Homogeneous hydrogenation of ketones in the presence of H 2 Ru(CO) 2 (PPh 3 ) 2

Abstract

The hydrogenation of ketones to alcohols in the presence of a hydrido ruthenium triphenylphosphine complex, H 2 Ru(CO) 2 (PPh 3 ) 2 , has been investigated. Acetophenone was hydrogenated at 120 °C with a conversion of 69% after 24 h. The yield is 85% working at 140°C. A kinetic investigation of the reaction was also performed: the reaction is first order with respect to the hydrogen pressure, substrate and catalyst concentration. The rate of hydrogenation depends on the nature of the substrate: steric and electronic effects have a strong influence on the hydrogenation of ketones. 2,2,2-Trifluoroacetophenone is almost completely hydrogenated at 120 °C after 3 h. A mechanism has been tentatively suggested for the hydrogenation of ketones in which the coordination of the substrate to the metal is followed by its insertion into the ruthenium-hydrido bond with formation of the corresponding alkoxide. This complex reacts with hydrogen, restoring the catalyst and forming the alcohol, We have also tested the activity of this catalyst in the hydrogenation of an α,β-unsaturated ketone (trans-4-phenylbut-3-en-2-one). The C=C double bond is preferentially hydrogenated with a high chemoselectivity (92.4% at 100°C). The data collected on the hydrogenation of trans-4-phenylbut-3-en-2-one suggest a different mechanism in which the rate-determining step is the interaction between the C=C double bond of the ketone with the metal-hydrido bond. Experimental evidence is reported to support this different hypothesis.