Mechanism and stereoselectivity of the Rh(II)-catalyzed cyclopropanation of diazooxindole: a density functional theory study

The mechanism and origin of stereoselectivity of rhodium(II)-catalyzed cyclopropanation reactions with diazooxindole and styrene has been studied using density functional theory calculations. The catalyzed reactions by achiral Rh-2(OAc)(4) and chiral Rh-2(S-PTTL)(4) as well as the uncatalyzed model were comparatively studied. The computational results indicate that the cyclopropanation step in both Rh-2(OAc)(4) and Rh-2(S-PTTL)(4) models is a single concerted but asynchronous process. The nitrogen extrusion step is found to be the rate-limiting step of the catalytic cycle, whereas the cyclopropanation step is the stereoselectivity-determining step. The diastereomeric ratios (dr) and the enantiomeric excess (ee) values are successfully predicted, which are in good agreement with the experimental values. The high trans-diastereoselectivity might be governed by the pi-pi interactions between the syn indole ring in carbenoid ligand and the phenyl group in styrene, whereas the good enantioselectivity can be ascribed to the steric interaction between the phenyl ring in styrene and the phthalimido group in the catalyst as well as the aromatic interactions (pi-pi and CH-pi) in the transition states. Additionally, the methodological study using different functionals demonstrated the importance of considering the dispersion interactions in the current reaction systems. This theoretical study will help in understanding the mechanism of the asymmetric cyclopropanations of olefins through carbene-transfer reactions.