Silicon−Carbon Unsaturated Compounds. 75. Thermal Isomerization of 2-Alkyl- and 2-Aryl-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silacyclohex-4-enes

The thermolysis of 2-tert-butyl-, 2-adamantyl-, and 2-phenyl-4,5-dimethyl-2-(trimethylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclohex-4-ene (1−3), synthesized by the reactions of acylpolysilanes with 2,3-dimethylbutadiene, in a sealed glass tube at 250 °C for 24 h gave the products, arising from dyotropic rearrangement, cis-2-tert-butyl-, cis-2-adamantyl-, and cis-2-phenyl-4,5-dimethyl-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclohex-4-ene (5−7), respectively, in high yields. The theoretical calculations using of 2-methyl-2-siloxy-1,1-bis(silyl)-1-silacyclohex-4-ene as a starting compound and cis-2-methyl-1-siloxy-1,2-bis(silyl)-1-silacyclohex-4-ene as a product indicated that a pentacoordinate silicon species plays an important role for the present dyotropic rearrangement. The thermolysis of the 4,5-diphenyl-1-silacyclohex-4-ene derivative 8, prepared by the reaction of pivaloyltris(trimethylsilyl)silane with 2,3-diphenylbutadiene, also afforded cis-2-tert-butyl-4,5-diphenyl-1-(trimethylsiloxy)-1,2-bis(trimethylsilyl)-1-silacyclohex-4-ene (9), whose structure was confirmed by X-ray crystallographic analysis. The thermolysis of 4,5-dimethyl-2-mesityl-2-(trimethylsiloxy)-1,1-bis(trimethylsilyl)-1-silacyclohex-4-ene (4) at 250 °C for 24 h, however, produced a ring-contracted compound, 3,4-dimethyl-1-mesityl-1-[trimethylsiloxybis(trimethylsilyl)silyl]cyclopent-3-ene (10).