Highly Diastereoselective Hydrogenation of Imines by a Bimetallic Pd-Cu Heterogeneous Catalyst

An efficient and practical heterogeneous bimetallic Pd-Cu/C catalyst was identified as an alternative to Raney nickel for the highly diastereoselective hydrogenation of imines prepared from prochiral ketones and alpha-phenylethylamines. Chiral amines were obtained with diastereomeric excess (de) up to 94% using Pd-Cu/C, while conventional Pd-C catalysts afforded only 72% de. Optimization showed that a robust process required a palladium/copper ratio of 4:1. Evidence for the influence of catalyst pretreatment which may change the structure of the catalyst and/or metal oxidation states on the selectivity of the reaction is discussed. The bimetallic catalyst system provided consistent results on scale and performed reliably on a variety of substrates.