Synthesis of azo compounds

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A highly efficient, metal-free, chemical oxidation of hydrazines using environmentally friendly TCCA as oxidant provides a broad range of azo compounds in THF in excellent yield. This step-economical process offers mild reaction conditions, operational simplicity, high reaction efficiency, and easy scale-up.
Y. Su, X. Liu, J. Yu, G. Cao, R. Zhang, Y. Zhao, D. Huang, K.-H. Wang, C. Huo, Y. Hu, Synthesis, 2020, 52, 1103-1112.

The synthesis of alkyl 2-phenylazocarboxylates largely depended on the stoichiometric use of toxic oxidants. The use of CuCl and DMAP as catalysts enables an environment-friendly aerobic oxidation of alkyl 2-phenylhydrazinecarboxylates to alkyl 2-phenylazocarboxylates under mild conditions.
M. H. Kim, J. Kim, J. Org. Chem., 2018, 83, 1673-1679.

A straightforward, convenient, and efficient oxidative dimerization of aromatic amines enables an easy access to symmetrical and unsymmetrical azobenzenes under extremely mild conditions using a unique and cost-effective iodinating reagent.
Y. Takeda, S. Okumura, S. Minakata, Synthesis, 2013, 45, 1029-1033.

Unsymmetrical and symmetrical bis(hetaryl)azo dyes were prepared by diazotisation-coupling and oxidation reactions in moderate yields. Their absorption maxima were observed in the range of 427-631 nm, being more bathochromic than the corresponding non-hetarylazo derivatives.
M. Wang, K. Funabiki, M. Matsui, Dyes and Pigments, 2003, 57, 77-86.

Treatment of anilines with N-chlorosuccinimide and 1,8-diazabicyclo[5.4.0]undec-7-ene enables a convenient one-step procedure for the synthesis of symmetrical azobenzenes in good yields in minutes.
A. A. John, Q. Lin, J. Org. Chem., 2017, 82, 9873-9876.

A green dehydrogenation of hydrazo compounds using basic alumina or KF/alumina under solvent-free conditions afforded azo compounds in good to excellent yields.
M. Mihara, T. Nakai, T. Iwai, T. Ito, T. Mizuno, Synlett, 2007, 2124-2126.