科研之友
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摘要
A series of phosphazenium hydrofluorides, P-1(tBu)center dot[18/19F] HF, P-1(tOct)center dot[F-18/19] HF, P-2(Et)center dot[F-18/19]HF, and P-4(tBu)center dot[F-18/19] HF, was synthesized. The radioactive phosphazenium [F-18] hydrofluorides were obtained by the one-step formation and trapping of gaseous [F-18]HF with the respective phosphazene bases. The [F-19] isotopomers were prepared from the corresponding phosphazene bases and Et3N center dot 3HF. Under the design of experiment (DoE)-optimized conditions, P-2(Et)center dot HF and P-4(tBu)center dot HF fluorinated alkyl chlorides, bromides, and pseudo-halides 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 P-2(Et)center dot HF and P-4(tBu)center dot HF is dominated by solvent polarity but is insensitive to water to at least 2 equiv. In contrast, P-1(tBu)center dot HF and P-1(tOct)center dot 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 (P1H+)-H-R. 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 [F-18]KF/Kryptofix methods, particularly in the areas for which nonpolar reaction conditions are required.
- 出版日期2011-7
