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Methods of Generation of Silyl Radicals 3 (t-BuMe 2 Si) 3 SiBr Na n-heptane (t-BuMe 2Si) 3Si (1.7) Processes involving photoinduced electron transfer of organosilanes [3,14,15] have not been covered in this book with the exception of the following method that was successfully applied to various radical reactions, such as cyclizations, intermolecular additions and tandem annulations (see Chapters 4, 5 and 7). Silyl radicals have been obtained by a complex but efficient method using PhSeSiR3 as the reagent. The strategy is based on the mesolysis of PhSeSiR : 3 to give R3Si: radical and PhSe [16–18]. Indeed, the selective formation of PhSeSiR : 3 is accomplished by visible-light irradiation (410 nm) of solutions containing PhSeSiR3, 9,10-dimethoxyanthracene (DMA) as the electron donor, and ascorbic acid (H2A) as the co-oxidant. Scheme 1.1 shows the photoinduced electron transfer with the formation of PhSeSiR : 3 and DMAþ: , together with the regeneration of DMA at the expense of ascorbic acid. The choice of the substituents is limited by their stabilities. Trialkyl substituted derivatives are highly sensitive to air and prone to hydrolysis, whereas the t-BuPh2Si derivative was found to be the most stable. no radical products DMA HA PhSeSiR 3 H2 A DMA hν (410 nm) PhSeSiR 3 mesolysis PhSe – + SiR 3 Scheme 1.1 Generation of silyl radicals by a photoinduced electron transfer method PhSeSiR3 reacts with Bu3SnH under free radical conditions and affords the corresponding silicon hydride (Reaction 1.8) [19,20]. This method of generating R3Si: radicals has been successfully applied to hydrosilylation of carbonyl groups, which is generally a sluggish reaction (see Chapter 5). AIBN, 80 �C PhSeSiEt3 + Bu3SnH PhSeSnBu3 + Et3SiH benzene Although a detailed mechanistic study is still lacking, it is reasonable to assume that the formation of R3Si: radicals occurs by means of the mesolysis of reactive intermediate PhSeSiR : 3 , by analogy with the mechanistic information reported above. Indeed, an electron transfer between the initially (1.8)
4 Formation and Structures of Silyl Radicals formed stannyl radical and the silyl selenide is more plausible (Reaction 1.9), than a bimolecular homolytic substitution at the seleno moiety. PhSeSiEt3 þ Bu3Sn: !PhSeSiEt3: þ Bu3Si þ 1.2 STRUCTURAL PROPERTIES OF SILYL RADICALS (1:9) Trisubstituted carbon-centred radicals chemically appear planar as depicted in the p-type structure 1. However, spectroscopic studies have shown that planarity holds only for methyl, which has a very shallow well for inversion with a planar energy minimum, and for delocalized radical centres like allyl or benzyl. Ethyl, isopropyl, tert-butyl and all the like have double minima for inversion but the barrier is only about 300–500 cal, so that inversion is very fast even at low temperatures. Moreover, carbon-centred radicals with electronegative substituents like alkoxyl or fluorine reinforce the non-planarity, the effect being accumulative for multi-substitutions. This is ascribed to ns bonds between n electrons on the heteroatom and the bond to another substituent. The degree of bending is also increased by ring strain like in cyclopropyl and oxiranyl radicals, whereas the disubstituted carbon-centred species like vinyl or acyl are ‘bent’ s radicals [21]. C Si 1 2 For a long time, this knowledge on carbon-centred radicals has driven the analysis of spectroscopic data obtained for silicon-centred (or silyl) radicals, often erroneously. The principal difference between carbon-centred and silyl radicals arises from the fact that the former can use only 2s and 2p atomic orbitals to accommodate the valence electrons, whereas silyl radicals can use 3s, 3p and 3d. The topic of this section deals mainly with the shape of silyl radicals, which are normally considered to be strongly bent out of the plane (s-type structure 2) [1]. In recent years, it has been shown that a-substituents have had a profound influence on the geometry of silyl radicals and the rationalization of the experimental data is not at all an extrapolation of the knowledge on alkyl radicals. Structural information may be deduced by using chemical, physical or theoretical methods. For better comprehension, this section is divided in subsections describing the results of these methods. 1.2.1 CHEMICAL STUDIES The pyramidal structure of triorganosilyl radicals (R3Si:) was first indicated by chirality studies on optically active compounds containing asymmetric silicon.
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: 2 Formation and Structures of Silyl
Page 15 and 16: 6 Formation and Structures of Silyl
Page 17 and 18: 8 Formation and Structures of Silyl
Page 19 and 20: 10 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
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Tris(trimethylsilyl)silane 57 A goo
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Tris(trimethylsilyl)silane 59 C(O)C
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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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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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