Some reactions catalysed by copper and its derivatives

The reductive coupling of aryl halides by copper (TJllman conditions) may well involve the formation of intermediate ArCu species [120]. 

It is thought that the decomposition of aryldiazonium halides by copper salts, which gives aromatic halides (Sandmeyer reaction), proceeds via a free radical mechanism and that Cu—Ar intermediates arc probably not important. It is however proposed that copper-diazo intomediates such as Ar—N=N—Cu are formed [121]. These may be compared with the aryl-diazomolybdenum and tungsten complexes jr-C5H5M(CO)2N2Ar [122]. 

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