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Streamlining Free Radical Green Chemistry Streamlining Free Radical Green Chemistry V. Tamara Perchyonok VTPCHEM PTY LTD, Melbourne, Victoria, Australia Ioannis Lykakis Department of Chemistry, University of Crete, Voutes-Heraklion, Greece AI Postigo Faculty of Science, University of Belgrano, Buenos Aires, Argentina RSC Publishing Contents and Chapter 1 Development of Free Radical Green Chemistry Technology: Journey through Times, Solvents, Causes, Effects and Assessments 1.1 Introduction 1 1.2 The Major Use of Free Radical Green Chemistry from the Beginning 4 4 1.3 Alternative Feedstocks 5 1.3.1 Innocuous or More Innocuous 1.3.2 Renewable 5 1.3.3 Light 5 1.3.4 Solve Other Environmental Problems 6 1.3.5 Biocatalysis 6 7 1.4 Benign Reagents/Synthetic Pathways 7 1.4.1 Innocuous or More Innocuous 1.4.2 Generates Less Waste 7 1.4.3 Selective 8 1.4.4 Catalytic 8 8 1.5 Biomass: Utilization and Sustainability 9 1.6 Green Chemical Syntheses and Processes 1.7 Basic Radical Chemistry: Structure, Reactions and Rates 10 1.7.1 General Aspects of Synthesis with Radicals: Advantages and Traditions 10 1.7.2 Reactions Between Radicals 10 1.7.3 Reaction Between a Radical and a Non-radical 10 1.7.4 Reactivity and Selectivity 11 11 1.7.5 Enthalpy: In Brief 13 1.7.6 Entropy 1.7.7 Steric Effects 14 1.7.8 Stereoelectronic Effects 15 Streamlining Free Radical Green Chemistry V. Tamara Perchyonok, Ioannis Lykakis and Al Postigo 2012 © V. Tamara Perchyonok, Ioannis Lykakis and AI Postigo Published by the Royal Society of Chemistry, www.rsc.org ix X Contents 1.7.9 Polarity 17 1.8 Solvent Effect and Free Radical Transformations: General Understanding 17 22 1.9 Why Water as a Solvent? Reasons and Advantages 1.9.1 Solubility of Organic Compounds in Water 23 1.9.2 Organic Cosolvents 23 1.9.3 Ionic Derivatization (pH Control) 24 1.9.4 Surfactants 24 1.9.5 Hydrophilic Auxiliaries 25 1.9.6 Summary 25 1.10 Classical Synthesis in Modern Solvents 25 1.10.1 Perfluorinated Solvents—a Novel Reaction Medium in Organic Chemistry: General Introduction 26 1.10.2 Benzotrifluoride and Derivatives: Useful Solvents for Organic Synthesis and Fluorous Synthesis 28 1.10.3 Reactions in Supercritical Carbon Dioxide 29 (SCCO2) as a Novel Reaction Medium 1.10.4 Solvent-free Reactions as an Alternative: General Interest for Solvent-free Processes 29 1.11 Methods of Generating Free Radicals 31 1.11.1 Thermal Cracking 31 1.11.2 Homolysis of Peroxides and Azo Compounds 32 1.11.3 Photolytic Bond Homolysis 32 1.11.4 Electron Transfer 32 1.11.5 Hydrogen and Halogen Atom Abstraction 33 1.11.6 The Configuration of Free Radicals 33 1.11.7 Elementary Reaction Steps between Radicals and Non-radicals: Reactions of Free Radicals 34 1.12 Sustainable Chemistry Metrics and Radical Chemistry: Comparative Approach 35 1.12.1 Classical Metrics of Chemical Reactions 36 1.12.2 How do Contemporary Free Radical Transformations Hold Up? Focus on Sustainability, Atom Efficiency and Advantages 38 1.13 Classics and Catalysis in Free Radical Chemistry: Reagents, Reactants and Protocols 45 1.14 Radical cascades and Free Radical Green Chemistry 46 1.15 Artificial Enzymes in Free Radical Synthetic Chemistry: the Chemist's Perspective 47 1.16 Future Challenges and Opportunities for the Chemical Profession and the Science of Chemistry 48 Streamlining Free Radical Green Chemistry xi 1.17 An Environmentally Friendly Economy from Green Chemistry 49 1.17.1 Renewable Energy Sources 49 1.17.2 Renewable Feedstocks 50 1.17.3 Pollution Reduction 50 1.17.4 Interdisciplinary Approach 50 1.18 Conclusion and Future Direction 51 References 51 in Chapter 2 Classical Synthetic Free Radical Transformations Alternative Media: Supercritical C02, Ionic Liquids and Fluorous Media 2.1 Introduction 58 2.2 Radicals in Synthetic Chemistry in the Nutshell 58 2.3 Reactions between Radicals 59 2.4 Elementary Reaction Steps between Radicals and Non-radicals 60 2.4.1 Additions 61 2.4.2 Substitution (Abstraction) Reactions 63 2.4.3 Elimination Reactions 64 2.4.4 Rearrangement Reactions 64 2.4.5 Termination/Electron Transfer Reactions 65 2.5 Reactivity and Selectivity 65 Processes: 2.6 Chain vs. Non-chain Free Radical 66 Reasons, Relevance and Outlook 2.7 Radical Reactions in Supercritical Fluids 66 2.7.1 Radical Reactions and Supercritical C02: Is 66 There a Hidden Advantage? 2.7.2 Radical Reactions in Supercritical Carbon Dioxide in Detail 68 2.7.3 Future Directions 70 2.8 Radical Reactions in Ionic Liquids 72 2.8.1 Ionic Liquids and Alternative Media: General Introduction 72 2.8.2 Radical Chain Reactions in Ionic Liquids: Triethylborane-induced Radical Reactions 72 2.8.3 Radical Additions of Thiols to Alkenes and Alkynes in Ionic Liquids 73 2.9 Radical Non-Chain Reactions in Ionic Liquids 75 2.9.1 Formation of Radicals by Oxidation with Transition Metal Salts: General Perspective 75 2.9.2 Oxidations involving Mn(in) in Ionic Liquids 75 xii Contents 2.9.3 Supported Ionic Liquids: Versatile Reaction and Separation Media—the Latest Developments 79 2.9.4 Conclusions and Future Directions 80 2.10 Fluorous Chemistry as an Alternative Reaction Medium for Free Radical Transformations 81 2.10.1 Fluorous Separation Techniques: from "Liquid-Liquid" to "Solid-Liquid" and "Light Fluorous" 81 2.10.2 Fluorous Chemistry and Radicals— Combined Efforts to the Rescue 83 2.10.3 Fluorous Radical Carbonylation Reactions: from Synthetic Approach to Practical Applications 83 2.11 Ishii Oxidation in Detail 89 2.12 From Phase-separation to Phase-vanishing Methods based on Fluorous-phase Screen: a Simple Way for the Efficient Execution of Organic Synthesis 91 2.13 Conclusions and Future Directions in Fluorous Chemistry 93 2.14 General Conclusion 93 References 94 Chapter 3 Solvent-Free Carbon-Carbon Bond Formations in Ball Mills and in the Solid State 3.1 Introduction 99 3.2 Radical Additions to Imines Mediated by Mn(m) 102 3.3 Solid-phase Homolytic Substitution in Action 102 3.4 Future Directions 104 References 104 Chapter 4 Microwaves in Synthesis: How do Microwaves Promote the Reaction in Conventional and Alternative Media? 4.1 Introduction 106 4.2 Microwave-assisted Fluorous Synthesis 107 4.3 Nitroxide-mediated Radical Cyclization and Intramolecular Addition Reactions In Microwaves 107 4.3.1 The Persistent Radical Effect: General Introduction 107 4.4 Radical Addition to C=N bonds in the Microwave 110 4.5 Microwave-assisted Generation of Alkoxyl Radicals and their Use in Additions, P- Fragmentations and Remote Functionalization 113 Streamlining Free Radical Green Chemistry xiii 4.6 Atom-transfer Reactions as Efficient and Novel Benzannulation Reactions in the Microwave 114 4.7 Conclusions and Future Directions 115 References 115 Chapter 5 Asymmetric Free-Radical Reductions Mediated by Chiral Stannanes, Germanes, and Silanes 5.1 Introduction 117 5.2 Stoichiometric Free Radical Reductions 118 5.3 Scope and Limitations 120 5.4 Examples Relevant to the Fine Chemical Industry 121 5.5 Strategies for the Avoidance of Tin Waste 121 5.6 Immobilization of Tin Reagents 122 5.7 Catalytic Reductions in Tin 123 5.8 Reducing Agents based on Germanium and Silicon 123 5.9 Summary 125 References 125 Chapter 6 Organic Radical Reductions in Water: Water as a Hydrogen Atom Source 6.1 Introduction 127 6.2 Water-soluble Organosilanes and Synthesis 128 6.3 rrw(trimethylsilyl)silane in Water and "on Water" 129 6.4 Triethylborane-Water Complex as a Reducing Agent 134 6.5 Titanium(m)-Water as a Reducing Agent 135 6.6 Summary 136 References 137 Chapter 7 Tin-Free Radical Reactions Mediated by Organoboron Compounds 7.1 Introduction 140 7.2 Organoboranes as Radical Initiators 141 7.3 In Reductive Processes 141 7.3.1 Reduction of Halides and Related Compounds 141 7.3.2 Reductive Addition of Heteroatom-centered Radicals to Alkynes and Alkenes 143 7.3.3 In Fragmentation Processes 144 7.4 In Atom-transfer Processes 145 7.4.1 Iodine Atom Transfer 145 7.4.2 Bromine Atom Transfer 148 7.4.3 Chlorine Atom Transfer 149 7.5 Organoboron Compounds as a Source of Carbon- centered Radicals 150 7.5.1 Conjugated Additions to Enones and Enals 150 xiv Contents 7.5.2 Conjugate Addition to Activated Olefins 155 7.5.3 Addition to Imine Derivatives 158 7.5.4 C-C Bond Formation via P-Fragmentation Processes 158 7.6 Organoboranes as Chain-transfer Reagents 162 7.6.1 Via Iodine Atom Transfer 163 7.6.2 Via Hydrogen Atom Transfer 165 7.7 Organoboron Compounds as Radical-reducing Agents 166 7.7.1 Complexes with Tertiary Amines 166 7.7.2 Complexes with Water and Alcohols 166 7.8 Conclusions 167 References 169 Chapter 8 Thiols as Efficient Hydrogen Atom Donors in Free Radical Transformations in Aqueous Media 8.1 Introduction 175 8.2 The Tris(trimethylsilyl)silane (TMS3SiH)/thiol System is an Efficient Radical Hydrogen Donor "on Water" 176 8.3 Thiol/Azo Initiator System in cis-trans Isomerization of Double Bonds in Aqueous Media 177 8.4 Thiols in Peptides: Degradation in Aqueous Media 181 8.5 Thiols in C-C Bond Formation in Water 182 8.6 Thiol-Ene Coupling as a Click Process for Materials and Bioorganic Chemistry 184 8.7 Hydrogen Sulfide in Oxidation and/or Reduction of Organic Compounds 185 8.8 Thiyl Radicals and the Influence of Antioxidants/Vitamins 186 8.9 Conclusions 189 References 189 Chapter 9 Advances in the Use of Phosphorus-centered Radicals in Organic Synthesis in Conventional Flasks: Advantages, Reasons and Applications 9.1 Introduction 195 9.2 Physical Organic Aspects 196 9.3 Use of P-centered Radicals as Mediators 197 9.4 Synthetic Applications of P-centered Radical Additions 202 9.4.1 Phosphinyl Radicals 203 9.4.2 Phosphonyl Radicals 204 9.5 Radicals
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