The Introduction of Fluorine into Aromatic Rings

The Introduction of Fluorine Into Aromatic Rings.— The direct fluorination (F2-N2) of nitrobenzene, benzotrifluoride, benzoic acid, and toluene, usually in trifluoroacetic acid at 5 C, has been described by Russian workers. Where direct comparisons can be made, the isomer distributions of the monofluoroarenes obtained are qualitatively similar to those formed in acetonitrile during concurrent American work, sununarized in Vol. 1 (p. 189). However, the Russians favour a homolytic mechanism, the preference for attack at the /wc/o-position in nitrobenzene being attributed to the electrophilic character of the attacking fluorine atom. Electrophilic displacement of a proton by F, via the usual addition-elimination mechanism, was preferred in the American paper, and this view has been emphasized in a recent review of the field by the same author.  

The mechanism for the fluorination of benzene and its derivatives with xenon difluoride in the presence of hydrogen fluoride has been discussed independently in more detail, but similar conclusions about the participation of radical-cations in the early stages of the reaction have been reached. However, the effects of added chlorine and of hydrogen chloride on fluori-nations by this method indicate that free radical-cations are involved only in the formation of biaiyls, the halogenobenzenes themselves being formed directly from a complex of the substrate and fluorinating agent. 

Radical-cations may also be involved in the formation of phenyl esters from aromatic carboxylic acids and benzene in the presence of xenon difluoride, and can apparently be detected by e.s.r. spectroscopy during the reaction of iViV-dimethylaniline with perfluoropiperidine. The last reagent gives a low yield of o- and p-fluorophenol in the ratio 3.5 1 in its reaction with sodium phenoxide, and the predominance of ortho substitution is ascribed to a cage mechanism of the type shown in (3). However, towards 2,4,6-tri-t-butylphenoxyl radicals the piperidine acts as a source of fluorine atoms to give the dienone (4).  

The effects of different alkali-metal cations on the vapour-phase fluorination of benzene with the complex fluorides M+CoFi have been described. An appreciable proportion of the product from CSC0F4 at 250 °C consists of aromatic compounds, including hexafluoro- (4%) and pentafluoro-benzene(9%). The lithium compound yields mainly 3,3,6,6-tetrafluorocyclo-hexa-1,4-diene, but the sodium and rubidium salts have no particular advantages over the potassium compound described previously. The results obtained from the reaction of 1,3-bistrifluoromethylbenzene with KC0F4, 

Further examination of the products arising from the conventional electrochemical fluorination (Simons cell anhydrous HF electrolyte) of benzoyl chloride, which yields cyclo-C,Fn COF (up to 65% yield) as the major product, leads to the conclusion that the acid fluorides (5) (mainly) and (6) are also formed. Some ring-contraction also occurs during similar fluorination of benzenesulphonyl chloride or fluoride, when (7) cis and trans) (ca. 10%) is formed together with cyclo-C Fu-SOjF (up to %).  

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