Structure-reactivity correlations in the aminolysis of ethyl S-aryl thiolcarbonates

The reactions of secondary alicyclic amines (saa) with ethyl S-4-X-phenyl t hiolcarbonates (X = Cl, H, Me, and MeO) are subjected to a kinetic analysis in water, 25.0 degrees C, ionic strength 0.2 (KCI). By following the leavi ng groups spectrophotometrically (260-270 nm), under amine excess, pseudo f irst-order rate coefficients (K-obsd) are obtained. Plots of k(obsd) agains t free-amine concentration at constant, pH are linear, with slope (k(N)) in dependent of pH. The Bronsted-type plots (log k(N) against amine pK(a)) are linear for the aminolysis of the four substrates, with slopes beta(N) = 0. 7-0.8. The magnitudes of the slopes are consistent with a stepwise mechanis m through a zwitterionic tetrahedral intermediate (T+/-) whose breakdown to products is rate-determining (k(2) step). This mechanism is simpler than t hat for the same aminolysis of the corresponding ethyl S-aryl dithiocarbona tes, which includes proton transfer from T+/- to the amine (k(3) step), due to the fact that for the title reactions k(2) > k(3)[amine]. With the resu lts of the present work and those for the reactions of saa with ethyl S-4-n itrophenyl thiolcarbonate, a dual parametric equation is deduced for kN as a function of amine and leaving group basicity, with both beta(N) and -beta (lg) = 0.8. Another dual parametric equation is deduced from literature dat a for the pyridinolysis of ethyl S-aryl thiolcarbonates, Comparison of both multiparametric expressions shows that pyridines are more reactive than is obasic saa toward thiolcarbonates when breakdown of T+ to products is rate- limiting.