Novel hydrido-ruthenium(II) complexes with histidine derivatives and their application in the hydrogenation of ketones.

The complexes RuHCl((R)-binap)(L-NH2) with L-NH2 = (S)-histidine-Me-ester (1), histamine (3), (S)-histidinol (4) or 1-Me-(S)-histidine-Me-ester (5), and RuHCl((S)-binap)(L-NH(2)) with L-NH2 = (S)-histidine-Me-ester (2) have been prepared in 60-81% overall yields in a one-pot, three-step procedure from the precursor RuCl2(PPh3)3. Their octahedral structures with hydride trans to chloride were deduced from their NMR spectra and confirmed by the results of a single crystal X-ray diffraction study for complex 3. Under H2 and in the presence of KOtBu, complexes 1-5 in 2-propanol form moderately active catalyst precursors for the asymmetric hydrogenation of acetophenone to 1-phenylethanol. Complex 5 is more active and enantioselective than complexes 1-4, allowing complete conversion to 1-phenylethanol in 46% e.e. (R) in 72 h at 20 degrees C under 1 MPa of H2 with substrate : catalyst : base = 2000 : 1 : 30. Complex 5, when activated, also catalyzes the hydrogenation of trans-4-phenyl-3-buten-2-one to exclusively the allyl alcohol 4-phenyl-3-buten-2-ol under 2.7 MPa of H2 at 50 degrees C in 2-propanol. This selectivity for C=O versus C=C hydrogenation is consistent with a mechanism involving the outer sphere transfer of hydride and proton to the polar bond. Further extensions to complexes with peptides with N-terminal histidine groups appear feasible on the basis of the current work.