Toward an understanding of the molecular mechanism of the reaction between1-methylpyrrole and dimethyl acetylenedicarboxylate. An ab initio study

The molecular mechanism for the reaction between 1-methylpyrrole and dimeth yl acetylenedicarboxylate (DMAD) has been studied using ab initio methods. Two alternative reaction pathways have been considered, both of which corre spond to stepwise processes with initial, rate-determining formation of a c ommon zwitterionic intermediate. This intermediate is formed by nucleophili c attack of the pyrrole ring to the carbon-carbon triple bond of DMAD, Clos ure of this intermediate (pathway A) affords a [4 + 2] cycloadduct, whereas intramolecular proton transfer (pathway B) affords a Michael adduct. The m uch larger potential energy barrier of the second step in pathway B relativ e to pathway A is responsible for the nonoccurrence of the former. Inclusio n of solvent effects, by means of a polarizable continuum model, does not m odify the electronic nature of this molecular mechanism.