AM1 semiempirical study of benzopyrroles as dienes for Diels-Alder reaction

A qualitative approach for quick evaluation of the reactivity of dienes such as cyclopentadiene, furan, pyrrole, indole, and isoindole for Diels–Alder reactions was applied. As qualitative methods, the frontier molecular orbital (FMO) energy gap and the Hammond postulate through bond orders and energy of the reaction were evaluated for such dienophiles as ethylene, 1,3-dioxa-4-cyclopentene, and maleic anhydride. It was demonstrated that pyrrole and benzopyrroles are poor dienes in comparison with cyclopentadiene and furan. Because the reaction is controlled by the HOMO of the diene there are special cases when isoindole can react as a diene, while the loss of indole aromaticity in the course of the reaction precludes the cycloaddition. Geometries of transition structures and the accompanying activation energies were calculated for pyrrole, indole, and isoindole as dienes. The stereoselectivity of the reaction is discussed and compared with experimental results. Key words: benzopyrroles as dienes, Diels–Alder cycloaddition, Hammond postulate.