Anomalous Rapid Reduction of Salicylaldehyde by Pyridine-Borane. Mechanism and Application to Selective Aldehyde Reduction

Article abstract of DOI:10.1021/jo00082a006

The reduction of salicylaldehyde by pyridine-borane complex (PB) is much faster than that of other substituted benzaldehydes and ketones (seconds vs hours).Experiments reveal that this acceleration is due to an autocatalytic process involving a pyridinium boronate salt, a component of the equilibrating product mixture from PB reduction of salicylaldehyde.This pyridinium salt behaves as a mild Broensted acid and effectively accelerates aldehyde but not ketone reductions by PB.The observation that mild Broensted acids are catalysts for PB reductions led to the development of a method using AcOH in CH2Cl2 to promote the selective reduction of aldehydes in the presence of ketones.