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1.3. Anodic fluorination of the organic compound with CH-fragment containing electron-seeking substitutes.

High yield of fluoroderivatives is achieved by anodic fluorination of substituted derivatives both aromatic and heterocyclic compounds, having protons movable enough in -position to the main cycle. As a rule, most effective  Et₃N* 3HF system  and platinum  electrode  are used [89,90].

Table 7. Electrochemical oxidization of caffeine and similar compounds (MeCN, Et₃N * 3HF) [88].

If in benzene ring there are electron-donating substituents, then during anodic fluorination active methene group of substituent will be  affected mostly [75,91-94].

It is shown, that the reaction goes via  initial generation of cation-radical of  substituent atom neighboring to -CH₂- fragment  with following fluoride-ion attack of  cationoid centre of  intermediate carbcation, which is in -position to carbonyl group [92,94].

Anodic fluorination of sulfides in Et₃N*3HF medium results in formation of monofluoro- and hem-difluoroderivatives [95]. At that elimination of SPh group occurs .

Anodic oxidization of benzylalkyl ketones, carboxylic acids esters, nitriles, benzyl derivatives in the system Et₃N*3HF/MeCN at platinum anode enables to obtain monofluoroderivatives, some of which are biologically active [64,70,91,96-100].

Substituent in benzene ring influences greatly on the ratio of resulting products [91].

At the same time electrophilic substituents in -position to methene group have a light influence on the anodic fluorination process, while the increasing of voltage results in increasing of difluoro-derivative yield.

The presence of methyl group in benzene ring results in its fluorination. Thus, p-methylbenzylsulfonate during anodic fluorination in Et₃N*3HF/MeCN system forms the mixture of electrolysis products 27-30 [93].

Electrochemical oxidization of aliphatic aldehydes  in Et₃N*5HF or Py* nHF (n = 3-6) in acetonitrile or sulpholane results in formation of corresponding carboxylic acids fluoroanhydrides with high yield [101,102].

2,2-disubstituted cyclic ketones in Et₄N*5HF at anodic fluorination produce fluoroanhydrides of appropriate fluorocontaining carboxylic acids due to selective splitting of Ñ-Ñ bond between carbonyl atom of carbon and -substituted carbon atom (table 8) [103-105]. Thus, electrolysis of 2,2-dimethylcyclohexanone  in electrolyte Et₄N*5HF at 0^oC with subsequent addition of methyl alcohol produces methyl 6-fluoro-6-methylheptanoic acid ester.

Probably, the reaction path is the  following:

At the same time anodic fluorination of unsaturated carboxylic acids esters having cylcopentane and cyclohexane fragments in Et₃N*5HF system results in forming of expanded by one carbon atom cycle.  2-2-Difluorocycloalkanecarboxylic acids esters with high selectivity and good yield are produced (table. 9) [106,107].

Table 8. Synthesis of carboxylic acids esters using electrosynthesis (electrolyte Et₃N*5HF, 0^oC) of cyclic ketones [103].

Table 9. Electrochemical oxidization of unsaturated cyclic esters [103].

Anodic fluorination of different N-substituted lactams using electrolyte Et₃N*3HF results in formation of corresponding monofluorocontaining products, in which fluorine atom is in -position to nitrogen atom [105].

During electrolysis of ,-unsaturated esters the hem-,-difluoroderivatives of esters were formed in electrolyte due to rearrangement  of alkyl group from to -position [107]. If alkenyl group is in this position, for example in case of carboxylic acids esters of dienes , then the fluorination with formation of vicinal difluoroderivatives will occur (table 10) [102].

Table 10. Anodic fluorination of dienes esters  [102].

In case of compounds, containing two carb-ethoxy groups at multiple bonds of diene and electron-donating substituents (for example, methyl groups), anodic fluorination produces mixture of  hem-difluoro- and vicinal difluoroderivatives (ratio 3:1) [102]. This shows the realization of two fluorination processes pathways.

Olah's reagent (HF/Py) is most widely used as electrolyte, and methylene chloride is used as solvent [49,50]. Another path is connected with use of  Et₃N*3HF as electrolyte , and acetonitrile as solvent  [31]. Carbonyl compounds, containing enol form of  -CH₂- fragment in -position are fluorinated with high selectivity into -position, producing as a rule monofluoroderivatives. It is important, that phenyl substituent should be at carbonyl group or this fragment should be in cyclic system (table 11) [44]. This circumstance results in the fact that anodic fluorination is widely used for selective introduction of fluorine atom into compounds, having electron-seeking groups. 

To be continued

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