Catalytic asymmetric hydrogenation of heterocyclic ketone-derived hydrazones, pronounced solvent effect on the inversion of configuration

An enantioselective hydrogenation of hydrazones derived from heterocyclic ketones was developed with up to 85% ee. The enantiomeric purity was enriched to >99% ee by crystallization from EtOAc in >80% yield. Optimization studies have revealed a notable solvent effect that resulted in inversion of enantioselectivity from 85% ee in MeOH to −27% ee in DCE. The hydrazone geometry and possible hydrogenation via endocyclic alkene were examined as possible factors for the inversion of enantioselectivity.