Pericyclic Reactions vs Polar Reactions. Use of Heterocycles as Electrophiles. Microwave Irradiation

Considering the possibility of analyzing polar cycloaddition reactions, in this work we studied a series
Diels-Alder reaction using different heterocycles derivatives acting as electrophiles joint to dienes of
different nucleophilicity, analyzing the effect of the microwave irradiation in these processes. For this,
two reaction conditions have been worked: toluene as solvent and solvent free. The last one presents
the better yield in shorter time of reaction. Using microwave heating the aromatic cycloadducts are
clearly predominant. The reactions that will be discussed could be considered of the pericyclic type,
however taken account the effect of the solvent on them, they could also be considered polar.
Consequently, it would be in the presence of a borderline situation. The methodology explored could
be used in commercial form. However, it would be necessary to have the required equipment for this
purpose. From a formal point of view the reactions that are discussed could be considered polar.
Considering the dynamics of the reactions, in only one case it was possible to verify the presence of
an intermediate state, while in the rest of them there was observed the presence of concerted and
markedly asynchronic reactions. For all these reasons in this work we try to discuss the scope of this
reactive model and its location in the panorama of the ways in which it proceeds according to the
reaction mechanism. That to say if for this kind of process, the concept of pseudo-pericyclic reaction
is applicable. The important solvent effect observed in these persuades us to consider these
processes as PD-A 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).