Abstract
Structural and kinetic aspects of the retro-cheletropic ene reactions of N-phenyl-1-methyl-6-methylenecyclohexa-2,4-dienylmethanimine (R) and its deuterated derivative were investigated using a variety of computational methods. Theoretical calculations were carried out with DFT methods at the B3LYP and M06 levels of theory, using the 6-311++G(d,p) basis set. Vibrational frequency analysis confirmed the stationary states including the transition state (TSs) structures. The mechanistic studies on the retro-cheletropic ene processes rejected the stepwise mechanism and confirmed a five-membered cyclic TS for the reaction. The conversion of the reactant to the products proceeds through an asynchronous concerted mechanism. Theoretical calculations indicate that the reaction displays a primary kinetic isotope effect of 8.75 at 417 K. Calculated activation parameters, especially M06 results, are in good agreement with the experimental data. The natures of bond making and breaking during the reactions have been investigated using the concepts of donor–acceptor, electron charge density and Laplacian in NBO and AIM methods, respectively.