Aminolysis of 2,4-dinitrophenyl X-substituted benzoates and Y-substituted phenyl benzoates in MeCN: Effect of the reaction medium on rate and mechanism

Abstract

Second-order rate constants (kN) have been determined spectrophotometrically for the reactions of 2,4-di-nitrophenyl X-substituted benzoates (1a-f) and Y-substituted phenyl benzoates (2a-h) with a series of alicyclic secondary amines in MeCN at 25.0 ± 0.1°C. The kN values are only slightly larger in MeCN than in H2O, although the amines studied are approximately 8 pKa units more basic in the aprotic solvent than in H2O. The Yukawa-Tsuno plot for the aminolysis of 1a-f is linear, indicating that the electronic nature of the substituent X in the nonleaving group does not affect the ratedetermining step (RDS) or reaction mechanism. The Hammett correlation with a constants also exhibits good linearity with a large slope (ρY = 3.54) for the reactions of 2a-h with piperidine, implying that the leaving-group departure occurs at the rate-determining step. Aminolysis of 2,4-dinitrophenyl benzoate (1e) results in a linear Brønsted-type plot with a βnuc value of 0.40, suggesting that bond formation between the attacking amine and the carbonyl carbon atom of le is little advanced in the transition state (TS). A concerted mechanism is proposed for the aminolysis of 1a-f in MeCN. The medium change from H2O to MeCN appears to force the reaction to proceed concertedly by decreasing the stability of the zwitterionic tetrahedral intermediate (T ±) in aprotic solvent. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.