Billion-fold Acceleration of the Methanolysis of Paraoxon Promoted by La(OTf)3 in Methanol

The methanolysis of the insecticide paraoxon (2) was investigated in methanol solution containing varying [La(OTf)3] (OTf = -OS(O)2CF3) as a function of at 25 °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 La3+-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 La3+ 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 La3+ 2(-OCH3)1, La3+ 2(-OCH3)2, and La3+ 2(-OCH3)3. The catalysis of the methanolysis of 2 is spectacular: a 2 × 10-3 M solution of [La3+]total, at neutral , affords a 109-fold acceleration relative to the base reaction (t 1/2 ≈ 20 s at 8.2) with excellent turnover. A mechanism of the catalyzed reaction involving the La3+ 2(-OCH3)2 species is proposed.