One pot selective hydrogenation and dynamic kinetic resolution over Cu/Al2O3: A way to (-)-menthol starting from low value mint oils

Different Cu catalysts have been used for the hydrogenation of very low value mint oils, namely, dementholized cornmint oil and Spanish pennyroyal oil. During the hydrogenation of dementholized oil Cu/Al2O3 showed the best activity and selectivity towards a mixture of menthols, while stereoselectivity toward the valuable isomer (-)-menthol can be improved by selectively dehydrogenating the products mixture. In the hydrogenation of pulegone and pennyroyal oil Cu/Al2O3 showed an unprecedented activity allowing complete transformation of the substrate into menthols under very mild experimental conditions (1 atm H-2, 90 degrees C). Moreover, a dynamic kinetic resolution process taking place during the hydrogenation step, allows to enrich the mixture in (-)-menthol.