The Diels-Alder reaction is considered to be a pericyclic process. However, depending on the structure of the diene and the dienophile the nature of this reaction can change. In the recent years we have worked with aromatic nitro-substituted compounds as electrophiles in cycloaddition reactions with different dienes of diverse nucleophilicity. In this context, carbocyclic compounds such as nitronaphthalenes and heterocyclic compounds such as nitropyrroles, nitrofuranes, nitrothiophenes, nitroselenophenes, nitroindoles, nitrobenzofuranes, and nitrobenzothiophenes, in both cases mono and disubstituted, were used. The dienes used were Danishefky’s diene, Rawal’s diene, 1-trimethylsyliloxy-1,3-butadiene, and isoprene. The monosubstituted electrophiles whose aromatic character is higher, for e.g. nitronaphthalene and nitrothiophenes, follow a heterocyclic cycloaddition channel and present, in the reactions with the less nucleophilic dienes, a pyrrolyl derivative as principal product while the other electrophiles follow a normal cycloaddition channel. The solvent effect observed in these reactions was important. In thermal conditions we observed a substantial influence of ionic liquids (ILs) on reaction times and product yields comparing to molecular solvents. This fact persuades us to consider these processes as polar Diels-Alder reactions in which an electrophile reacts with a nucleophile. On the other hand when microwave irradiation was used, the best results were obtained in free solvent conditions. We confirm this approach using theoretical calculations based on Density Functional Theory (DFT).
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