Reprinted from the Journal of the American Chemical Society, 115, 7531-7532 (1993).
©American Chemical Society, 1993.
William J. Leigh* and Gregory W. Sluggett
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Abstract: Irradiation of acetonitrile or cyclohexane solutions of 1,1,1-trimethyl-2,2,2-triphenyldisilane (1) in the presence of acetone and chloroform yields products resulting from acetone-trapping of a 1,3,5-(1-sila)hexatriene species and silyl chlorides from reaction of the halocarbon with trimethyl- and triphenylsilyl radicals. The reaction of the silatriene with acetone in cyclohexane yields two products; the minor one is the silyl ether arising from formal ene-addition, while the major product has been assigned a 1,2-siloxetane structure on the basis of ¹H, ¹³C, and silicon-29 NMR spectra of the crude photolysis mixtures. Attempts to isolate the siloxetane by distillation or chromatographic methods result in its decomposition. Steady state triplet sensitization and quenching experiments and nanosecond flash photolysis techniques have been employed to demonstrate that the formation of silyl free radicals results from a reaction of the disilane triplet state, which is populated by intersystem crossing from a low-lying excited singlet state in direct photolyses. While the yield of silyl radicals and their trapping products is quite low in hydrocarbon solvents, they are the major products in acetonitrile. This is attributed to a solvent effect on the quantum yield of intersystem crossing. The lifetimes of the reactive triplet states of 1 in tetrahydrofuran and 1,2-di-tert-butyl-1,1,2,2-tetraphenyldisilane in cyclohexane solution have been estimated to be 6±3 and 1.4±0.3 ns, respectively, at 21°C from Stern-Volmer quenching studies. The formation of the 1,3,5-(1-sila)hexatriene species in the photolysis of 1 is shown to be due predominantly to reaction from the lowest excited singlet state via [1,3]-trimethylsilyl migration.