Kinetic Studies on the Structure-Reactivity of Aryl Dithiomethylacetates

Abstract

Kinetic studies of the pyridinolysis (XC 5 H 4 N) of aryl dithiomethylacetates (CH 3 CH 2 C(=S)SC 6 H 4 Z, 1) are carried out in acetonitrile at 60.0 o C. A biphasic Brönsted plot is obtained with a change in slope from a large (β X ≅ 0.8) to a small (β X ≅ 0.2) value at pK a o = 5.2, which is attributed to a change in the rate limiting step from breakdown to formation of a zwitterionic tetrahedral intermediate, T ± , in reaction path as the basicity of the pyridine nucleophile increases. This mechanism is supported by the change of the cross-interaction constant ρXZ from a large positive (ρ XZ = +1.36) for the weakly basic pyridines to a small negative (ρ XZ = -0.22) value for the strongly basic pyridines. The magnitudes of ρZ and activation parameters are also consistent with the proposed mechanism.