Perfluorinated heteroaromatic compounds

The standard method for the synthesis of perfluorinated heteroaromatic compounds is most commonly by halogen exchange of the corresponding perchlorinated system using molten potassium fluoride in an autoclave at very high temperature (300-480°C) in the absence of solvent. In many cases, the synthesis of the perchlorinated precursors is the most difficult part of the synthetic sequence and this is usually accomplished by chlorination of the parent hydrocarbon by using a... 

The products of the electrochemical perfluorination of aromatic and heteroaromatic compounds are the corresponding perfluorinated cyclic and heterocyclic alkanes.28 and also per-fluorinated derivatives of the heteroaromatic compounds. Perfluorocyclohexane is the principal product from the electrochemical fluorination of benzene and fluorobenzene. Chloro derivatives of perfluorocyclohexane are produced from chlorobenzenes. Anisoles give fully saturated per-fluoro ethers, together with cleavage products. Extensive cleavage is observed in the fluorination of benzenethiols. Chloropyridines, fluorocarbons and sulfur hexafluoride or nitrogen trifluoride are characteristic byproducts from the above scries of reactions. 

This route is especially valuable for the transformation of electron-rich heteroaromatic compounds into their fluorinated analogues, which are not suitable for the nucleophilic exchange route. The method has been extended by addition of fluorinated olefins. The fluoroolefins add in a radical process to the 2-position of tetrahydrofuran, followed by perfluori-nation to give the perfluorinated 2-alkyl-substituted tetrahydrofurans in excellent yields [84JFC(25)523 85JFC(29)323] (Scheme 3). 

The 1,4-dinucleophilic building blocks used most are 1,2-disubstituted ethanes of the type HXCHiCHjYH, semicarbazides, thiosemicarbazides, hydroxamic acid amides, amidrazones, and 1,2-disubstituted aromatic and heteroaromatic compounds. 1,1-Dielectrophilic building blocks preferentially used are perfluorinated carboxylic acids and their derivatives, such as acid halides, anhydrides, imidoesters, nitriles, perfluoroalkyl chlorosul-... 

Perfluoroheteroaromatic compounds [25, 26], in which all the hydrogen atom substituents of the heterocyclic ring have been replaced by fluorine atoms, were first synthesized in the 1960s by reaction of potassium fluoride with appropriate perchlorinated heteroaromatic precursors [25, 26] and a range of perfluorinated heteroaromatic systems may be accessed by halogen exchange (Halex) techniques as shown in Figure 11.3. 

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