Chemoselective transfer hydrogenation of α,β-unsaturated carbonyl compounds using potassium formate over amine-grafted Ru/AlO(OH) catalysts

Grafting of 3-(2-aminoethylamino)propyltrimethoxysilane onto Ru/AlO(OH) resulted in an active and highly chemoselective heterogeneous catalyst for the transfer hydrogenation of alpha,beta-unsaturated carbonyl compounds to the corresponding allylic alcohols. Potassium formate was used as a sustainable hydrogen donor. A range of substrates including cinnamaldehyde, alpha-amylcinnamaldehyde, citral, 3-methyl-2-butenal, trans-2-pentenal, and trans-hexenal were selectively hydrogenated at the C=O moiety with >96% selectivity. In comparison, the unmodified 1 wt% Ru/AlO(OH) catalyzed hydrogenation of cinnamaldehyde at the C=C bond, yielding 3-phenylpropanal as the product. Higher loaded samples with 2-10 wt% Ru exhibited 20-25% selectivity to cinnamyl alcohol. The results show that low coordination sites were more selective to hydrogenation of the internal C=C than the terminal C=O bond. Immobilization of the amine via chemical bonding with hydroxyl groups of the AlO(OH) support blocks adjacent exposed metal sites, increasing the chemoselective reduction of C=O. Optimum results were achieved at an amine/Ru ratio of 6. The catalyst maintained high activity and chemoselectivity even after five cycles.