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Radicals, Ion Radicals, and Triplets the Spin-Bearing Intermediates of Organic Chemistry

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Radicals, Ion Radicals, and Triplets the Spin-Bearing Intermediates of Organic Chemistry Radicals, Ion Radicals, and Triplets The Spin-Bearing Intermediates of Organic Chemistry Nathan L. Bauld Department of Chemistry and Biochemistry The University of Texas at Austin Austin, Texas WILEY-VCH New York • Chichester • Weinheim • Brisbane • Singapore • Toronto Contents Preface xiii Acknowledgment xiv 1. Basic Concepts of Free Radicals 1 1.1 Nomenclature 2 1.2 Generation of Free Radicals 2 1.2.1 Redox Cleavage 4 1.2.2 Photochemical Cleavage 4 1.3 The First Identified Free Radical 5 1.4 Detection of Reactive Radicals 6 1.5 Radical Intermediates in Solution 6 1.6 Hyperconjugative Stabilization of Radicals 1.7 Conjugative Stabilization 8 1.8 Three-Electron Bonds 9 1.9 Radical Reaction Modes 9 1.10 Stable Radicals 11 1.11 An "Aromatic" Radical 12 1.12 Radical Scavengers 13 Vlll CONTENTS 1.13 Spin Traps 14 1.14 Radical Inhibitors 15 1.15 Radical Probes and Clocks 15 1.16 Hypersensitive Mechanistic Probes 17 1.17 The Uniqueness of the Radical Probe Reaction 18 1.18 Radical Rearrangements 20 1.19 Nonclassical Radicals 22 1.20 Anchimeric Assistance for Homolysis 23 1.21 Polar Effects in Radical Reactions 23 1.22 Frontier-Orbital Interpretation of Polar Effects in Radical Additions 26 1.23 Radical Substituent Constants 27 1.24 General Methods for the Generation of Specific Carbon Radicals 29 References 32 Exercises 34 2. Radical Reactions 41 2.1 Nonchain Radical Reactions 41 2.1.1 Reactions of Caged Radical Pairs 41 2.1.2 Cage Reactions of Peroxides 42 2.1.3 Concerted versus Stepwise Decomposition 43 2.1.4 Cage Reactions: Azo Compounds 44 2.1.5 Formation of Grignard Reagents 44 2.1.6 Reduction of Organomercury Derivatives 45 2.1.7 The Kolbe Coupling Reaction 45 2.1.8 Homolytic Aromatic Substitution 45 2.1.9 Diradicals 48 2.1.10 Trimethylenemethane Diradicals 49 2.1.11 1,4-Diradicals 49 2.1.12 Diradical Cycloadditions and Cycloreversions 51 2.1.13 Dehydroaromatic Diradicals 51 2.2 Radical Chain Reactions 52 2.2.1 Homolytic Additions (AdH) 53 2.2.2 Stereochemistry 56 2.2.3 Energetics 57 2.2.4 The Scope of Radical Chain Additions 59 2.2.5 Thiol Additions 59 CONTENTS IX 2.2.6 Vinyl Polymerization 61 2.2.7 More General Aspects of Radical Chain Reaction Kinetics 64 2.2.8 Radical Cyclizations 65 2.2.9 Homolytic Substitution (5H) 67 2.2.10 Chlorination 67 2.2.11 Bromination 69 2.2.12 Bromination by N-Bromosuccinimide 70 2.2.13 Autoxidation 71 2.2.14 Inhibition 74 2.2.15 Hydrogen Abstraction from Electronegative Atoms by Electronegative Radicals 75 References 76 Exercises 78 3. The Characterization of Radicals and Radical Pairs by ESR and CIDNP 85 3.1 Electron Spin Resonance 85 3.1.1 The Hydrogen Atom 86 3.1.2 The Methyl Radical; a Hyperfine Splittings 88 3.1.3 The Ethyl Radical; ß Hyperfine Splittings 89 3.1.4 The Allyl Radical; The McConnell Equation 90 3.1.5 Cyclic Delocalized Radicals 92 3.1.6 Conformational Dependence of ß Hyperfine Splittings 93 3.1.7 Reinforcement/Interference Effects upon ß Hyperfine Splittings 94 3.1.8 Long-Range Hyperfine Splittings 95 3.1.9 Nitrogen Splittings 96 3.1.10 13C Hyperfine Splittings; The Hybridization of the Radical Site 97 3.2 Electron-Nuclear Double Resonance (ENDOR) Spectroscopy 99 3.3 Chemically Induced Dynamic Nuclear Polarization (CIDNP) 100 3.3.1 CIDNP Exemplified 101 3.4 Chemically Induced Dynamic Electron Polarization (CIDEP) 104 References 104 Exercises 106 4. Anion Radicals 113 4.1 Formation of Anion Radicals 114 4.2 Simple TT-Type Anion Radicals 114 X CONTENTS 4.3 The Butadiene Anion Radical 116 4.4 The Tetracyanoethylene Anion Radical: A Stable Anion Radical 117 4.5 Anion Radicals of Aromatic Systems 118 4.6 Anion Radicals of Nonbenzenoid Cyclic Conjugated Systems: The Cyclooctatetraene Anion Radical 121 4.7 Multianion Radicals 122 4.8 Birch Reduction: Protonation of Anion Radicals 124 4.9 The Pinacol Coupling Reaction 127 4.10 The Acyloin Condensation 129 4.11 Semidiones and Semiquinone 129 4.12 Fragmentation Reactions and Their Reversal 132 4.13 The SRA,1 Reaction 132 4.14 Pericyclic Reactions 133 References 135 Exercises 137 5. Cation Radicals 141 5.1 Analogy to Anion Radicals: The Pairing Theorem 142 5.2 Historical 143 5.3 Scope of Cation Radical Formation 144 5.4 Simple Inorganic Cation Radicals 145 5.5 Classification of Cation Radicals 146 5.6 Structure 147 5.7 Reactivity 147 5.8 Chemical Methods of Preparation 148 5.9 Physical, Photochemical, and Electrochemical Methods 149 5.10 Cation Radicals of Small Organic Molecules 149 5.11 Cation Radical Reactions: Electron (Hole) Transfer 151 5.12 Acidity: Thermodynamic and Kinetic 152 5.13 Reactions with Nucleophiles 154 CONTENTS XI 5.14 Radical Coupling 155 5.15 Cation Radical-Radical Coupling: The ET Mechanism for Aromatic Nitration 155 5.16 Mesolytic Cleavages 157 5.17 Abstractions 158 5.18 Rearrangements 159 5.19 Chain versus Catalytic Mechanisms 163 5.20 Cycloadditions 164 5.21 Role Selectivity in Diels-Alder Additions 166 5.22 Cyclobutanation 169 5.23 Periselectivity 169 5.24 Cation Radical versus Br0nsted Acid-Catalyzed, Carbocation-Mediated Reactions 170 5.25 Reactions with Dioxygen 171 References 172 Exercises 175 6. Ion Radical Pairs and Electron Transfer 181 6.1 The Energetics of Electron Transfer 182 6.2 Rates of Electron Transfer 182 6.3 The Marcus Equation: Derivation 183 6.4 Electrostatic Effects: The Füll Marcus Equation 185 6.5 Limitations of the Marcus Equation 186 6.6 Back Electron Transfer in Contact Ion Radical Pairs 187 6.7 Back Electron Transfer in Solvent-Separated Ion Radical Pairs 188 6.8 Production of Triplets via Back Electron Transfer 189 6.9 Ion Radical Chemistry via Photosensitized Electron Transfer 191 6.10 Thermal Electron Transfer 193 6.11 Stable Cation Radical-Anion Radical Pairs 195 6.12 Intramolecular Electron Transfer 196 Ml CONTENTS 6.13 Electron Transfer via Tunneling 197 References 198 Exercises 199 7. Triplets and Higher Multiplets 203 7.1 Spin Functions of the Triplet State 203 7.2 Relative Stability of Triplets and Singlets 205 7.3 Noninteracting or Weakly Interacting Triplets 207 7.4 Excited-State Triplets 207 7.5 Dioxygen: A Stable Triplet Ground State 208 7.6 Stable Organic Triplets 209 7.7 Persistent Triplets in Antiaromatic Systems 210 7.8 Carbenes: Reactive Ground-State Triplets 211 7.9 Reactions of Triplet Carbenes 212 7.10 Diphenylcarbene 213 7.11 Triplet Carbenes That Are Not Ground States 214 7.12 Nitrenes 214 7.13 Triplet Ground States of Non-Kekule Hydrocarbon Diradicals 215 7.14 Triplet Sensitization: Diene Triplets 216 7.15 Geometrie Isomerization of Triplet States 218 7.16 Thermally Generated Excited Triplets 219 7.17 Triplet ESR Spectra 219 7.18 Triphenylene Triplet 222 7.19 Naphthalene Triplet 222 7.20 Trimethylenemethane (TMM) Triplet 223 7.21 Higher Multiplets 223 References 224 Exercises 225 Index 229 .
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