A DFT study on the reaction mechanism of enantioselective reduction of ketones with borane catalyzed by a B-methoxy-oxazaborolidine catalyst derived from (–)-β-pinene

Theoretical studies on enantioselective reduction of ketone with borane catalyzed by a B-methoxy-oxazaborolidine catalyst derived from pinene have been performed by means of the density functional theory (DFT) method. All the structures were optimized completely using wB97XD/6-31G(d,p) level and the mechanism of the enantioselective reduction is studied. The formation of the M4s complexes via transition state TS3s was the rate-determining step and the chirality-limiting step for this enantioselective reduction; the dominant reaction is the attack of proton from the Si surface of M3a, which provides the corresponding secondary alcohols in 98% ee. NCI analysis of the four computed transition states associated with stereoselectivity-determining step discloses that TS3a(S) is the stable conformation with respect to TS3a(R), TS3b(S), and TS3b(R). The produced reaction pathway takes place through: M1 → M2a → M3a → M4a(S) → M5a(S) → M6a(S) → M7a(S). The chirality of the reduced product is of S type, which is in agreement with the experiment.