"Front Matter". In: Organosilanes in Radical Chemistry - Index of

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Structural Properties of Silyl Radicals 9 towards aromatic substitution (see Section 5.1.1), limited the detection of this type of transients by EPR spectroscopy. For example, PhH2Si:, Ph2HSi: and Ph3Si: radicals have not been observed in solution whereas the corresponding perdeuterated silyl radicals have been detected in a solid matrix [35]. Two sterically hindered analogous radicals, trimesitylsilyl and tris(3,5-di-tertbutylphenyl)silyl have been observed by EPR in solution and appear to be partially delocalized species according to the ring proton hfs constants [36,37]. Similar considerations and analogous experiments have been extended to avinyl substituted silyl radicals and the results are in line with the a-phenyl substituted case [38]. The spectra of Me3Si-substituted silyl radicals are of particular interest. Thus, when Me3Si groups progressively replace methyl groups, the 29Si hfs constants decrease from 181 G in the Me3Si: radical to 64 G in the (Me3Si) 3Si: radical (Table 1.1). This trend is due mainly to the spin delocalization onto the Si C b-bond and in part to the decrease in the degree w of pyramidalization at the radical centre caused by the electron-releasing Me3Si group [39]. Kinetic information from the line width alterations of EPR spectra by changing the temperature has been obtained for a number of silacycloalkyl radicals [40,41]. For example, silacyclopentyl radical exists at low temperature ( 119 8C) in two equivalent twist conformations (11 and 12), which interconvert at higher temperature (15 8C). The Arrhenius parameters for such interconversion are log A=s 1 ¼ 12:0 and Ea ¼ 21:3kJ=mol. H H H Si H H H H Si H H H 11 12 (1.13) Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si: radical contrary to the stable Ph3C: radical is mentioned above. The decay of the trimesitylsilyl radical at 63 C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-butyl groups to decrease pyramidalization [45]. The most persistent trialkyl-substituted silyl radical is [(Me3Si) 2CH] 3Si:, which at 20 8C follows a first-order decay with a half-life of 480 s [36]. An exceptionally stable diradical was isolated by reaction of 1,1-dilithio-2,3,4,5tetraphenylsilole with 1,1-dichloro-2,3-diphenylcyclopropene, for which the structure 13 was suggested on the basis of EPR data and theoretical calculations [46]. The remarkable unreactivity of this diradical has been explained by steric hindrance, as well as delocalization of the unpaired electrons over the silole ring.
10 Formation and Structures of Silyl Radicals Ph Ph Ph Ph Si Ph Ph Ph Si On the other hand, bulky trialkylsilyl substituents have a profound effect on the structure of silyl radicals. Indeed, by increasing the steric effect with more crowded trialkylsilyl substituents the persistency of silyl radicals increases substantially. Table 1.2 reports the EPR data for a variety of tris(trialkylsilyl)silyl radicals in comparison with the prototype (Me3Si) 3Si:. Inspection of the data shows the a- 29 Si hfs constants tend to decrease and the b- 29 Si hfs slightly increase when the methyl group is progressively replaced by a bulkier group, the effect being cumulative, e.g., along the series (Me3Si) 3Si:, (Et3Si) 3Si:, (i Pr3Si) 3Si: and (Me3Si) 3Si:, (Et2MeSi) 3Si:, (t-Bu2MeSi) 3Si:. These trends have been associated with an increase of the polysilane skeleton flattening through the series [12,13,48–50]. Indeed, the half-lives of the radicals increase within the series and the (t-Bu2MeSi) 3Si: radical is found to be stable and isolable in a crystal form. Therefore, the radicals (Et3Si) 3Si:, (i-Pr3Si) 3Si:,(t-BuMe2Si) 3Si: and (t-Bu2 MeSi) 3Si: have a practically planar structure due to the steric repulsions among the bulky silyl substituents. The small differences of their a- 29 Si hfs constants are presumably due to different degrees of spin delocalization onto the Si w C b-bond, as a consequence of conformational effects in order to minimize the steric hindrance. Persistent silyl radicals have also been formed upon Table 1.2 EPR data for a variety of tris(trialkylsilyl)silyl radicals Silyl radical a(a- 29 Si)(G) a(b- 29 Si)(G) a(others) (G) g factor Reference (Me3Si) 3Si: 63.8 7.1 0.43 (27 H) 2.0053 [47] (EtMe2Si) 3Si: 62.8 7.1 0.37 (18 H) 2.0060 [48] 0.14 (6 H) (Et2MeSi) 3Si: 60.3 7.3 0.27 (12 H) 2.0060 [48] 0.15 (9 H) 3:2 (3 13 C) (Et3Si) 3Si: 57.2 7.9 0.12 (18 H) 2.0063 [48] 3:0 (3 13 C) (i-Pr3Si) 3Si: 55.6 8.1 2:2 (3 13 C) 2.0061 [49] (t-BuMe2Si) 3Si: 57.1 8.1 0.33 (27 H) 2.0055 [12] 0.11 (18 H) (Me3SiMe2Si) 3Si: 59.9 7.4 2.0065 [50] (t-Bu2MeSi) 3Si: 58.0 7.9 2.0056 [13] 13 Ph Ph Ph Ph Ph
Page 1 and 2: Organosilanes in Radical Chemistry
Page 3 and 4: DEDICATION To my parents XrZstoB an
Page 5 and 6: viii Contents 3.6 Hydrogen Atom: An
Page 7 and 8: PREFACE A large number of papers de
Page 9 and 10: ACKNOWLEDGEMENTS I thank Keith U. I
Page 11 and 12: 2 Formation and Structures of Silyl
Page 17: 8 Formation and Structures of Silyl
Page 21 and 22: 12 Formation and Structures of Sily
Page 27 and 28: 2 Thermochemistry 2.1 GENERAL CONSI
Page 29 and 30: Bond Dissociation Enthalpies 21 Tab
Page 31 and 32: Bond Dissociation Enthalpies 23 2.2
Page 33 and 34: Ion Thermochemistry 25 Table 2.4 Re
Page 35 and 36: Ion Thermochemistry 27 Table 2.5 El
Page 37 and 38: References 29 3. Goumri, A., Yuan,
Page 39 and 40: 32 Hydrogen Donor Abilities of Sili
Page 55 and 56: 4 Reducing Agents 4.1 GENERAL ASPEC
Page 57 and 58: General Aspects of Radical Chain Re
Page 59 and 60: Tris(trimethylsilyl)silane 53 Therm
Page 61 and 62: Tris(trimethylsilyl)silane 55 4.3.1
Page 63 and 64: Tris(trimethylsilyl)silane 57 A goo
Page 65 and 66: Tris(trimethylsilyl)silane 59 C(O)C
Page 67 and 68: Tris(trimethylsilyl)silane 61 radic
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Tris(trimethylsilyl)silane 63 86% y
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Tris(trimethylsilyl)silane 65 RO RO
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Tris(trimethylsilyl)silane 67 O AcO
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Tris(trimethylsilyl)silane 69 Tris(
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Other Silicon Hydrides 71 The reduc
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Other Silicon Hydrides 73 The decre
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Other Silicon Hydrides 75 Ph MeS O
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Other Silicon Hydrides 77 Table 4.6
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Silicon Hydride / Thiol Mixture 79
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Silylated Cyclohexadienes 81 and (4
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References 83 34. Kawashima, E., Uc
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References 85 104. Gimisis, T., Bal
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88 Addition to Unsaturated Bonds te
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90 Addition to Unsaturated Bonds ap
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92 Addition to Unsaturated Bonds 5.
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94 Addition to Unsaturated Bonds R
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96 Addition to Unsaturated Bonds Ph
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98 Addition to Unsaturated Bonds EP
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100 Addition to Unsaturated Bonds 5
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102 Addition to Unsaturated Bonds T
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104 Addition to Unsaturated Bonds R
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106 Addition to Unsaturated Bonds a
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110 Addition to Unsaturated Bonds S
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112 Addition to Unsaturated Bonds T
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114 Addition to Unsaturated Bonds I
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120 Unimolecular Reactions 1 Si(H)M
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122 Unimolecular Reactions t-Bu t-B
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124 Unimolecular Reactions MeO Si O
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126 Unimolecular Reactions t-Bu t-B
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128 Unimolecular Reactions R 1 R 2
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130 Unimolecular Reactions From the
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132 Unimolecular Reactions rearrang
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134 Unimolecular Reactions H 3 C +
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136 Unimolecular Reactions Br O Si(
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138 Unimolecular Reactions R3Si C C
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140 Unimolecular Reactions Me3Si O
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142 Unimolecular Reactions 43. Albe
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144 Consecutive Radical Reactions c
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146 Consecutive Radical Reactions O
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148 Consecutive Radical Reactions a
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150 Consecutive Radical Reactions d
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152 Consecutive Radical Reactions T
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154 Consecutive Radical Reactions M
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156 Consecutive Radical Reactions F
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162 Consecutive Radical Reactions i
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164 Consecutive Radical Reactions A
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168 Consecutive Radical Reactions r
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170 Consecutive Radical Reactions E
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174 Consecutive Radical Reactions A
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176 Consecutive Radical Reactions P
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178 Consecutive Radical Reactions f
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182 Consecutive Radical Reactions 1
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184 Consecutive Radical Reactions 8
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186 Silyl Radicals in Polymers and
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Page 225 and 226:
220 List of Abbreviations PED photo
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222 Index Carbonyl sulfide 111 Carb
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224 Index Organosilicon boranes 2,
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226 Index Triethylsilane as mediato
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