Taming halonium metathesis

• Published in: International journal of mass spectrometry Vol. 222; no. 1; pp. 451 - 463
• Main Authors: Leblanc, Danielle, Kong, Jennie, Mayer, Philip S., Morton, Thomas Hellman
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 2003; Elsevier
• Subjects: Allylic cation; Cyclopentadienyl cation; Hammond postulate; Ion–molecule reaction; Monofluorinated carbocation; Unsaturated ketone; Cyclopentadienyl cation; Allylic cation; Hammond postulate; Unsaturated ketone; Ion–molecule reaction; Monofluorinated carbocation
• DOI: 10.1016/S1387-3806(02)00993-4
• ISSN: 1387-3806

The halonium metathesis reaction of ketones and aldehydes,  C O+ XF +→ C F ++ X O is difficult to harness for three reasons: (1) the interchange of F + for O is so exothermic that the CF + 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 CF 3 +, 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 CF 3 +. Deuterium substitution at the carbon adjacent to the carbonyl (α-position) does not restrain further decomposition, but deuteration of the next carbon (β-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 CF 3-containing products (such as m/ z 125 from the reaction of cycloalkanones with CF 3 +) and direct elimination of HF (as conjectured for the reaction of CFO + with diethyl ketone, with concurrent loss of CO 2). Structures of ion–molecule reaction products can be reliably inferred only when the expected mechanism yields the minimum energy structure for that molecular formula.