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

The second-order rate constants of the [2 + 2] cycloaddition reaction betwe en diethyl azodicarboxylate and ethyl vinyl ether were obtained spectrophot ometrically in various solvents and aqueous solutions of 1,4-dioxane and me thanol at 30 +/- 0.1 degrees C. Zn all media except aqueous solutions, a ve ry good linear correlation of logk(2) vs E-T(N) (normalized polarity parame ter) was obtained (n = 11, r = 0.991, s = 0.086). Because of the higher pol arity of the activated complex relative to the reactants, the rate of the r eaction increase with increasing solvent polarity parameter. Dual-parameter correlation of logk(2) vs pi* (dipolarity/polarizability) and a (hydrogen bonding acidity) also gives good results in various solvents (n = 11, r = 0 .985, s = 0.118). Both pi* and alpha have approximately equal effects on th e reaction rate. In aqueous solutions of 1,4-dioxane and methanol, the seco nd-order rate constants of the reaction increase dramatically with increasi ng mole fraction of water. A dual-parameter correlation of logk(2) vs Sp (s olvophobicity parameter) and E-T(N) 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 E-T(N). This model represents a significant improv ement in regression coefficient with respect to the single parameter correl ation vs Sp or E-T(N) (r = 0.954 and 0.854, respectively). Similarly to aqu eous solutions, a dual-parameter correlation of logk(2) vs Sp and E-T(N) wa s obtained in all media (n = 21, r = 0.987, s = 0.177). Copyright (C) 2000 John Wiley & Sons, Ltd.