Methanolysis La3+-catalyzed

Fig. 2 Speciation diagram for 2.0 x 10" 3 M La3+ in methanol as a function of pH. Data superimposed on the figure as ( ) are second-order rate constants for La3 +-catalyzed methanolysis of p-nitrophenyl acetate (2) as a function of pH.

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 initial study of the La3 +-catalyzed methanolysis of carboxylate esters163 reported the apparent second-order rate constant for La2 + ( OCH3)2-catalyzed methanolysis of some representative examples of aryl esters (2, 5 and 2,4-dinitrophenyl acetate (14)), phenyl benzoate (15) and three aliphatic esters, ethyl acetate, isopropyl acetate (16) and tert-butyl acetate (17). Given in Table 6 are the rate constants for the La3+ and methoxide-catalyzed methanolysis of these esters along with... 

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)[ ...

Our initial studies of the La3 + -catalyzed methanolysis of HPNPP81 immediately indicated that there were important features of the reaction which are not seen in the... 

Table 6 Maximal second-order rate constants for (La3+)2(CH30 )2-catalyzed methanolysis and second-order rate constants for methoxide attack on various esters, T = 25 °C...

Fig. 7 Bronsted plot of the log second-order rate constant for La3 + - and 9 Zn2 + ( OCH3)-catalyzed methanolysis of phosphates 20 (including 1) vs. the JpA a values for the corresponding phenols linear regressions through the La3 + and 9 Zn +( OCH3) data (excluding 20b) give gradients of (1.43 0.08) (solid line, ) and —(1.12 0.13) (dashed line, O), respectively. Note that points on lower right for the p-methoxy derivative (20f) are coincident. Reproduced from ref. 17c with permission.

Metal-methoxide reactions of phosphates, phosphorothioates, phosphonates and phosphonothioates. The rate constant data for the (La3 + ( OCH3))2 and 9 Zn2 + ( OCH3)-catalyzed methanolysis of the phosphates and phosphorothioates are given in Tables 9 and 10, respectively, while those for the phosphonates (22a-f)17e and phosphonothioates (23a-e)1 s are given in Tables 11 and 12 along with the corresponding methoxide data. The latter two series data are displayed in... 

Fig. 10 Bronsted plots of the second-order rate constants for methoxide and metal ion-catalyzed methanolysis of 22a-f at 25 °C. (La3 + ( OCH3))2 (A, gradient = —1.26 + 0.06), 9 Zn2 + rOCH3) ( , gradient =+ -1.06 0.09), OCH3 (T, gradient = -0.76 0.06). Reproduced with permission from ref. 17e.

In the case of La3 + - and Zn2+-catalyzed methanolysis of the phosphorothioate esters the observed / ig values of —0.87 and —0.74 also signify an associative mechanism with some departure of the leaving group, but it is difficult to assign the extent of the bond cleavage since the /ieq value is not known for the phosphoryl transfer between thiol and oxygen nucleophiles. 

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... 

Table 12 Second-order rate constants for the methanolysis of phosphonates 23a-e catalyzed by methoxide, (La3 + ( OCH3))2, and 9 Zn2 1 ( OCH3) in methanol at T — 25°C...

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