The methanolyses of a series of O,O-diethyl O-aryl phosphates (2,5) and O,O-diethyl S-aryl phosphorothioates (6) promoted by methoxide and two metal ion systems, (La3+)2(−OCH3)2 and 4:Zn2+:−OCH3
(4
= 1,5,9-triazacyclododecane) has been studied in methanol at 25 °C. Brønsted plots of the log
k2 values vs.sspKa for the phenol leaving groups give βlg values of −0.70, −1.43 and −1.12 for the methanolysis of the phosphates and −0.63, −0.87 and −0.74 for the methanolysis of the phosphorothioates promoted by the methoxide, La3+ and Zn2+ systems respectively. The kinetic data for the metal-catalyzed reactions are analyzed in terms of a common mechanism where there is extensive cleavage of the P–XAr bond in the rate-limiting transition state. The relevance of these findings to the mechanism of action of the phosphotriesterase enzyme is discussed.
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