Heterogeneous enantioselective hydrogenation of isophorone over proline modified Pd catalysts

Enantioselective hydrogenation of isophorone (3,5,5-trimethyl-2-cyclohexenone) over (S)-proline modified Pd catalysts has been studied. Both the Pd particle size and the acid–base properties of the support influenced the hydrogenation reaction of isophorone and the subsequent kinetic resolution of 3,3,5-trimethylcyclohexanone (TMCH). This was explained by assuming the reaction pathway as a consequential process. The rapid hydrogenation of isophorone initially yielded racemic TMCH, and then the slow kinetic resolution consumed the (R)-TMCH, leaving (S)-TMCH in excess. Most promisingly, the Pd/MgO catalyst with enhanced adsorption of proline and moderate Pd particle sizes resulted in high enantioselectivity (ee of 95%) and 43% yield of TMCH.