Comparison of the Reactivities of Dimethylsilylene (SiMe2) and Diphenylsilylene (SiPh2) in Solution by Laser Flash Photolysis Methods

Laser flash photolysis studies have been carried out with the goal of comparing the reactivities of dimethyl- and diphenylsilylene (SiMe2 and SiPh2, respectively) toward a comprehensive series of representative substrates in hexane solution under common conditions. The silylenes are generated using dodecamethylcyclohexasilane (1) and 1,1,3,3-tetramethyl-2,2-diphenyl-1,2,3-trisilacyclohexane (9), respectively, as photochemical precursors. The reactions examined include O−H, O−Si, and M−H (M = Si, Ge, Sn) bond insertions, Lewis acid−base complexation with tetrahydrofuran, (1+2) cycloaddition to CC and C⋮C multiple bonds, chlorine atom abstraction from CCl4, and reaction with molecular oxygen; the kinetic data for SiPh2 are accompanied by product studies in most cases. Further insight into the mechanisms of these reactions is provided by the identification of reaction intermediates and/or transient products in some cases; notable examples include the reactions of SiPh2 with methoxytrimethylsilane, carbon tetrachloride, and molecular oxygen. With several of the substrates that were studied, comparison of the kinetic data for SiPh2 to previously reported rate constants for reaction of dimesitylsilylene in cyclohexane allows an assessment of the role of steric effects in affecting silylene reactivity. Rate constants could also be determined for quenching of tetramethyldisilene (Si2Me4) with molecular oxygen, CCl4, and several other reagents that were studied.