A Study of the Kinetics of La3+-Promoted Methanolysis of S-Aryl Methylphosphonothioates: Possible Methodology for Decontamination of EA 2192, the Toxic Byproduct of VX Hydrolysis

The kinetics of the La3+-catalyzed methanolysis of a series of S-aryl methylphosphonothioates (4a−e, phenyl substituents = 3,5-dichloro, 4-chloro, 4-fluoro, 4-H, 4-methoxy) were studied at 25 °C with sspH control. The reaction involves saturation binding of the anionic substrates to dimeric La3+/methoxide catalysts formulated as La23+(−OCH3)x, where x = 2−5 depending on the solution sspH. Cleavage of the La3+-bound methylphosphonothioates is fast, ranging from 5 × 10−3 s−1 to 5.5 × 10−5 s−1 for substrates 4a−e at a sspH of 8.4 and 1.6 × 10−1 s−1 to 4 × 10−3s−1 at a sspH of 11.7. The rate accelerations for the methanolysis of substrates 4a−e, relative to their background methoxide-promoted reactions, average 7 × 1010 and 1.5 × 109, respectively, at sspH’s of 8.4 and 11.7. The catalytic system is predicted to cleave EA 2192 (S-2(N,N-di-iso-propylaminoethyl) methylphosphonothioate), a toxic byproduct of the hydrolysis of VX, with a t1/2 between 4 and 8 min at a sspH of 8.4, and 27 min at a sspH of 11.7.