Intramolecular Fluorine Migration via Four-Member Cyclic Transition States

Abstract

Gaseous CF(3)(+) interchanges F(+) for O with simple carbonyl compounds. CF(3)(+) reacts with propionaldehyde in the gas phase to produce (CH(3))(2)CF(+) via two competing pathways. Starting with 1-(13)C-propionaldehyde, the major pathway (80%) produces (CH(3))(2)CF(+) with the carbon label in one of the methyl groups. The minor pathway (20%) produces (CH(3))(2)CF(+) with the carbon label in the central position. The relative proportions of these two pathways are measured by (19)F NMR analysis of the neutral CH(3)CF=CH(2) produced by deprotonation of (CH(3))(2)CF(+) at <10(-)(3) Torr in an electron bombardment flow (EBFlow) reactor. Formation of alkene in which carbon is directly bonded to fluorine means that (in the minor product, at least) an F(+) for O transposition occurs via adduct formation followed by 1,3-atom transfer and then isomerization of CH(3)CH(2)CHF(+) to the more stable (CH(3))(2)CF(+). Use of CF(4) as a chemical ionization (CI) reagent gas leads to CF(3)(+) adduct ions for a variety of ketones, in addition to isoelectronic transposition of F(+) for O. Metastable ion decompositions of the adduct ions yield the metathesis products. Decompositions of fluorocycloalkyl cations formed in this manner give evidence for the same kinds of rearrangements as take place in CH(3)CH(2)CHF(+). Density functional calculations confirm that F(+) for O metathesis takes place via addition of CF(3)(+) to the carbonyl oxygen followed by transposition via a four-member cyclic transition state. A computational survey of the effects of different substituents in a series of aldehydes and acyclic ketones reveals no systematic variation of the energy of the transition state as a function of thermochemistry, but the Hammond postulate does appear to be obeyed in terms of progress along the reaction coordinate. Bond lengths corresponding to the central barrier correlate with overall thermochemistry of the F(+) for O interchange, but in a sense opposite to what might have been expected: the transition state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postulate is accounted for by the relative positions of the potential energy wells that precede and follow the central barrier.

Gas-phase fluorine migration reactions in the radical cations of pentafluorosulfanylbenzene (Aryl―SF5) and benzenesulfonyl fluoride (Aryl―SO2F) derivatives and in the 2,5-xylylfluoroiodonium ion

“…Mass spectrometric research on polyfluorinated compounds can provide important information for their structural assignment and enrich the knowledge of their gas‐phase reactivity for the chemists. Previous work showed that the unique mass spectrometric behaviors of these compounds include F‐atom migration, double F‐atom migration, F‐atom ring walk, and trifluoromethyl (CF 3 ) group rearrangement . Tajima et al .…”

“…Morton et al . reported ion–molecule reactions of CF 3 + with carbonyl compounds, followed by F‐atom migration via four‐membered cyclic transition states . Speranza et al .…”

The Cationic CF+ Stretching Vibration in the Gas Phase

“…The structure of ion 4 in the gas phase has been confirmed by collection and identification of the neutral products from its subsequent ion-molecule reactions. [11] Because of the small number of atoms in 4, resonant IRMPD on this ion in the gas phase presents a challenge. For that reason, we have examined the perdeuterated analogue 5, for which the vibrational levels are more closely spaced.…”

Collision‐induced fluorine atom migration in atmospheric pressure ionization mass spectrometry

“…Furthermore, it has been demonstrated that 1,3-fluorine transfer operates in the reaction of CF 3 C with propionaldehyde in the gas phase, via a four-membered cyclic transition state. 8 In order to examine whether the F migration phenomenon occurs in other trifluoromethoxy-containing compounds, 4-(trifluoromethoxy)phenol was studied using the LCQ ion trap mass spectrometer and a similar CID fragmentation pattern was observed. Upon CID in the negative ion mode, 4-(trifluoromethoxy)phenol gave rise to three major fragments at m/z 155, 111, and 108 (Fig.…”