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.
N. G. Nair et al., "1,3-Sigmatropic Fluorine Migration to Boron in McLafferty Type of Rearrangements: Observation of Tetrafluorobenzyne Radical Cation and Trifluorobenzyne Cation by CID-mass Spectrometry," Journal of Organometallic Chemistry, Elsevier, Jun 2013.
The definitive version is available at http://dx.doi.org/10.1016/j.jorganchem.2013.06.006
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