Phosphonate Ester Hydrolysis Catalyzed by Two Lanthanum Ions. Intramolecular Nucleophilic Attack of Coordinated Hydroxide and Lewis Acid Activation
作者:Akira Tsubouchi、Thomas C. Bruice
DOI:10.1021/ja00133a013
日期:1995.7
(8-Hydroxy-2-quinolyl)methyl (8-hydroxy-2-quinolyl)methyl phosphonate (I) has been synthesized as a model compound and investigated in terms of catalysis of hydrolysis by two metal ions in concert. The divalent metal ions, Zn2+, Ni2+, Co2+ Cu2+ as well as Al3+, form only 1:1 complexes in which the metal ion is sequestered by coordination with the two 8-hydroxyquinoline moieties of I. In these complexes the >PO2- linker points away from the metal ion, and the latter does not act as a catalyst for hydrolysis. The spectrophotometric titration of I with La3+ indicates that the complexes (I)La and (I)La-2 exist in solution with association constant (log K-as1, 6.54; log K-as2, 4.81) greater than that (log K-as, < 4.0) for La3+ complexation of 8-hydroxy-2-methylquinoline. The large values of K-as with I suggest that La3+ interacts with both the 8-hydroxyquinoline ligands and the negative charge of the phosphonate linkage (>PO2-). La3+ is a very effective catalyst for hydrolysis of I. Studies of the rates of hydrolysis of I as a function of [La3+] establish that the hydrolysis proceeds via two reaction paths:.(i) spontaneous hydrolysis of a complex with (I)La-2 stoichiometry and (ii) La3+ promoted hydrolysis of the (I)La-2 complex. On the other hand, removal of one of two 8-hydroxyquinoline ligands of I to provide (8-hydroxy-2-quinolyl)methyl methylphosphonate (II) leads to the formation of the 1:1 complex (II)La, which is hydrolytically inert but subject to catalysis by free La3+. Thus, the remarkable spontaneous hydrolysis of (I)La-2 is due to double metal ion cooperativity in catalysis. The pH-rate profile supports a mechanism for the spontaneous hydrolysis of (I)La-2 which involves the intramolecular attack of HO- ligated to one of the two La3+ in the complex (I)La-2(OH2)(n-1)(OH). The pK(a) value of the water bound to La3+ in (I)La-2(OH2)(n) and rate constant for the hydrolysis of (I)La-2(OH2)(n-1)(OH) are determined as 7.19 and 1.36 x 10(-3) s(-1), respectively. The [La3+] and [HO-] dependence of La3+ promoted hydrolysis of (I)La-2(X)(n) and (II)La(X)(n) X = H2O or HO-} is in accord with the transition state compositions of (I)La-2(X)n}La(OH)(2+)} and (II)La(X)(n)}La(OH)(2+)}, respectively. The bimolecular rate constant for the hydrolysis of (I)La-2(X)(n) and (II)La(X)(n) by La(OH)(2+) were calculated as 2.62 x 10(-1) and 7.61 x 10(-2) M(-1) s(-1), respectively. The similarity of these second-order rate constants suggests that one 8-hydroxyquinoline probably in leaving group} ligated La3+ is effectively associated with the leaving group oxyanion and negative charge of the >PO2- to activate I and II by Lewis-acid catalysis. Based on an estimation of the rate constant for hydroxide-catalyzed hydrolysis of I with P-O bond cleavage, a rate enhancement of similar to 10(13) fold has been observed on the spontaneous hydrolysis of (I)La-2(OH2)(n-1)(OH).This is attributed to a combination of Lewis-acid activation La3+ ligated to negative >PO2- and leaving oxygen} and intramolecular nucleophilic attack of hydroxide bound to the second La3+.
(8-羟基-2-喹啉基)甲基 (8-羟基-2-喹啉基)甲基膦酸酯(I)已被合成作为一种模型化合物,并通过双金属离子协同催化水解进行了研究。二价金属离子,如Zn²+、Ni²+、Co²+、Cu²+,以及三价铝离子Al³+,仅形成1:1的复合物,其中金属离子通过与I的两个8-羟基喹啉基团的配位作用而被隔离。在这些复合物中,>PO₂⁻连接基团指向远离金属离子的方向,因此金属离子并不催化水解反应。
分光光度滴定法测定I与La³+的相互作用表明,溶液中存在(I)La和(I)La₂两种复合物,其结合常数对数值分别为(log K_as1=6.54,log K_as2=4.81),大于La³+与8-羟基-2-甲基喹啉形成复合物的结合常数对数值(log K_as<4.0)。结合常数K_as的显著值提示La³+与I的两个8-羟基喹啉配基及膦酸酯连接基团的负电荷>PO₂⁻均发生相互作用。La³+是I水解的有效催化剂。
研究I在不同[La³+]浓度下的水解速率表明,水解过程经由两种反应路径进行:
(i) 具有(I)La₂化学计量的复合物的自发水解;
(ii) La³+促进的(I)La₂复合物水解。
另一方面,去除I的两个8-羟基喹啉基团中的一个,生成(8-羟基-2-喹啉基)甲基甲基膦酸酯(II),形成1:1的(II)La复合物。此复合物在水解上呈惰性,但能被游离的La³+催化。因此,(I)La₂的显著自发水解归因于双金属离子在催化中的协同作用。
pH-速率关系支持(I)La₂自发水解的机制,即复合物(I)La₂(OH₂)ₙ⁻¹(OH)内在分子内羟基配位的La³+进行攻击。已测定(I)La₂(OH₂)ₙ中水的pKa值为7.19,及(I)La₂(OH₂)ₙ⁻¹(OH)水解的速率常数为1.36×10⁻³ s⁻¹。
La³+促进的(I)La₂(X)ₙ(II)La(X)ₙ(X=H₂O或HO⁻)水解对[La³+]和[HO⁻]的依赖性,与过渡态组成(I)La₂(X)ₙ}La(OH)(2+)}和(II)La(X)ₙ}La(OH)(2+)}一致。
(I)La₂(X)ₙ和(II)La(X)ₙ经由La(OH)(2+)催化的二分子水解速率常数分别为2.62×10⁻¹和7.61×10⁻² M⁻¹ s⁻¹。这些二级速率常数的相似性表明,一个8-羟基喹啉基团(可能偏离的配基)与La³+配位,有效关联离去基团的氧阴离子及>PO₂⁻的负电荷,通过路易斯酸催化激活I和II。
基于对I在P-O键断裂条件下水解速率常数的估算,(I)La₂(OH₂)ₙ⁻¹(OH)的自发水解速率提高约10¹³倍,归因于路易斯酸活化La³+配位负电荷>PO₂⁻和离去氧}与第二La³+配位羟基的分子内亲核攻击的协同作用。