Origins of the Unfavorable Activation and Reaction Energies of 1-Azadiene Heterocycles Compared to 2-Azadiene Heterocycles in Diels-Alder Reactions

The reactivities of butadiene, cyclopentadiene, furan, thiophene, pyrrole, and their 1-aza- and 2-aza-derivatives in Diels-Alder reactions with ethylene and fumaronitrile were investigated with density functional theory (M06-2X/6-311G-(d,p)). The activation free energies for the Diels-Alder reactions of cyclic 1-azadienes are 10-14 kcal mol(-1) higher than those of cyclic 2-azadienes, and the reaction free energies are 17-20 kcal mol(-1) more endergonic. The distortion/interaction model shows that the increased activation energies of cyclic 1-azadienes originate from increased transition state distortion energies and unfavorable interaction energies, arising from addition to the nitrogen terminus of the C = N bond.