Aminolyses of 2,4-Dinitrophenyl 2-Furoate and Benzoate: Effect of Nonleaving Group on Reactivity and Mechanism

摘要

Second-order rate constants (k_N) have been determined spectrophotometrically for reactions of 2,4-dintrophenyl 2-furoate (2) with a series of alicyclic secondary amines in 80 mol % H₂O/20 mol % dimethyl sulfoxide (DMSO) at 25.0 oC. The furoate 2 is more reactive than 2,4-dintrophenyl benzoate (1) toward all the amines studied. The higher acidity of 2-furoic acid (pKa = 3.16) compared with benzoic acid (pK_a = 4.20) has been suggested to be responsible for the reactivity order, at least in part. The Br┆nsted-type plots for the reactions of 1 and 2 are curved downwardly, indicating that the aminolyses of both 1 and 2 proceed through a zwitterionic tetrahedral intermediate (T【) with a change in the rate-determining step on changing the amine basicity. Dissection of the k_N values into their microscopic rate constants has revealed that the pK_ao and k₂/k_-1 ratios for the reactions of 1 and 2 are identical, indicating that the nature of the nonleaving group (i.e., benzoyl and 2-furoyl) does not affect the reaction mechanism. The k₁ values have been found to be larger for the reactions of 2 than for those of 1, which is fully responsible for the fact that the former is more reactive than the latter.