Fluorine from allylic position

Interestingly, the photolysis of methyl 3-azido-2.6-difluorobenzoate (61) in diethylamine yields, in addition to the two expected 3//-azepines 62 and 63, a third azepine 64 formed by nucleophilic displacement of allylic fluorine from the 3-position by diethylamine.188 Displacement of allylic halogen from haloazepines has been noted previously.177... 

A nucleophilic mechanism can be applied in reductions with complex hydrides of highly fluori-nated aliphatic and alicyclic fluoroalkenes with electron-deficient C = C bonds the hydride anion adds as a strong nucleophilic agent to the more electrophilic carbon atom the intermediate anion can then lose a fluoride ion either from the original C = C bond, or from the allylic position finishing an SN2 displacement of the fluorine. Thus, the reductions of vinylic C-F bonds with hydrides proceed by a nucleophilic addition-elimination mechanism. Displacement of fluorine in highly fluorinated aromatic compounds proceeds by the same mechanism ... 

Due to the much weaker acidity effect of fluorine compared to chlorine, proton abstraction from different positions may compete. Furthermore, a consecutive second hydrogen fluoride elimination may occur resulting in the formation of the corresponding alkyne (pathway A). Moreover, if the formation of an allyl-type fluoride is possible it may be attacked by the base to give dienes and allylic ethers (pathway B). 

The authors explained the formation of these compounds as follows. The O-nucleophile initially attacks the C=C bond of the perfluoroolefin, forming a carbanion. The latter may be stabilized by elimination of the fluoride ion from the y-position of the CF2 fragment, giving a multiple bond. The olefin has a mobile fluorine atom in the allyl position, leading to intramolecular nucleophilic cyclization induced by the O- or N-nucleophiles. This process may be regarded as an example of the important role of the intermediate from the highly active perfluoroolefin, determining the direction of the subsequent attack of the O- and N-nucleophiles. 

The first step in the hydrolysis in tert-butyl alcohol is probably nucleophilic displacement of fluorine in the reactive allylic position. Spontaneous elimination of hydrogen fluoride from the geminal fluorohydroxy compound yields perfluorocyclopent-2-en-l-one. 

Thus, both perfluoroalkyl olefins 3 and 4 incorporated CO2 on one of the initial vinyl carbons, regioselectively on the site of the hydrocarbon chain. The carboxylation process involves a shift of the double bond, with the loss of one fluorine atom from the allylic position. 

Another powerful approach to prepare a-amino acids bearing an aromatic or unsaturated side chain in /I (but also many other compounds) is based on the reactivity of 5-fluoro-4-trifluoromethyloxazole, a starting material easily accessible from hexafluoroacetone. The fluorine atom in the 5 position is easily displaced by an allylic or benzylic alcohol. Then, the obtained ethers spontaneously undergo a Claisen rearrangement to afford, after acidic hydrolysis, an a-trifluoromethyl amino acid... 

Fluorinated vinylzinc reagents readily react (particularly with Cu(I)X catalysis) with allyl halides at room temperature (equation 63)57. With a substituted allyl halide, the 2-propenyl reagent gave products of both a- and -attack. The predominant product is derived from attack at the less hindered position (equation 64)5. 

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