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

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Abstract
Lithium‐powered: A kinetic investigation into the asymmetric transfer hydrogenation of non‐activated aryl alkyl ketones, catalyzed by N‐Boc‐protected α‐amino acid hydroxyamide ruthenium–arene complexes, has revealed that the reactions proceed through an unprecedented bimetallic outer‐sphere mechanism. Under optimized conditions, these catalysts provide access to secondary alcohols in high yields and with excellent enantioselectivities (>99 % ee). magnified image The combination of N‐Boc‐protected α‐amino acid hydroxyamides (pseudo‐dipeptides) and [{Ru(p‐cymene)Cl2}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 an 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 the 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.

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