Effects of Si substitution on the Ei reaction of alkyl sulfoxides.

A computational study has been carried out on the effects of SiH3 substitution on the Ei reaction of alkyl sulfoxides. SiH3 substitution in the beta-position (relative to the sulfur atom) is found to lower the barrier to elimination about 5 kcal mol(-1), in line with qualitative experimental observations. However, SiH3 substitution at the alpha-position has a similar, if smaller, effect. It is argued that the barrier-lowering in the latter case is mainly correlated with changes in the overall deltaH of the elimination reaction. In contrast, electronic effects in the transition state overcome an unfavorable delta(deltaH) when SiH3 is in the beta-position. On the basis of estimated magnitudes of the various energy changes and the fractional change in bond orders in the transition state, it is argued that both hyperconjugative stabilization and inductive effects of Si substitution contribute. Finally, it is shown that the elimination reaction by SiH3 transfer from Cbeta to O has a surprisingly high enthalpic barrier, given its modest endothermicity, and it is suggested that this is due to a forced unfavorable geometry at the Si center in the transition state.