Kinetics and mechanism for the reaction of 4-nitrophenyl 2-thiophenecarboxylate with secondary alicyclic amines

Abstract

Second-order-rate constants (k(N)) have been measured spectrophotometrically for the reactions of 4-nitrophenyl 2-thiophenecarboxylate (1a) with a series of secondary alicyclic amines in H2O containing 20 mole % DMSO at 25.0degreesC. The ester la is less reactive than 4-nitrophenyl 2-furoate (1b) but more reactive than 4-nitrophenyl benzoate (1c) except towards piperazinium ion. The Bronsted-type plots for the aminolyses of 1a, 1b and 1c are linear with a beta(nuc) value of 0.92, 0.84 and 0.85, respectively, indicating that the replacement of the CH=CH group by a sulfur or an oxygen atom in the benzoyl moiety of 1c does not cause any mechanism change. The reaction of piperidine with a series of substituted phenyl 2-thiophenecarboxylates gives a linear Hammett plot with a large rho(-) value (rho(-) = 3.11) when sigma(-) constants are used. The linear Bronsted and Hammett plots with large beta(nuc) and rho(-) values suggest that the aminolysis of la proceeds via rate-determining break-down of the addition intermediate to the products.