Pseudo-Pericyclic Reactions: Aromatic Compound Derivatives as Electrophiles in Polar Diels-Alder Reactions. An Experimental and Theoretical Study

Abstract

The Diels-Alder reaction is considered a pericyclic procces. However, depending of the structure of the diene and the dienophile the nature of this reaction can change. In the last years we have worked with aromatic nitro substituted compounds as electrophile in cycloaddition reactions with different dienes of diverse nucleophilicity. In that direction, it was used carbocyclic -nitronaphthalenes- and heterocyclic -nitropyrroles, nitrofuranes, nitrothiophenes, nitroselenophenes, nitroindoles, nitrobenzofuranes, and nitrobenzothiophnes-, in both cases mono and disubstituted. The dienes used were Danishefkys´diene, Rawals?diene, 1-trimethylsyliloxy-1,3-butadiene, and isoprene. Those monosubstituted electrophiles which aromatic character is higher -e.g. nitronaphthalene and nitrothiophenes- follow an heterocyclic cycloaddition channel and offer in its rections with the less nucleophilic dienes a pyrrolyl derivative as principal product, instead the others electrophiles follow a normal cycloaddition channel. In all these reactions the solvent effect was important. In thermal conditions we observed a great influence of ionic liquids (ILs) respect to the molecular solvents.This fact persuade us to consider these processes as polar Diels-Alder reactions in which and in a simple interpretation react an electrophile with a nucleophile. On the other hand when it was used microwave irradiation the best resold was obtained in free solvent conditions. We confirm this approach using theoretical calculations based on Density Functional Theory (DFT).