Biaryl halides, electroreduction

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 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]. 

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 ... 

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. 

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... 

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. 

Yet another procedure for the preparation of biaryls is Pd-catalyzed electroreductive homocoupling of aryl and heteroaryl halides (Scheme 7 and Table 4 ). 

Many electrochemical approaches to biaryls, e.g., electroreductive dimerization of aryl halides [12, 13] and electrooxidative dimerization of arylboronic acids [14], have been investigated. Substituted or non-substituted benzoic acids are electrochemically reduced to the corresponding alcohols or aldehydes. For instance, electroreduction of /M-phenoxybenzoic add proceeds smoothly to afford m-phenoxybenzaldehyde, an intermediate for insecticide [2,15]. 

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