Mechanism of Rh(III)-catalyzed cyclopropanation using N-enoxyphthalimides and alkenes: Insights from DFT calculations

Abstract

The Rh(III)-catalyzed cyclopropanation reaction using N-enoxyphthalimides and alkenes developed by Rovis group [J. Am. Chem. Soc.2014, 136, 11292−11295] provided an efficient method for the constructions of trans 1,2-disubstituted cyclopropanes in which an elegant control of the diastereoselectivities was achieved. In the current report we aimed at uncovering the mechanism and diastereoselectivity of the reactions using density functional theory (DFT) calculations. By comparing the energies of all possible pathways, we found that a novel mechanism involving a four-membered Rh(V) species is the energetically most favorable one. In this pathway, the four-membered Rh(V) intermediate is formed by sequential CH activation, alkene insertion and NO bond cleavage steps, and the final cyclopropane product is formed via an reductive elimination process. The NO bond cleavage was found to be the diastereoselectivity-determining, which was reproduced well the experimentally observed selectivity. By analyzing using the distortion/interaction model, it was found that the distortion energy plays a main role in determining the diastereoselectivity.