Divergent Synthesis of Multi‐Substituted Aminotetralins via [4+2] Annulation of Aldimines with Alkenes by Rare‐Earth‐Catalyzed Benzylic C(sp3)−H Activation

The search for efficient and selective methods for the divergent synthesis of multi‐substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2‐methyl aromatic aldimines with alkenes via benzylic C(sp3)−H activation by half‐sandwich rare‐earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi‐substituted 1‐aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5Me4SiMe3 or C5Me5 exclusively afforded the trans‐selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis‐selective annulation products. This protocol features 100% atom‐efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates.