Multiparameter correlation of the rate of a [2?+?2] cycloaddition reaction versus solvophobicity parameter and normalized polarity parameter in aqueous solutions

The second-order rate constants of the [2 + 2] cycloaddition reaction between diethyl azodicarboxylate and ethyl vinyl ether were obtained spectrophotometrically in various solvents and aqueous solutions of 1,4-dioxane and methanol at 30 ± 0.1 °C. In all media except aqueous solutions, a very good linear correlation of logk2 vs ETN (normalized polarity parameter) was obtained (n = 11, r = 0.991, s = 0.086). Because of the higher polarity of the activated complex relative to the reactants, the rate of the reaction increase with increasing solvent polarity parameter. Dual-parameter correlation of logk2 vs π* (dipolarity/polarizability) and α (hydrogen bonding acidity) also gives good results in various solvents (n = 11, r = 0.985, s = 0.118). Both π* and α have approximately equal effects on the reaction rate. In aqueous solutions of 1,4-dioxane and methanol, the second-order rate constants of the reaction increase dramatically with increasing mole fraction of water. A dual-parameter correlation of logk2 vs Sp (solvophobicity parameter) and ETN was found in aqueous solutions (n = 13, r = 0.988, s = 0.165), in which Sp plays an important role in determining the reaction relative to ETN. This model represents a significant improvement in regression coefficient with respect to the single parameter correlation vs Sp or ETN (r = 0.954 and 0.854, respectively). Similarly to aqueous solutions, a dual-parameter correlation of logk2 vs Sp and ETN was obtained in all media (n = 21, r = 0.987, s = 0.177).