Asymmetric hydrogenation of alpha-keto acid derivatives by rhodium-{amidophosphine-phosphinite} catalysts

The enantioselective hydrogenation of several alpha-keto esters (3a-f, 5a-j), alpha-keto amides (7a-e) and isatine derivatives (9a-d) with a set of four representative neutral homogeneous rhodium-amidophosphine-phosphinite catalysts has been investigated. Trifluoroacetato-Rh-AMPP catalytic precursors promoted the rapid, efficient synthesis of aliphatic oc-hydroxy esters 4a-f in moderate to high enantioselectivities (66-95% ee), in contrast to most aromatic alpha-hydroxy esters 6a-j (8-81% ee). Best enantioselectivities for alpha-hydroxy amides 8a-e (85-95% ee) and dioxindoles 10a-d (80-94% ee) were obtained with chloro-Rh-AMPP precursors. It is proposed that, contrary to alpha-keto amides, alpha-keto esters do not chelate onto the rhodium center and that, in such circumstances, the asymmetric induction is mainly controlled by the steric hindrance around the C=O function. (C) 1997 Elsevier Science Ltd.