Using Computational Chemistry to Rationalize the Diastereoselectivity of the Borohydride Reduction of Benzoin

Benzoin, an [alpha]-hydroxy ketone, is stereoselectively reduced by sodium borohydride to yield hydrobenzoin, the stereochemistry of which is determined by acetalization and analysis of the derivative by NMR spectroscopy. This classical experiment has been enhanced by modern spectroscopic and computational analysis to enable students to rationalize the stereoselectivity via a simple H-bond model. Students explore the conformational potential energy surface of benzoin and use it to rationalize how the hydride nucleophile is likely to approach the carbonyl carbon atom. This H-bond model is highly successful in rationalizing the observed stereochemical induction in the reduction product.