Cathodic dehalogenation

Scheme 94 Cathodic dehalogenation with immobilized vitamin Bn-...

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. 

Cathodic dehalogenation has been applied for the reduction of dichloroacetic acid to monochloroacetic acid. Thus, in the synthesis of chloroacetic acid, the always formed dichloroacetic acid can be recycled to give the desired product and chlorine, which is used again for the chlorination [21] ... 

The cathodic dehalogenation can be carried out by an indirect route for example, by reducing aromatic azo compounds 436 439). 

Golinske, D. and Voss, J. (2005) Electroreduction of organic compounds, 35. Quantum chemical calculations of reaction pathways for the cathodic dehalogenation of chloro-dibenzo-furans and oligo-chlorobenzenes. Zeit. Natur. B Chem. Sci. 60, 780-786. 

The //-doping of carbon during cathodic dehalogenation is a common side-reaction (see Tables 4.1, 4.2 and the text below). Kijima et al. [39,42,43] reduced diiodoacetylene to carbon at a platinum electrode in dimethylformamide media ... 

Advantages of the cathodic dehalogenation are (i) treatment at ambient temperature, (ii) no additional chemicals, and (iii) selective removal of chlorine while the organic skeleton remains to be digested by the biological route. This is still the cheapest way. 

Due to cathodic dehalogenation as well as dehydrohalogenation of /-BuCl and subsequent C-C bond coupling, 2,2,3,3-tetramethylbutane, 2,2,4-trimethylpentane and homologues were detected as the only reaction products (Eq. 2). 

Another example of the use of ion-exchange membranes in electrosynthesis is cathodic dehalogenation. Dichloroacetic acid has been obtained with almost quantitative current yields from trichloroacetic acid by elimination of chlorine at controlled potential [21]. The overall reaction which takes place in the electrolyzer can be written as follows ... 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

Coordinatively unsaturated complexes and those giving easily such species by ligand dissociation favor pathways related to that described in Eqs. (10) and (13). Coordinatively saturated complexes reduce halocarbons via outer-sphere ET [193, 194]. In cases of electrochemical dehalogenations, the species formed by one-electron reduction of the mediators on the cathode often react in this way [156, 157, 198], For example (Eq. (14)) [157, 166] ... 

Cleavage Gem-dihalides and monohalides have been dehalogenated to chiral monohalides in the presence of alkaloids [397, 398]. l,l-Diphenyl-2-bromo-2-carboxyl (bromo or methyl carboxylate) cyclopropanes are cathodically debromi-nated in the presence of alkaloid cations with enantioselectivities up to 45% ee. A mechanism is proposed whereby the alkaloid is adsorbed at the Hg cathode, which protonates face selectively the carbanion generated by 2e reduction from the bromide [399]. 

Dehalogenations, including dehalodefluorinations (e. g., formation of l)159 or didefluorinations to give fluoroarenes,160 can be accomplished electrochemically on a cathode. The method is very sensitive to the electrolyte composition. 

The cathodic substitution of halides has been applied in the synthesis of antiinflammatory agents. Arylpropionic acids are formed by reductive dehalogenation of the corresponding benzylic halides in the presence of carbon dioxide as electrophile [22] ... 

By cathodic 1,6-dehalogenation p-xylylenes were prepared. When OLfxpifil,d-- hexachloro-p-xylene was electrolyzed at low temperatures the tetrachloro-p-xy-lene could be isolated 3231. Cpe of a, a+dibromo-p-xylene at the plateau of the first or second polarographic wave yields 5-10% [2.2] -paracyclophane and 90% poly-p-xylylene as products from the reactive unsubstituted p-xylylene (Eq. 

The electroreduction of trichloroethylene (0.4 g L 1) on Cu in 0.05 M NaOH was found to be more efficient than on Ag or Cd cathodes [4], with the current efficiency increasing when the applied current density decreased. At a current density of 4 mA cm-2, the current efficiencies for the dehalogenation of monochloroacetic acid, dichloroacetic acid, chloroform, and trichloroethylene were 2%, 10%, 87%, and 29%, respectively. 5-Chlorosalicylic acid could not be dechlorinated on Cu. Nagaoka et al. [17]... 

Zhang and Rusling [66] employed a stable, conductive, bicontinuous microemulsion of surfactant/oil/water as a medium for catalytic dechlorination of PCBs at about 1 mA cm-2 on Pb cathodes. The major products were biphenyl and its reduced alkylbenzene derivatives, which are much less toxic than PCBs. Zinc phthalocyanine provided better catalysis than nickel phthalocyanine tetrasulfonate. The current efficiency was about 20% for 4,4 -DCB and about 40% for the most heavily chlorinated PCB mixture. A nearly complete dechlorination of 100 mg of Aroclor 1260 with 60% Cl was achieved in 18 hr. Electrochemical dehalogenation was thus shown to be feasible in water-based surfactant media, providing a lower-cost, safer alternative to toxic organic solvents. 

Finally, the indirect dehalogenation, via electrolytic production of atomic hydrogen, represents a possible alternative, especially when operating in protonated solvents, under background current conditions, on cathodes activated with noble metal particles (e.g. Pd, Ru, Rh, Ir, Au) (Cheng et al. 1997 Tsyganok et al. 1998 Tsyganok and Otsuka 1999 Iwakura et al. 2004). 

Polyhaloacetic acids and their partially hydrodehalogenated products represent a second important family of herbicide-/pesticide-derived substrates. In their review on the environmental applications of industrial electrochemistry, Juttner and co-authors (Juttner et al. 2000) documented the electroreductive dechlorination of dichloroacetic acid (a by-product of monochloroacetic acid), a way to recover the valuable compound and avoid wastes. The electrochemical reduction of polychloro- and polybromo-derivatives was performed by Korshin and Jensen (2001) on Cu and Au cathodes. Complete dehalogenation was obtained for all substrates, but for monochloroacetic acid. To overcome the intrinsic poor reactivity of the monochloro-derivative the photoelectrochemical properties of a p-doped SiC electrode were investigated (Schnabel et al. 2001) however, the dehalogenation stopped at monochloroacetic acid. 

SET promoted dehalogenation is occasionally of interest. Treatment of 2-chloro-, 2-bromo- or 2-iodopyridine with sodium in liquid ammonia gives directly the EPR-detectable radical anion of the parent molecule, dehalogenation being rapid on the time-scale of the experiment [141]. The same is true for a variety of dichloro- and dibromopyridines and pyrimidines, but with 2-fluoropyridine the radical anion is more persistent and only the spectrum of the starting material is registered. For the cathodic reduction of pentachloropyridine, see Section 6.4.6. 

---