The elucidation of exact reaction mechanisms, especially for the purpose of determining the origin of regio-and diastereo-selectivity, 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 diastereo-selectivity, 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 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 forms during the alkene insertion process. These results provide valuable insights into the reactions catalyzed by rare-earth metals.

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