Edwards, David R. Neverov, Alexei A. Brown, R. Stan 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 <i>bis</i>(<i>N,N</i>-dimethylbenzylamine-2C,N)palladium(II) (<b>3</b>), are shown to promote the methanolytic transesterification of <i>O</i>-methyl <i>O</i>-4-nitrophenyl phosphorothioate (<b>2b</b>) at 25 °C with impressive rate accelerations of 10<sup>6</sup>−10<sup>11</sup> over the background methoxide promoted reaction. A detailed mechanistic investigation of the methanolytic cleavage of <b>2a</b>−<b>d</b> having various leaving group aryl substitutions, and particularly the 4-nitrophenyl derivative (<b>2b</b>), catalyzed by Pd-complex <b>3</b> is presented. Plots of <i>k</i><sub>obs</sub> versus palladacycle [<b>3</b>] demonstrate strong saturation binding to form <b>2b</b>:<b>3</b>. Numerical fits of the kinetic data to a universal binding equation provide binding constants, <i>K</i><sub>b</sub>, and first order catalytic rate constants for the methanolysis reaction of the <b>2b</b>:<b>3</b> complex (<i>k</i><sub>cat</sub>) which, when corrected for buffer effects, give corrected (<i>k</i><sub>cat</sub><sup>corr</sup>) rate constants. A sigmoidal shaped plot of log(<i>k</i><sub>cat</sub><sup>corr</sup>) versus <sub>s</sub><sup>s</sup>pH (in methanol) for the cleavage of <b>2b</b> displays a broad <sub>s</sub><sup>s</sup>pH independent region from 5.6 ≤ <sub>s</sub><sup>s</sup>pH ≤ ∼10 with a <i>k</i><sup>minimum</sup> = (1.45 ± 0.24) × 10<sup>−2</sup> s<sup>−1</sup> and a [lyoxide] dependent wing plateauing above a kinetically determined <sub>s</sub><sup>s</sup>p<i>K</i><sub>a</sub> of 12.71 ± 0.17 to give a <i>k</i><sup>maximum</sup> = 7.1 ± 1.7 s<sup>−1</sup>. Brønsted plots were constructed for reaction of <b>2a</b>−<b>d</b> at <sub>s</sub><sup>s</sup>pH 8.7 and 14.1, corresponding to reaction in the midpoints of the low and high <sub>s</sub><sup>s</sup>pH plateaus. The Brønsted coefficients (β<sup>LG</sup>) are computed as −0.01 ± 0.03 and −0.86 ± 0.004 at low and high <sub>s</sub><sup>s</sup>pH, respectively. In the low <sub>s</sub><sup>s</sup>pH plateau, and under conditions of saturating <b>3</b>, a solvent deuterium kinetic isotope effect of <i>k</i><sup>H</sup>/<i>k</i><sup>D</sup> = 1.17 ± 0.08 is observed; activation parameters (Δ<i>H</i><sub>Pd</sub><sup>⧧</sup> = 14.0 ± 0.6 kcal/mol and Δ<i>S</i><sub>Pd</sub><sup>⧧</sup>= −20 ± 2 cal/mol·K) were obtained for the <b>3</b>-catalyzed cleavage reaction of <b>2b</b>. Possible mechanisms are discussed for the reactions catalyzed by <b>3</b> at low and high <sub>s</sub><sup>s</sup>pH. This catalytic system is shown to promote the methanolytic cleavage of <i>O,O-</i>dimethyl phosphorothioate in CD<sub>3</sub>OD, producing (CD<sub>3</sub>O)<sub>2</sub>PO(S<sup>−</sup>) with a half time for reaction of 34 min. Methyl Aryl Phosphorothioates;sspH;rate constants;3O;group aryl substitutions;methanolytic cleavage;2 b displays;3OD;binding;LG 2011-03-07
    https://acs.figshare.com/articles/journal_contribution/Study_on_the_Transesterification_of_Methyl_Aryl_Phosphorothioates_in_Methanol_Promoted_by_Cd_II_Mn_II_and_a_Synthetic_Pd_II_Complex/2686162
10.1021/ic102220m.s001