A new, comprehensive insight on the competition between (4+2) and (2+4) cycloaddition schemes and molecular mechanism of 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 and largely synchronous bond formation, with minor asynchronicity arising from local electronic effects and weak non-covalent interactions.