Aryl halides electroreduction

The reduction of organic halides is of practical importance for the treatment of effluents containing toxic organic halides and also for valuable synthetic applications. Direct electroreduction of alkyl and aryl halides is a kinetically slow process that requires high overpotentials. Their electrochemical activation is best achieved by use of electrochemically generated low-valent transition metal catalysts. Electrocatalytic coupling reactions of organic halides were reviewed in 1997.202... 

Recently, chloro-, bromo-, and iodoben-zenes have been subjected to electroreduction using Ni(0) complex mediators to yield biphenyl. NiCl2L2 and NiBr2L2 [L= P(Ph)3, (Ph)2PCH2CH2P(Ph)2] have been used as catalysts [259-265]. Pro-tic media such as alcohols, that is, methanol, ethanol or alcohol-water mixtures are found to be suitable solvents for achieving the electrosynthesis of biaryls from aryl halides according to a procedure that involves a catalytic process by nickel-2,2 -bipyridine complexes [266]. Electrochemical cross-coupling between... 

Scheme 63 Electroreductive coupling of aryl halides to biaryls.

The electroreductively produced ArNi(II) XL2 (162) can lead to the corresponding aryl-carboxylates (163) [284]. Electroreduction of the arylnickel complexes (162), formed by insertion of an electrogenerated [Ni(0)L2] into aryl halide (161) (X = Br, Cl) in the presence of CO2 in a THE/HMPA-LiCl04 (or BU4NBE4) system, affords the carboxylate (163) (Scheme 66) [271-273]. 

Scheme 64 Poly(l,4-phenylene) polymers by electroreduction of aryl halides.

The Pd(0)-catalyzed electroreductive coupling of aryl halides (303) is a currently relevant topic. In the electroreduction of aryl halides (307) the replacement of the halogen atom by hydrogen predominantly takes place giving (306). Difficulties are encountered, however, when aryl-aryl coupling products (305) via (304) are wanted (Scheme 116). An efficient electroreductive coupling of aryl bromides (307) (X = Br) and iodides (307) (X = I) into biaryls (310) has been shown to occur in a DMF/Et4NOTs/(Pb cathode) system in the presence of Pd(0) and/or Pd(II) catalysts (Scheme 117) [440]. 

Electroreductive carboxylation of aryl halides, /J-bromostyrene, and allyl acetates... 

Scheme 1. Electroreductive dimerisation of aryl halides catalyzed by Ni-dppe complexes...

This was done, for example, with a Pd-PPha complex used stoichiometri-cally as outlined in Eq. 7 [28, 37, 54]. The electrochemically generated Pd° complex first reacts with one aryl halide. The electroreduction of this cr-Pd-complex in the presence of the other aryl halide affords the unsymmetrical biaryl in good yield ... 

Another two-step approach was reported using Ni-bpy as catalyst. The electroreduction of an aryl halide catalyzed by a Ni-bpy complex, when carried... 

Table 4. Ni-bpy catalyzed electroreductive cross-coupling between aryl halides and a-chloroesters...

In the early 1980s it was shown that the electroreduction of aryl halides catalyzed by Ni-PPh3 [97] or Ni-dppe [98] and in the presence of COj mainly leads to the arylcarboxylate instead of the biaryl. An electroanalytical study of the Ni-dppe system has resulted in the proposal of a catalytic cycle [99,100]. In this mechanism CO2 is involved in a reaction with the aryl-nickel(I) formed by electroreduction of the cr-aryl-nickel II) as indicated in Scheme 1. 

The catalyst is not necessary either for the electrocarboxylation of aryl halides or various benzylic compounds when conducted in undivided cells and in the presence of a sacrificial anode of aluminum [105] or magnesium [8,106], Nevertheless both methods, i.e., catalysis and sacrificial anode, can be eventually associated in order to perform the electrocarboxylation of organic halides having functional groups which are not compatible with a direct electroreductive process. 

Very few transition-metal catalyzed electroreductive carbon-heteroatom bond formations have been described. The electrochemical silylation of allylic acetates was carried out in the presence of Pd-PPha [131]. The electrosynthesis of arylthioethers from thiophenol and aryl halides [132] and the coupling of bromobenzene with dichlorophenylphosphine [133] were performed with Ni-bpy as catalyst. 

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

The formation of arylzinc reagents can also be accomplished by using electrochemical methods. With a sacrificial zinc anode and in the presence of nickel 2,2-bipyridyl, polyfunctional zinc reagents of type 36 can be prepared in excellent yields (Scheme 14) . An electrochemical conversion of aryl halides to arylzinc compounds can also be achieved by a cobalt catalysis in DMF/pyridine mixture . The mechanism of this reaction has been carefully studied . This method can also be applied to heterocyclic compounds such as 2- or 3-chloropyridine and 2- or 3-bromothiophenes . Zinc can also be elec-trochemically activated and a mixture of zinc metal and small amounts of zinc formed by electroreduction of zinc halides are very reactive toward a-bromoesters and allylic or benzylic bromides . ... 

It has been reported that the electroreductive coupling between aryl halides and 2-chloro- or 2-bromopyridines in the presence of NiBr2(bpy) as the catalyst led to the cross-coupling product in better yields (55-80%) (equation 52)49. 

When the electroreduction of CoX2 is carried out in the presence of an aromatic halide (FG-C6H4X), a reaction occurs between the electrogenerated cobalt(I) species and the aryl halide (equation 56). 

After the electrochemical preparation of ZnBr2 and the introduction of both 0 2 and aryl halide, it was seen that arylzinc compounds were detected in small amounts without engaging electricity. This phenomena was interpreted as follows the zinc stemming from electroreduction of ZnBr2 can reduce cobalt(II) hahde to form low-valent cobalt Co(I), which can activate aryl bromides to form arylzinc compounds via ArConX. The mechanism would be similar to that proposed by the electrochemical approach. In this case, the reduced zinc becomes reactive and can replace electricity. From this electrochemical... 

Ni°(bpy)n complexes, with n = 1 or 2, formed by electroreduction of NiX2(bpy)n in DMF rapidly add oxidatively to aryl halides. 

Electrochemical reduction of aryl halides in the presence of olefins (94), (equation 54) leads to the formation of arylated products (95). Electroreduction of several aralkyl halides at potentials ranging from -1.24 V to -1.54 V (see) gives products which involve dimerization, cyclization, and reduction to the arylalkanes. Carbanions and/or free radicals were again postulated as intermediates79. Aryl radicals generated from the electrochemical reduction of aryl halides have been added to carbon-carbon double bonds80,81. Electrochemical reduction of aryl halides in the presence of olefins leads to the formation of arylated products78. Preparative scale electrolyses were carried out in solvents such as acetonitrile, DMF and DMSO at constant potential or in liquid ammonia at constant current. The reaction is proposed to involve an S l mechanism. 

Ni(0) obtained from electrochemical reduction of NiBr2 in THF/HMPA acts as an efficient catalyst for the electroreductive coupling of ethylene with aryl halides to give 1,1-diarylethanes 217 (equation 109). By proper control of reaction conditions, such as reduction potential, solvent and supporting electrolyte, it can be shown that substituted olefins can be prepared from aromatic halides and alkenes164. 

Nickel-2,2-bipyridine complexes are also used for the preparation of unsymmetrical biaryls such as 4-methoxy-4 -trifluoromethylbiphenyl by electroreduction of two aryl halides, one of which has electron-donating and the other electron-withdrawing groups in the aromatic ring as shown in equation 110. The reaction was carried out in N-methylpyrrolidinone at constant current in an undivided cell fitted with a sacrificial magnesium anode and excess of 2,2 -bipyridine167. 

A highly asymmetric induction in the nickel-catalyzed electroreductive couplings of aryl halides with a-chloropropionic acid derivatives was reported by Nedelec and coworkers [Eq, (65)] [495]. 

Aromatic Acids. - Aryl halides undergo electroreductive carboxylation... 

As a very rare example, electroreductive cross-coupling of two different aryl halides was achieved in the presence of a stoichiometric amount of a palladium complex (Scheme 12). 

The electroreductive coupling process can also been performed with heteroaryl halides as 2- or 3-bromothiophene or 3-bromofuran instead of aryl halides. 

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