Density functional theory and atoms-in-molecule study on the role of two-electron stabilizing interactions in retro Diels-Alder reaction of cycloadducts derived from substituted cyclopentadiene and p-benzoquinone

A systematic investigation on the cycloreversion reaction of the cycloadduct formed between substituted cyclopentadiene and p-benzoquinone ( 1 - 19) is reported at the B3LYP/6-311+G**// B3LYP/6-31G* level of theory. The computed activation barrier exhibits a fairly high sensitivity to the nature of substituents at the C-7-position. Gibbs free energy of activation for 1 and 19 are found to be 20.3 and 30.1 kcal mol(-1), respectively, compared to 7, which is estimated to be 24.7 kcal mol(-1). Quantitative analysis of the electronic effects operating in both the cycloadduct as well as the corresponding transition state for the retro Diels - Alder (rDA) reaction performed using the natural bond orbital (NBO) and atoms in molecule (AIM) methods have identified important two-electron stabilizing interactions. Among four major delocalizations, sigma(C-7 -X) to sigma*(C-1 - C-5) [and to sigma*(C-2 - C-6)] is identified as the key contributing factor responsible for ground state C-1 - C-5 bond elongation, which in turn is found to be crucial in promoting the rDA reaction. A good correlation between the population of antibonding orbital [sigma*(C-1 - C-5)] of the ground state cycloadduct and Gibbs free energy of activation is observed. The importance of factors that modulate ground state structural features in controlling the energetics of rDA reaction is described.