Mercury cathodes, arylation

Dialkyl sulphones are not reducible at a mercury cathode. Aryl alkyl and diaryl sulphones are however reduced with cleavage of a carbon-sulphur bond. Polaro-graphic half-wave potentials for this process are given in Table 5.6. One-electron addition in aprotic media to phenyl methyl sulphone [66] and to diphenyl sulphone [67] leads in both cases to a delocalised radical-anion in which the sulphone grorqj can be described as contributing a vacant symmetrical dx-orbital to the conjugated system. Phenyl methyl sulphone radical-anion is prepared and characterised in liq-... 

Several studies confirm an ortho effect leading to a dominating aryl sulphur bond cleavage. For example, the introduction of a bulky ortho substituent17 will provide the formation of the aliphatic sulphinic acid. A series of cyclic sulphones was studied18,19 at the mercury cathode and the results (see Table 2) appear to be fully in agreement with those expected when considering the preliminary works presented above. 

The principles outlined above are, of course, important in electro-synthetic reactions. The pH of the electrolysis medium, however, also affects the occurrence and rate of proton transfers which follow the primary electron transfer and hence determine the stability of electrode intermediates to chemical reactions of further oxidation or reduction. These factors are well illustrated by the reduction at a mercury cathode of aryl alkyl ketones (Zuman et al., 1968). In acidic solution the ketone is protonated and reduces readily to a radical which may be reduced further only at more negative potentials. 

The subsequent steps involve, among several proposed reactions, arylation of the mercury cathode, and release of lead, but most importantly they indicate [ArPbH] as evidence for polymer formation by the proposed route ... 

Some aryl iodides are known to generate the diaryImercury at a mercury cathode. In the case of 4-iodoaniso e, reduction at more negative potentials in dimeth-ylfonnamide leads to the formation of less di(4-methoxyphenyl)mercury. At glassy carbon, anisole is the only reduction product. 4-Bromoanisole gives only anisole at either mercury or carbon [143]. Mercur> has been used as cathode material for many preparative experiments with aryl halides but glassy carbon and also stainless steel are very satisfactory alternatives. 

Electrochemical reduction, though a little-used method, defluorinates aryl fluorides to the corresponding hydrogeno derivatives. 1,3-Difluorobenz.ene is reduced at a mercury cathode to give a mixture of fluorobenzene and benzene,54 while the reduction of perfluorobenzene at an... 

Aryldiazonium salts are reducible at a mercury cathode. Polarograms in acid solution show a pH-independent one-electron wave followed by a larger pH-dependent wave [194,195]. Reduction on the plateau of the first wave [196] showed n = 1 and only aryl-mercury salts were isolated, indicating the initial formation of a radical. 

Thiol esters of aromatic acids, ArC(0)SR, are cathodically reduced, with fission of the S-CO bond generating the arylcarbonyl radical and thiolate as in Eq. (25) [101]. The thiol may be obtained in good yield [102]—for example, L-cysteine in 83% yield from S-benzoyl-L-cysteine at a mercury cathode in DMF-water-tetramethylammonium chloride. The electroreduction of aryl thioesters [containing -C(0)SR, -C(S)OR, and -C(S)SR functions] has been studied, and some interesting products have been formed [103-105], as in Eqs. (26) through (29). Trithiocarbonates are reduced to the tetraalkyltetrathioethylenes [106]. 

Many sulfones, e.g., diaryl, alkyl aryl, and alkyl benzyl sulfones, but not dialkyl sulfones, are cleaved to sulfinates and compounds RH with surprising ease and under mild conditions and in good yield when treated with tetra-alkylammonium generated from the tetramethylammonium ion on a mercury cathode.648... 

Organomercury compounds can also be obtained in preparatively useful quantitites by the action of radicals on metallic mercury. For example, aryl-mercuric compounds are formed on decomposition of arenediazonium salts in the presence of metallic mercury 28 and when 3-iodopropionitrile is reduced electrolytically at a mercury cathode, the radicals produced react with the cathode material forming bis-(2-cyanoethyl)mercury.202 Organomercury compounds are also obtained analogously on reduction of ketones at a mercury cathode.203... 

Some examples of electrosyntheses based on the reduction of monohalobenzenes are now cited. Reduction of iodobenzene and bromobenzene at mercury in carbon dioxide-saturated DMF containing TEAP affords benzoic acid in up to 40% yield [178]. Bromobenzene, 3-methoxybromobenzene, 4-bromofluorobenzene, 4-(trifluoromethyl)-chlorobenzene, and 1,2,4-trichlorobenzene can be converted into aldehydes at a cadmium-coated stainless-steel cathode in DMF containing TBABr [179]. In the presence of trimethylchlorosilane, silylation products arise from the reduction of various aryl halides at platinum in DMF containing TEAOTs [100]. Reduction of p-iodoanisole at... 

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