Additivity of Diene Substituent Gibbs Free Energy Contributions for Diels-Alder Reactions between Me₂C=CMe₂ and...

Systematic computational studies of pericyclic Diels-Alder reactions between (H₃C)₂C=C(CH₃)₂, 1, and all permutations of substituted cyclopentadienes c-C₅R¹R²R³R⁴R^5aR^5b (R = H, CH₃, CF₃, F) allowed isolation of substitutional effects on Gibbs free energy barrier heights and reaction Gibbs free energies. "Average Substitution Gibbs Free Energy Correction" ΔG _ASC# ^‡/ΔG _ASC# values for each substituent in each position appeared to be additive. Substituent effects on barriers showed interesting contrasts. Methyl substitution at positions 5a and 5b increased barriers significantly, while substitution at all other positions had essentially no impact. In contrast, fluoro substitution at positions 5a and 5b lowered barriers more than substitution at other positions. Trifluoromethyl substitution mixed these effects, in that substitution at positions 5a and 5b increased barriers, but substitution at other positions lowered them. Despite the variances, ΔG _ASC# ^‡/ΔG _ASC# values allowed reliable prediction of barriers and exergonicities for reactions between 1 and highly substituted cyclopentadienes, and between 1 and cyclopentadienes with random mixtures of CH₃/CF₃/F substituents. ΔG _ASC# ^‡/ΔG _ASC# values were correlated with steric considerations and quantum theory of atoms in molecules (QTAIM) calculations. Overall, the ASC values provide a resource for predicting which Diels-Alder reactions of this type should occur at rapid rates and/or give stable bicyclic products.