Reduction of Carbonyl Compounds via Hydrosilylation. 3. Asymmetric Reduction of Keto Esters via Hydrosilylation Catalyzed by a Rhodium Complex with Chiral Phosphine Ligands

Asymmetric reduction of α-, β-and γ-keto esters via hydrosilylation catalyzed by rhodium complexes with chiral phosphines, (+)-BMPP, (+)-or (-)-DIOP, was investigated. High optical yields around 80% (72–85% ee) were attained in the case of pyruvates. Clear double asymmetric induction was observed in the asymmetric hydrosilylation of (-)-menthyl phenylglyoxylate. Under optimum conditions, the optical purity of the mandelate was 77%. Transfer hydrogenation instead of hydrosilylation is proposed to be involved in the reaction of an acetoacetate and a benzoylacetate. Asymmetric hydrosilylation of various levulinates followed by acid hydrolysis afforded 4-methyl-γ-butyrolactone (76–85% ee) in nearly quantitative yield. A combination of DIOP ligand with α-naphthylphenylsilane was found to be quite effective for the asymmetric induction. High efficiency of asymmetric induction for the keto esters compared with simple prochiral ketones may be due to an attractive interaction between the ester carbonyl and rhodium center in the transition state. Possible mechanisms for the induction of asymmetry are discussed using Dreiding models of the intermediate α-silyloxyalkyl-rhodium complexes.