DFT study on the cycloaddition reactions of [c]-annelated carbo- and heterocyclic five-membered dienes with ethylene

Abstract

B3LYP/6-31G* calculations were done on a series of [c]-annelated heterocyclic five-membered dienes and their cycloaddition transition state structures and products. Cyclobutano[c]-, cyclobuteno[c]- and benzo[c]-fused five-membered rings represent non-aromatic, antiaromatic and aromatic ring fused dienes, respectively, and their reactivity was studied with ethylene as dienophile. In the cases of cyclobutano- and cyclobuteno-fused dienes, the fused portion remains a mere spectator and no significant geometric variations are seen along the reaction coordinate. In contrast, in the benzo-fused systems, the fused benzene ring witnesses significant changes in the bond lengths along the reaction coordinate highlighting the active participation of the π-framework in the reaction. Thus, this may be classified as an [8 + 2] cycloaddition. The benzo[c]-fused rings exhibit the lowest activation energies followed by cyclobutano and cyclobuteno analogues, and the exothermicities decrease in the same order. A linear relationship between the reaction exothermicity and activation energy is obtained for the systems under consideration. The percentage bond length alternation on going from reactant to transition state, the Fukui function indices and frontier orbital analysis, extent of charge transfer from diene to dienophile at the transition state, and deformation energies were estimated to explain the reactivity of the dienes with ethylene.