Fluorine 1,2-shift

The fluorine shifts of a-aldohexopyranosyl fluorides, having an axially oriented fluorine atom, are at higher field than those of the corresponding, equatorially oriented /8 counterparts, which, in turn, were found to display temperature-dependent shifts. ... 

Chemical shifts and chemical-shift differences between pairs of ano-mers in the monodeoxymonofluoro-D-hexopyranose series were observed to be strongly dependent on the stereochemistry of electronegative substituents in the molecule. In consequence, no a priori distinction between anomers can be made on the sole basis of their respective fluorine shifts. The use of fluorine shifts as a probe for structural and conformational... 

The three shifts for thianthrene, determined in deuterochloroform solution, are shown in 10 (84CB107), and the fluorine shifts (in ppm relative to trichlorofluoromethane) for perfluorothianthrene, determined in acetone solution, are shown in 11 (68T2783, 68T3997). Other C determi-... 

Perfluorocyclopentadienes can be photochemically rearranged by a [1,5] sigmatropie fluorine shift. Thus, pcrfluoro(tricyclo[5.2.2.02 ( ]undeca-2,5,8-tiiene) (19) has been converted into the 2(6).3,8-triene 20,54 and perfluoroindenc isomerized to perfluoroisoindene. which can be trapped as its Diels-Alder adduct with ethylene.55... 

The proposed mechanism of the isomerization of polyfluorooxiranes to ketones does not involve a 1,2-fluorine shift, but rather coordination of the Lewis acid with the epoxide oxygen, ring opening to the carbocation, and intermolecular addition and elimination of fluoride.29 45... 

Ring closure to a bicyclic product could also be expected in the irradiation of peril uoroazacyclohexadiene 25, but the product obtained, in 87% yield, is peril uoro[l-isopropyl-3-(methylimino)cyclobutene] (27), which is presumed to be formed via ring opening to azatriene 26, followed by electrocyclic ring closure to the cyclobutene and a 1,3-fluorine shift.20... 

The mechanism for the cyclization of these perfluoro ketones, proposed by German and coworkers5 and discussed in the review by Krcspan and Petrov.4 involves initial activation of the carbonyl with antimony V) fluoride and a 1.4-fluorine shift from the /1-trifluoromethyl group. The resultant difluoromethylene carbocation then cyclizes with the carbonyl oxygen to give the tetrahydrofuran. 

The result of acid-catalyzed isomerisation of F-dienes depends on several factors structure of substrate, catalyst, and temperature. Action of SbF5 on terminal dienes under mild conditions causes a 1,3 fluorine shift occurring stereoselectively to give trans-, frans-, and cis-, trans- isomers of the corresponding internal dienes [160] ... 

When it comes to chemical shifts for bridgehead hydrogens, as was the case with the fluorine shifts, there does not seem to be any understandable trend (Scheme 3.13). On the other hand, the carbon data for l-fluoro-bicyclo[l.l.l]pentane are unique, with both bridgehead carbons being much more highly shielded than their counterparts... 

Perfluoroallylic fluorides show 1,3-fluorine shifts upon direct irradiation126. 

Acetylenes 1 react smoothly with oxygen difluoride to produce a,a-difluoro ketones 2. " The intermediates of this addition are a-fluorovinyl hypofluorites, which rearrange with a fluorine shift to the observed difluoro ketones. In all cases small amounts of acyl fluorides are formed as byproducts. 

Thermolysis of both F3CSiH3 and F2HCSiH3 in the presence of efficient carbene trapping agents have shown that they decompose predominantly by a-fluorine shift to give fluorosilane and the fluorocarbenes CF2 and CHF, respectively [24]. Secondary reactions at elevated temperature in the absence of any... 

Rapid 1,2-fluorine shifts have been shown by Olah and Bollinger (1967) to be responsible for the equivalence of the four methyl groups in 2-fluoro-2,3-dimethyl-3-butyl cation. 

Indeed, NMR studies suggest that ionisation of 2,3-difluoro-2,3-dimethylbutane gives a rapidly equilibrating mixture in which methyl 1,2-shifts, rather than fluorine shifts, occur [75] (Figure 4.28). 

Conversely, cleavage with a Lewis acid catalyst gives a ketone [191, 192] (Figure 8.63). These are interesting reactions because they involve a 1,2-fluorine shift to a positive centre (Figure 8.64), a process that is, of course, very well known for hydride shifts. The conversion of hexafluoropropene oxide to hexafluoroacetone is probably the... 

Summary of F shift trends and other NMR properties. Fluorine shifts in inorganic solids are affected by both nearest and next-nearest neighbour effects. It is often difficult to make unequivocal assignments of F NMR resonances of samples which are either genuinely disordered or consist of low-coverage surface phases... 

Shaler, T. A., Morton, T. H. Fluorine Shifts in Gaseous Cations. Analogs of Wagner-Meerwein Rearrangements. J. Am. Chem. Soc. 1994, 116, 9222-9226. 

Breakage of the P-O bond or the P-C bond in the cyclic intermediate (above) leads to reaction (13.7), whereas reaction (13.8) requires a series of fluorine shifts from the carbon to the phosphorus atom, to give CO and PF [1300]. 

Si-F bond or as a [1,2]-fluorine shift in H3SiSiH2F (see equation 40). The authors note that the insertions of H2Si into the Si-F bond of H3SiF295 and into HF289 are quite similar. 

Empirical correlations of the fluorine shift with structure are relatively sparse when compared with information concerning proton behavior. It seems probable, however, that the future will see further developments in this field, particularly for structural investigation of organic lluorinc compounds. 

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