Tsang Josephine S W, Neverov Alexei A, Brown R S
Department of Chemistry, Queen's University, Kingston, Ontario, Canada, K7L 3N6.
J Am Chem Soc. 2003 Feb 12;125(6):1559-66. doi: 10.1021/ja021176z.
The methanolysis of hydroxypropyl-p-nitrophenyl phosphate (HPNPP, 1) promoted by La(OTf)(3) under buffered conditions was studied in methanol as a function of pH at 25 degrees C. (31)P NMR studies at -90 degrees C indicate that there are at least three La/1 complexes formed at pH approximately 5.3 of 1:1, 2:2, and 1:2 stoichiometry. Kinetic studies of the observed pseudo-first-order rate constants for the methanolysis of 1 as a function of [La(3+)] at 4.5 < pH < 10.5 indicate there are two general pH regimes. In the low pH regime between 4.5 and 7.6, the plots of k(obs) versus [La(3+)] exhibit saturation behavior with very strong 1:1 binding, with a plateau rate constant that depends on [OCH(3)(-)]. The catalytically productive species is shown to be a 2:2 complex of La(3+) and 1, where the phosphate is proposed to be doubly activated, thereby promoting the methoxide reaction by some 4.6 x 10(10)-fold. In the high pH regime from 7.9 to 10.5, 1:1, 2:2, and 2:1 La(3+)/1 complexes are formed with the La(3+) coordinated in the form of La(3+)(OCH(3)(-)). Throughout this pH regime at high [La(3+)], a saturation complex, (La(3+)OCH(3)(-))(2)/1, is formed that spontaneously decomposes with a rate constant of (5-10) x 10(-)(3) s(-)(1), leading to an acceleration of 10(9)-fold at pH 8.0.
在25℃下，研究了在缓冲条件下，三氟甲磺酸镧（La(OTf)₃）促进的对硝基苯磷酸羟丙酯（HPNPP，1）在甲醇中的甲醇解反应随pH的变化。在-90℃下的³¹P NMR研究表明，在pH约为5.3时，至少形成了化学计量比为1:1、2:2和1:2的三种La/1配合物。在4.5＜pH＜10.5范围内，对1的甲醇解反应观察到的准一级速率常数随[La³⁺]变化的动力学研究表明存在两种一般的pH区域。在4.5至7.6的低pH区域，k(obs)对[La³⁺]的曲线呈现饱和行为，具有非常强的1:1结合，平台速率常数取决于[OCH₃⁻]。催化活性物种被证明是La³⁺和1的2:2配合物，其中磷酸盐被认为是双重活化的，从而使甲醇反应速率提高了约4.6×10¹⁰倍。在7.9至10.5的高pH区域，形成了1:1、2:2和2:1的La³⁺/1配合物，其中La³⁺以La³⁺(OCH₃⁻)的形式配位。在整个高[La³⁺]的该pH区域内，形成了一种饱和配合物(La³⁺OCH₃⁻)₂/1，其以(5 - 10)×10⁻³ s⁻¹的速率常数自发分解，在pH 8.0时导致反应加速10⁹倍。
