Electrofluorination

The electrofluorinated organics of principal industrial importance are perfluorooctanoic acid [335-67-1/, CE2(CE2)gCOOH, and perfluorooctanesulfonic acid [1763-23-1/, CE2(CE2)yS02H. Typical conditions for preparation of the latter compound involve batch electrolysis of a 10 wt... 

The final route to fluorine compounds is electrofluorination (anodic fluorination) usually in anhydrous or aqueous HF. The preparation of NF tFl3 j (x = 1, 2, 3) has already been described (p. 818). Likewise a reliable route to OF2 is the electrolysis of 80% FIF in the presence of dissolved MF (p. 638). Perchloryl fluoride has been made by electrolysing NaC104 in FIF but a simpler route (p. 879) is the direct reaction of a perchlorate with fluorosulfuric acid ... 

Electrofluorination of aliphatic carboxylic and sulfonic acid chlorides or fluorides to perfluorinated products [31]. 

Electrofabrication, 9 833 Electrofluorinated organics, 9678-679 Electrofluorination system, improved,... 

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. 

The electrofluorination of acetophenone and benzophenone takes place in anhydrous HF and in the presence of solvents such as chloroform and acetonitrile [38]. The fluorination of the aromatic rings occurred to various extent. Further uses of anhydrous hydrogen fluoride as a liquid environment for electrofluorination processes have been reported, for example, by Matalin etal. [39]. In particular, systems with low conductivity in liquid hydrogen fluoride and nonselective processes have been studied and optimized. The fluorination of benzene and halobenzenes in the presence of Et4NF—(HF) in an undivided cell has been studied by Horio et al. [40] Cathodic dehalogenation is observed to accompany the anodic fluorination process. 

The three principal electrochemical methods are described by which fluorine can be directly introduced into organic compounds, namely electrolysis in molten salts or fluoride ion solutions, electrolysis in molten potassium fluoride/hydrogen fluoride melts at porous anodes, and electrolysis in anhydrous hydrogen fluoride at nickel anodes. Using examples from the past decade, it is aimed to demonstrate that electrofluorination in its various forms has proved to be an increasingly versatile tool in the repertoire of the synthetic chemist. Each method is described in terms of its essential characteristics, reaction parameters, synthetic utility, advantages and disadvantages, patent protection, and potential for commercial exploitation. The different mechanisms proposed to explain each process are critically reviewed. 

Electrofluorination of carbon-carbon double bonds using the versatile Et3N- 3HF/CH3CN system [4] produces mixtures of cis-/trans-isomeric vicinal difluorides, often together, however, with products resulting from solvent incorporation. 

In the absence of an organic solvent, electrofluorination in neat Et3N 3HF of alkenes like styrene, stilbene, 2,3-dimethylbut-2-ene, or 2-methyl-2-butene yield the corresponding vicinal difluorides in 22 - 55 % yield. Butadiene produces a 1 2 mixture of 1,2- and 1,4- difluorides [5]. 

Further problems can exist in the electrofluorination of aromatics arising from polymer formation on the anode surface, causing increased cell impedance and low current efficiencies. 

Thus, electrofluorination of benzene in neat Et3N-3HF yields a mixture of fluorobenzene, p-difluorobenzene, trifluorocyclohexadiene, and tetrafluoro-cyclohexadiene [13]. 

In further studies [16], using a group of new electrolytes, R4NF mHF (where R=CH3, C2H5, n-C3H7, and m >3.5), said to have beneficial properties in terms of viscosity, electrolytic conductivity, and electrochemical stability, the same workers have published a series of papers in which they have studied the electrofluorination of benzene, fluorobenzene, and 1,4-difluorobenzene, (Part I) [16], at high current densities, and high current efficiencies without any film formation at the anode. 

In parallel studies, this group has also studied the electrofluorination of bro-mobenzene [21] under similar conditions. 

Meurs et al. have also studied the electrofluorination of phenol [13] to produce 4,4-difluorocyclohexadienone in 20% yield this was then hydrogenated to give 4-fluorophenol (90%). 

Meurs et al. have shown the electrofluorination of ethylbenzene in neat Et3N 3HF gives 1-fluoro-l-phenylethane in 42% yield [13]. 

Selective electrofluorinations of activated -positions in thioethers, sulphides, selenides and heterocycles have been reported by a number of workers [34 - 38]. 

Sono and co-workers have electrofluorinated a number of complex nitrogen-containing compounds in the presence of Et3N 3HF/CH3CN [43]. Caffeine afforded 8-fluorocaffeine as the sole product in 40.3% yield. 

The preparation of new fluorine containing pyridazinones, prepared by electrofluorination in various base. HF systems has been patented by Rikagaku Kenkyusho as a route to herbicidal, insecticidal or bactericidal compounds... 

Laurent et al. have shown that 2,2-diphenyl-1,3-dithiane is electrofluorinated in Et3N 3HF to the gem difluoride in good yield [47]. 

Likewise, in a later publication, the same workers found that electrofluorination of a series of diarylketone hydrazones in Et3N 3HF/CH2C12 electrolyte produced almost solely the monofluoro-derivative [49]. 

The process is characterised by the electrofluorination of volatile organic substrates within the matrix of pores of a carbon anode immersed in molten KF 2HF as electrolyte (as in a mid-temperature fluorine generator cell), and depends on the phenomenon that the anodically charged porous carbon is not wetted by the electrolyte. The fluorination probably takes place at the three phase interface of organic vapour, solid carbon, and liquid electrolyte in close proximity to, or at the sites where fluorine is being evolved. 

Thus 3 M Co. have patented a process for the production of 2-hydrofluorina-ted ethers by the electrofluorination of perfluoroisobutene - alcohol adducts. The products range, depending upon operating conditions, from mixtures of the... 

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