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

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1 Formation and Structures of Silyl Radicals 1.1 METHODS OF GENERATION OF SILYL RADICALS The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X: are centred at carbon, nitrogen, oxygen, or sulfur atoms depending on the objective. R3SiH þ X: !R3Si: þ XH (1:1) For example, photochemically produced t-BuO: radicals have been mainly used for the generation of silyl radicals to be studied by spectroscopic techniques (see Chapters 1 and 2). Carbon-centred X: radicals are of great importance in chemical transformations under reducing conditions, where an appropriate silane is either the reducing agent or the mediator for the formation of new bonds (see Chapters 4, 5 and 7). Chapter 3 is entirely dedicated to the hydrogen donor abilities of silicon hydrides towards a variety of radicals. In particular, a large number of available kinetic data are collected and analysed in terms of the substituent influence on the Si w H moiety and on the attacking radical. Several methods for generating of silyl radicals exist using direct interaction of silanes with light (Reaction 1.2). However, none of them is of general applicability, being limited to some specific application [3]. R3Si w Y ! hv R3Si: þ Y: (1:2) The best example is the photochemistry of aryldisilanes, which undergo essentially three principal photoprocesses [4–6]. These include the silylene extru- Organosilanes in Radical Chemistry C. Chatgilialoglu # 2004 John Wiley & Sons, Ltd ISBN: 0-471-49870-X
2 Formation and Structures of Silyl Radicals sion, 1,3-Si shift to the ortho position of the aryl group to afford silatrienes and homolytic cleavage of Si w Si bond to give silyl radicals. Silenic products are derived from the lowest excited singlet state and are the major products in nonpolar solvents, while silyl radicals are derived from the lowest excited triplet state and are the major products in polar solvents such as acetonitrile [5]. The homolytic cleavage can also be promoted when the 1,3-Si migration is sterically hindered as shown in Reaction (1.3) [7]. Regarding the alkyl substituted oligo- and polysilanes, the silylene extrusion is the principal photoprocess in the far-UV photochemistry whereas reductive elimination of silylsilylene and homolytic Si w Si scission is also detected [8,9]. Ph t-Bu Si Si Bu-t Ph Ph Ph hν Ph t-Bu Si + Si Bu-t (1.3) Organosiliconboranes having bulky substituents on the boron, e.g. R3SiB[N(CHMe) 2] 2, exhibit UV absorption at wavelengths longer than 300 nm. Photolysis of this band afforded a pair of silyl and boryl radicals that can be trapped quantitatively by nitroxide (TEMPO) as shown in Reaction (1.4) [10]. N(CHMe2) 2 hν PhMe2Si B N(CHMe2) 2 TEMPO O SiMe 2Ph N Ph Ph Ph O N B N(CHMe2) 2 N(CHMe2 ) 2 (1.4) Silyl radicals have been produced by one-electron oxidation of silyl metals [11]. This is found to be the method of choice for the generation of persistent silyl radicals and allowed the preparation of the first isolable silyl radical (see later in this chapter). Reactions (1.5) and (1.6) show two sterically hindered silyl anions with Na þ as the counter-cation, and their oxidation by the nitrosyl cation [12] and the complex GeCl2=dioxane [13], respectively. (t-BuMe 2Si) 3SiNa (t-Bu 2MeSi) 3SiNa NO + BF 4 − n-heptane GeCl 2 /dioxane Et 2 O (t-BuMe 2 Si) 3 Si• (t-Bu 2 MeSi) 3 Si Silyl radicals are also involved as the reactive intermediates during oneelectron reduction of bromosilanes. As an example, Reaction (1.7) shows the reduction by sodium of a silyl bromide to produce a persistent radical, which has been characterized by EPR spectroscopy [12]. (1.5) (1.6)
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: ACKNOWLEDGEMENTS I thank Keith U. I
Page 13 and 14: 4 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
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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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