title: Kinetics and mechanism of the pyridinolysis of aryl furan-2-carbodithioates in acetonitrile.
abstract: 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.
authors.affiliation: Department of Chemistry, Chonbuk National University, Chonju 560-756, Korea.
authors.forename: Hyuck Keun
authors.initials: HK
authors.lastname: Oh
authors.forename: Myoung Hwa
authors.initials: MH
authors.lastname: Ku
authors.forename: Hai Whang
authors.initials: HW
authors.lastname: Lee
authors.forename: Ikchoon
authors.initials: I
authors.lastname: Lee