Study on the Transesterification of Methyl Aryl Phosphorothioates in Methanol Promoted by Cd(II), Mn(II), and a Synthetic Pd(II) Complex
2011-03-07T00:00:00Z (GMT) by
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  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 displays a broad sspH independent region from 5.6 ≤ sspH ≤ ∼10 with a kminimum = (1.45 ± 0.24) × 10−2 s−1 and a [lyoxide] dependent wing plateauing above a kinetically determined sspKa of 12.71 ± 0.17 to give a kmaximum = 7.1 ± 1.7 s−1. Brønsted plots were constructed for reaction of 2a−d at sspH 8.7 and 14.1, corresponding to reaction in the midpoints of the low and high sspH plateaus. The Brønsted coefficients (βLG) are computed as −0.01 ± 0.03 and −0.86 ± 0.004 at low and high sspH, respectively. In the low sspH plateau, and under conditions of saturating 3, a solvent deuterium kinetic isotope effect of kH/kD = 1.17 ± 0.08 is observed; activation parameters (ΔHPd⧧ = 14.0 ± 0.6 kcal/mol and ΔSPd⧧= −20 ± 2 cal/mol·K) were obtained for the 3-catalyzed cleavage reaction of 2b. Possible mechanisms are discussed for the reactions catalyzed by 3 at low and high sspH. This catalytic system is shown to promote the methanolytic cleavage of O,O-dimethyl phosphorothioate in CD3OD, producing (CD3O)2PO(S−) with a half time for reaction of 34 min.