Biaryl nickel

Biaryls Nickel(O). Thallium(III) trifluoroac-etate-Palladium(II) acetate. 

Biaryls. Nickel complexes of the type NiCl2L2 catalyze the cross-coupling of sterically hindered aryl Grignard reagents with an aryl bromide to give biaryls in high yield as shown in the example. Thus 2-phenylmesitylene can be... 

In 1956 it was found that when pyridine is refluxed with a modified Raney-nickel catalyst, 2,2 -bipyridine (1) is formed in satisfactory yield. The isomeric bipyridines could not be detected, and the product was readily purified. Similar heterocyclic biaryls have been formed in the same way from substituted pyridines and from some related compounds, the yield being dependent on the nature of the compound. The reaction has become the method of choice for the preparation of 2,2 -bipyridine, and it is now used on an industrial scale. Bipyridyls are of particular importance as chelating agents. 

Reaction of -picoline with a nickel-alumina catalyst has been reported to give a mixture of four isomeric dimethylbipyridines, one of which has been identified at 6,6 -dimethyl-2,2 -bipyridine. With palladium-on-carbon, 2,4-lutidine was found to be more reactive than pyridine,and the isolated biaryl has been assigned the structure (2). However, some confusion arises from the statement that this... 

Several substituted pyridines have been examined using the degassed Raney nickel, and the results are summarized in Table I. As all the biaryls obtained formed colored chelates with either ferrous or cuprous ions, they must be derivatives of 2,2 -bipyridine. Structural ambiguities cannot arise with 2,2 -bipyridines derived from 2- and 4-substituted pyridines but 3-substituted pyridines could conceivably give three isomeric 2,2 -bipyridines (e.g., 3, 4, 5). In fact, however, each 3-substituted pyridine so far examined has given only one 2,2 -bipyridine. 

Reaction of -picoline over degassed Raney nickel was found to give 5,5 -dimethyl-2,2 -bipyridine (5), the structure of which was established by its synthesis from 2-bromo-5-methylpyridine. Oxidation of this dimethyl-2,2 -bipyridine, and similar oxidation of the diethyl-2,2 -bipyridine derived from 3-ethylpyridinc, gave the corresponding dicarboxylic acid and the same acid was produced by the action of degassed Raney nickel on sodium nicotinate (in water) or on ethyl nicotinate. These transformations established the 5,5 -substitution pattern for three 2,2 -bipyridines derived from 3-substituted pyridines but such evidence is not available for the biaryls... 

Table II shows that, at least for the reactions with quinoline and with 4-methylquinoline (lepidine), nickel-alumina and degassed Raney nickel catalysts are of similar efficiency but better yields have been obtained with degassed Raney nickel, and only this catalyst produces the biaryl from 7-methyIquinoIine.

Several products other than 2,2 -biaryls have been isolated following reaction of pyridines with metal catalysts. From the reaction of a-picoline with nickel-alumina, Willink and Wibaut isolated three dimethylbipyridines in addition to the 6,6 -dimethyl-2,2 -bipyridine but their structures have not been elucidated. From the reaction of quinaldine with palladium-on-carbon, Rapoport and his co-workers " obtained a by-product which they regarded as l,2-di(2-quinolyl)-ethane. From the reactions of pyridines and quinolines with degassed Raney nickel several different types of by-product have been identified. The structures and modes of formation of these compounds are of interest as they lead to a better insight into the processes occurring when pyridines interact with metal catalysts. 

In a Kumada-Corriu reaction, an aryl halide is oxidatively coupled with a homogeneous nickel(ll)-phosphine catalyst [2], This species reacts with a Grignard reagent to give biaryl or alkylaryl compounds. Later, palladium-phosphine complexes were also successfully applied. By this means, stereospecific transformations were achieved. 

Although the copper mediated Ullmann reaction is a well known method for biaryl synthesis, drastic conditions in the range of 150-280 °C are required. Zerovalent nickel complexes such as bis(l,5-cyclooctadiene)nickel or tetrakis(triphenylphosphine)nickel have been shown to be acceptable coupling reagents under mild conditions however, the complexes are unstable and not easy to prepare. The method using activated metallic nickel eliminates most of these problems and provides an attractive alternative for carrying out aryl coupling reactions(36,38). 

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... 

Biaryl synthesis from aryl halides is a more interesting reaction due to the value of these molecules and their difficult access by chemical methods. The first electrosyntheses were simultaneously done in 1979-80 by three groups [21-23] who used NiCljPPha (1-20%) as catalyst precursor in the presence of excess PPhs. Later, several groups investigated the use of bidentate phosphines like dppe associated with nickel in the synthesis of various biaryls, and notably 2,2 -bipyridine and of 2,2 -biquinoline from respectively 2-chloropyridine and 2-chloroquinoline [24], More recently new nickel complexes with l,2-bis(di-2-alkyl-phosphino)benzene have been studied from both fundamental and synthetic points of view [25]. They have been applied to the coupling of aryl halides. 

The Pd-PPh3 system (Scheme 3) is characterized by a two-electron reduction step of the cr-aryl-palladium intermediate [37], as also proposed previously for aryl-nickel complexes ligated to PPha [23, 38]. The formation of the biaryl proceeds by reductive elimination from the diarylpalladium and regeneration of Pd°. 

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. 

Diaryltellurium dichlorides are detellurated by heating with Raney nickel in appropriate solvents, giving the corresponding biaryls. ... 

In the absence of the olefin, the corresponding biaryls are formed in moderate yields (like the Raney nickel-promoted detellurations (see Section 4.6.1.1)). 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

The ferrocenylphosphine-nickel catalysts are also applied to asymmetric synthesis of axially chiral biaryl compounds through the cross-coupling reaction. Although initial attempts to this... 

On the other hand, treatment of phenoxatellurine with degassed Raney nickel gives rise to benzofuran [75CS(A) 116]. In the same way biaryls were formed from diaryl tellurides and diaryltellurium dichlorides (72T3323). 

Atropisomeric biaryl bisphosphines, in hydrogenation, 10, 2 Atropisomerism, dynamic NMR studies, 1, 421 ATRP, see Atom transfer radical polymerization type reactions rj3-l-Azaallyl complexes, with nickel, 8, 158 6-Aza[3.2.1]bicyclooct-3-ene, via ring-closing metathesis,... 

R = alkenyl, aryl, allyl, benzyl, propargyl R = alkenyl, aryl, alkynyl, alkyl, benzyl, allyl The Negishi Coupling was the first reaction that allowed the preparation of unsymmetrical biaryls in good yields. The versatile nickel- or palladium-catalyzed coupling of organozinc compounds... 

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