Kinetics and Reaction Mechanism of Aminolyses of Benzyl 2-Pyridyl Carbonate and t-Butyl 2-Pyridyl Carbonate in Acetonitrile

Second-order rate constants (k(N)) have been measured spectrophotometrically for the reactions of benzyl 2-pyridyl carbonate 3 and t-butyl 2-pyridyl carbonate 4 with a series of alicyclic secondary amines in MeCN at 25.0 +/- 0.1 degrees C. Substrate 4 is much less reactive than 3 and the steric hindrance exerted by the bulky t-Bu group in 4 has been attributed to its decreased reactivity. The Bronsted-type plots for the reactions of 3 and 4 are linear with beta(nuc) = 0.57 and 0.45, respectively. Thus, the reactions have been concluded to proceed through a concerted mechanism, although the current reactions were expected to proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate T-+/-. It has been proposed that the rate of leaving-group expulsion is accelerated by the intramolecular H-bonding interaction in T-+/- and the "push" provided by the RO group through the resonance interaction. Thus, the enhanced nucleofugality forces the reactions to proceed through a concerted mechanism. The reactivity-selectivity principle (RSP) is not applicable to the current reaction systems, since the reaction of the less reactive 4 results in a smaller beta(nuc) than that of the more reactive 3. Steric hindrance exerted by the bulky t-Bu group in 4 has been suggested to be responsible for the failure of the RSP.