Silicon−Carbon Unsaturated Compounds. 66. Photolysis of cis- and trans-1,2-Dimethyl-1,2-diphenyl-1,2-disilacyclohexane in the Presence of Isobutene

A mixture of cis- and trans-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane (1a and 1b) was prepared by the reaction of 1,4-bis(chloromethylphenylsilyl)butane with lithium metal in the presence of a catalytic amount of dimethylphenylsilyllithium in THF. The photolysis of pure 1a in the presence of isobutene afforded cis-2,3-benzo-1-isobutyl-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (2a) in 63% yield, in addition to a small amount of a formal [2 + 2] cycloadduct (3a) with cis-configuration, arising from addition of the rearranged silene with isobutene. Similar photolysis of 1b with isobutene gave a trans-adduct (3b) formed by formal [2 + 2] cycloaddition of the rearranged silene arising from 1b to isobutene as the main product, together with a small amount of trans-2,3-benzo-1-isobutyl-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene (2b). The structure of the [2 + 2] cycloadduct 3b, was verified by X-ray crystallography. The thermolyses of 3a and 3b at 150 °C for 24 h produced 2a and 2b, respectively, in quantitative yields.