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Theoretical Studies of Rare-Earth-Catalyzed [3 + 2] Annulation of Aromatic Aldimine with Styrene: Mechanism and Origin of Diastereoselectivity.

The synthesis of multisubstituted 1-aminoindanes through catalyst-controlled diastereodivergent [3 + 2] annulation of aromatic imines with alkenes is of great interest and importance. An understanding of the exact reaction mechanism, especially for the origin of diastereoselectivity, is an essential aspect for further development of such reactions. In this study, density functional theory calculations have been carried out on the rare-earth-catalyzed diastereodivergent [3 + 2] annulation of benzaldimine with styrene. The results show that the reaction mainly involves generation of active species, olefin insertion, cyclization, and protonation steps. The noncovalent interactions, such as C-H···π and metal···π interactions, play an important role in stabilizing the key transition state or intermediate. Both steric and electronic factors account for the diastereoselectivity. The preferred cis-diastereoselectivity could be ascribed to more efficient orbital interaction, while the crowded space will induce the formation of a C-H···π interaction between the NtBu group and benzene ring in a trans-diastereoselectivity manner, thus stabilizing the trans-selective transition state. Therefore, the stereospecific product could be obtained by fine-tuning the ligand/metal combination of the catalysts.