Toward an understanding of the molecular mechanism of the reaction between 1-methylpyrrole and dimethyl acetylenedicarboxylate.: An ab initio study

The molecular mechanism for the reaction between 1-methylpyrrole and dimethyl acetylenedicarboxylate (DMAD) has been studied using ab initio methods. Two alternative reaction pathways have been considered, both of which correspond to stepwise processes with initial, rate-determining formation of a common zwitterionic intermediate. This intermediate is formed by nucleophilic attack of the pyrrole ring to the carbon-carbon triple bond of DMAD, Closure of this intermediate (pathway A) affords a [4 + 2] cycloadduct, whereas intramolecular proton transfer (pathway B) affords a Michael adduct. The much larger potential energy barrier of the second step in pathway B relative to pathway A is responsible for the nonoccurrence of the former. Inclusion of solvent effects, by means of a polarizable continuum model, does not modify the electronic nature of this molecular mechanism.