Theoretical study of silylene substituent effects on the abstraction reactions with oxirane and thiirane.

The potential energy surfaces for the abstraction reactions of silylenes with oxirane and thiirane have been characterized in detail using density functional theory (B3LYP) as well as the ab initio method (QCISD), including zero-point corrections. Five silylene species including SiH(2), Si(CH(3))(2), Si(NH(2))(2), Si(OH)(2), and SiF(2) have been chosen in this work as model reactants. All the interactions involve the initial formation of a donor-acceptor ylide-like complex followed by a heteroatom shift via a two-center transition state. The complexation energies, activation barriers, and enthalpies of the reactions were used comparatively to determine the relative silylenic reactivity, as well as the influence of substituents on the reaction potential energy surface. As a result, our theoretical investigations suggest that, irrespective of deoxygenation and desulfurization, the alkyl-substituted silylene abstractions are much more favorable than those of the pi donor-substituted silylenes. Moreover, for a given silylene, while both deoxygenation and desulfurization are facile processes, the deoxygenation reaction is more exothermic as well as more kinetically favorable. Furthermore, a configuration mixing model based on the work of Pross and Shaik is used to rationalize the computational results. The results obtained allow a number of predictions to be made.