Diazonium salts, coupling carbonylation

Since A,A -disubstituted hydrazines are readily available from a variety of sources (see Volume I, Chapter 14), their dehydrogenation constitutes a widely applicable route to both aliphatic and aromatic azo compounds. Such oxidative procedures are of particular value in the aliphatic series because so many of the procedures applicable to aromatic compounds, such as the coupling with diazonium salts, have no counterpart. The oxidation reactions permit the formation not only of azoalkanes, but also of a host of azo compounds containing other functional groups, e.g., a-carbonyl azo compounds [83], a-nitrile azo compounds [84], azo derivatives of phosphoric acid [85], phenyl-phosphoric acid derivatives [86],... 

The preparation of unsymmetrical biaryls,6 the Sandmeyer reaction,6 the replacement of an aromatic diazonium group by hydrogen,7 and the reaction of diazonium salts with ,0-unsaturated carbonyl compounds 8 appear to proceed by free radical mechanisms and will not be discussed. Diazonium coupling reactions, however, have the characteristics of ionic processes. 

Scheme 4.10 Pd-catalyzed carbonylative coupling of aryl diazonium salts...

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