Task-specific hexaethylene glycol bridged di-cationic ionic liquids as catalysts for nucleophilic fluorination using potassium fluoride

A tailor-made di-cationic ionic liquid (DCIL), hexaethylene glycol (hexaEG) bridged bis(2-hydroxy-2-methyl-n-propylimidazolium) dimesylate (hexaEG-D^tOHIM), was prepared in 94% yield. We investigated the catalytic activity of hexaEG-D^tOHIM and hexaEG-bridged bis(3-hexaEGyl imidazolium) dimesylate (hexaEG-DHIM) in nucleophilic fluorination using an alkali metal fluoride (MF) and compared their activities with a variety of mono-cationic ILs. In this reaction, these two task-specific DCILs exhibited much higher catalytic activity than mono-cationic ILs. Moreover, the hexaEG functionalized IL (e.g. hexaEG-DHIM) more effectively enhanced the reactivity of KF compared with the t-alcohol functionalized IL (e.g. hexaEG-D^tOHIM). In particular, the combination of KF/hexaEG-DHIM in t-alcohol media provided excellent chemoselectivity and high chemical yields in the fluorination reaction of base sensitive substrates such as 1-(2-mesyloxyethyl)naphthalene. This protocol was shown to be generally applicable to fluorination reactions with a variety of substrates.