Demethylation reactions of phosphate esters catalysed by complexes of polyether ligands with metal iodides
作者:Dario Landini、Angelamaria Maia、Cristina Pinna
DOI:10.1039/b105877k
日期:2001.11.29
Metal ion catalysis has been observed in demethylation reactions of methyl di(para-substituted phenyl)phosphate esters 1–4 promoted by complexes of polyether ligands 5–7 with alkali metal iodides MI (M = Li, Na, K) in low polarity media (chlorobenzene, 1,2-dichlorobenzene). The catalytic effect is found to depend on both the metal ion charge density and the ligand topology. Rate constants increase, in the order K+ < Na+ < Li+, with the complexes of crown ether 5 and PEG 6, whereas they are much lower and independent of the cation with the cryptates of 7. Reaction rates are favored by electron-withdrawing substituents, with Hammett ρ-values
that increase with decreasing Lewis acid character of the cation (Li+ < Na+ < K+) or when the charge of the latter is shielded by a good complexing agent like cryptand 7. Kinetic data are explained on the basis of a concerted “push–pull” mechanism.
在低极性介质(氯苯、1,2-二氯苯)中,聚醚配体 5-7 与碱金属碘化物 MI(M = Li、Na、K)的络合物促进了二(对位取代苯基)磷酸甲酯 1-4 的去甲基化反应,观察到了金属离子催化作用。催化作用取决于金属离子的电荷密度和配体的拓扑结构。冠醚 5 和 PEG 6 的络合物的速率常数按 K+ < Na+ < Li+ 的顺序增加,而 7 的隐色酸酯的速率常数则低得多,且与阳离子无关。当阳离子的路易斯酸特性降低时(Li+ < Na+ < K+),或当阳离子的电荷被像隐色酸酯 7 这样的良好络合剂屏蔽时,反应速率会受到抽电子取代基的影响,其 Hammett ρ 值会增加。动力学数据是根据协同 "推拉 "机制来解释的。