Deoxygenation and desulfurization of oxiranes and thiiranes by carbenes: a theoretical study

The potential-energy surfaces for the abstraction reactions of carbenes with oxirane and thiirane have been characterized in detail by using density functional theory (B3LYP/6-31G*), which include zero-point corrections. Six carbene species: dimethylcarbene, cyclobutylidene, cyclohexylidene, phenylchlorocarbene, methoxyphenylcarbene, and dimethoxycarbene have been chosen in this work is model reactants. All the interactions involve the initial formation of a loose donor-acceptor 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 carbenic reactivity, as well as the influence of substituents on the reaction potential-energy surface. As a result, our theoretical investigations indicate that, irrespective of deoxygenation and desulfurization, the relative carbenic reactivity decreases in the order: cyclobutylidene > dimethylcarbene approximately equals cyclohexylidene > phenylchlorocarbene > methoxyphenylcarbene >> dimethoxycarbene. Namely, the alkyl-substituted carbene abstractions are much more favorable than those of the pi-donor-substituted carbenes. Moreover, for a given carbene, while both deoxygenation and desulfurization are facile processes, the deoxygenation reaction is more exothermic but less 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 are in good agreement with available experimental observations, and allow a number of predictions to be made.