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 degrees C 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 (beta(X) = 0.7-0.8) to formation (beta(X) = 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, rho(XZ), from a positive (rho(XZ) = +0.86) to a small negative (rho(XZ) = -0.11) value at the breakpoint supports the mechanistic change proposed. The magnitudes of rho(Z) and activation parameters are also consistent with the proposed mechanism.
Oh
Hyuck Keun
HK
Department of Chemistry, Chonbuk National University, Chonju 560-756, Korea.
Ku
Myoung Hwa
MH
Lee
Hai Whang
HW
Lee
Ikchoon
I