Taming halonium metathesis

Abstract : The halonium metathesis reaction of ketones and aldehydes is difficult to harness for three reasons: (1) the interchange of F+ for O is so exothermic that the product ion often decomposes further; (2) the adduct of XF+ to the carbonyl oxygen frequently undergoes side reactions to the virtual exclusion of metathesis; and (3) the metathesis ion is liable to rearrange to a mixture of isomers. Several approaches have been explored to make this reaction a useful source of gaseous fluoroalkyl cations. DFT calculations reinforce the intuition that cyclopentenone should give a good yield of metathesis ions in its ion-molecule reaction with CF3+, an expectation that is borne out by experiment. Other approaches have met with less dramatic success. Variation of XF+ gives a number of interesting results, but little improvement over CF3+. Deuterium substitution at the carbon adjacent to the carbonyl (a-position) does not restrain further decomposition, but deuteration of the next carbon (ß-position) does appear to do so. The isotopic labeling experiments indicate that the prevalent mode of decomposition requires hydrogen shift followed by a 1,3-elimination of HF. Observed side reactions of adduct ions include alkene expulsion to yield CF3-containing products (such as m/z 125 from the reaction of cycloalkanones with CF3+) and direct elimination of HF (as conjectured for the reaction of CFO+ with diethyl ketone, with concurrent loss of CO2). Structures of ion-molecule reaction products can be reliably inferred only when the expected mechanism yields the minimum energy structure for that molecular formula. © 2002 Published by Elsevier Science B.V.
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Danielle Leblanc, J. Kong, P.S. Mayer, T.H. Morton. Taming halonium metathesis. International Journal of Mass Spectrometry, Elsevier, 2003, 222 (1-3), pp.451-463. ⟨10.1016/S1387-3806(02)00993-4⟩. ⟨hal-00917382⟩

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