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.