Heterogeneous Organophosphate Ethanolysis: Degradation of Phosphonothioate Neurotoxin by a Supported Molybdenum...

A polystyrene-supported molybdenum peroxo material [Mo-Y(s)] was applied toward the oxidative degradation of the organophosphate neurotoxin O,S-diethylphenyl phosphonothioate (1) through ethanolysis. In addition to the operational advantages of the heterogeneous reactivity, oxidative ethanolysis with a 10-fold excess of hydrogen peroxide yields only P-S bond scission to produce diethylphenyl phosphonate and ethyl sulfate. This is the first report of a molybdenum solid support that promotes the degradation of sulfur-containing organophosphate with the turnover benefits of heterogeneous catalysis. The activation parameters of 1 ethanolysis by Mo-Y(s) (E_a = 57 ± 6 kJ/mol and ΔS^⧧ = -124 ± 21 J/mol·K) and by the model compound oxodiperoxo(pyridine-2-carboxylato)molybdate(VI) bis(pyridine-2-carboxylic acid) monohydrate (3; E_a = 55 ± 5 kJ/mol and ΔS^⧧ = -154 ± 15 J/mol·K) are almost identical for the oxidation of thioanisole by 3. This suggests that the rate-determining step for 1 ethanolysis is sulfur oxidation to form an intermediate phosphonothioate S-oxide, which subsequently undergoes nucleophilic attack by the ethanol solvent to form diethylphenyl phosphonate and ethyl sulfate. Evidence for the formation of this S-oxide intermediate and the postulated ethanolysis mechanism is provided.