Demethylation reactions of phosphate esters catalysed by complexes of polyether ligands with metal iodides

Abstract

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.