Structure-reactivity correlations in the aminolysis of aryl chloroformates

The reactions of a series of secondary alicyclic amines with 4-methylphenyl and 4-methoxyphenyl chloroformates are subjected to a kinetic investigation in water, at 25.0 degreesC, ionic strength 0.2 M (KCI). Under amine excess, pseudo-first-order rate coefficients (k(obs)) are found for all reactions, Plots of k(obs) vs [NH] (NH is the free amine) are linear, with the slope (k(N)) pH independent, except the reactions of 1-(2-hydroxyethyl)piperazine with both substrates at pH 6.2-73. The Bronsted-type plots for the k(N) values for the aminolysis of both chloroformates are linear, with slopes ca. 0.3, which is consistent with rate-determining formation of a zwitterionic tetrahedral intermediate (T+/-). With the pK(a) and log k(N) data For the present reactions. together with those for the same aminolysis of phenyl and 4-nitrophenyl chloroformates. two dual parametric equations are found for log k(N) as a function of pK(a) of the nucleophile, Hammett sigma of the "nonleaving" group, and pK(a) of the "nonleaving" group, with coefficients beta (N) = 0.3, rho (nlg) = 0.7, and beta (nig) = -0.2, respectively. (C) 2001 John Wiley & Sons. Inc.