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

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 pH in methanol solvent. In both cases the kinetics of catalyzed methanolysis maximize at pH 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(s)pH 9.1, a 2 mmol dm-3 solution of La(OTf)3 with equimolar NaOCH3 provides accelerations of 2.2x10(8)-fold, 9.7x10(6)-fold and 9.3x10(6)-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 approximately 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 t1/2 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.