OXIDATIVE DIMERIZATION OF AROMATIC AMINES TO AZO COMPOUNDS

Copper complexes in pyridine solution have been shown to be effective catalysts for the oxidative dimerization of aromatic amines to azo compounds [169]. A thorough treatment of oxidative dimerization reactions is beyond the scope of this review. However, recent reports give evidence for copper peroxo complexes [154] in these systems and for the intermediacy of a species of the type, 

Terentev and Mogilyansky [171] first reported that the air oxidation of aniUne catalyzed by cuprous chloride in pyridine gave good yields of azobenzene. Kinoshita [172,173] found that the actual oxidant was produced by air oxidation for a cuprous chloride pyridine complex. Table 8 lists some representative reactants and yields of azo compounds formed. Reaction occurs at room temperature when air or oxygen is bubbled into the reaction mixture. 

It can be seen that yields of azo compounds produced from p-toluidine and p-anisidine are almost quantitative, whereas yields of azo compounds from o-toluidine and c -anisidine are poor. Kinoshita rationalized this difference on the basis of steric hindrance to copper amine complex formation exerted by the ortho substituents. It is of interest to note that the yield of azo compounds obtained in the free radical initiated autoxidation of aromatic amines are not significantly affected by the position of substituent groups on the aromatic ring [175,176]. 

Another interesting observation was that hydrazobenzene could be almost quantitatively converted to azobenzene under reaction conditions. These facts led Kinoshita to propose a mechanism in which a copper aniline complex is formed. Oxidation then occurs in the coordination sphere to give the coordinated radical, [CeHsNH] which dimerizes to hydrazobenzene. 

In a related system, EPR has been used to study the oxidative coupling reaction of aniline to give azobenzene catalyzed by [Cu(py)4(N03)2] [170]. Addition of aniline to the catalytic complex results in progressive reduction of Cu(II) to Cu(I) 

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