Aminolysis of 2,4-Dinitrophenyl 2-Furoate and 2-Thiophenecarboxylate: Effect of Modification of Nonleaving Group from Furoyl to Thiophenecarbonyl on Reactivity and Mechanism

Abstract

Second-order rate constants have been determined spectrophotometrically for reactions of 2,4-dinitrophenyl 2thiophenecarboxylate (2) with a series of alicyclic secondary amines in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 o 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 pKa at the center of the Bronsted curvature, defined as pKa 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 k2/k–1 ratio is not influenced upon changing the nonleaving group from furoyl to thiophenecarbonyl. However, k1 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 k1 for the reactions of 2 is fully responsible for the fact that 2 is less reactive than 1.