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Copper Catalysts [Cross-coupling Reaction using Transition Metal Catalysts]

 Copper-mediated coupling reactions have been known as a traditional reaction. For instance, homocouplings of aryl halides and alkynes are called the Ullmann reaction and the Glaser coupling respectively, and both of these were discovered over a hundred years ago. Around the same time, carbon-heteroatom bond forming reactions such as the Ullmann ether synthesis and the Goldberg amination were also reported. In early studies of these classical copper-mediated coupling reactions, there were a number of disadvantages needing improvement such as use of over stoichiometric amounts of copper reagents, harsh reaction conditions, and low substrate generality. However, copper-mediated coupling reactions can be carried out at a lower cost compared with using palladium catalysts. So, various modified methods have been studied and more practical reactions have been developed.
 Recent advances in the Ullmann-type reactions using aryl halides have been achieved under milder reaction conditions and reducing the amount of copper catalyst by the choice of suitable solvents, bases, and ligands. Also, investigation of effective ligands for these reactions has been developed and found that the diamine and dicarbonyl compounds effectively play as a ligand in preventing side reactions and deactivating a monovalent copper catalyst.
 For instance, CuTc (copper(I) 2-thiophenecarboxylate) catalyzed the Ullmann coupling and can proceed at room temperature without having to apply heat. CuTc also acts as a co-catalyst of palladium catalyzed reactions. In a case of the Liebeskind-Srogl cross coupling reaction, CuTc assisted activation of thioesters plays an important role in promoting the coupling reaction.
 Furthermore, the coupling reaction of aryl boronic acids with amines, phenols, and thiols promoted by a divalent copper and an oxygen from the air as a reoxidant, is known as the Chan-Lam-Evans coupling. This is a halogen-free coupling reaction and the stirring efficiency is important for the reaction to proceed due to reoxidizing the copper catalyst by an oxygen in the air. The rate of this reaction is generally slow and requires several days to complete the reaction, while it is carried out at room temperature.

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