Fluoride Sensor Also Fluorinates

A sulfonium borane complex that selectively traps fluoride ions under aqueous conditions can turn around and hand off the fluoride in nucleophilic substitution reactions under anhydrous conditions, a feat that should open new possibilities in fluorine chemistry, according to Haiyan Zhao and François P. Gabbaï of Texas A&M University (Org. Lett., DOI: 10.1021/ol200129q). Gabbaï’s group previously prepared a sulfonium borane salt in which dimethylsulfonium and arylborane groups on adjacent rings of naphthalene work together to trap a fluoride ion and function as a fluoride sensor. The researchers figured out that they can demethylate the sulfur using a phenylthiolate salt, untethering the fluoride to form an anionic fluoroborate that is an efficient room-temperature nucleophilic fluorinating reagent. This new reagent works on a variety of substrates, including alkyl halides and aromatic compounds bearing nitro (shown), sulfonyl chloride, and other electron-rich substituents. “The benefit of our approach is that we can start from aqueous fluoride solutions and after one simple capture step carry out fluorination reactions in dry organic solvents,” Gabbaï says. This capability might benefit ¹⁸F radiolabeling of organic reagents used in positron emission tomography, he suggests, because the short-lived ¹⁸F^– is first prepared in aqueous solution and requires fast processing.