Origin of the Minor Enantiomeric Product in a Noyori Asymmetric Hydrogenation: Evidence for Pathways Different to the Major Mechanism

摘要

In a typical asymmetric catalytic process,a prochiral substrate (Sub) is converted into a major (Prod_S) and a minor (Prod_R) enantiomeric product via diastereomeric catalyst-substrate (CatSub) and catalyst-product (CatProd) complexes (Scheme 1).The two cycles are diastereomorphic to each other,but they take place through a single mechanism.As such,the Arrhenius equation can be applied confidently to correlate the Prod_S/Prod_R ratio to the relative stabilities of the diastereomeric transition states at the first irreversible step from CatSub to CatProd.The energy profile thus obtained can be utilized to estimate the structural difference between the two transition states.In most cases,however,this premise of a single mechanism has not been confirmed.