Methanolysis of paraoxon

Table 5 Kinetic constants for the methanolysis of paraoxon 1 and fenitrothion 7 catalyzed by Zn2+ and Cu2+ in the absence and presence of ligands 8 and 9, T= 25 °C...

Fig. 6 Plot of the predicted k"bs vs. (pH rate profile for La3 +-catalyzed methanolysis of paraoxon (1) (solid line) based on the kinetic contributions of (left to right), La( + ( OCH3)[ ...

Table 8 Computed second-order rate constants (k2n) for the La2 + COCH3) -catalyzed methanolysis of paraoxon 1 and its thioate derivative 3 obtained through fits of k2bs at each (pH to Equation (8)a...

Having established the speciation, we now have a very powerful tool for analyzing the kinetic data for the pH dependence of the La3 + catalysis of the alcoholysis of various substrates. Included in the Figs 1 and 2 plots are the second-order rate constants for La3 + -catalysis of the ethanolysis of paraoxon (1) and the methanolysis of /xnitrophenyl acetate (PNPA, 2) as a function of pH in ethanol and methanol, respectively. The kinetic data mainly follow the rise/fall behavior of the Lal+( OR)2 species with some involvement of the other species, La2 + ( OR)i, La2 1 ( OR)3 and Lal+(-OR)4. 

Fits for the La3 +-catalyzed methanolysis of /xnitrophenyl acetate (2),7 paraoxon (1), its thio derivative, 0,0-Diethyl. S -p-nitrophcnyl phosphorothioate (3)10b and IV-p-nitrophenyl /1-lactam (4)32 have also been computed, with the various ky" the values being given in Table 4. 

The catalysis afforded by the La3 + system for the transesterifications of paraoxon in ethanol and methanol is quite spectacular relative to the background reactions that are assumed to be promoted by the lyoxide. The reaction rate constant of ethoxide with paraoxon in ethanol at 5.1 x 10-3 dm3 mol-1 s-133 is roughly a factor of two lower than the rate constant of methoxide with paraoxon in methanol (1.1 x 10 2dm3mol 1 s-1).17a However a solution 2mmoldm-3 in total [La3 + ], which contains 1 mmol dm-3 of Lal+, has a maximum rate constant of 7 x 10-4s-1 for decomposition of 1 in ethanol at pH of 7.3, and accelerates the rate of ethanolysis of paraoxon by a factor of 4.4 x 10n-fold relative to the ethoxide reaction at the same pH.34 By way of comparison, the acceleration afforded by a 1 mmol dm-3 solution of the La + dimer catalyzing the methanolysis of 1 at the maximal pH of 8.3 (kobs = 0.0175 s 1) is 109-fold greater than its background methoxide reaction. On this simple basis La2+ in ethanol appears to be catalytically superior to La2+ in methanol, but this stems almost exclusively from the pH values... 

Fig. 12 Plots of log/ 2Me vs. log( ) or logk29 Zn(OMe> (O) for the methanolysis of phosphate esters (20, including paraoxon 1) at 25°C gradient for La3+ plot is 1.94 + 0.10 slope for 9 Zn(OMe) plot is 1.49 + 0.11 all data included. Note data points for the p-methoxy derivative (20f) in lower left corner are coincident. Reproduced with permission from ref. 17c.

Fig. 21 A plot of the observed pseudo-first-order rate constants (kabs) for methanolysis of 2.5 x 10 SM paraoxon vs. [35 2Zn(II)] in the presence of 1 equivalent of added CH30 per complex, pH = 9.5, T = 25 + 0.1 °C. Dotted line is presented as a visual aid directed through all actual data (O) solid line is a linear fit through the data ( ) corrected for inhibition by triflate counterions. Reproduced with permission from ref. 95.

Given that the autoprotolysis constants of ethanol and methanol are 10 191 and 10-16-77, respectively, the background kQbs values for the lyoxide-catalyzed ethanolysis and methanolysis reactions of paraoxon at pH 7.3 (ethanol) and 8.3 (methanol) are 8.1 x 10-15 and 3.5 x 10 us 1, respectively... 

It was found that La -catalysed methanolysis of hydroxy-p-nitrophenyl phosphate (HPNPP) (76) is a model for the RNA transestrification reaction (Scheme 16). The same authors proposed catalytic methanolysis promoted by La as a new method for controlled decomposition of paraoxon (74) (Scheme 17). They found that methanolysis of (74) promoted by La(OTf)3 (Tf=0S(0)2CF3) in a methanol medium is billion-fold accelerated. Investigation of the reaction of oximate ot-nucleophiles with diisopropylphosphoro-fluoridate (DFP) (77) and two model phosphonates (78) and (79) has been... 

The aminolysis of 0,0-diethyl 4-nitrophenyl phosphate (Paraoxon) by piperidine in 10 ILs was compared to results in MeCN, dioxane, and DMSO. Generally, rates and selec-tivities in the ILs were similar to those in DMSO. P NMR analysis of the aminolysis products in the ILs showed that piperidine attacked P=0 (5n2(P)), the C( 1) aromatic carbon ( nAt) and the ethyl group (5 2(0)), the latter pathway not having been observed previously. A theoretical study of the effect of sulfur substitution on the methanolysis... 

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