Hexakis(trialkylsilyl)cyclotrisilanes and photochemical generation of bis(trialkylsilyl)silylenes

Hexakis(trialkylsilyl)cyclotrisilanes 1a (trialkylsilyl = tert-butyldimethylsilyl) and 1b (trialkylsilyl = diisopropylmethylsilyl) were synthesized by the reductive coupling of the corresponding 2,2-dibromotrisilanes. Their molecular structures were confirmed by X-ray crystallography. In the absence of trapping reagents, photolysis of cyclotrisilanes 1a and 1b gave the corresponding stable tetrakis(trialkylsilyl)disilenes quantitatively. Irradiation of cyclotrisilanes 1a and 1b in (Z)-2-butene and (E)-2-butene afforded exclusively the corresponding (Z)- and (E)-2,3-dimethyl-1-silacyclopropanes, respectively. Stereospecific formation of the cycloadducts indicates that the bis(trialkylsilyl)silylenes generated from cyclotrisilanes 1a and 1b would be singlets in the ground state.