Kinetics and mechanism of the pyridinolysis of aryl furan-2-carbodithioates in acetonitrile

Kinetic studies on the pyridinolysis of aryl furan-2-carbodithioates 1 are carried out at 60.0 degreesC in acetonitrile. The biphasic rate dependence on the pyridine basicity with a breakpoint at pK(a)degrees = 5.2 is interpreted to indicate a change of the rate-limiting step from breakdown (betax = 0.7-0.8) to formation (betax = 0.2) of the tetrahedral intermediate, T+/-:, at the breakpoint as the basicity of the pyridine nucleophile is increased. Observation of the breakpoint is possible with pyridines since the expulsion rate of the pyridine (k-(a)) from T+/- is sufficiently low, with the low k(-a)/k(b) ratio leading to a low breakpoint, pK(a)degrees. The clear-cut change in the cross-interaction constants, pxz, from a positive (rhoxz = +0.86) to a small negative (rhoxz = -0.11) value at the breakpoint supports the mechanistic change proposed. The magnitudes of rhoz and activation parameters are also consistent with the proposed mechanism.