Theoretical Study on the Aza-Diels-Alder Reaction Catalyzed by PHCl Lewis Acid via Pnicogen Bonding.

The reaction mechanism of the Aza-Diels-Alder (A-D-A) cycloaddition reaction between XC═NNH, where X = H, F, Cl, Br, and 1,3-butadiene catalyzed by a PHCl Lewis acid was characterized using density functional theory calculations. The influences of various substituents of X on the studied reaction were analyzed using the activation strain model (ASM), which is also termed as the distortion-interaction model. Calculations showed that the smallest and largest values of the activation energies belong to the substituents of F and Br, respectively. The activation energy of the studied reactions was decreased within 8.6 kcal·mol in the presence of PHCl catalyst. Investigations showed that the pnicogen bonding is adequately capable of activating the A-D-A reaction. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis were implemented to understand the nature of C···C and C···N bonds at the TS structures. Additionally, the energy decomposition analysis (EDA) based on the ETS-NOCV scheme was used to characterize the nature of C···C and C···N bond. The results of the study mirror the fact that the PHCl Lewis acid may be suggested as a simple suitable catalyst for experimental studies on the A-D-A reactions.