Study on the Transesterification of Methyl Aryl Phosphorothioates in Methanol Promoted by Cd(II), Mn(II), and a Synthetic Pd(II) Complex

Methanol solutions containing Cd(II), Mn(II), and a palladacycle, (dimethanol bis(N,N-dimethylbenzylamine-2C,N)palladium(II) (3), are shown to promote the methanolytic transesterification of O-methyl O-4-nitrophenyl phosphorothioate (2b) at 25 °C with impressive rate accelerations of 106−1011 over the background methoxide promoted reaction. A detailed mechanistic investigation of the methanolytic cleavage of 2a−d having various leaving group aryl substitutions, and particularly the 4-nitrophenyl derivative (2b), catalyzed by Pd-complex 3 is presented. Plots of kobs versus palladacycle [3] demonstrate strong saturation binding to form 2b:3. Numerical fits of the kinetic data to a universal binding equation provide binding constants, Kb, and first order catalytic rate constants for the methanolysis reaction of the 2b:3 complex (kcat) which, when corrected for buffer effects, give corrected (kcatcorr) rate constants. A sigmoidal shaped plot of log(kcatcorr) versus sspH (in methanol) for the cleavage of 2b di...