Ruthenium-catalyzed oxidative kinetic resolution of secondary alcohol with molecular oxygen

Abstract

Ruthenium-Catalyzed Oxidative Kinetic Resolution of Secondary Alcohol with Molecular Oxygen Oxidation of alcohol to ketone is one of the most useful transformations in organic reactions and natural product synthesis. To carry out these types of oxidation, several oxidants are devised and, among those, catalytic methods using transition metal species are regarded as the most efficient procedure. In our laboratory, we have reported the oxidation of sec-alcohol with ruthenium catalysis. The significance of these experiments is to separate the enantiomers based on the difference of the oxidation rate, not just simply converting alcohol to ketone. Separation of enantiomers by rate difference of reaction is known in the case of palladium system. Because ruthenium catalyst has various oxidation states, we used ruthenium catalyst for the oxidation of alcohols under oxygen atmosphere. Ruthenium is able to carry out the oxidation reaction of alcohol efficiently. However, separation of enantiomers based on the difference in the rate of reaction was somewhat difficult with ruthenium. This limitation is due to the fact that when ruthenium catalyst participates in the oxidation reaction, the catalyst remains in reduced form, unable to carry out next catalytic cycle. Also, the reversibility of the reaction under the catalysis causes racemization of the enantiomers. We tried to design an efficient system with ruthenium which has various oxidation states, however, ended up being retarded by the oxo-philic characteristic and racemization ability of ruthenium catalyst.