title: Billion-fold acceleration of the methanolysis of paraoxon promoted by La(OTf)3 in methanol.
abstract: The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)(3)] (OTf = (-)OS(O)(2)CF(3)) as a function of at 25 degrees C. Plots of the pseudo-first-order rate constants (k(obs)) for methanolysis as a function of [La(OTf)(3)](total) were obtained under buffered conditions from 5.15 to 10.97, and the slopes of the linear parts of these were used to determine the second-order rate constants (k(2)(obs)) for the La(3+)-catalyzed methanolysis of 2. Detailed analysis of the potentiometric titration data of La(OTf)(3) in methanol through fits to a multicomponent equilibrium mixture of dimers of general stoichiometry La(3+)(2)((-)OCH3)n, where n assumes values of 1-5, gives the equilibrium distribution of each as a function of. These data, when fit to a second expression describing k(2)(obs) in terms of a linear combination of individual rate constants k(2)(2:1), k(2)(2:2).k(2)(2:)n for the dimers, allow one to describe the overall catalytic profile in terms of the individual contributions. The most catalytically important species are the three dimers La(3+)(2)((-)OCH3)1, La(3+)(2)((-)OCH3)2, and La(3+)(2)((-)OCH3)3. The catalysis of the methanolysis of 2 is spectacular: a 2 x 10(-3) M solution of [La(3+)](total), at neutral, affords a 10(9)-fold acceleration relative to the base reaction (t(1/2) approximately 20 s at 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La(3+)(2)((-)OCH3)2 species is proposed.
authors.affiliation: Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
authors.forename: Josephine S
authors.initials: JS
authors.lastname: Tsang
authors.forename: Alexei A
authors.initials: AA
authors.lastname: Neverov
authors.forename: R S
authors.initials: RS
authors.lastname: Brown
mesh_terms: D002619:Chemical Warfare Agents / Q000737:chemistry*; D007306:Insecticides / Q000737:chemistry*; D007700:Kinetics; D007811:Lanthanum / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008698:Mesylates / Q000737:chemistry*; D000432:Methanol / Q000737:chemistry*; D010261:Paraoxon / Q000737:chemistry*
chemical_list: D002619:Chemical Warfare Agents; D007306:Insecticides; D008698:Mesylates; D007811:Lanthanum; C012077:trifluoromethanesulfonic acid; D010261:Paraoxon; D000432:Methanol