Syntheses of elusive unsaturated silacycles

Abstract

<p>In an attempt to stabilize a silicon carbon (p-p)(pi) bond by incorporating it into a benzenoid framework, it was found that the gas phase pyrolysis of 1-allyl-1-methyl-1-silacyclohexa-2,4-diene resulted in the formation of 1-methyl-1-silabenzene. In the presence of acetylenes, 1-methyl-1-silabenzene affords the 4+2 Diels-Alder adduct. In the absence of added traps, 1-methyl-1-silabenzene dimerizes in a 4+2 Diels-Alder fashion;The thermolysis of 4-allyl-4-methyl-4-silacyclopentene afforded the dimer of 1-methylsilole, the first C-unsubstituted silole. The monomeric species was generated and trapped in situ from the thermolysis of the 1-methylsilole dimer. The first isolation of a C-unsubstituted silole, 1,1-dimethylsilole, was achieved with the thermolysis of 5-benzoyloxy-3,3-dimethyl-3-silacyclopentene. At elevated temperatures and in the absence of added traps, it was found that 1,1-dimethylsilole dimerizes in a 4+2 Diels-Alder fashion. This dimer was shown to undergo a thermal retro Diels-Alder to provide an in situ source of monomeric silole;Vacuum pyrolysis of either allylcyclopentadienyldimethylsilane or 5-trimethylsilyl-5-dimethylmethoxysilylcyclopentadiene affords 6,6-dimethyl-6-silafulvene. In the absence of added traps, a dimer arising from an initial head-to-tail dimerization followed by two 1,5 silyl migrations is obtained;A beta elimination of trimethylmethoxysilane to generate a silicon-carbon double bond was extended to the syntheses of siletenes. Vacuum pyrolysis of 1,3-bis(trimethylsilyl)-3-dimethmethoxysilylpropene afforded 1,1-dimethyl-4-trimethylsilyl-1-silacyclobutene. Likewise, the thermolysis of 1,3-bis(trimethylsilyl)-3-dimethoxymethylsilylpropene was shown to afford 1-methoxy-1-methyl-4-trimethylsilyl-1-silacyclobutene;In the course of the preparation of silacycloheptatriene, it was found that 4,4-dimethyl-4-silaheptadial undergoes cyclization of 5,5-dimethyl-5-silacycloheptene with titanium metal. Further unsaturation to a silacycloheptadiene was achieved. It was also found that 1,1-dimethyl-4-trimethylsilyl-7,7-dichloro-1-silabicyclo(4.1.0)hept-2-ene underwent ring expansion to 3-chloro-1,1-dimethylsilepin under the influence of fluoride ion, although in low yields.</p>