Kinetic studies on the pyridinolysis of aryl dithiocyclopentanecarboxyaltes 2 were carried out at 60.0 o C in acetonitrile. In the aminolysis of 2, the βX values were 0.5 -0.8 with anilines, and there was no breakpoint. However, in the pyridinolysis of 2, biphasic Brönsted plots were obtained, with a change in slope from a large value (βX ≅ 0.7) to a small value (βX ≅ 0.4) at pKa 0 = 5.2. This was attributed to a change in the rate-limiting step from breakdown to the formation of a zwitterionic tetrahedral intermediate, T± , in the reaction path, with an increase in the basicity of the pyridine nucleophile. An obvious change in the cross-interaction constant ρXZ from a large positive (ρXZ = +1.02) value to a small negative value (ρXZ = -0.17) supports the proposed mechanistic change.

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