Highly enantioselective ruthenium-catalyzed reduction of ketones employing readily available Peptide ligands.

Highly efficient and selective catalysts for the asymmetric reduction of aryl alkyl ketones under hydrogen-transfer conditions (2-propanol) were obtained by combining a novel class of pseudo-dipeptide ligands with [[RuCl(2)(p-cymene)](2)]. A library of 36 dipeptide-like ligands was prepared from N-Boc-protected alpha-amino acids and the enantiomers of 2-amino-1-phenylethanol and 1-amino-2-propanol. The catalyst library was evaluated with the reduction of acetophenone and excellent enantioselectivity of 1-phenylethanol was obtained with several of the novel catalysts. A ligand based on the combination of N-Boc-L-alanine and (S)-1-amino-2-propanol (ligand A-(S)-4) was found to be particular effective. When the situ formed ruthenium complex of this ligand was employed as the catalyst in the hydrogen-transfer reaction of various aryl alkyl ketones, the corresponding alcohol products were achieved in excellent enantioselectivity (up to 98 % ee).