Fluorine radical interception with radical traps: An experimental and DFT study

The interception of fluorine radical produced in the electrophilic fluorination with a series of radical traps (5,5-dimethyl-l-pyrroline N-oxide (DMPO), 2,2,6,6-tetramethyl piperidine N-oxide (TEMPO), 2-methyl-2-nitropropane (MNP), phenyl-tert-butynitrone (PBN)) has been investigated by electrospray ionization-mass spectrometry (ESI-MS) and density functional theory (DFT). The interception experiments by ESI-MS indicate that only DMPO possesses the capacity of trapping fluorine radical, while others (TEMPO, PBN and MNP) do not. Meanwhile, ESI-MS and tandem mass spectrum (MS/MS) experiments manifest that DMPO can trap fluorine radical to produce two kinds of adducts, namely ion B at m/z 132 containing one fluorine atom and ion C at m/z 152 containing two fluorine atoms, which process has been reasonably explained by DFT calculations. Ionization principle in ESI-MS suggests that ions B and C originate from the precursor compounds IM1 and IM2 respectively. The formation mechanisms of IM1 and IM2 are described as follows: first, the addition of fluorine radical to DMPO yields the pivotal intermediate M1, whose hydrogen is then abstracted by fluorine radical to form IM1. Furthermore, the addition of fluorine radical to IM1 can produce the intermediate M17, which subsequently abstracts hydrogen from other compounds to form IM2. The current work is to prove that ESI-MS and DFT provide a suitable tool for studying the interception of the radical.

► DMPO can succeed in trapping fluorine radical.
► Two kinds of adducts of DMPO and fluorine radical have been observed by ESI-MS.
► The structures of adducts are explored by MS/MS technique and DFT calculations.