Au0 nanocolloids as recyclable quasihomogeneous metal catalysts in the chemoselective hydrogenation of α,β-unsaturated aldehydes and ketones to allylic alcohols

In the catalytic hydrogenation of α,β-unsaturated aldehydes and ketones, highly selective allylic alcohol formation can be achieved by application of Au0 nanocolloids dispersed in amide solvents. The polyvinylpyrrolidone protected Au0 nanoparticles prefer CO reduction over CC saturation and act as chemoselective quasihomogeneous metal catalysts in the hydrogenation of trans-2-butenal (crotonaldehyde), 2-methyl-2-propenal (methacrolein), 4-methyl-3-penten-2-one (mesityl oxide) and 3-methyl-3-penten-2-one. An extensive solvent screening revealed the superiority of amides as media for both synthesis and application of the Au0 nanocolloids. In comparison with the widely used alcohol solvents, amides offer enhanced colloidal stability for the Au0 nanosol and increased hydrogenation chemoselectivity. Control over the Au0 cluster formation provided the opportunity to investigate the size-dependency of the catalytic performance and to determine the optimum gold cluster size for a maximization of the allylic alcohol yields. The most successful Au0 clusters, with a typical diameter of 7 nm and synthesized in N,N-dimethylformamide, lead to a crotyl alcohol selectivity of 73% at 93% crotonaldehyde conversion and a 58% allylic alcohol yield in the hydrogenation of mesityl oxide at a molar substrate/Au catalyst ratio of 200. Analogous Pt0 and Ru0 sols are more active than the Au0 nanosols, but substantially less chemoselective for allylic alcohols. The Au0 nanocolloids can be recycled efficiently by ultrafiltration over custom-made, cross-linked polyimide membranes. In the recycling experiments the gold nanodispersion was well retained by the solvent-resistant ultrafiltration membranes and the performance of the colloidal gold catalyst was satisfactorily preserved in successive hydrogenation runs.