Fluoronium metathesis and rearrangements of fluorine-stabilized carbocations

The ion-molecule reaction of gaseous trifluoromethyl cation with the conjugated enone 3-methylcyclopentenone yields the C(6)H(8)F(+) product from metathesis of F(+) with the ketone oxygen, along with concomitant formation of neutral carbonyl fluoride. Comparison of the infrared multiple photon dissociation (IRMPD) spectrum of the product ion with that predicted by anharmonic DFT shows a good match. The IRMPD action spectrum, monitored by expulsion of hydrogen fluoride to form C(6)H(7)(+) (m/z 79), shows that the ion-molecule reactions of CF(3)(+) with two other isomeric, conjugated enones, cyclohexenone and 2-methylcyclopentenone, also yield the same product ion as results from 3-methylcyclopentenone. Although CF(3)(+) is a comparatively soft Lewis acid (by comparison to the isoelectronic BF(3)), formation of the most stable C(6)H(8)F(+) isomer (the one observed experimentally) cannot be accounted for by attachment of CF(3)(+) to the C=C bond of a neutral enone. IRMPD of the C(6)H(7)(+) ion formed by reaction of CF(3)(+) with 2-methylcyclopentenone, monitored by loss of H(2), indicates that the C(6)H(7)(+) ion consists of a mixture of protonated benzene and protonated fulvene. Neither of those C(6)H(7)(+) ions can be produced by unimolecular loss of HF from C(6)H(8)F(+) without prior rearrangement. Plausible pathways for reversible skeletal isomerization of C(6)H(8)F(+), probed by DFT, suggest the intervention of nonclassical, edge-protonated cyclopropane transition states, analogous to the rearrangement of the cyclohexyl cation to the 1-methylcyclopentyl cation. (C) 2011 Elsevier B.V. All rights reserved.