"Front Matter". In: Organosilanes in Radical Chemistry - Index of
"Front Matter". In: Organosilanes in Radical Chemistry - Index of "Front Matter". In: Organosilanes in Radical Chemistry - Index of
Formation of Carbon–Heteroatom Bonds 169 has been proposed as a versatile alternative to phenylseleno esters for the generation of acyl radicals [73]. Acyl radicals can in turn give addition to azides affording the cyclized products in satisfactory yields (Reaction 7.63) [74]. O I S N3 n 59 (TMS) 3SiH AIBN, 80 �C n = 1 n = 2 O 81% 75% NH n + S 60 (7.63) Similarly, C w Se bonds are formed by an internal homolytic substitution of aryl radicals at selenium, with the preparation of selenophenes and benzeneselenophenes [75]. Scheme 7.7 illustrates the reaction of aryl iodides 61 with (TMS) 3SiH, which afforded benzeneselenophenes in good yields. The presence of (TMS) 3SiI after the reduction induced the final dehydration of the intermediate 3-hydroxyselenophenes, presumably through an intermediate silyl ether. R Se OH R' R OH R' 61 I + SeCH 2 Ph (TMS) 3 SiI (TMS) 3SiH AIBN, 80 �C Scheme 7.7 Internal homolytic substitution at selenium R Se 80-93% R Se R' OSi(TMS) 3 An example of C w Si bond formation concludes this overview of carbon– heteroatom bond formation. Reflux of bromide 62 in benzene and in the presence of small amounts of (TMS) 3SiH and AIBN afforded the silabicycle 63 in 88 % yield (Reaction 7.64) [76]. The key step for this transformation is the intramolecular homolytic substitution at the central silicon atom, which occurred with a rate constant of 2:4 10 5 s 1 at 80 8C (see also Section 6.4). The reaction has also been extended to the analogous vinyl bromide (Reaction 7.65) [49]. R'
170 Consecutive Radical Reactions EtO 2 C EtO 2 C 62 Br Si(TMS) 3 Br Si(TMS) 3 (TMS) 3SiH (cat.) AIBN, 80 �C (TMS) 3 SiH AIBN, 80 �C EtO 2 C EtO 2C 63, 88% Si(TMS) 2 75% Si(TMS) 2 (7.64) (7.65) 7.5 OTHER USEFUL RADICAL REARRANGEMENTS b-Silyl substituted carbon-centred radicals, which are produced when adding R3Si: to unsaturated bonds can participate in consecutive reactions other than cyclization. A simple example is given in Reaction (7.66) where the adduct of silyl radical to b-pinene rearranged by opening the four-membered ring prior to H atom transfer [33,77]. Me t-BuMe 2Si (TMS) 3 SiH MeO MeO AIBN, 85 �C 82% 70% SiR 3 (7.66) Useful bicyclic ring systems are obtained by (TMS) 3Si: radical-mediated fragmentation of strained ketoalkene precursors. For example, the ketoalkene 64 reacted with 1.5 equiv of silane to give 95 % of hydrindanone 65 (Reaction 7.67) [78]. (TMS) 3Si: radical adds first to the terminal alkene and the carboncentred radical can relieve the strain by cleaving the adjacent C w C bond. O 64 (TMS) 3SiH AIBN, 80 �C (TMS) 3Si O H H 65, 95% (7.67)
- Page 123 and 124: 118 Addition to Unsaturated Bonds 7
- Page 125 and 126: 120 Unimolecular Reactions 1 Si(H)M
- Page 127 and 128: 122 Unimolecular Reactions t-Bu t-B
- Page 129 and 130: 124 Unimolecular Reactions MeO Si O
- Page 131 and 132: 126 Unimolecular Reactions t-Bu t-B
- Page 133 and 134: 128 Unimolecular Reactions R 1 R 2
- Page 135 and 136: 130 Unimolecular Reactions From the
- Page 137 and 138: 132 Unimolecular Reactions rearrang
- Page 139 and 140: 134 Unimolecular Reactions H 3 C +
- Page 141 and 142: 136 Unimolecular Reactions Br O Si(
- Page 143 and 144: 138 Unimolecular Reactions R3Si C C
- Page 145 and 146: 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 163 and 164: 158 Consecutive Radical Reactions O
- Page 165 and 166: 160 Consecutive Radical Reactions O
- Page 167 and 168: 162 Consecutive Radical Reactions i
- Page 169 and 170: 164 Consecutive Radical Reactions A
- Page 171 and 172: 166 Consecutive Radical Reactions O
- Page 173: 168 Consecutive Radical Reactions r
- Page 177 and 178: 172 Consecutive Radical Reactions O
- 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 185 and 186: 180 Consecutive Radical Reactions O
- 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 193 and 194: 188 Silyl Radicals in Polymers and
- Page 195 and 196: 190 Silyl Radicals in Polymers and
- Page 197 and 198: 192 Silyl Radicals in Polymers and
- Page 199 and 200: 194 Silyl Radicals in Polymers and
- Page 201 and 202: 196 Silyl Radicals in Polymers and
- Page 203 and 204: 198 Silyl Radicals in Polymers and
- Page 205 and 206: 200 Silyl Radicals in Polymers and
- Page 207 and 208: 202 Silyl Radicals in Polymers and
- Page 209 and 210: 204 Silyl Radicals in Polymers and
- Page 211 and 212: 206 Silyl Radicals in Polymers and
- Page 213 and 214: 208 Silyl Radicals in Polymers and
- Page 215 and 216: 210 Silyl Radicals in Polymers and
- Page 217 and 218: 212 Silyl Radicals in Polymers and
- Page 219 and 220: 214 Silyl Radicals in Polymers and
- Page 221 and 222: 216 Silyl Radicals in Polymers and
- Page 223 and 224: 218 Silyl Radicals in Polymers and
170 Consecutive <strong>Radical</strong> Reactions<br />
EtO 2 C<br />
EtO 2 C<br />
62<br />
Br<br />
Si(TMS) 3<br />
Br<br />
Si(TMS) 3<br />
(TMS) 3SiH (cat.)<br />
AIBN, 80 �C<br />
(TMS) 3 SiH<br />
AIBN, 80 �C<br />
EtO 2 C<br />
EtO 2C<br />
63, 88%<br />
Si(TMS) 2<br />
75%<br />
Si(TMS) 2<br />
(7.64)<br />
(7.65)<br />
7.5 OTHER USEFUL RADICAL REARRANGEMENTS<br />
b-Silyl substituted carbon-centred radicals, which are produced when add<strong>in</strong>g<br />
R3Si: to unsaturated bonds can participate <strong>in</strong> consecutive reactions other than<br />
cyclization. A simple example is given <strong>in</strong> Reaction (7.66) where the adduct <strong>of</strong><br />
silyl radical to b-p<strong>in</strong>ene rearranged by open<strong>in</strong>g the four-membered r<strong>in</strong>g prior to<br />
H atom transfer [33,77].<br />
Me<br />
t-BuMe 2Si<br />
(TMS) 3 SiH<br />
MeO<br />
MeO<br />
AIBN, 85 �C<br />
82%<br />
70%<br />
SiR 3<br />
(7.66)<br />
Useful bicyclic r<strong>in</strong>g systems are obta<strong>in</strong>ed by (TMS) 3Si: radical-mediated<br />
fragmentation <strong>of</strong> stra<strong>in</strong>ed ketoalkene precursors. For example, the ketoalkene<br />
64 reacted with 1.5 equiv <strong>of</strong> silane to give 95 % <strong>of</strong> hydr<strong>in</strong>danone 65 (Reaction<br />
7.67) [78]. (TMS) 3Si: radical adds first to the term<strong>in</strong>al alkene and the carboncentred<br />
radical can relieve the stra<strong>in</strong> by cleav<strong>in</strong>g the adjacent C w C bond.<br />
O<br />
64<br />
(TMS) 3SiH<br />
AIBN, 80 �C<br />
(TMS) 3Si<br />
O<br />
H<br />
H<br />
65, 95%<br />
(7.67)
Change language