A theoretical study of acetylene, ethylene, and cyclopropene additions to benzo[b]- and benzo[c]-fused heterocycles

The reactivity of benzo[b]- and benzo[c]-fused heterocycles as dienes for Diels-Alder reactions were evaluated on the basis of Frontier Molecular Orbital (FMO) energy gaps between reactants, necessary FMO changes for the transformation of reactants into transition state structures, uniformity of bond orders of heterocyclic rings, and uniformity of bond orders of the six-membered cyclic transition state structures. All of these values were obtained with AM1 semiempirical methods. The selectivity of the cycloaddition reaction was also judged with Secondary Orbital Interactions (SOI) between two reactant moieties in isomeric transition state structures. For every possible reaction pathway, AM1 and B3LYP/AM1 energies were evaluated. The relative reactivity and selectivity obtained by computing activation barriers for these reactions were in full agreement with our qualitative studies as well as with experimental results. The usefulness of the FMO and bond order uniformity approach for the study of the Diels-Alder reaction was discussed. (C) 1997 Elsevier Science Ltd.