Homogeneous Nucleophilic Radiofluorination and Fluorination with Phosphazene Hydrofluorides

A series of phosphazenium hydrofluorides, P1tBu⋅[18/19F]HF, P1tOct⋅[18/19F]HF, P2Et⋅[18/19F]HF, and P4tBu⋅[18/19F]HF, was synthesized. The radioactive phosphazenium [18F]hydrofluorides were obtained by the one‐step formation and trapping of gaseous [18F]HF with the respective phosphazene bases. The [19F] isotopomers were prepared from the corresponding phosphazene bases and Et3N⋅3HF.

Under the design of experiment (DoE)‐optimized conditions, P2Et⋅HF and P4tBu⋅HF fluorinated alkyl chlorides, bromides, and pseudohalides in 76–98 % yield, but gave lower yields with iodides and electron‐deficient arenes. DoE models showed that fluorination can be performed in glass vessels, and that the reactivity of P2Et⋅HF and P4tBu⋅HF is dominated by solvent polarity but is insensitive to water to at least 2 equiv.

In contrast, P1tBu⋅HF and P1tOct⋅HF were unstable towards autofluorolysis. DFT calculations were performed to rationalize this finding in terms of diminished steric bulk, higher Parr’s electrophilicity, and chemical hardness of P1RH+. The corresponding radiofluorination reaction gave no valid DoE model but displayed similar substrate scope.

High specific activity and excellent radiochemical yields with various pseudohalides (81–91 %) suggest that the proposed radiofluorination methodology can complement the current [18F]KF/Kryptofix methods, particularly in the areas for which nonpolar reaction conditions are required.