Absolute rate measurements for some gas-phase addition reactions of dimethylsilylene

Abstract

Time-resolved studies of the room-temperature reactions of dimethylsilylene, SiMe₂, with a series of olefins, acetylenes and buta-1,3-diene are reported. SiMe₂ was generated by 193 nm laser flash photolysis of pentamethyldisilane or octamethyltrisilane and was monitored in real time by c.w. laser absorption techniques. Bimolecular rate constants ranging from 0.072 to 17.0 × 10^–11 cm³ molecule^–1 s^–1 were obtained for the reactions of SiMe₂ with the thirteen unsaturated compounds studied. The trends with substrate class and substituent are consistent with a mechanism in which the electrophilic character of SiMe₂ is important in the rate-controlling stage of the reaction and fit in well with theoretical models of the corresponding carbene addition reactions developed from Holfmann's original calculations. Although it is probable that the three-membered ring siliranes and silirenes are the primary products of these addition reactions, they appear to be too reactive to be characterised by gas chromatography.