About the Volume Editor ...................................... xiii
Preface to Series .............................................. xv
Introduction ................................................. xvii
Contributors .................................................. xxi
1 A Brief History of Carbon Radicals ........................... 1
Malcolm D.E. Forbes
2 Intermolecular Radical Additions to Alkynes: Cascade-Type
Radical Cyclizations ......................................... 9
Uta Wille
2.1 Introduction ............................................ 9
2.2 Cascade Reactions Involving Radicals of Second Row
Elements ............................................... 11
2.2.1 Cascade Reactions Initiated by Addition of
C-Centered Radicals to Alkynes .................. 11
2.2.2 Cascade Reactions Initiated by Addition of
O-Centered Radicals to Alkynes (Self-
Terminating Radical Oxygenations) ............... 16
2.2.3 Cascade Reactions Initiated by Addition of
N-Centered Radicals to Alkynes .................. 24
2.3 Cascade Reactions Initiated by Addition of Higher
Main Group (VI)-Centered Radicals to Alkynes ........... 27
2.3.1 Cascade Reactions Initiated by Addition of
Sn-Centered Radicals to Alkynes ................. 27
2.4 Cascade Reactions Initiated by Addition of Higher
Main Group (VI)-Centered Radicals to Alkynes ........... 30
2.4.1 Cascade Reactions Initiated by Addition of
S-Centered Radicals to Alkynes .................. 30
2.4.2 Cascade Reactions Initiated by Addition of
Se-Centered Radicals to Alkynes ................. 36
2.5 Cascade Reactions Initiated by Addition of Higher
Main Group (V)-Centered Radicals to Alkynes ............ 37
2.5.1 Cascade Reactions Initiated by Addition of
P-Centered Radicals to Alkynes ................... 37
3. Radical Cation Fragmentation Reactions in Organic
Synthesis ................................................... 43
Alexander J. Poniatowski and Paul E. Floreancig
3.1 Introduction ........................................... 43
3.1.1 Oxidative Carbon-Carbon Bond Cleavage ........... 44
3.1.2 Thermodynamic and Kinetic Considerations ........ 46
3.1.3 Reactive Intermediate Lifetime .................. 49
3.2 Electron Transfer-Initiated Cyclization Reactions ...... 49
3.2.1 Rate Enhancement and Mechanistic Studies ........ 50
3.2.2 Development of a Catalytic Aerobic Protocol ..... 50
3.2.3 Oxidative Cascade Reactions ..................... 52
3.3 Oxidative Acyliminium Ion Formation .................... 52
3.4 Carbon-Carbon Bond Formation ........................... 54
3.4.1 Chemoselectivity and Reactivity ................. 54
3.4.2 Reaction Scone .................................. 55
3.5 Summary and outlook .................................... 58
4 Selectivity in Radical Cation Cycloadditions ................ 61
Christo S. Sevov and Olaf Wiest
4.1 Introduction ........................................... 61
4.2 Mechanism and the Origin of the Rate Acceleration ...... 62
4.3 Selectivity in Radical Cation Cycloadditions ........... 63
4.4 Chemoselectivity ....................................... 64
4.4.1 Effect of Dienophile Substituents on
Chemoselectivity ................................ 64
4.4.2 Effect of Sensitizers and Solvents on
Chemoselectivity ................................ 66
4.4.3 Effect of Concentrations on Chemoselectivity .... 67
4.4.4 Effect of Electron-Rich Dienophiles on
Chemoselectivity ................................ 67
4.5 Regioselectivity .......................................
4.6 Periselectivity ........................................
4.6.1 Effects of Solvent and Concentration on
Periselectivity ................................. 70
4.6.2 Effect of Diene/Dienophile Redox Potentials
on Periselectivity .............................. 71
4.6.3 Substituent and Steric Effects on
Periselectivity ................................. 72
4.6.4 Quantifying Periselectivity Through Ion Pair
Association ..................................... 74
4.7 EndolExo Selectivity ................................... 75
4.7.1 Effects of Secondary Orbital Interaction and
Solvents on Endo/Exo Selectivity ................ 75
4.7.2 Effect of Sensitizer on Endo/Exo Selectivity .... 76
4.7.3 Ion Pairs and Endo/Exo Selectivities ............ 77
4.8 Conclusions ............................................ 79
5 The Stability of Carbon-Centered Radicals ................... 83
Michelle L. Coote, Ching Yeh Lin, and Hendrik Zipse
5.1 Introduction ........................................... 83
5.1.1 The Consequences of Different Stability
Definitions: How Stable Are Ethyl and
Fluoromethyl Radicals? ........................... 85
5.2 Theoretical Methods .................................... 86
5.2.1 Testing the Performance of Different
Theoretical Approaches: How Stable Are Allyl
and Benzyl Radicals? ............................ 87
5.2.2 The Application of IMOMO Schemes: How Stable
Are Benzyl and Diphenylmethyl Radicals? ......... 89
5.3 RSE Values for Carbon-Centered Radicals ................ 91
5.4 Use of RSE Values in Practical Applications ............ 91
5.4.1 Susceptibility to Hydrogen Atom Abstraction ..... 91
5.4.2 Assessment of Radical Stability in Other
Types of Reactions ............................. 100
5.5 Conclusions ........................................... 102
6 Interplay of Stereoelectronic Vibrational and
Environmental Effects in Tuning Physicochemical
Properties of Carbon-Centered Radicals ..................... 105
Vincenzo Barone, Malgorzata Biczysko, and Paola Cimino
6.1 Introduction .......................................... 105
6.2 EPR Spectroscopy ...................................... 107
6.2.1 Theoretical Background ......................... 107
6.2.2 Environmental Effects .......................... 108
6.2.3 Vibrational Effects ............................ 108
6.2.4 Dynamical Effects .............................. 109
6.3 Calculation of EPR Parameters ......................... 110
6.3.1 Geometric Parameters ........................... 112
6.3.2 EPR Parameters ................................. 113
6.3.3 Case Studies: Glycine and Glycyl Radicals ...... 117
6.3.3.1 Glycine Radical ....................... 117
6.3.3.2 Glycyl Radical ........................ 119
6.3.4 Case Studies: Vibrationally Averaged
Properties of Vinyl and Methyl Radicals ........ 120
6.4 Vibrational Properties Beyond the Harmonic
Approximation ......................................... 122
6.4.1 Case Studies: Anharmonic Frequencies of
Phenyl and Naphthyl Cation Radicals ............ 122
6.4.2 Case Studies: Gas and Matrix Isolated IR
Spectra of the Vinyl Radical ................... 125
6.5 Electronic Properties: Vertical Excitation
Energies, Structure, and Frequencies in Excited
Electronic States ..................................... 126
6.5.1 Theoretical Background ......................... 126
6.5.2 Case Studies: Vertical Excitation Energies
of the Vinyl Radical ........................... 126
6.5.3 Case Studies: Structures and Frequencies of
the Vinyl Radical in First Three Doublet
Excited Electronic States ...................... 129
6.6 Vibronic Spectra ...................................... 129
6.6.1 Theoretical Background ......................... 132
6.6.2 Computational Strategy ......................... 134
6.6.3 Case Studies: Electronic Absorption Spectrum
of Phenyl Radical .............................. 134
6.7 Concluding Remarks .................................... 137
7 Unusual Structures of Radical Ions in Carbon Skeletons:
Nonstandard Chemical Bonding by Restricting Geometries ..... 141
Georg Gescheidt
7.1 Introduction .......................................... 141
7.2 The Tools ............................................. 142
7.2.1 Cyclovoltammetry ............................... 143
7.2.2 EPR Parameters: Experimental and Calculated .... 143
7.3 Pagodane and Its Derivatives .......................... 144
7.4 Different Stages of Cycloaddition/Cycloreversion
Reactions Within Confined Environments ................ 151
7.5 Extending the "Cage Concept" .......................... 152
7.6 Summary ............................................... 154
8 Magnetic Field Effects on Radical Pairs in Homogeneous
Solution ................................................... 157
Jonathan R. Woodward
8.1 Introduction .......................................... 157
8.2 The Spin-Correlated Radical Pair ...................... 158
8.2.1 Radical Pair Interactions ...................... 159
8.2.2 Intraradical Interactions ...................... 159
8.2.3 Interradical Interactions ...................... 160
8.3 Application of a Magnetic Field ....................... 162
8.3.1 The Zeeman Effect .............................. 162
8.4 Spin-State Mixing ...................................... l63
8.4.1 Coherent Spin-State Mixing ..................... l63
8.4.2 The Life Cycle of a Radical Pair ............... 165
8.4.3 Incoherent Spin-State Mixing ................... 167
8.5 The Magnetic Field Dependence of Radical Pair
Reactions ............................................. 167
8.5.1 "Normal" Magnetic Fields ....................... 167
8.5.2 Weak Magnetic Fields ........................... 169
8.5.3 Strong Magnetic Fields ......................... 171
8.6 Theoretical Approaches ................................ 172
8.6.1 General Approaches ............................. 172
8.6.2 Modeling Diffusion ............................. 173
8.6.3 The Semiclassical Approach ..................... 173
8.6.4 The Stochastic Liouville Equation .............. 174
8.6.5 Monte Carlo Approaches ......................... 174
8.7 Experimental Approaches ............................... 174
8.7.1 Fluorescence Detection ......................... 175
8.7.2 Optical Absorption Detection ................... 176
8.7.3 Rapid Field Switching .......................... 176
8.8 The Life Cycle of Radical Pairs in Homogeneous
Solution .............................................. 176
8.8.1 Differentiating G-Pairs and F-Pairs ............. 177
8.9 Summary ............................................... 180
9 Chemical Transformations Within the Paramagnetic World
Investigated by Photo-CIDNP ................................ 185
Martin Goez
9.1 Introduction .......................................... 185
9.2 CIDNP Theory .......................................... 186
9.3 Experimental Methods .................................. 190
9.4 Radical-Radical Transformations During Diffusive
Excursions ............................................ 191
9.5 Radical-Radical Transformations at Reencounters ....... 196
9.6 Interconversions of Biradicals ........................ 199
9.7 Conclusions ........................................... 203
10 Spin Relaxation in Ru-Chromophore-Linked Azine/Diquat
Radical Pairs .............................................. 205
Matthew T. Rawls, Ilya Kuprov, С. Michael Elliott, and
Ulrich E. Steiner
10.1 Introduction .......................................... 205
10.2 EPR for the Isolated Ions ............................. 209
10.3 Calculation Methods for EPR of the Isolated Ions ...... 211
10.3.1 Calculation of g Tensor Components ............. 212
10.3.2 Calculation of Hyperfine Coupling Constants .... 213
10.3.2.1 Ab Initio Hyperfine Coupling
Constants: General Notes .............. 213
10.3.2.2 Theoretical Values of Isotropic and
Anisotropic Hyperfine Coupling
Constants ............................. 214
10.4 Implications for Spin-Relaxation in Linked Radical
Pairs ................................................. 216
11 Reaction Dynamics of Carbon-Centered Radicals in Extreme
Environments Studied by the Crossed Molecular Beam
Technique .................................................. 221
Ralf I. Kaiser
11.1 Introduction .......................................... 221
11.2 The Crossed Molecular Beam Method ..................... 223
11.3 Experimental Setup .................................... 224
11.3.1 The Crossed Beam Machine ....................... 224
11.3.2 Supersonic Beam Sources ........................ 227
11.3.2.1 Ablation Source ....................... 227
11.3.2.2 Pyrolytic Source ...................... 228
11.3.2.3 Photolytic Source ..................... 228
11.4 Crossed Beam Studies .................................. 229
11.4.1 Reactions of Phenyl Radicals ................... 229
11.4.2 Reactions of CN and C2H Radicals ............... 236
11.4.3 Reactions of Carbon Atoms, Dicarbon
Molecules, and Tricarbon Molecules ............. 237
11.5 Conclusions ........................................... 240
12 Laser Flash Photolysis of Photoinitiators: ESR, Optical,
and IR Spectroscopy Detection of Transients ................ 249
Igor V. Khudyakov and Nicholas J. Turro
12.1 Introduction .......................................... 249
12.2 Photodissociation of Initiators ....................... 250
12.2.1 Quantum Yields of Free Radicals in
Nonviscous Solutions ........................... 250
12.2.2 Cage Effect Under Photodissociation ............ 252
12.2.3 The Magnetic Field Effect on
Photodissociation .............................. 253
12.3 TR ESR Detection of Transients ........................ 254
12.3.1 CIDEP Under Photodissociation of Initiators .... 254
12.3.2 Addition of Free Radicals to the Double
Bonds of Monomers .............................. 260
12.3.3 Electron Spin Polarization Transfer from
Radicals of Photoinitiators to Stable
Nitroxyl Polyradicals .......................... 268
12.4 Optical Detection of Transients ....................... 270
12.4.1 UV-vis Spectra of Representative Radicals ...... 270
12.4.2 Representative Kinetic Data on Reactions of
Photoinitiator Free Radicals ................... 270
12.5 IR Detection of Free Radicals and Monitoring Their
Reactions ............................................. 274
12.6 Concluding Remarks .................................... 274
13 Dynamics of Radical Pair Processes in Bulk Polymers ........ 281
Carlos A. Chesta and Richard G. Weiss
13.1 Introduction .......................................... 281
13.1.1 General Considerations ......................... 281
13.1.2 Escape Probability of an Isolated, Intimate
Radical Pair in Liquids and Bulk Polymers ...... 283
13.2 Singlet-State Radical Pairs from Irradiation of
Aryl Esters and Alkyl Aryl Ethers ..................... 286
13.2.1 General Mechanistic Considerations From
Solution and Gas-Phase Studies ................. 286
13.2.1.1 Photo-Fries Reactions of Aryl
Esters ................................ 287
13.2.1.2 Photo-Claisen Reactions of Alkyl
Aryl Ethers ........................... 289
13.3 Photo-Reactions of Aryl Esters in Polymer Matrices.
Kinetic Information from Constant Intensity
Irradiations .......................................... 289
13.3.1 Relative Rate Information from Irradiation
of Aryl Esters in Which Acyl Radicals Do Not
Decarbonylate Rapidly .......................... 290
13.3.2 Absolute and Relative Rate Information from
Constant Intensity Irradiation of Aryl
Esters in Which Acyl Radicals Do
Decarbonylate Rapidly .......................... 293
13.4 Rate Information from Constant Intensity
Irradiation of Alkyl Aryl Ethers ...................... 297
13.4.1 Rate Information from an Optically Active
Ether .......................................... 299
13.4.1.1 Results from Irradiation in
n-Alkane Solutions .................... 299
13.4.1.2 Results from Irradiation in
Polyethylene Films .................... 304
13.5 Comparison of Calculated Rates to Other Methods for
Polyethylene Films .................................... 306
13.6 Triplet-State Radical Pairs ........................... 308
13.6.1 Triplet-State Radical Pairs from the
Photoreduction of Benzophenone by Hydrogen
Donors ......................................... 308
13.6.2 Triplet-State Radical Pairs from Norrish
Type I Processes ............................... 311
13.7 Concluding Remarks .................................... 318
14 Acrylic Polymer Radicals: Structural Characterization
and Dynamics ............................................... 325
Malcolm D.E. Forbes and Natalia V. Lebedeva
14.1 Introduction .......................................... 325
14.2 The Photodegradation Mechanism ........................ 326
14.3 Polymer Structures .................................... 327
14.4 The Time-Resolved EPR Experiment ...................... 329
14.5 Tacticity and Temperature Dependence of Acrylate
Radicals .............................................. 332
14.6 Structural Dependence ................................. 334
14.6.1 d3-Poly(methyl methacrylate), d3-PMMA .......... 335
14.6.2 Poly(ethyl methacrylate), РЕМА ................. 337
14.6.3 Poly(ethyl cyanoacrylate), PECA ................ 337
14.6.4 Poly(ethyl acrylate), PEA ...................... 337
14.6.5 Poly(fluorooctyl methacrylate), PFOMA .......... 338
14.6.6 Polyacrylic Acid, PAA .......................... 339
14.6.7 Polymethacrylic Acid, PMAA ..................... 340
14.7 Oxo-Acyl Radicals ..................................... 340
14.8 Spin Polarization Mechanisms .......................... 343
14.9 Solvent Effects ....................................... 344
14.9.1 pH Effects on Poly (acid) Radicals ............. 344
14.9.2 General Features for Polyacrylates ............. 345
14.10 Dynamic Effects ...................................... 347
14.10.1 The Two-Site Jump Model ....................... 348
14.10.2 Simulations and Activation Parameters ......... 350
14.11 Conclusions .......................................... 352
Index ......................................................... 359
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