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

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Radical Chemistry on Silicon Surfaces 209 H w Si(111) undergoes radical-activated reactions with a variety of terminal olefins to afford densely packed monolayers [48,57,58]. Reactions were carried out in neat deoxygenated alkenes using thermal decomposition of diacyl peroxides as the radical initiation. In the absence of radical initiation, it was found that the hydrosilylation process occurs either by heating at temperatures >150 8C or by UV irradiation, probably through a homolytic cleavage of the Si w H bond. ATR-FTIR and XPS spectroscopies as well as scanned-energy photoelectron diffraction (PED) provide evidence for a covalent bond between carbon and silicon. The modified surface could withstand exposure to boiling water, boiling CHCl3 and sonication in CH2Cl2, which also suggests chemisorption and not physisorption of the monolayer to the silicon. Approximately 50 % of the H w Si(111) groups reacted with 1-alkene and in agreement with the outcome of molecular modelling [48,59]. The alkyl chains are tilted ca 308 with respect to surface normal and are mainly in the all-trans conformation [60]. It is also worth mentioning that similar monolayers on H w Si(111) surface were obtained by Lewis acid-catalysed hydrosilylation of alkenes or by direct reaction with alkylmagnesium bromide [61]. The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H w Si(111) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si w H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the Si Si H Si Si Si Si R Si Si H Si Si Si H––Si(111) H Si Si Si H Si H Si Si Si Si Si radical initiation radical translocation Si Si Si Si Si Si R Si Si Scheme 8.11 Hydrosilylation of an alkene by hydrogen terminated Si(111) H Si H Si Si Si Si Si H Si H Si R Si Si Si Si
210 Silyl Radicals in Polymers and Materials allylic position, may be competitive. However, lack of significant polymerization when styrene is used as 1-alkene, indicates the efficiency of the 1,5 hydrogen shift with respect to carbon–carbon bond formation [48]. Electrons from a scanning tunnelling microscope in ultrahigh vacuum have been used to create surface isolated silyl radical on Si(111) and their exposure to styrene leads to the formation of compact islands containing multiple styrene adsorbates bonded to the surface through individual C w Si bonds [62]. These observations support the radical chain reaction mechanism of Scheme 8.11 and indicate that the chain reaction does not propagate in a single direction on H w Si(111). The fact that UV irradiation of O2-saturated terminal olefins and H w Si(111) leads to the formation of oxidized silicon and a partial hydrocarbon monolayer [48] indicates competitive reactions of oxygen and olefin with surface silyl radical. Terminal acetylenes, such as 1-octyne or phenylacetylene, also form monolayers on H w Si(111) when initiated by diacyl peroxide decomposition or UV irradiation [48,58]. Evidence that a vinyl group is attached to the Si surface is obtained from the ATR-FTIR and XPS spectra. Also in this case, the proposed mechanism is a surface propagation chain reaction in which a vinyl radical, formed by the addition of alkyne to a surface silyl radical, abstracts a hydrogen atom from the adjacent site of silicon backbone. The efficiency of this hydrogen transfer is expected to depend also on the shape of the intermediate carboncentred radical. Figure 8.5 shows the shapes of radical-adducts derived from 1-octyne (60) and phenylacetylene (61) being a s-type and p-type radical, respectively (see also Section 5.2.1). The decrease in packing density observed on going from 1-octyne to phenylacetylene [48] could also be due to different radical chain efficiencies and, in particular, of the hydrogen transfer step. The functionalization of H w Si(111) surfaces has been extended to the reaction with aldehydes. Indeed, H w Si(111) reacts thermally (16 h at 85 8C) with decanal to form the corresponding Si w OCH2R monolayer that has been characterized by ATR-FTIR, XPS and atomic force microscopy (AFM) [63]. The reaction is thought to proceed either by a radical chain mechanism via adventitious radical initiation or by nucleophilic addition/hydride transfer. On the other hand, the reaction of H w Si(111) with octadecanal activated by irradiation with a 150 W mercury vapour lamp (21 h at 20–50 8C) afforded a Si Si R Si Si H Si Si Si Si Si Si Si 60 61 Figure 8.5 s-type (60) and p-type (61) vinyl radicals. Ph H Si Si Si
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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
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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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Page 169 and 170: 164 Consecutive Radical Reactions A
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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
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Page 187 and 188: 182 Consecutive Radical Reactions 1
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Page 191 and 192: 186 Silyl Radicals in Polymers and
Page 213: 208 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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