Selective anodic fluorination compounds

Selective anodic fluorination of aldehyde and cyclic ketones can be successfully carried out in Et3N-5HF to give the corresponding acyl fluorides and fluoroacyl fluorides, respectively, in good yields, as shown in Eqs. (10) and (11) [41]. In these reactions, fluoride salts such as Et3N-3HF lead to lower yields, reflecting the fact that this salt is discharged prior to oxidation of the carbonyl compounds. 

Fuchigami and coworkers have developed conditions for highly selective anodic fluorination reactions of various heterocyclic compounds including sulfur-containing heterocycles, as discussed later [3,6]. 

Anodic oxidation of organotellurium compounds in the presence of fluoride ion results in difluorination at the tellurium atom selectively, in excellent yields and with high current efficiencies (Eq. 100). This is the first example of anodic fluorination of tellurides. Even a trifluoroethyl telluride does not give an a-fluorotelluride but rather reacts by way of Te-F bond formation. 

Phthalides have an extremely high oxidative potential (2.86 V) (03GC512). Selective fluorination also takes place in the case of carbonyl-containing heterocyclic compounds. Thus mono- and difluoro derivatives of 4-(metho-xyphenyl)acetone and ethyl 4-(methoxyphenyl)acetate (87TL2359, 98JFC(87)215) and fluoroindanes (90TL3137) are produced by anodic fluorination of hydrocarbon substrates in the... 

Fuchigami T, Konno A, Nakagawa K, Shimojo M (1994) Electrolytic partial fluorination of organic compounds. 12. Selective anodic monofluorination of fluoroalkyl and alkyl sulfides. J Org Chem... 

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

In a recent review, Tao etal. [34] describe the partial fluorination and the perfluorination of organics with particular emphasis on medically important compounds and pharmaceuticals. The selective electrofluorination (SEF) of olefins and active methylene groups is reviewed by Noel et al. [35] In the case of heterocycles, nuclear fluorination is known to be the predominant process. However, in aromatic compounds, nuclear substitution as well as addition proceeds simultaneously, leading to the formation of a mixture of products. The influence of solvents, supporting electrolytes, and adsorption on product yield and selectivity is summarized and evaluated. Dimethoxyethane is found to be a superior solvent for SEF processes. Redox mediators have been employed to minimize anode passivation and to achieve better current efficiencies. 

In contrast, Rozhkov and coworkers found that Et3N HF salt and Et4NF-3HF were highly effective for conducting selective fluorination, particularly for nuclear fluorination of aromatic compounds (see Sec. II) [13]. This seems to be the first major breakthrough in anodic partial fluorination. 

Anodic oxidation of organic compounds containing group 15 elements in the presence of fluoride ions provides the corresponding fluorinated products [Eq. (22)] [56-61]. Fluorination occurs at the heteroatoms selectively. 

Anodic monofluorination of the side chain of various heterocyclic compounds has been systematically studied [82-84]. The active methylenethio group attached to heterocycles is selectively fluorinated to give the corresponding a-fluorinated products in moderate to good yields [Eqs. (36) and (37)] [82-87]. In these reactions, the choice of solvent and supporting fluoride salt is of great importance in order to achieve efficient fluorination reaction [84-87]. 

Aromatic compounds such as benzene, substituted benzenes, and naphthalene are selectively fluorinated by constant potential anodic oxidation in Et4N-3HF/MeCN [5, 6] (Scheme 2). 

Another pioneering electrosynthesis was the soft fluorination developed by I.N. Rozhkov, I.L. Knunyants, etc. [64, 65]. Unlike the known hard fluorination in liquid HF [66], which leads to perfluorinated compounds, often accompanied by side reactions, the soft fluorination is a selective electrosynthesis. The process is performed on a Pt anode in MeCN at the oxidation potential of the substrate RH. The -eajv-e-p mechanism is suggested for this reaction [67-69] (Figs. 9.4 and 9.5) ... 

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