Enantioselective silyl protection of alcohols promoted by a combination of chiral and achiral Lewis basic catalysts

Catalytic enantioselective monosilylations of diols and polyols furnish valuable alcohol-containing molecules in high enantiomeric purity. These transformations, however, require high catalyst loadings (20–30 mol%) and long reaction times (2–5 days). Here, we report that a counterintuitive strategy involving the use of an achiral co-catalyst structurally similar to the chiral catalyst provides an effective solution to this problem. A combination of seemingly competitive Lewis basic molecules can function in concert such that one serves as an achiral nucleophilic promoter and the other performs as a chiral Brønsted base. On the addition of 7.5–20 mol% of a commercially available N-heterocycle (5-ethylthiotetrazole), reactions typically proceed within one hour, and deliver the desired products in high yields and enantiomeric ratios. In some instances, there is no reaction in the absence of the achiral base, yet the presence of the achiral co-catalyst gives rise to facile formation of products in high enantiomeric purity. A counter-intuitive strategy that combines a chiral Lewis base catalyst with an achiral Lewis base co-catalyst results in an exceptionally large increase in the facility of catalytic enantioselective silylation of polyols. The catalytic ensemble drives such reactions to completion within a few hours, rather than the usual two–five days, without loss of enantioselectivity.