Pericyclic Reactions vs Polar Reactions. Use of Heterocycles as Electrophiles. Microwave Irradiation | Chapter 5 | Theory and Applications of Chemistry Vol. 2
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).
Author(s) Details
Maria N. Kneeteman
IQAL (UNL-CONICET), Laboratorio Fester –QUIMICA ORGANICA (FIQ), Universidad Nacional del Litoral, Santiago del Estero 2829, 3000 Santa Fe, Argentina.
IQAL (UNL-CONICET), Laboratorio Fester –QUIMICA ORGANICA (FIQ), Universidad Nacional del Litoral, Santiago del Estero 2829, 3000 Santa Fe, Argentina.
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