Cation participation in nucleophilic substitution reactions promoted by complexes of polyether ligands with alkali and alkaline-earth metal salts

Abstract

Cation electrophilic catalysis has been studied in nucleophilic substitution reactions of octyl methanesulfonate (1) promoted by complexes of macrocyclic polyethers 2–5 with alkali and alkaline-earth metal salts in chlorobenzene. The catalytic effect increases with increasing Lewis acid character of the cation, in the order Cs⁺∼ Rb⁺∼ K⁺ < Na⁺ < Li⁺ and Ba²⁺ < Sr²⁺ < Ca²⁺, and is mainly related to the ability of the polyether to shield the metal ion charge. Kinetic data clearly indicate that crown ethers 2-4 allow the metal ion participation in the activation process, whereas the cryptand [2.2.2,C₁₀](5), which more effectively binds the cation, inhibits catalysis. The lower reactivity of the complexes of alkaline-earth metal salts with both crown ethers and cryptands is most likely due to a higher interaction of the divalent cation with the anion in the ion-pair.