Role of Orbital Interactions and Activation Strain (Distortion Energies) on Reactivities in the Normal and Inverse Electron-Demand Cycloadditions of Strained and Unstrained Cycloalkenes.

The Diels-Alder reactivities of a series of cycloalkenes, from the highly strained cyclopropene to the unstrained cyclohexene, have been studied with density functional theory using the M06-2X functional. The normal electron-demand Diels-Alder reactions with cyclopentadiene and the inverse electron-demand Diels-Alder reactions with 3,6-bis(trifluoromethyl)tetrazine were analyzed using the distortion/interaction-activation strain model. Previous studies showed that activation strain computed from the distorted reactants in the transition structures are larger for unstrained than strained cycloalkenes, and that most of the activation energy differences are accounted for by this difference. We have now analyzed the strain and interaction energy curves for the series of cycloalkenes along the reaction coordinate. Our analyses reveal that the strain curves associated with the distortion of the reactants in the Diels-Alder reactions are nearly identical and that the reactivity differences originate from differences in interaction energies. Analysis of the diene-dienophile interactions reveal that the reactivity trends result from differences in the strength of the primary and secondary orbital interactions.