The Diels–Alder Reaction from the EDA-NOCV Perspective: A Re-Examination of the Frontier Molecular Orbital Model

We report calculated activation barriers for the Diels–Alder reaction between substituted butadienes and ethenes with density functional theory at the M06–2X level using a basis set with TZ2P quality. The interactions between the dienes and dienophiles at the transition state are analyzed with the EDA-NOCV method. There is a modest correlation between the activation barriers and the strength of the frontier-orbital interactions. The calculations suggest that the effect of the substituents on the reactivity of the Diels–Alder reactions depends on both, the deformation (“strain”) energies of the reactants and the intrinsic orbital interactions at the transition states. Surprisingly, the frontier-orbital interactions exhibit a better correlation with the reaction barriers than the total orbital interactions.