Mechanistic Investigations into the Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Pseudo-Dipeptide Ruthenium Complexes

The combination of N-Boc-protected alpha-amino acid hydroxyamides (pseudo-dipeptides) and [{Ru(p-cymene)Cl-2}(2)] resulted in the formation of superior catalysts for the asymmetric transfer hydrogenation (ATH) of non-activated aryl alkyl ketones in propan-2-ol. The overall kinetics of the ATH of acetophenone to form 1-phenylethanol in the presence of ruthenium pseudo-dipeptide catalysts were studied, and the individual rate constants for the processes were determined. Addition of lithium chloride to the reaction mixtures had a strong influence on the rates and selectivities of the processes. Kinetic isotope effects (KIEs) for the reduction were determined and the results clearly show that the hydride transfer is rate-determining, whereas no KIEs were detected for the proton transfer. From these observations a novel bimetallic outer-sphere-type mechanism for these ATH process is proposed, in which the bifunctional catalysts mediate the transfer of a hydride and ail alkali metal ion between the hydrogen donor and the substrate. Furthermore, the use of a mixture of propan-2-ol and THF (1:1) proved to enhance the rates of the ATH reactions. A series of aryl alkyl ketones were reduced under these conditions in tile presence of 0.5 mol % of catalyst, and the corresponding secondary alcohols were formed in high yields and with excellent enantioselectivities (> 99 % ee) in short reaction times.