Reduction of azo compounds

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An efficient Fe/CaCl₂ system enables the reduction of nitroarenes and reductive cleavage of azo compounds by catalytic transfer hydrogenation in the presence of sensitive functional groups including halides, carbonyl, aldehyde, acetyl, nitrile, and ester substituents with excellent yields. The simple experimental procedure and easy purification make the protocol advantageous.
S. Chandrappa, T. Vinaya, T. Ramakrishnappa, K. S. Rangappa, Synlett, 2010, 3019-3022.

Nickel catalyzes a semihydrogenation of azoarenes with NH₃BH₃ to provide hydrazoarenes with good functional group tolerance and a high turnover frequency at room temperature. Results of control and deuterium-labeling experiments indicate that the ethanol hydroxyl and BH₃ groups each donated one hydrogen to this transfer hydrogenation, and the main byproducts were B(OEt)₃ and H₂.
D. Gong, D. Kong, Y. Li, C. Gao, L. Zhao, Org. Lett., 2023, 25, 4168-4172.

Thioacetic acid mediates a mild catalyst- and metal-free hydrogenation of azobenzenes to hydrazobenzenes in very good yields under visible light irradiation. The method is compatible with a variety of substituents and tolerates other unsaturated functionalities (carbonyl, alkenyl, alkynyl, etc.). Preliminary mechanistic study indicated that the transformation could be a radical process.
Q. Li, Y. Luo, J. Chen, Y. Xia, J. Org. Chem., 2023, 88, 2443-2452.

N,N-Diisopropylethylamine mediates a cathodic reduction approach for the hydrogenation of azobenzenes in dichloromethane as hydrogen source. The reaction proceeded smoothly in a simple undivided cell under constant-current electrolysis. A series of azobenzenes were successfully reduced to the corresponding hydrazobenzenes in good yields at room temperature.
H. Zhou, R. Fan, J. Yang, X. Sun, X. Liu, X.-C. Wang, J. Org. Chem., 2022, 87, 14536-14543.

A visible-light-promoted transfer hydrogenation of azobenzenes proceeds smoothly in methanol at ambient temperature in the presence of B₂pin₂ through a radical pathway. The reaction reduces a broad range of azobenzenes to the corresponding hydrazobenzenes in very good yields.
M. Song, H. Zhou, G. Wang, B. Ma, Y. Jiang, J. Yang, C. Huo, X.-C. Wang, J. Org. Chem., 2021, 86, 4804-4811.

A bidentate Ru(II)-NC complex catalyzes a transfer hydrogenation of azoarenes to hydrazoarenes using ethanol as a hydrogen source in the presence of a weak base. Control experiments and density functional theory calculations suggest a Meerwein-Ponndorf-Verley mechanism with ethyl acetate as the byproduct.
D. Gong, D. Kong, N. Xu, Y. Hua, B. Liu, Z. Xu, Org. Lett., 2022, 24, 7339-7343.

Various azobenzenes have been reduced to the corresponding hydrazines by using an aqueous solution of sodium dithionite. The yield is generally excellent, but two compounds, viz. 4,4-dimethoxyazobenzene and 2,2,4,4,6,6-hexamethylazobenzene, gave no hydrazine at all.
L. K. Sydnes, S. Elmi, P. Heggen, B. Holmelid, D. Malthe-Sørensen, Synlett, 2007, 1695-1696.