Theoretical study of intramolecular anion radical cycloaddition of the phenyl-substituted bis(enone)

Abstract

DFT (U)B3LYP calculations with the 6-31G(d,p), 6-31+G(d,p), and 6-311+G(d,p) basis sets were carried out to investigate the mechanisms of the three reactions from the phenyl-substituted bis(enone) anion radical to the cis-bicyclobutane anion radical (cis-P −) [ R - → cis- P - (1)], to the trans-bicyclobutane anion radical (trans-P −) [ R - → trans- P - (2)], and to the Diels–Alder cycloaddition anion radical (da-P −) [ R - → da- P - (3)], respectively, in both the vacuum and the tetrahydrofuran (THF) solvent. The calculations indicate that the reactions occurring in both phases have the same skeleton of the energy profile for each of the three channels. The three reaction processes all consist of two reaction steps. The first step for reaction (1) is to form a trans-intermediate having a five-atom ring via a trans-transition state and the second step is to form the cis-P − product via a cis-transition state. The first steps of reactions (2) and (3) are both to form a cis-intermediate having a five-atom ring via a cis-transition state and the second steps are to form the trans-P − product via a trans-transition state for reaction (2) and to form the da-P − product via a transition state for reaction (3). The B3LYP calculations indicate that trans-P − is the dominant product in both the vacuum and the THF solvent. The possibilities of the electron transfers from the three kinds of aromatic anion radicals (the chrysene, anthracene, and naphthalene anion radicals) to the neutral reactant and from the trans-P −, cis-P −, and da-P − products to the neutral reactant were explored using the B3LYP/6-311+G(d,p) method by means of calculating the electron affinities of the respective neutral species. The calculations indicate that the chain reaction mechanism for the anion radical cyclobutanation of the phenyl-substituted bis(enone) suggested by experimentalists is feasible thermodynamically.