radical cyclization reactions carbon-centered radicals 1,3-dicarbonyl carbon, electrochemical oxidation of, 14, 16 Mn-catalyzed electrochemical transformations, 19–20 redox catalysis, 14 sulfonyl radicals, 18 electrochemical transformations, 24 halide-mediated ionic cyclization reactions alkenes and indoles, 19, 22 hypervalent iodine species, 19 iodobenzenes, anodic oxidation of, 19, 22 ketones, iodide-mediated electrochemical dehydrogenative cyclizations of, 23 heteroatom-centered radicals aza-Wacker-type cyclization reactions, 8–9 halides, 8 KBr or KI, 10 NCRs, 4, 6–7 N–I bond, 10 P-centered radicals, 14 TEMPO, 4 redox-mediated electrochemical cyclization reactions, 4–5

384

redox-active molecules Li-ion batteries, organic active materials for, 229 basic concepts, 230–231 capacity increase, 230, 232 cyclability increase, 232, 234–240 voltage increase, 241–242 redox flow batteries, organic active materials for aqueous electrolyte, 242–244 nonaqueous electrolyte, 244–246 redox-active polymeric materials, 270 conjugated polymers, 249 electrochemical polymer reaction, 252–259 oxidative and reductive electropolymerization, 250–252 polythiophene, doping and dedoping behavior of, 250 redox-active moieties, 268–270 two- and threedimensional, 259–260 nonconjugated polymers, 249 block copolymers, 261–263 dendrimers, periphery of, 265–266 polymeric materials mimicking metalloproteins, 263–265 redox-active inorganic polymers, 266–268 side chain, 261 redox flow batteries, organic active materials for aqueous electrolyte, 242–244 nonaqueous electrolyte, 244–246 redox-mediated electrochemical cyclization reactions, 4–5 restricted intramolecular rotation (RIR), 312

Subject Index

reversible hydrogen electrode (RHE), 184 RHE. See reversible hydrogen electrode (RHE) RIR. See restricted intramolecular rotation (RIR) SAEP. See spatially addressable electrolysis platform (SAEP) SAMs. See self-assembled monolayers (SAMs) saturated calomel electrode (SCE), 140 scalable artificial photosynthesis via sustainable redox processes artificial photosynthetic reactions, 176, 178 frame chemical processes, 176–177 metal halide perovskites, photosynthetic and photocatalytic reduction by Cs2AgBiBr6, 200 CsPbBr3 QD/GO, 196–197 HDA chains, 198 organic ammonium lead halide perovskites, 196 X-ray photoelectron spectra, 196–197 mining plastics waste, 178 nonfood biomass oxidation electrocatalytic and photoelectrocatalytic, 184–188 photocatalytic, 178–184 plastics pollution problem, 175–176, 200 synthetic polymer oxidation heterogeneous photocatalytic oxidation, 189–192 homogeneous photocatalytic oxidation, 192–196 SCE. See saturated calomel electrode (SCE)

Subject Index

SDGs. See Sustainable Development Goals (SDGs) self-assembled monolayers (SAMs), 275, 284 singly occupied molecular orbitals (SOMOs), 334 SOMOs. See singly occupied molecular orbitals (SOMOs) spatially addressable electrolysis platform (SAEP), 61 sugar machine 1,2-trans glycosidic linkages of hexoses via automated electrochemical assembly, 93–95 biologically active oligosaccharides, synthesis of Myc-LCOs, 92–93 TMG-chitotriomycin, 89–92 chemical methods, types of, 80–81 chemoenzymatic method, 80 conventional carbohydrate building blocks, 81 cyclic oligosaccharides via automated electrochemical assembly, 96–97 electrochemical glycosylation glycosyl sulfonium ion intermediates, 84–86 glycosyl triflate intermediates, generation of, 82–84 phenyl glycoside, 81–82 thioglycosides, 81–82 glycosyl imidate, 81 oligosaccharides, automated electrochemical solutionphase synthesis of one-pot iterative glycosylation, principle of, 86–88 tetrasaccharide, 89 thioglycosides, 81 Sustainable Development Goals (SDGs), 301 synthetic polymer oxidation

385

heterogeneous photocatalytic oxidation, 189–192 homogeneous photocatalytic oxidation, 192–196 TAC. See trisaminocyclopropenium cation (TAC) TBS. See tert-butyldimethylsilyl (TBS) TEMPO. See 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) TEOA. See triethanolamine (TEOA) tert-butyldimethylsilyl (TBS), 172 tetrahydrofuran (THF), 112 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 4, 125, 184 TfO . See triflate anion (TfO ) THF. See tetrahydrofuran (THF) TiO2. See titanium oxide (TiO2) titanium oxide (TiO2), 210–212 TMEDA. See N,N,N’,N’-tetramethylethylenediamine (TMEDA) TOTs. See trioxotriangulenes (TOTs) TPrA. See tri-n-propylamine (TPrA) triethanolamine (TEOA), 353 triflate anion (TfO ), 93 tri-n-propylamine (TPrA), 362 trioxotriangulenes (TOTs), 236 trisaminocyclopropenium cation (TAC), 145 vanadium(V)-induced oxidative cross-coupling of enolate species boron and silyl enolates oxidative cross-coupling, 107, 110 oxovanadium(V)-catalyzed oxidative crosscoupling, 110, 112–116 oxovanadium(V)-induced intermolecular selective oxidative crosscoupling, 105–109 intermolecular oxidative crosscoupling, 101–102 stoichiometric and catalytic amounts, 105

386

water, fluorescent sensors for, 328 AIEE, 302 anthracene–amino acid structure, 303 fluorescent sensor-doped polymer films, 326–327 HOMO, 303 ICT-based fluorescent sensors 9-MP, 319–320 9-MP-BF3, 320–321 D–p–A dyes, 319 ET-1-BF3, 322 ET-1-H2O, 322 ST-3-BF3, 324, 326 YNI-2-BF3, 322, 324 Karl Fischer titration method, 300–301 OM-1a, OF-1a, or OF-2a, 307–308 OS-1, 303–305 OS-1, DL values of, 305–307 PET/AIEE-based fluorescent sensors advantage, 316

Subject Index

DPDBF–anthracene– AminoMeCNPhenylBPin structure RS-2, 313–314 RIR, 312–313 RN-1, 313–314 TPE–anthracene–AminoMeCNPhenylBPin structure RS-1, 313 PET-based fluorescent sensors, 301–302 PET/FRET-based fluorescent sensors BODIPY–AminoMePhenylBPin skeleton, 310 disadvantage, 309 DJ-1 and DJ-2, 309–310, 312 PhenylBPin, para-position on, 308 SFC/AIEE-based fluorescent sensors, 316–319