Mechanistic study of [4 + 3] cyclization of N,N′-cyclic azomethine imines with isatoic anhydrides under Brønsted acid catalysis

The possible mechanisms of [4 + 3] cyclization of N,N′-cyclic azomethine imine with isatoic anhydride catalyzed by a Brønsted acid catalyst have been investigated by performing DFT calculations. The calculated results show that the most energetically favorable pathway mainly includes (1) nucleophilic attack on isatoic anhydride, (2) the decarboxylation process, (3) formation of seven-membered N-heterocycle, (4) rearrangement of C−N bond, (5) transfer of hydrogen bond, (6) breaking of C−N bond, (7) formation of a new C−N bond. Furthermore, the chemoselectivity of reaction involving the transformation between two isomer products is controlled by a thermodynamical equilibrium of the reassembly of the C−N bond, which can be promoted by Brønsted acid catalyst in kinetics. This work should be useful for understanding the detailed mechanism and chemoselectivity of the Brønsted acid-catalyzed reactions.