Systematic computational studies of pericyclic Diels–Alder reactions between 2,3-dimethyl-2-butene and all permutations of substituted cyclopentadienes c-C5R1R2R3R4R5aR5b (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 Corrections” △G‡ASC#/△GASC# 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, △G‡ASC#/△GASC# values allowed accurate predictions of barriers and exergonicities for reactions between 2,3-dimethyl-2-butene and highly substituted cyclopentadienes, and between 2,3-dimethyl-2-butene and cyclopentadienes with random mixtures of CH3/CF3/F substituents. △G‡ASC#/△GASC# values were correlated with steric considerations and 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.

Additivity of diene substituent Gibbs free energy contributions for Diels–Alder reactions between 2,3-dimethyl-2-butene and substituted cyclopentadienes

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