Density functional theory study of the Diels–Alder reactivities of [b]-annelated cyclic five-membered dienes

Abstract

B3LYP/6–31G* calculcations were performed to model the Diels–Alder reaction of cyclobutano-, cyclobuteno- and benzo[b]- fused cyclic five-membered ring dienes with two prototypical dienophiles, ethylene and acetylene. The cyclobuteno[b]-fused dienes, which loose the unfavorable antiaromaticity upon cycloaddition, are the most reactive; in contrast, the benzo[b]-fused dienes, which encounter disruption of aromaticity in the course of the reaction, are the least reactive. Synchronicity calculations based on bond order analysis indicate that the reactions of benzo[b]-annelated dienes show significant deviations from synchronicity, indicating highly unsymmetrical transition states. Deformation energies were employed to corroborate the computed activation energies. Although there are some deviations in the quantitative results between ethylene and acetylene reactions, the trends are essentially the same with all the dienes considered, with acetylene showing a slightly higher reactivity. The comparison with the corresponding [c]-fused dienes further strengthens the argument that product stability controls the reactivity in these reactions, as exemplified by excellent linear fits between the activation and reaction energies.