Computational Investigation of the Competition between the Concerted Diels–Alder Reaction and Formation of Diradicals in Reactions of Acrylonitrile with Nonpolar Dienes

The energetics of the Diels–Alder cycloaddition reactions of several 1,3-dienes with acrylonitrile, and the energetics of formation of diradicals, were investigated with density functional theory (B3LYP and M06–2X) and compared to experimental data (Hall et al., J. Org. Chem. 1993, 58, 7049–7058). For the reaction of 2,3-dimethyl-1,3-butadiene with acrylonitrile, the concerted reaction is favored over the diradical pathway by 2.5 kcal/mol using B3LYP/6-31G­(d); experimentally, this reaction gives both cycloadduct and copolymer. The concerted cycloaddition of cyclopentadiene with acrylonitrile is preferred computationally over the stepwise pathway by 5.9 kcal/mol; experimentally, only the Diels–Alder adduct is formed. For the reactions of (E)-1,3-pentadiene and acrylonitrile, both cycloaddition and copolymerization were observed experimentally; these trends were mimicked by the computational results, which showed only a 1.2 kcal/mol preference for the concerted pathway. For the reactions of (Z)-1,3-pentadiene and acrylonitrile, the stepwise pathway is preferred by 3.9 kcal/mol, in agreement with previous experimental findings that only polymerization occurs. M06–2X is known to give more accurate activation and reaction energetics (Pieniazek, et al., Angew. Chem. Int. 2008, 47, 7746–7749), but the energies of diradicals are too high.