Kinetics and mechanism of the morpholine–borane reduction of substituted acetophenones and benzaldehydes

Abstract

Rates of reduction of substituted acetophenones and benzaldehydes by morpholine–borane in aqueous ethanol are enhanced by the introduction of electron-withdrawing groups in the carbonyl compound are are relatively independent of the water : ethanol ratio. Reduction by morpholine–[²H₃]borane, is somewhat slower (k_H/k_D= 1·23 and 1·47 for acetophenone and benzaldehyde respectively) and a pronounced retardation of rate is observed for reduction by morpholine–cyanoborane. A four-centre transition state is proposed and it is suggested that hydride transfer from boron to the carbonyl carbon atom may be energetically important in forming the activated complex. No significant intramolecular catalysis occurs on substitution of a hydroxy-group ortho to the carbonyl carbon atom.