Aminolysis of 2,4-dinitrophenyl 2-furoate and 2-thiophenecarboxylate: Effect of modification of nonleaving group from furoyl to thiophenecarbonyl on reactivity and mechanism

Second-order rate constants have been determined spectrophotometrically for reactions of 2,4-dinitrophenyl 2-thiophenecarboxyl ate (2) with a series of alicyclic secondary amines in 80 mol% H2O mol% DMSO at 25.0 +/- 0.1 degrees C. The Bronsted-type plot exhibits a downward curvature, i.e., the slope decreases from 0.74 to 0.34 as the amine basicity increases. The pK(a) at the center of the Bronsted curvature, defined as pK(a)(o), has been determined to be 9.1. Comparison of the Bronsted-type plot for the reactions of 2 with that for the corresponding reactions of 2,4-dinitrophenyl 2-furoate (1) suggests that reactions of 1 and 2 proceed through a common mechanism, although 2 is less reactive than 1. The curved Bronsted-type plot has been interpreted as a change in RDS of a stepwise mechanism. The replacement of the O atom in the furoyl ring by an S atom (1 -> 2) does not alter the reaction mechanism but causes a decrease in reactivity. Dissection of the apparent second-order rate constants into the microscopic rate constants has revealed that the k(2)/k(-1) ratio is not influenced upon changing the nonleaving group from furoyl to thiophenecarbonyl. However, k(1) has been calculated to be smaller for the reactions of 2 than for the corresponding reactions of 1, indicating that the C=O bond in the thiophenecarboxylate 2 is less electrophilic than that in the furoate 1. The smaller k(1) for the reactions of 2 is fully responsible for the fact that 2 is less reactive than 1.