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In the catalytic hydrogenation of α,β-unsaturated aldehydes and ketones, highly selective allylic alcohol formation can be achieved by application of Au 0 nanocolloids dispersed in amide solvents. The polyvinylpyrrolidone protected Au 0 nanoparticles prefer C O reduction over C C 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 Au 0 nanocolloids. In comparison with the widely used alcohol solvents, amides offer enhanced colloidal stability for the Au 0 nanosol and increased hydrogenation chemoselectivity. Control over the Au 0 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 Au 0 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 Pt 0 and Ru 0 sols are more active than the Au 0 nanosols, but substantially less chemoselective for allylic alcohols. The Au 0 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. |
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