First application of ionic liquids in electrophilic fluorination of arenes; Selectfluor™ (F-TEDA-BF4) for “green” fluorination

Abstract

The NF fluorinating agent F-TEDA-BF4 dication salt (Selectfluor™) 1 dissolves in imidazolium-based ionic liquids [emim][OTf] 7, [emim][BF4] 8, [bmim][PF6] 9 and [bmim][BF4] 10 (assisted by sonication), providing a convenient medium for fluorination of arenes under essentially acid-free conditions in a simple set-up (no volatile solvent; simple extraction of the aromatics without aqueous work-up), from which the ionic liquid can be easily recycled and reused. Comparative studies in [emim][OTf] 7 with anisole as substrate show that 1 is superior to NFTh-BF4 (Accufluor®) 2 and that the N-fluoropyridinium salt NFPy-B2F74 is least effective. The scope of the reaction has been surveyed. Substrate selectivity (kmesitylene ∶ kdurene = 10) measured in competitive experiments in 7 is clearly indicative of a conventional polar mechanism. Substrate selectivity measured without the ionic liquid in MeCN solvent is also indicative of a polar mechanism but exhibits lower magnitude (kmesitylene ∶ kdurene = 6).