A new, comprehensive insight into the competition between [4+2] and [2+4] cycloaddition schemes and molecular mechanism of the Diels–Alder reaction of 2-nitrofuran with isoprene

Abstract

A comprehensive mechanistic study of the competing [4+2] and [4+2] cycloadditions between 2-nitrofuran and isoprene was performed using molecular electron density theory at the ωB97XD/6-311G(d,p) theory level. Reactivity analysis of the reagents indicates that the reaction is governed by the nucleophilic attack of the diene isoprene on the electrophilic dienophile 2-nitrofuran. Systematic exploration of all possible pathways revealed a one-step reaction mechanism, in which the [4+2] cycloaddition pathways are favored under thermodynamic control. Interconversion between the [4+2] and [4+2] cycloadducts was not observed. Analyses of the transition states revealed a weakly polar character, largely synchronous bond formation, with minor asynchronicity arising from local electronic effects and weak non-covalent interactions.