Transition states of binap-rhodium(I)-catalyzed asymmetric hydrogenation: Theoretical studies on the origin of the enantioselectivity

By using the hybrid IMOMM(B3LYP:MM3) method, we examined the binap-Rh-I-catalyzed oxidative-addition and insertion steps of the asymmetric hydrogenation of the enamide 2-acetylamino-3-phenylacrylic acid. We report a path that is energetically more favorable for the major enantiomer than for the minor enantiomer. This path follows the lock-and-key motif and leads to the major enantiomeric product via an energetically favorable binap-dihydride-Rh-III-enamide complex. Our theoretical results are consistent with the mechanism that takes place via Rh-III dihydride formation, that is, oxidative addition of H-2 followed by enamide insertion.