Quantum chemical modeling of chiral catalysis. Part 13. On the role of borane O-adducts in the enantioselective reduction of ketones catalyzed by chiral oxazaborolidines

Plausible reactions of Lewis bases (ketones and ethers) with borane (O-adducts of chiral oxazaborolidines used as catalysts in the enantioselective reduction of ketones were investigated by means of ab initio MO methods. Properties of the O-adducts were found to be different from those of the corresponding N-adducts. The O-adducts were not able to form complexes with ketones and ethers similar to those of the corresponding N-adducts. Distances between the Lewis acidic and basic centers were much longer in the O-adducts than in the corresponding N-adducts.