Categories: N-H Bond Formation >
Reduction of azo compounds
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.
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.