Structure−Reactivity Correlations in the Aminolysis of Ethyl S-Aryl Thiolcarbonates

Abstract

The reactions of secondary alicyclic amines (saa) with ethyl S-4-X-phenyl thiolcarbonates (X = Cl, H, Me, and MeO) are subjected to a kinetic analysis in water, 25.0 °C, ionic strength 0.2 (KCl). By following the leaving groups spectrophotometrically (260−270 nm), under amine excess, pseudo-first-order rate coefficients (k obsd) are obtained. Plots of k obsd against free-amine concentration at constant pH are linear, with slope (k N) independent of pH. The Brönsted-type plots (log k N against amine pK a) are linear for the aminolysis of the four substrates, with slopes βN = 0.7−0.8. The magnitudes of the slopes are consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T±) whose breakdown to products is rate-determining (k 2 step). This mechanism is simpler than that for the same aminolysis of the corresponding ethyl S-aryl dithiocarbonates, 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 results of the present work and those for the reactions of saa with ethyl S-4-nitrophenyl thiolcarbonate, a dual parametric equation is deduced for k N as a function of amine and leaving group basicity, with both βN and −βlg = 0.8. Another dual parametric equation is deduced from literature data for the pyridinolysis of ethyl S-aryl thiolcarbonates. Comparison of both multiparametric expressions shows that pyridines are more reactive than isobasic saa toward thiolcarbonates when breakdown of T± to products is rate-limiting.

Kinetics and Mechanism of Pyridinolysis of Aryl Dithiocyclopentanecarboxylates in Acetonitrile

“…For example, in the aminolysis of ethyl S-aryl thiolcarbonates with secondary alicyclic amines in water, the slopes were βx = 0.7 -0.8, 9 whereas in the pyridinolysis of S-phenyl 4-nitrobenzoates in acetonitrile, the slopes were βx = 0.6 -0.7, 10 both of which were consistent with those stepwise mechanisms where the breakdown of a zwitterionic tetrahedral intermediate T ± determines the rate. The value of 0.35 obtained for the more basic pyridines, as listed in Table 1, is also consistent with those obtained for stepwise mechanisms in which the formation of T ± limits the rate.…”

“…8 but it is well within the range (βX ≥ 0.7 -0.8 in water 9 and βX ≥ 0.6 -0.7 in acetonitrile 10 ) of the corresponding values for stepwise reactions with rate-limiting expulsions of the leaving group. For example, in the aminolysis of ethyl S-aryl thiolcarbonates with secondary alicyclic amines in water, the slopes were βx = 0.7 -0.8, 9 whereas in the pyridinolysis of S-phenyl 4-nitrobenzoates in acetonitrile, the slopes were βx = 0.6 -0.7, 10 both of which were consistent with those stepwise mechanisms where the breakdown of a zwitterionic tetrahedral intermediate T ± determines the rate.…”

Kinetic Studies on the Structure-Reactivity of Aryl Dithiomethylacetates

“…The magnitude of β X is somewhat smaller than those (β X ≥ 0.8) 19 normally obtained but is well within the range (β X ≥ 0.7-0.8 in water 20 and 0.6-0.7 in acetonitrile 21 ) of the corresponding values for the stepwise reactions with rate-limiting expulsion of leaving group. For example, in the aminolysis of ethyl S-aryl thiolcarbonates with secondary alicyclic amines in water the slopes were βx = 0.7-0.8, 20 and in the pyridinolysis of Sphenyl 4-nitrobenzoates in acetonitrile the slopes were βx = 0.6-0.7, 21 both which were consistent with a stepwise mechanism where the breakdown of a zwitterionic tetrahedral intermediate, T ± , is rate-determining. The βx = 0.25 obtained for more basic pyridines in Table 1 is also consistent with a stepwise mechanism in which the formation of T ± is rate-limiting.…”

Kinetics and mechanism of the reactions of methyl 2,4,6-trinitrophenyl carbonate with anilines

“…7 Similarly, Brønsted slope values of 0.8 or slightly lower have been reported for the stepwise aminolysis of 2,4-dinitrophenyl acetate and 1-acetoxy-4-methoxypyridinium ion. 8 On the other hand, the β value found in this work is near the upper limit found for the aminolysis proceeding by concerted-type reactions.…”