STEPWISE, ZWITTERIONIC COURSE OF HETERO-DIELS–ALDER REACTION BETWEEN 1,2,4-TRIAZINE MOLECULAR SYSTEMS AND 2-CYCLOPROPYLIDENE-1,3-DIMETHYLIMIDAZOLINE

Abstract

The computational wb97xd/6-311+G(d)(PCM) study confirms without any doubt the polar, stepwise mechanism of hetero-Diels–Alder reactions as illustrated on the example of 5,6-diphenyl-1,2,4-triazine and 2-cyclopropylidene-1,3-dimethylimidazoline. The first reaction stage is the formation of a prereaction molecular complex, which can next convert to two possible zwitterionic intermediates characterized by ''cyclic'' and ''extended'' conformation. The first one cyclizes in the second reaction step to the expected six-membered cycloadduct.