Unexpected Behavior of the Heaviest Halogen Astatine in the Nucleophilic Substitution of Aryliodonium Salts

International audienceAryliodonium salts have become precursors of choice for the synthesis of 18 F-labeled tracers for nuclear imaging. However, little is known on the reactivity of these compounds with heavy halides, that is, radioiodide and asta-tide, at the radiotracer scale. In the first comparative study of radiohalogenation of aryliodonium salts with 125 I À and 211 At À , initial experiments on a model compound highlight the higher reactivity of astatide compared to iodide, which could not be anticipated from the trends previously observed within the halogen series. Kinetic studies indicate a significant difference in activation energy (E a = 23.5 and 17.1 kcal mol À1 with 125 I À and 211 At À , respectively). Quantum chemical calculations suggest that astatination occurs via the monomeric form of an iodonium complex whereas iodi-nation occurs via a heterodimeric iodonium intermediate. The good to excellent regioselectivity of halogenation and high yields achieved with diversely substituted aryliodonium salts indicate that this class of compounds is a promising alternative to the stannane chemistry currently used for heavy radiohalogen labeling of tracers in nuclear medicine