Theoretical Studies of the Asymmetric Binary-Acid-Catalyzed tert-Aminocyclization Reaction: Origins of the Csp3-H Activation and Stereoselectivity

A mechanistic study of the tert-aminocyclization reaction was performed by using DFT calculations and labeling experiments. The results showed that the reaction proceeded through a rate-limiting-, stereospecific-, and suprafacial 1,5-H-transfer pathway, followed by a barrier-less C?C bond formation. The mode of stereocontrol for facial selection could be ruled out owing to the high activation energy of C?N bond rotation. The intrinsic feature of this Lewis acid activation was found to be the activation of the LUMO, as well as an intermediate-stabilization effect. The catalytically active species was believed to be a 1:1 complex of phosphoric acid and MgCl2, which was stabilized by a H...Cl hydrogen bond. The chiral catalytic complex selectively recognizes and activates one of the two helical conformations of substrate A, required for 1,5-suprafacial H-transfer, which dictates the stereoselectivity of the forming products.