Steric Effects in the Reduction of Ketones with Sodium Borohydride

The rates for the reduction of ketones with sodium borohydride are interpreted in terms of two parameters, both derived from force-field calculations; i.e. the strain difference between alcohol and ketone (Δ strain) and the steric hindrance towards approach of the hydride R. Models for the evaluation of R are discussed. With this approach reduction rates over a range of 10⁸ can be rationalized within a factor of 6–10.