Unraveling polar Diels-Alder reactions with conceptual DFT analysis and the distortion/interaction model.

The reaction energetics of 280 polar Diels-Alder (DA) reactions between 70 dienophiles and 4 dienes have been studied in detail using the B3LYP/6-31G* level of theory, combining conceptual density functional theory (DFT) analysis and the distortion/interaction model. The barrier heights are governed by a fine balance between the energy required to distort the reactants from their initial to their transition state geometries (ΔE++d) and the binding energy between the deformed reactants in the TS (ΔE++i). The ΔE++i values strongly correlate with the electrophilicity index, ω, which measures the stabilization energy when the system acquires an additional electronic charge from the environment, whereas the ΔE++d was found to depend mainly on the nature of the diene, structural parameters of the dienophile (degree of substitution and ring size) and the asynchronicity of the TS. A detailed analysis to account for the geometrical parameters of the strained diene and dienophile moieties that influence the energy strain of the distorted fragments is also reported.