Shedding light on the factors controlling the mechanism, selectivity and reactivity of the Diels–Alder reactions between substituted pyridinones and ethylenes: a MEDT study

Abstract

A Molecular Electron Density Theory (MEDT) study of the mechanism, the regioselectivity and the stereoselectivity of the Diels–Alder reactions between electron-rich and electron-deficient pyridinones and ethylenes using DFT methods at the B3LYP/6-31G(d) theoretical level has been performed. The CDFT reactivity indices show that pyridinone 1 and benyloxyethylene 4 are strong nucleophiles, while nitroethylene 3 and pyridinone 2 are strong electrophiles. The change of the electronic character of the substituent of pyridinones or ethylenes provokes a change in their reactivity and the electrophilic and nucleophilic behaviours. These DA reactions proceed through a relatively asynchronous polar mechanism and favour kinetically the formation of the cycloadduct generated from ortho-exo approach, in agreement with the experimental data. The van der Waals’ favourable interactions at ortho-exo approaches are the reasons for the stabilisation of these systems and explain well the observed experimental ortho-exo selectivity.