Mechanism and origins of selectivity in rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes
The rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes represent an efficient method for the synthesis of biologically and pharmaceutically relevant chiral methylene pyrrolidines. In this report, the reaction mechanism was investigated by means of density functional theory calculations. The results show that the Rh–allyl complex was formed through an initial C–N oxidative addition. The subsequent proximal-insertion of allene into the Rh–N bond/C(sp2)–C(sp3) reductive elimination or the distal-insertion of allene into the Rh–C bond led to the formation of 3-methylene-pyrrolidine products or 2-methylene-pyrrolidine products, respectively. The calculations reproduced well the experimentally observed high regio-, E/Z, and diastereo-selectivity. In particular, it was found that the substituent of the allene has a dramatic impact on the proximal-insertion of allene into the Rh–N bond, which enables the selectivity switch between the proximal and distal CC bond insertion upon the change of the substituent. Finally, the high E/Z and diastereo-selectivity is rationalized in terms of the steric repulsion between the allene and the Rh–allyl moiety in the corresponding transition states.