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Structural Properties of Silyl Radicals 5 For example, the a-naphthylphenylmethylsilyl radical (3) generated by hydrogen abstraction from the corresponding chiral silane reacts with CCl4 to give optically active chlorosilane that has retained, at least in part, the configuration of the starting material [22]. Thus, the silyl radical is chiral and exists in a pyramidal form with considerable configurational stability, and it abstracts a chlorine atom from CCl4 faster than its inversion (Reaction 1.10). Moreover, it was observed that the a-naphthylphenylmethylsilyl radical gave varying degrees of optical purity in the products as the concentration of CCl4 was progressively diluted with benzene or cyclohexane. Analysis of these results by using a Stern–Volmer type of approach, yielded kinv=k ¼ 1:30 M at 80 8C, where kinv is the rate constant for inversion at the silicon centre (Reaction 1.10) and k is the rate constant for the reaction of silyl radical with CCl4 [23]. From these data, kinv ¼ 6:8 10 9 s 1 at 80 8C is obtained which corresponds to an activation barrier of ca 23.4 kJ/mol if a normal preexponential factor of inversion is assumed, i.e., log (A=s 1 ) ¼ 13:3. A number of other optically active organosilanes behave similarly, when the a-naphthyl group in a- NpSi*(Ph)(Me)H, is replaced by neo-C5H11, C6F5or Ph2CH [22]. Under the same conditions, however, Ph3SiSi (Ph)(Me)H gave a chloride that was racemic indicating either that the inversion rate of the disilyl radical is much faster than its rate of reaction with CCl4, or that the radical centre is planar. α-Np Si Me Ph 3 k inv Si α-Np Me Ph (1.10) Analogous competitive kinetic studies have been reported for the inversion of silyl radicals 4 and 5 generated from corresponding silanes (Reaction 1.11) [24]. Rate constants for the interconversion of the two isomer radicals were estimated to be k1 9 10 9 s 1 and k 1 4 10 9 s 1 at 0 C [23]. The equilibrium is slightly shifted to the right (K 2:3) which suggests that radical 5 is a few hundred calories more stable than radical 4. Activation energies for the forward and reverse inversion processes can be estimated to be ca 17–21 kJ/mol by assuming log (A=s 1 ) ¼ 13:3. t-Bu 4 Me Si k 1 k −1 t-Bu 5 Si Me (1.11) For comparison, it is worth mentioning that in the gas phase H3Si: is bent out of the plane by (16:0 2:0)8, corresponding to an H w Si w H bond angle of (112:5 2:0)8 and with an inversion barrier of 22.6 kJ/mol [25]. Structural information on silyl radicals has also been obtained from the isomerization of 9,10-dihydro-9,10-disilaanthracene derivatives 6 and 7 [26,27].
6 Formation and Structures of Silyl Radicals Indeed, irradiation of a pentane solution of either the cis isomer 6 or the corresponding trans isomer 7 in the presence of di-tert-butyl peroxide as radical initiator affords the same cis/trans mixture. For R ¼ Me or Ph, a ratio of 47/53 is observed whereas for the more sterically hindered R ¼ t-Bu a ratio of 81/19 is obtained. It was proposed that the radicals 9 and 10 generated by hydrogen abstraction from 6 and 7, respectively, undergo inversion of the radical centre (Reaction 1.12) followed by hydrogen abstraction from the parent silanes (an identity reaction, see Chapter 3) [27]. Interestingly, the analogous 9-silaanthracene derivative 8 does not isomerize under identical conditions [8], suggesting that the disilaanthracene skeleton plays an important role either in lowering the activation energy of the identity reaction or fastening the inversion of silyl radical in Reaction (1.12). H R Si Si H R Si H R H R Si Si 6 7 8 Si R 9 10 1.2.2 ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTRA H R R H Si Si H Ph R Si H Ph (1.12) EPR spectroscopy is the most important method for determining the structures of transient radicals. Information obtained from the EPR spectra of organic radicals in solution are: (i) the centre position of the spectra associated with g factors, (ii) the number and spacing of the spectral lines related to hyperfine splitting (hfs) constants, (iii) the total absorption intensity which corresponds to the radical concentration, and (iv) the line widths which can offer kinetic information such as rotational or conformational barriers. The basic principles as well as extensive treatments of EPR spectroscopy have been described in a number of books and reviews and the reader is referred to this literature for a general discussion [28–30]. Generally, the EPR spectra of silyl radicals show a central set of lines due to 1 H hfs constants and weaker satellites due to the coupling with 29 Si(I ¼ 1=2, 4:7 %). The data for silyl radicals, presented in Table 1.1, have
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 13: 4 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
Page 63 and 64: 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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