Elucidating the mechanism and origin of diastereoselectivity in scandium-catalyzed β-C(sp3)–H activation and transformation of an aliphatic aldimine†

The elucidation of exact reaction mechanisms, especially for the purpose of determining the origin of regio- and diastereoselectivity, is essential for furthering the development of substituent divergent reactions. In this study, density functional theory calculations were carried out to elucidate the mechanism and, thereby, the origin of regio- and diastereoselectivity, of the divergent [3 + 2] annulation of an aldimine with alkenes catalyzed by the rare-earth metal scandium (Sc). The reaction mechanism was found to consist of the generation of active species, C(sp³)–H activation, alkene insertion, ring closure, and protonation processes. The alkene insertion process was found to determine the regio- and diastereoselectivity of the reaction. The main role of the additive amine is to lower the positive charge of the Sc atom, thereby weakening and cleaving the Sc–C(sp³) bond, before forming an Sc–C(sp²) bond during the alkene insertion process. These results provide valuable insights into the reactions catalyzed by rare-earth metals.