Kinetics and mechanism of the aminolysis of ethyl aryl carbonates in acetonitrile

The aminolysis reactions of ethyl aryl carbonates with benzylamines in acetonitrile at 25.0°C are investigated. The base-catalyzed path, k2, disappears when strong nucleophiles (X = p-CH3O and p-CH3) react with a substrate activated by a strong nucleofuge (Z = p-NO2). The large magnitude of rho x (-1.7 to -2.5), rho z (3.4 to 4.3), and rho xz (1.4) values, and relatively large kH/kD (1.6 to 1.8) found for the uncatalyzed path (k1) can be accounted for in terms of a stepwise mechanism with rate-limiting expulsion of the phenoxide leaving group. The catalyzed process (k2) is characterized by the much smaller magnitude of rho x (-1.0 to -1.7), rho z (0.4 to 0.7), and rho xz (0.2), the larger kH/kD (2.1 to 2.5) values, and the lower Δ Hdouble dagger values (1.8-1.9 kcal mol-1) than those of the uncatalyzed process (k1) with large negative Δ Sdouble dagger values (-65 to -67 cal K-1 mol-1). These results are consistent with four- and six-centered transition states for the two processes, k1 and k2, respectively.