Intramolecular fluorine migration via four-member cyclic transition states

Gaseous CF3+ interchanges F+ for O with simple carbonyl compounds. CF3+ rea cts with propionaldehyde in the gas phase to produce (CH3)(2)CF+ via two co mpeting pathways. Starting with 1-C-13-propionaldehyde, the major pathway ( 80%) produces (CH3)(2)CF+ with the carbon label in one of the methyl groups . The minor pathway (20%) produces (CH3)(2)CF+ with the carbon label in the central position. The relative proportions of these two pathways are measu red by F-19 NMC analysis of the neutral CH3CF=CH2 produced by deprotonation of (CH3)(2)CF+ at <10(-3) Torr in an electron bombardment flow (EBFlow) re actor. Formation of alkene in which carbon is directly bonded to fluorine m eans 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 CH3CH2CNF+ to the more stable (CH3)(2)CF+. Use of CF4 as a chemical ioni zation (CI) reagent gas leads to CF3+ adduct ions for a variety of ketones, in addition to isoelectronic transposition of F+ for O, Metastable ion dec ompositions of the adduct ions yield the metathesis products. Decomposition s of fluorocycloalkyl cations Termed in this manner give evidence for the s ame kinds of rearrangements as take place in CH3CH2CHF+. Density functional calculations confirm that F+ for O metathesis takes place via addition of CF3+ to the carbonyl oxygen followed by transposition via a four-member cyc lic transition state. A computational survey of the effects of different su bstituents in a series of aldehydes and acyclic ketones reveals no systemat ic variation of the energy of the transition state as a function of thermoc hemistry, but the Hammond postulate does appear to be obeyed in terms of pr ogress along the reaction coordinate. Bond lengths corresponding to the cen tral barrier correlate with overall thermochemistry of the F+ for O interch ange, but in a sense opposite to what might have been expected: the transit ion state becomes more product-like as the metathesis becomes increasingly exothermic. This reversal of the naive interpretation of the Hammond postul ate is accounted for by the relative positions of the potential energy well s that precede and follow the central barrier.