Liu Tony, Neverov Alexei A, Tsang Josephine S W, Brown R Stan
Department of Chemistry, Queen's University, Kingston, Ontario, Canada.
Org Biomol Chem. 2005 Apr 21;3(8):1525-33. doi: 10.1039/b502569a. Epub 2005 Mar 18.
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 degrees C. Brønsted plots of the logk2 values vs. pKa for the phenol leaving groups give beta(lg) 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.
