Mechanism and Origins of Ligand-Controlled Selectivity of Rhodium-Catalyzed Intermolecular Cycloadditions of Vinylaziridines with Alkynes

In this report, the mechanism of the rhodium-catalyzed intermolecular cycloaddition of vinylaziridines with alkynes is investigated by means of density functional theory calculations. The calculations show that the reaction is initiated by a C-N oxidative addition to form the Rh-allyl complex, from which the migratory insertion of the alkyne into the Rh-N bond was found to take place in a 1,2-fashion. The subsequent competing direct C-C reductive elimination and alkenyl isomerization/ C-C reductive elimination led to the [3+2] and [5+2] cycloadditions, respectively. The experimentally observed ligand-controlled selectivity is reproduced quite well by the calculations. Importantly, it turns out that the steric repulsion between the ligand and the alkenyl moiety has a profound impact on the direct C-C reductive elimination and the alkenyl isomerization, which enables the selectivity switch between the [3+2] and [5+2] cycloaddition upon change of ligand.