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Nucleophilic 18F-fluorination of heteroaromatic iodonium salts with no-carrier-added [18F]fluoride.

Diaryliodonium salts containing the 2-thienyl group as an example of an electron-rich heteroaromatic moiety proved to be very potent precursors for the nucleophilic, regioselective no-carrier-added (nca) radiofluorination of various arenes. It even allowed the nucleophilic introduction of nca [18F]fluoride into electron-rich arene compounds in one step. The influences of the substitution pattern, of counteranions, and of different reaction conditions were studied. Effects of counterions could be explained by the influence of solvent on ion pair separation of precursor salts. Different aryl(2-thienyl)iodonium salts were used as precursors, where the homoaromatic group systematically varied from bearing electron-deficient to electron-rich substituents. Relative rates of exchange kinetics correlated linearly with Hammett constants of the appropriate substituents confirming a nucleophilic aromatic substitution reaction of high reactivity and low selectivity.