Catalytic asymmetric conjugate reduction with ethanol: A more reactive system Pd(II)-Pr-DUPHOS complex with molecular sieves 4A

We describe herein the catalytic asymmetric conjugate reduction of α,β-unsaturated carbonyl compounds using a novel cationic Pd-ⁱPr-DUPHOS complex. In this reaction, EtOH worked well as a solvent and a reducing agent, and the reaction was completed within several hours in most cases to afford the reduced compounds almost quantitatively with modest to good enantioselectivity (up to 72% ee). It was found that the Pd-ⁱPr-DUPHOS complex was more reactive than the previously reported Pd-BINAP complex when molecular sieves 4A was added as an additive. Based on an X-ray structural analysis of [Pd{(S,S)-ⁱPr-duphos}](OTf)₂ complex, a working hypothesis of the reaction mechanism is also described.