Computational investigation of the effect of alkoxy carbon substitution on the mechanism of carbonyl group reduction by 1-hydridosilatranes

Abstract

We undertook computational investigations to determine the effects of the electronic nature of the cage atoms on the energetics of the reduction of acetone by 1-hydridosilatranes. QTAIM analyses of the parent 1-hydridosilatrane 1H6 and hexasubstituted silatranes 1Me6 and 1F6 showed that the change in charge induced by the donor/acceptor properties of the substituents remained almost entirely localized on the alkoxy carbon; the hydridic nature of the silicon-bound hydrogen as gauged by the charge remained essentially unchanged. This shows resilience to the electronic effects of the substituents at this position. The energetics of acetone reduction to 2-propanol were mapped out for all three cases. Calculations showed favorable Gibbs free energies for each reduction, with considerable exergonicities and modest barriers. Reductions using 1H6 and 1Me6 follow hydride transfer mechanisms without involvement of carbonyl oxygen that lead to an ion/neutral pair, while reduction using 1F6 seems to involve a concerted σ-metathesis-like mechanism that leads directly to a dialkoxysilatrane.