Understanding the intermolecular Diels-Alder cycloaddition promotion: Activation strain model/energy decomposition analysis model and conceptual density functional theory viewpoints.

The present work reports the computational study of the major Diels-Alder reaction between 2-bromocycloalkenone and a variety of mono- and di-substituted dienes. Through density functional theory (DFT) calculations and subsequent activation strain model/energy decomposition analysis/conceptual DFT (C-DFT) analyses, the key factors governing the activation barriers heights, and thus reactivity, are characterized. In contrast with a previous study, steric effects do not appear to control reactivity. Conversely, in all presented cases, a subtle interplay between deformation and interaction energies is evidenced at transition states. In the end, neither term alone is enough to explain or predict reactivity. Yet a simple C-DFT descriptor allows to predict with a reasonable efficiency the activation barriers: the excitation energy needed to observe a charge transfer from the diene to the dienophile. Theoretical elements are provided to support the use of this descriptor.