A highly enantioselective rhodium catalyst for the hydrogenation of aliphatic α‐keto esters

The enantioselective hydrogenation of several α‐keto acid derivatives with rhodium diphosphine catalysts has been investigated using a random screening approach. The neutral rhodium catalyst prepared in situ from bis(2,5‐norbornadiene rhodium chloride) and NORPHOS has been found to be an excellent catalyst for preparing aliphatic α‐hydroxy esters in high optical purities. The reaction parameters for the hydrogenation of ethyl 2‐oxo‐4‐phenyl‐butyrate, an intermediate for the ACE inhibitor Benazepril, were optimized and the best optical yields obtained were 96%.