Example 2 The target compound N,N'-di-p-tolylbenzidine (structural formula (2)-p) was synthesized according to the following reaction scheme. The steps were as follows:(c1) 4,4'-diiodo-1,1'-biphenyl (35 g, 86 mmol), p-toluidine (92 g, 86 mmol), copper powder (2.7 g, 43 mmol) and potassium carbonate (12 g, 86 mmol) were added to a reactor, and the reaction was stirred for 24 hours at 170 °C. After completion of the reaction, tetrahydrofuran (400 ml) was added to the reaction mixture and the insoluble material was removed by filtration. The filtrate was concentrated under reduced pressure and the residue obtained was washed sequentially with ethyl acetate, hexane and acetonitrile. Finally, the resulting crystals were dried to give (c2) N,N'-bis(4-methylphenyl)benzidine (13 g, 40% yield).
