Polar Diels-Alder reactions developed in a protic ionic liquid: 3-nitroindole as dienophile. Theoretical studies using DFT methods

Abstract

N-Tosyl-3-nitroindole reacts with moderate and strong nucleophilic dienes in a polar Diels-Alder reaction using protic ionic liquid (PIL) [HMIM][BF4] as reaction media. In the cases in which the diene has oxygenated substituents, the PIL favors the aromatization of the formal [4+2] cycloadducts. Although the presence of the PIL in the reactions of N-tosyl-3-nitroindole as electrophile shows a clear tendency to the aromatization of the [4+2] cycloadducts, we observed that the neoteric solvent did not increase the yields in relation to a molecular solvent. Part of this work is specifically concerned with theoretical studies using DFT reactivity indices. The global and local electrophilicity and nucleophilicity indices were calculated for N-tosyl-3-nitroindole, and the dienes used in this study. For the studied reactions, the regioselectivity involved in the polar Diels-Alder reaction is well predicted.