AZIDES AND NITRENES
PART 1 PROPERTIES AND REACTIONS OF CARBENES 1 1 Carbene Stability 3 11 Introduction 3 12 Background 4 121 Measures of Carbene Stability 4 122 Bonding and Orbital Interactions in Carbenes 6 123 Determining Carbene Stability 8 13 Carbene Stability 11 131 Hydrocarbon-Substituted Carbenes 11 132 Substituted Phenylcarbenes 16 133 Heteroatom-Substituted Carbenes 20 134 Conjugated Heterocyclic Carbenes 24 135 Carbenes with Other Electron-Withdrawing Groups 27 136 Carbenes versus Ylides 29 14 Correlations Involving Carbene Stability 30 141 CSE versus Singlet–Triplet Gap 30 142 CSE(Singlet) versus Proton Affinities 31 143 CSE(Singlet) versus Carbene Reaction Rates 32 15 Conclusion and Outlook 35 Suggested Reading 35 References 36 2 Stable Carbenes 40 21 Introduction 40 22 Types of Stable Carbenes 41 221 Push–Push Carbenes 42 222 Push–Pull Carbenes 48 223 Abnormal Carbenes 49 23 Spectroscopic Characteristics 50 24 Chemical Reactivity 52 241 Heteroallenes/Electrophilic Traps 55 242 Lewis Acids: Boryl Compounds 56 243 Alkenes, Alkynes, and Nitriles 56 244 Carbon Monoxide and Isonitriles 59 245 X–H Activation 60 246 Homodiatomic Bond Activation 63 247 Elemental Allotropes and other Reactive Species 65 25 Conclusions and Outlook 65 Suggested Reading 67 References 67 3 Acid–Base Chemistry of Carbenes 75 31 Introduction 75 32 Solution pKas of the Conjugate Acids of Carbenes 76 321 Experimental Methods in Aprotic Solvents 77 322 Experimental Methods in Aqueous Solution 80 323 Substituent Effects on pKa 85 324 Solvent Effects on pKa 91 33 Gas-Phase Basicities and Proton Affinities of Carbenes 92 331 Experimental Methods 93 332 Heteroatom-Stabilized Carbenes 94 333 Alkyl- and Aryl-Substituted Carbenes 100 334 Halocarbenes 102 34 Conclusion and Outlook 103 Suggested Reading 104 References 104 4 Computational Methods for the Study of Carbenes and their Excited States 107 41 Introduction 107 42 Carbenes 109 43 Rearrangement in Excited States (RIES) 111 44 Advances in Computational Investigations of Carbenes 115 441 Configuration Interaction 117 442 Complete Active Space Self-Consistent Field (CASSCF) Theory 118 443 Coupled-Cluster Theory with Resolution-of-the-Identity Approximation (RI-CC2) 121 444 Time-Dependent Density Functional Theory (TD-DFT) 121 445 Molecular Dynamics (MD) 123 45 Theoretical Studies of the Photochemistry of Carbene Precursors 123 46 Conclusion and Outlook 127 Suggested Reading 128 References 128 5 Dynamics in Carbene Reactions 131 51 Introduction 131 511 Carbenes 131 512 Nonstatistical Reaction Dynamics 133 52 Dynamics of Carbene Cycloadditions to Alkenes and Alkynes 136 521 Carbene Additions to Unstrained Alkenes 136 522 Carbene Additions to Strained ? Systems 144 523 Carbene Additions to Strained ? Systems 152 53 Dynamics of other Carbene-Mediated Reactions 153 531 Wolff Rearrangement 153 532 Photochemical Carbene Formation 156 54 Conclusion and Outlook 159 Suggested Reading 159 References 160 6 U ltrafast Kinetics of Carbene Reactions 166 61 Introduction 166 62 Ultrafast UV-Vis Studies of the Intermolecular Reactivity of p-Biphenylylcarbene (Bpch) 167 63 Rearrangements in the Excited State of the Carbene Precursor 171 64 Dynamics of Carbene Vibrational Cooling and Solvation 173 65 Influence of Solvent on Carbene Intersystem Crossing Rates 176 66 Electronically Excited (Open Shell) Singlet Carbenes 179 67 Parent Phenyldiazirine—Mechanistic aspects of Singlet Carbene Formation 179 68 Influence of Halo-Substituent Electron-Donating Capacity on Diazirine Decay in the First Excited Singlet State 183 69 The Influence of Excitation Wavelength on the Photochemistry of Diazirines 186 610 Conclusion and Outlook 189 Suggested Reading 190 References 191 7 Tunneling in the Reactions of Carbenes and Oxacarbenes 193 71 Introduction: Light- and Heavy-Atom Tunneling 193 72 Alkyl- and Halocarbenes 197 721 Cyclopropylcarbenes 197 722 Ethylidene and Phenylmethylcarbene 200 723 Methylchlorocarbene and Benzylchlorocarbene 201 724 tert-Butylchlorocarbene 202 725 1-Methylcyclobutylfluorocarbene 202 726 Noradamantylchlorocarbene 203 73 The Formose Reaction and Hydroxycarbenes 203 731 Hydroxymethylene 203 732 Methylhydroxycarbene and Tunneling Control 205 733 Arylhydroxycarbenes 206 734 Cyclopropylhydroxycarbene 209 74 Conclusion and Outlook 210 Suggested Reading 211 References 211 8 Carbodicarbenes 216 81 Introduction 216 82 Carbodicarbenes with N-Heterocyclic Ligands C(NHC)2 218 83 Tetraaminoallenes and “Hidden” Carbodicarbenes 225 84 Bent Allenes 228 85 Related Compounds 233 86 Conclusion and Outlook 234 Suggested Reading 234 References 234 9 Catalytic Reactions with N-Mesityl-Substituted N-Heterocyclic Carbenes 237 91 Introduction 237 911 Historical Background 238 912 State of the Art Prior to 2004 238 913 N-Mesityl Catalysts as the Key Innovation 239 92 The N-Mesityl Group: A Mechanistic Aspect 242 921 Catalytic Generation of Reactive Species via N-Mesityl NHCs 242 922 The Kinetic Effect of the N-Mesityl Group 243 93 NHC Catalysis by Class of Reactive Intermediates 246 931 Acyl Anion Equivalent 246 932 Homoenolate Equivalent 248 933 Enolate Equivalent 254 934 Activated Carboxylate Equivalent 257 935 a,b-Unsaturated Activated Carboxylate Equivalent 264 94 Conclusion and Outlook 268 Suggested Reading 268 References 268 10 Supramolecular Carbene Chemistry 274 101 Introduction 274 102 Types of Hosts used in Supramolecular Carbene Chemistry 276 1021 Cyclodextrins 276 1022 Hemicarcerands 276 1023 Resorcin[4]arene-Based Cavitands 278 1024 Octa Acid 278 1025 Cucurbit[n]urils 278 1026 Zeolites 280 103 Choosing the Right Carbene Guest 283 104 Diazirines as Suitable Supramolecular Carbene Precursors 285 105 Architecture of the Guest@Host Complex 287 106 Case Studies 291 1061 Carbenes with Available 1,2-H Shifts 291 1062 (2-[1,3-Dioxolan-2-yl]phenyl)(4-phenylphenyl)- carbene 294 1063 4-Oxocyclohexa-2,5-dienylidene 295 1064 Dichlorocarbene 296 1065 Adamantylidene 296 1066 3-Oxobicyclo[321]octan-8-ylidene 302 1067 endo-3-Hydroxybicyclo[321]octan-8-ylidene 304 1068 Ring Fragmentation and Expansion 305 1069 Aryl(halo)carbenes 308 10610 Fluoro(phenoxy)carbene 312 10611 Persistent Triplet Carbenes 312 107 Conclusion and Outlook 313 Acknowledgments 315 Suggested Reading 315 References 315 PART 2 Meta l Carbenes325 11 Modern Lithium Carbenoid Chemistry 327 111 Introduction 327 112 Structural Features of Lithium Carbenoids 328 113 Lithium Halide Carbenoids 329 1131 Thermal and Kinetic Stability 329 1132 Configurational Stability and Stereochemistry of the Coupling Reactions with Electrophiles 331 114 Structure–Reactivity Relationships 342 115 Lithium–Oxygen Carbenoids 344 116 Lithium–Nitrogen Carbenoids 352 117 Conclusion and Outlook 355 Suggested Reading 357 References 357 12 Rhodium Carbenes 363 121 Introduction 363 122 Overview of Rhodium–Carbenoid Intermediates and Chiral Catalysts 364 123 Enantioselective Cyclopropanation 367 124 Cascade Sequences Initiated by Rhodium-Catalyzed Cyclopropanation 370 125 Enantioselective Cyclopropenation 373 126 C–H Functionalization By Carbenoid-Induced C–H Insertion 375 127 Combined C–H Activation/Cope Rearrangement (CHCR) 380 128 Formation and Reactions of Rhodium-Bound Ylides 383 129 Vinylogous Reactions of Rhodium– Vinylcarbenoids 392 1210 Conclusions and Future Outlook 397 Suggested Reading 397 References 398