A conceptual DFT analysis of the plausible mechanism of some pericyclic reactions

Abstract

Out of several pericyclic reactions, Diels-Alder (DA) reaction is one of the most widely used synthetic processes. In the present work, several models and methodologies have been used to determine and to analyze the plausible mechanism of some representative DA cycloaddition reactions. A comparison between the dual descriptor and the bond reactivity indices corresponding to the natural bond orbital of the reagents is included, which provides a complete description of the plausible reaction mechanism. In the next step, two very recent models are used to determine the local electronic density transfer and redistribution between the reactants involved. The description of the local electronic density transfer has been made in two stages; first, the variation in the net charge on the atoms is obtained, and then, the electronic density transfer between the natural bond orbitals is calculated. The values obtained using the two models are correlated with the experimental rate constants of the reactions. Finally, the natural bond orbitals are obtained at several steps along the reaction path and the variation in their partial occupation is compared with the corresponding electron density transfer among these orbitals. Furthermore, frontier molecular orbital (FMO) approach has been employed to understand the more feasible way of interaction between the DA pair. Relative electrophilicity descriptors like net electrophilicity (∆ω±), net reactivity index (NRI, Δω±R), and electrophilicity difference (∆ω) between DA pairs have also been employed to describe the studied reaction mechanisms especially whether they follow non-polar-concerted/polar-stepwise pathway along with their classification in terms of normal or inverse electron demand. Furthermore, adaptive natural density partitioning method (AdNDP) and energy decomposition analyses (EDA) in conjunction with natural orbital for chemical valence (NOCV) have been made use of in order to analyze the actual bonding situation in the transition state (TS).