Palladium catalysts biaryl coupling reactions

Air-stable biaryl monophosphine ligands developed by Buchwald and co-workers have played a central role in the advancement of palladium-catalyzed cross-coupling reactions, including BHA chemistry the utility of appropriately configured ancillary ligands of this type in the selective monoarylation of ammonia has been examined. Following on preliminary experimentation that established the capability of the Pd2(dba)j/L3 catalyst system (dba=dibenzylideneacetone) to promote the monoarylation of ammonia at elevated temperature Tsvelikhovsky... 

The development of C-H activation for cross-coupling reactions has been advanced rapidly. Extensive efforts have been devoted to the mechanistic studies in this field to shed light on the mechanism. The use of oxygen or air as sole oxidant has been successfully realized in a palladium-catalyzed cross-coupling reaction involving C-H activation. We have the right to believe that palladium-catalyzed double C-H activation would be a good alternative for the synthesis of biaryls. With further improvements in catalyst turnover, selectivity, and oxidant, the method should ultimately be able to find industrial applications. 

The palladium-catalysed Suzuki coupling reaction of aryl halides with arylboronic acids has proved to be a general and convenient synthetic tool employed in organic chemistry to prepare biaryl compounds.[1] The discovery and development of active and efficient palladium-catalyst systems have been the focus of great interest recently. New catalytic systems based on paUadium-oxazolines, such as 2-aryl oxazolines and 2,2 -(l,3-phenylene)bisoxazoline (Figure 4.1) have been developed for the coupling reaction. These catalytic systems have the potential to overcome... 

There has been a review of palladium-catalysed carbonylative coupling reactions of aryl halides with carbon nucleophiles in the presence of carbon monoxide. It has been shown that rhodium is an efficient catalyst for the homocoupling reaction of arylzinc compounds in the presence of 1 atm of carbon monoxide to give diaryl ketones. Under similar conditions, palladium and nickel catalysts yield biaryls. The beneficial catalysis by rhodium is likely to derive from the ease of migration of the aryl ligand to carbon monoxide in the rhodium(III) intermediate. A rhodium catalyst has also been used in the formation of indole-3-carboxylates by reaction of indoles with alcohols in the presence of carbon monoxide. The catalytic cycle. Scheme 5, is likely to involve metallation of the indole at the 3-position, followed... 

Unactivated aryl iodides undergo the conversion Arl — ArCHj when treated with tris(diethylamino)sulfonium difluorotrimethylsilicate and a palladium catalyst.131 A number of methods, all catalyzed by palladium complexes, have been used to prepare unsymmetrical biaryls (see also 3-16). In these methods, aryl bromides or iodides are coupled with aryl Grignard reagents,152 with arylboronic acids ArB(OH)2,153 with aryltin compounds Ar-SnR3,154 and with arylmercury compounds.155 Unsymmetrical binaphthyls were synthesized by photochemically stimulated reaction of naphthyl iodides with naphthoxide ions in an SrnI reaction.156 Grignard reagents also couple with aryl halides without a palladium catalyst, by the benzyne mechanism.157 OS VI, 916 65, 108 66, 67. 

A similar reaction was reported by Kabalka et al. where ligandless and solvent-free Suzuki couplings were performed with potassium fluoride on alumina. This reaction is very interesting as the catalyst used was palladium powder, the least expensive form of palladium available32. The authors demonstrated the simplicity of the procedure by efficient isolation of the biaryl products via a simple filtration. This could be done as the palladium catalyst remains adsorbed on the alumina surface. A small amount of water in the matrix was beneficial for the outcome of the reactions. Recycling of the catalyst was possible by adding fresh potassium fluoride to the palladium/alumina surface and the catalytic system remained effective at least through six reaction cycles (Scheme 2.6). 

Palladium catalysts have been found which are effective in the Suzuki coupling reaction of arylboronic acids with aryl chlorides carrying electron-withdrawing groups.73 Biaryls may also be synthesized by cross-coupling of arylboronic acids with arenediazonium salts.74,75 There has been a report of the polymer-bound palladium-catalysed Suzuki coupling of aryl triflates with organoboron compounds.76 Arylbor-onates may themselves be synthesized by the palladium-catalysed reactions of... 

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