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

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Oxidation Studies on Silyl-Substituted Silicon Hydrides 193 This silylperoxyl radical undergoes an unusual rearrangement to 13 followed by a 1,2-shift of the Me3Si group to give 14. Hydrogen abstraction from the silane by radical 14 gives the desired product and another silyl radical 11, thus completing the cycle of this chain reaction. The reaction mechanism of (Me3Si) 3SiH oxidation has been studied in some detail [19]. The absolute rate constant for the spontaneous and strongly endothermic (ca 130 kJ/mol) reaction of (Me3Si) 3SiH with molecular oxygen (Reaction 8.7) has been determined to be (0:48 0:18) 10 4 M 1 s 1 at 70 8C by using an inhibitor-based (hydroquinone) method. (Me3Si) 3SiH þ O2 !(Me3Si) 3Si: þ HOO: (8:7) A rate constant of 8:3 10 3 s 1 at 70 8C for the rearrangement of radical 12 is obtained by kinetic analysis of the autoxidation of (Me3Si) 3SiH and based on the assumption that the termination rate constant for radical 12 is 2kt ¼ 2:4 10 8 M 1 s 1 at 70 8C, i.e., the corresponding value for Ph3SiOO: radical. It is worth mentioning that the hydrogen abstraction from (Me3Si) 3SiH of cumylperoxyl radical (Reaction 8.8) occurs with a rate constant of 66:3M 1 s 1 at 70 8C, which means that in pure silane the rearrangement of silylperoxyl radical will be ca 30 times faster. Obviously, this is the reason for the absence of silyl hydroperoxide in the product studies. ROO: þ (Me3Si) 3SiH !ROOH þ (Me3Si) 3Si: (8:8) The rearrangement 12!13 is not straightforward and two alternatives routes have been suggested. One of them involves a 1,3 transfer from silicon to oxygen to give the silyl radical 15 followed by SHi reaction on the peroxide moiety [15]. The other indicated by PM3 calculations is a dioxirane-like three-membered intermediate 16 having a pentacoordinated central silicon followed by a 1,2 silyl migration to afford radical 13 [19]. R Me3Si Si O SiMe3 O R Me3Si Si SiMe3 O O 15 16 The 1,2 shift of Me3Si group from silicon to oxygen (13 ! 14) is expected to be a very fast process ( >10 7 s 1 ) as demonstrated in analogous reactions (see Chapter 6). Interestingly, the hydrogen abstraction from (Me3Si) 3SiH by the disilyloxysilyl radical 14 is expected to be the slow step (i.e., kp in the oxidizability value kp=(2kt) 1=2 ) although it cannot be very slow. In fact, the silyl radical 14 rather prefers to abstract a hydrogen atom from (Me3Si) 3SiH than to add to another oxygen molecule. Product studies of (Me3Si) 3SiH oxidation under high dilution showed the formation of siloxane 19, which is probably derived through the intermediacy of peroxyl radical 18 (Scheme 8.5) [12]. Therefore, the rate constant for addition of radical 17 to
194 Silyl Radicals in Polymers and Materials molecular oxygen should be much slower than a diffusion-controlled rate constant. EPR investigation on the oxidation of (Me3Si) 2Si(H)Me in benzene at 50 8C showed some interesting features too [15]. First of all, the measured oxidizability value kp=(2kt) 1=2 ¼ 1:8 10 2 M 1=2 s 1=2 for this silane is similar to that of poly(hydrosilane)s when referring to each Si w H moiety. Assuming a SiMe3 Me3Si O Si O SiMe3 17 O 2 SiMe3 Me3Si O Si O O O SiMe3 18 SiMe3 O Me3Si O Si OH O SiMe3 Scheme 8.5 Reaction sequence for the complete oxidation of (Me3Si) 3SiH 2kt ¼ 1 10 9 M 1 s 1 for radical 14 (R ¼ Me), the rate constants (kp) for the hydrogen abstraction from (Me3Si) 2Si(H)Me by radical 14 is 6 10 2 M 1 s 1 . Oxygen consumption of (Me3Si) 2Si(H)Me initiated by photogenerated t-BuO: radicals in the presence of 14 mM of a-tocopherol, indicated that not all the silylperoxyl radicals 12 are trapped. Since the rate constant for the reaction of peroxyl radical with a-tocopherol (under similar experimental conditions) is 4:1 10 6 M 1 s 1 [20], we calculated a rate constant of ca 10 3 s 1 for the unimolecular reaction of silylperoxyl radical 12, where R ¼ Me. 8.3 FUNCTIONALIZATION OF POLY(HYDROSILANE)S 8.3.1 HALOGENATION It was reported that the reaction of poly(phenylhydrosilane) (Mw ¼ 3004, Mw=Mn ¼ 1:79) with CCl4 under room light selectively affords poly(phenylchlorosilane) along with CHCl3 and CH2Cl2 as by-products (Reaction 8.9) [21]. The replacement of Si w H moieties by Si w Cl is found to be >84 % or >95 % for reaction periods of 28 h and 5 days, respectively. Vapour 19
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Organosilanes in Radical Chemistry
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DEDICATION To my parents XrZstoB an
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viii Contents 3.6 Hydrogen Atom: An
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PREFACE A large number of papers de
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ACKNOWLEDGEMENTS I thank Keith U. I
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2 Formation and Structures of Silyl
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10 Formation and Structures of Sily
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2 Thermochemistry 2.1 GENERAL CONSI
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Bond Dissociation Enthalpies 21 Tab
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Bond Dissociation Enthalpies 23 2.2
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Ion Thermochemistry 25 Table 2.4 Re
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Ion Thermochemistry 27 Table 2.5 El
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References 29 3. Goumri, A., Yuan,
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32 Hydrogen Donor Abilities of Sili
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4 Reducing Agents 4.1 GENERAL ASPEC
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General Aspects of Radical Chain Re
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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
Page 147 and 148: 142 Unimolecular Reactions 43. Albe
Page 149 and 150: 144 Consecutive Radical Reactions c
Page 151 and 152: 146 Consecutive Radical Reactions O
Page 153 and 154: 148 Consecutive Radical Reactions a
Page 155 and 156: 150 Consecutive Radical Reactions d
Page 157 and 158: 152 Consecutive Radical Reactions T
Page 159 and 160: 154 Consecutive Radical Reactions M
Page 161 and 162: 156 Consecutive Radical Reactions F
Page 167 and 168: 162 Consecutive Radical Reactions i
Page 169 and 170: 164 Consecutive Radical Reactions A
Page 173 and 174: 168 Consecutive Radical Reactions r
Page 175 and 176: 170 Consecutive Radical Reactions E
Page 179 and 180: 174 Consecutive Radical Reactions A
Page 181 and 182: 176 Consecutive Radical Reactions P
Page 183 and 184: 178 Consecutive Radical Reactions f
Page 187 and 188: 182 Consecutive Radical Reactions 1
Page 189 and 190: 184 Consecutive Radical Reactions 8
Page 191 and 192: 186 Silyl Radicals in Polymers and
Page 197: 192 Silyl Radicals in Polymers and
Page 225 and 226: 220 List of Abbreviations PED photo
Page 227 and 228: 222 Index Carbonyl sulfide 111 Carb
Page 229 and 230: 224 Index Organosilicon boranes 2,
Page 231 and 232: 226 Index Triethylsilane as mediato
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"Front Matter". In: Organosilanes in Radical Chemistry - Index of

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