1,3-Sigmatropic fluorine migration to boron in McLafferty type of rearrangements: Observation of tetrafluorobenzyne radical cation and trifluorobenzyne cation by CID-mass spectrometry

The electron-impact mass spectra of the tris(pentafluorophenyl)boroxine (1) and triphenylboroxine (2) were analyzed to probe the McLafferty type of rearrangements involving 1,3-fluorine or 1,3-hydrogen migrations to boron from the adjacent aromatic rings. 1,3-Sigmatropic fluorine migration leading to the McLafferty rearrangement of 1 results in the formation of the tetrafluorobenzyne radical cation (1c), whereas the similar rearrangement involving 1,3-hydrogen rearrangement is not observed for the nonfluorinated analog 2. The DFT calculations show that the activation barrier for the 1,3-fluorine migration in 1 is significantly lower than that for the 1,3-hydrogen migration in 2 (ΔΔG^‡ ∼33 kcal/mol), which is in accordance with the observed 1,3-fluorine migration for the fluorinated boroxine and the lack of such rearrangement for the nonfluorinated boroxine. The 1:1 stoichiometry of the fluoride anion with 1 has also been demonstrated by high resolution electrospray ionization time-of-flight mass spectrometry.