La3+-catalyzed methanolysis of O,O-diethyl S-(p-nitrophenyl) phosphorothioate and O,O-diethyl S-phenyl phosphorothioate. Millions-fold acceleration of the destruction of V-agent simulants

Abstract

The La³⁺-catalyzed methanolysis of two phosphorothioate derivatives, O,O-diethyl S-(p-nitrophenyl) phosphorothioate (4a) and O,O-diethyl S-phenyl phosphorothioate (4b) were studied as a function of [La³⁺] and ^s_spH in methanol solvent. In both cases the kinetics of catalyzed methanolysis maximize at ^s_spH 9.1 and a detailed analysis indicates that the dominant species responsible for catalysis are dimers formulated as La³⁺₂(⁻OCH₃)₂ and La³⁺₂(⁻OCH₃)₄. The catalysis is compared with that seen for the corresponding phosphate esters, namely paraoxon (3a) and O,O-diethyl phenyl phosphate (3b) for which La³⁺ catalysis is slightly better and markedly worse than for 4a and 4b respectively. Overall, at ^s_spH 9.1, a 2 mmol dm⁻³ solution of La(OTf)₃ with equimolar NaOCH₃ provides accelerations of 2.2 × 10⁸-fold, 9.7 × 10⁶-fold and 9.3 × 10⁶-fold for methanolysis of 3a, 4a and 4b, relative to the background reaction of methoxide reacting with the three substrates. In each case, the P-containing product of the reactions is exclusively diethyl methyl phosphate. Turnover experiments with 6-fold and 100-fold excesses of 4a and 4b respectively, methanolyzed in the presence of ∼10 mmol dm⁻³ La³⁺ and equimolar NaOCH₃, indicate that the reactions are essentially complete within 103 s and 70 min respectively. The latter turnover experiment with 4b corresponded to 100 turnovers in 70 min and an overall reaction t_1/2 of 8 min. A common mechanism of reaction is postulated for each of the substrates which involves Lewis acid coordination of one of the La³⁺ to the P=O unit, followed by nucleophilic attack by the second La³⁺-⁻OCH₃.