Tsang Josephine S, Neverov Alexei A, Brown R S
Department of Chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6.
J Am Chem Soc. 2003 Jun 25;125(25):7602-7. doi: 10.1021/ja034979a.
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) 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+), 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.
在25℃下，研究了杀虫剂对氧磷（2）在含有不同浓度[La(OTf)₃]（OTf = (-)OS(O)₂CF₃）的甲醇溶液中的甲醇解反应。在缓冲条件下，获得了甲醇解的准一级速率常数（k(obs)）随[La(OTf)₃]（总浓度）变化的曲线，范围为5.15至10.97，这些曲线线性部分的斜率用于确定La³⁺催化2的甲醇解的二级速率常数（k₂(obs)）。通过对甲醇中La(OTf)₃的电位滴定数据进行详细分析，将其拟合为一般化学计量比La³⁺₂((-)OCH₃)ₙ的二聚体的多组分平衡混合物，其中n取值为1 - 5，得到了每种二聚体的平衡分布随其变化的情况。这些数据拟合为第二个表达式，该表达式根据二聚体的各个速率常数k₂(2:1)、k₂(2:2)、k₂(2:3)的线性组合来描述k₂(obs)，从而可以根据各个贡献来描述整体催化情况。催化作用最重要的物种是三种二聚体La³⁺₂((-)OCH₃)₁、La³⁺₂((-)OCH₃)₂和La³⁺₂((-)OCH₃)₃。2的甲醇解催化作用非常显著：在中性条件下，2×10⁻³ M的[La³⁺]（总浓度）溶液相对于基础反应（在8.2时t₁/₂约为20秒）能实现10⁹倍的加速，且具有出色的周转率。提出了一种涉及La³⁺₂((-)OCH₃)₂物种的催化反应机制。
