Recyclable Au0, Ag0 and Au0-Ag0 nanocolloids for the chemoselective hydrogenation of α,β-unsaturated aldehydes and ketones to allylic alcohols

A high-throughput evaluation of polymer-stabilized metal clusters, with varying composition and size, as unpromoted quasi homogeneous catalysts for the amide-phase hydrogenation of a,p-unsaturated aldehydes and ketones, allowed to identify the most appropriate metal clusters to achieve a high selectivity to allylic alcohols. Au-0 and Ag-0 based nanocolloids proved superior owing to their pronounced preference for C=O reduction over C=C saturation. Optimization of the metal nanoclusters; in terms of size and composition led to exceptionally high allylic alcohol selectivities. These robust metal nanocolloids could be recycled efficiently from the amidic reaction media by solvent-resistant nanofiltration over cross-linked polyimide membranes. The catalytic activity of the optimized bimetallic nanocolloids consisting of alloyed Au and Ag was well preserved in successive hydrogenation runs while the chemoselectivity was unaffected. The wide substrate scope enabled to study the effect of the C=C-C=O substitution pattern on the substrate reactivity and the allylic alcohol selectivity. (C) 2009 Elsevier B.V. All rights reserved.