Alkaline-earth metal complexes of thiol-pendant crown ethers as turnover catalysts of ester cleavage
作者:Perla Breccia、Roberta Cacciapaglia、Luigi Mandolini、Carla Scorsini
DOI:10.1039/a708392k
日期:——
Methanolysis of pNPOAc under moderately alkaline conditions is catalysed by the alkaline-earth metal ion (Ca, Sr, Ba) complexes of 2-(mercaptomethyl)-18-crown-6 according to biphasic kinetics in which an initial burst of pNPOH release is followed by a zeroth-order phase. Two competing catalytic mechanisms contribute to the overall process, (i) a double displacement mechanism in which the complexed metal ion enables the SH group of the catalyst to undergo an acylation–deacylation cycle, and (ii) a bypass mechanism in which a catalytically active complex-bound methoxide ion is involved and no covalently bound intermediate is formed. Calcium ion is much more efficient than its larger analogues both in the acylation–deacylation cycle of the double displacement mechanism, and in the direct delivery of methoxide ion to the ester substrate.
在中碱性条件下,2-(巯甲基)-18-冠醚-6 的碱土金属离子(钙、锶、钡)络合物可根据双相动力学催化对 NPOAc 的甲醇分解,在双相动力学中,最初的对 NPOH 释放爆发阶段之后是零阶阶段。在整个过程中有两种相互竞争的催化机制:(i) 双置换机制,即络合金属离子使催化剂的 SH 基团进行酰化-脱乙酰基循环;(ii) 旁路机制,即催化活性络合物结合甲氧离子,不形成共价结合的中间体。无论是在双置换机制的酰化-脱酰化循环中,还是在将甲氧基离子直接输送到酯底物中,钙离子都比其较大的类似物有效得多。