SN2 Fluorination reactions in ionic liquids: a mechanistic study towards solvent engineering

Abstract

In the catalysis of S_N2 fluorination reactions, the ionic liquid anion plays a key role as a Lewis base by binding to the counterion Cs⁺ and thereby reducing the retarding Coulombic influence of Cs⁺ on the nucleophile F⁻. The reaction rates also depend critically on the structures of ionic liquid cation, for example, n-butyl imidazolium gives no S_N2 products, whereas n-butylmethyl imidazolium works well. The origin of the observed phenomenal synergetic effects by the ionic liquid [mim-^tOH][OMs], in which t-butanol is bonded covalently to the cation [mim], is that the t-butanol moiety binds to the leaving group of the substrate, moderating the retarding interactions between the acidic hydrogen and F⁻. This work is a significant step toward designing and engineering solvents for promoting specific chemical reactions.