Organocatalysis of nucleophilic substitution reactions by the combined effects of two promoters fused in a molecule: oligoethylene glycol substituted imidazolium salts

Oligoethylene glycol substituted imidazolium salts were synthesized as promoters for a range of SN2 reactions, and their efficiency was examined. These tailor-made organic promoters enhanced the nucleophilicity of alkali metal salts significantly through the combined effects of two promoters (oligoethylene glycols and imidazolium salts) in a single molecule. The effects of the oligoethylene glycol side chain length, ionic liquid anions, nucleophiles, and substrates were investigated systematically. [hexaEGmim][OMs] and [dihexaEGim][OMs] showed the highest efficiency for SN2 reactions using alkali metal salts. The role of the terminal hydroxyl groups of the oligoethylene glycol moiety was assessed by examining the relative SN2 yields of chlorination and bromination. The results showed that the hydrogen bonding strength of the hydroxyl groups with the nucleophile is very important. The mechanism for the excellent promotion of SN2 reactions by oligoEGILs was examined by quantum chemical calculations. The results showed that the oxygen atoms in the oligoethylene glycol portion and the ionic liquid anion act on the counter cation K+ or Na+ as a Lewis base, to enhance the reactivity of the metal salts significantly.