Vinylic fluorine

The vinylic fluorine of perfluoroisobutylene can be replaced with chlorine by using phosphorus oxychloride or henzoyl chloride however, somewhat different mixtures result from these two chlorinating agents [72] (equation 59). 

Vinylic fluorines offluoralkenes are replaced with chlorine or bromine when treated with lithium halide salts in methoxyethanol, dimethylformamide, and pyridine [77]. 

Enolate anions of p-keto esters react with some fluoroolefms, initially by replacement of a vinylic fluorine atom, to give ultimately heterocyclic products [2S, 29] (equation 25). 

Vinyl fluorinated zinc reagents can be prepared by two different methods (1) capture of the corresponding vinyllithium reagent at low temperatures with a zinc salt and (2) direct insertion of zinc into a carbon-halogen bond. 

Diethylaminopropyne and trans octafluoro-2-butene give a single product from the addition of a vinylic fluorine across the triple bond [27] (equation 20)... 

For dienophiles with vinylic fluorine, such as perfluoropropene, the [2+4] reactivity is diminished so that [2-1-2] cycloaddition dominates [71] (equation 64). 

Displacement of Vinyl fluorine or chloride by secondary amines has given some unusual enamines as illustrated for the preparation of 1,1-difluoro-2-piperidino-3-phenyl-2-cyclobutene (151) (US), l,l-difluoro-2,4-dipiperidino-3-phenyl-2-cyclobutene (152) (114), and 2-phenyl-3-(l -aziridinyl)-2-cyclohexenone (153) (115). 

Single vinylic fluorine substituents absorb over quite a wide range of chemical shifts, with fluoroallene at the high field end (-169 ppm) and P-fluoroacrylate derivatives at the low field end (-75 ppm) (Scheme 3.36). 

The chemical shifts of terminal (1 °) vinylic fluorines are not affected significantly by conjugation of the fluorine-bearing double bond with either another C=C double bond or a benzene ring (Scheme 3.40). In this case, however, the fluorines of the (Z)-isomers are slightly deshielded relative to those of the (F)-isomers. 

Geminal chlorine or bromine substituents deshield vinylic fluorine significantly, whereas a vicinal chlorine substituent shields the fluorine, much as was the case for the saturated systems. Again similarly, a second vicinal chlorine substituent reverses the trend and shifts the fluorine signal downfield (Scheme 3.45). 

The conspicuously high speed of hydrogenolysis of allylic and vinylic halogens as compared with those removed further from the multiple bonds or halogens in alkanes implies that the multiple bond participates in a multicenter transition state [63]. Such a mechanism would account even for the surprisingly easy hydrogenolysis of allylic and vinylic fluorine [66, 528, 529, 530]. 

Addition-elimination processes of a vinylic fluorine, from gem-difluoroalkenes." ... 

Perfluorobuta-1,3-diene (14) can also be converted into perflnorobut-2-yne (15) by heating to 100 C for 30 minutes.42 This reaction is energetically favored by converting unfavorable vinylic fluorine atoms into exceptionally stable trilluoromethyl groups. Chambers and co-... 

Terminal vinylic fluorine bonds in substituted allylic alkenols are reduced selectively with lithium aluminum hydride in high yields. The reduction of l,l,2-trifluoroalk-l-en-3-ols in refluxing diethyl ether affords l,2-difluoroalk-l-en-3-olsas a mixture of ( )- and (Z)-isomers in 76-82% yield together with a small amount of l,l,2-trifluoroalk-2-enes (E- and Z-isomers).82 Similarly, lithium 3,3-difluoroalk-2-ene oxides 1 afford unstable l-fluoroalk-l-en-3-ols 2 (EjZ 95 5) in 80-90% crude yields.88 84... 

Catalytic hydrogenation of substituted 2-fluoroprop-2-enoates over palladium at room temperature under atmospheric pressure results in saturation and only partial fluorine hydrogenolysis.100 The replacement of vinylic fluorines with hydrogen by catalytic hydrogenation proceeds more readily in fluoromaleic, fluorofumaric, difluoromaleic, and difluorofumaric acid 10.no It is assumed that fluorine hydrogenolysis occurs before saturation of the C = C bond. Yields of defluorinated products are strongly dependent on the catalyst used.110... 

Vinylic fluorine in lithium 2,3,3-trifluoroprop-2-enoate is substituted with hydride ion by a nucleophilic addition-elimination mechanism using lithium aluminum hydride at low temperatures.113... 

In perfluorinated dihydrooxazines only vinylic fluorine bonds are hydrogenolyzed using hydrogen over palladium, while C-F bonds at a-positions to oxygen and nitrogen atoms remain unaffected (Table 4).154... 

Acidic hydrolysis of pentafluoro-l-(2-methoxyphenyl)prop-l-ene gives 2,3,3,3-tetrafluoro-l-(2-methoxyphenyl)propan-l-one (8) by hydrolysis of the vinylic fluorine concomitant cleavage of the ether group produces the corresponding phenol 9 in variable amounts, depending on the reaction conditions.15... 

Stereoselective reduction of vinylic fluorine is also accomplished with tri-butylphosphine [/ ] (equation 12)... 

Appropriately constructed fluoroallylic alcohols are attacked at the double bond by sodium borohydride with SN2 displacement of the hydroxyl group rather than the vinylic fluorine [47] (equation 36)... 

Acidic hydrolysis of l-(o-methoxyphenyl)pentafluoropropene gives o-hy-droxy-2,3,3,3-tetrafluoropropiophenone by hydrolysis of vinylic fluorine and cleavage of the ether to the phenol [4] (equation 4). 

CE/Z ratios were determined hy integration of the vinylic fluorine signals in F NMR spectra... 

Reactions of fluorinated dipolarophiles. Electron-deficient unsaturated species generally make better dipolarophiles, therefore, fluorinated alkenes become better dipolarophiles when vinylic fluorines are replaced by perfluoroalkyl groups For example, perfluoro-2-butene is unreactive with diazomethane, but more highly substituted perfluoroalkenes, such as perfluoro-2-methyl-2-pentene, undergo cycloadditions in high yields [5] (equation 2) Note the regiospecificity that is observed in this reaction... 

Substituting vinylic hydrogen in a,P-unsaturated carbonyl compounds with vinylic fluorine does not affect their dienophilic character negatively Indeed, 3,3-difluoroacrylic acid is more reactive toward furan than its nonfluonnated counterpart [95] (equation 81) Consistent with this observation is the fact that tetrafluorobenzoquinone forms only a bis-Diels-Alder adduct in 68% yield in its reaction with cyclopentadiene at room temperature [96, 97 ... 

Intermolecular transfer of trifluoromethyl has been demonstrated in the fluoride ion induced rearrangement of the perfluorinated alkene (77) to the isomer (78). Again, the driving force in this process is to produce an isomer (78) with fewer vinylic fluorine sites than in the starting isomer (77). The intermediate trifluoromethyl anion has been trapped with perfluoropyrimidine (79)... 

Heterocycles can be formed in a number of systems and perhaps the most surprising involves reaction of the diene (38) (Scheme 47) in moist ether Obviously, the process depends on the high reactivity of the vinylic fluorine atoms in (38) and the subsequent step is most likely an electrocyclisation, to form (88). Also, pyrroles, e.g. (89) and, more remarkably, pyrrolopyri-dines, e.g. (90) may be synthesised from (38) using aniline derivatives [44, 106],... 

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