On the Mechanism of Phosphonofluoridate and Phosphonic Acid Anhydride Syntheses Using Pentafluoropyridine.

A method for the fluorination of phosphonic acids is described that does not require anhydrous conditions. Mechanistically, we show that pentafluoropyridine (PFP) functions as an activator and fluoride source, quantitatively forming phosphonofluoridic acids (phosphonofluoridates). Mechanistic pathways were postulated using 31P and 19F NMR spectroscopy and mass spectrometry data. Furthermore, we show that either altering reaction conditions or the provision of a fluoride scavenger results in the formation of corresponding phosphonic acid anhydrides.