Abstract
This protocol describes the aerobic oxidation of aromatic anilines to aromatic azo compounds using gold (Au) nanoparticles supported on TiO2 as a catalyst. Yields above 98% are achieved under a few bars of oxygen pressure. It should be noted that the use of stoichiometric amounts of environmentally unfriendly reagents, e.g., transition metals and nitrites, commonly used in current syntheses of azo compounds, is avoided using this approach. The protocol is illustrated with the synthesis of parent azobenzene from aniline, and this reaction takes 22 h. Au on TiO2 can also be used as a hydrogenation catalyst, making it possible to prepare azo compounds directly from nitroaromatics through a two-step (hydrogenation followed by aerobic oxidation), one-pot, one-catalyst reaction. In addition, the catalytic process is efficient for the synthesis of symmetric and a range of asymmetric aromatic azo compounds from the mixtures of two anilines substituted with electron-donor and electron-acceptor substituents.
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Acknowledgements
Financial support by the Spanish MICINN (CTQ2009-11583) is gratefully acknowledged. We are also thankful to La Fundación Areces for generous funding in this project. A.G. thanks CSIC for a JAE contract as research associate.
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A.C. and H.G. designed the research work. A.G. carried out the experimental work and synthetic protocol. All authors discussed the results and commented on the manuscript.
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Grirrane, A., Corma, A. & Garcia, H. Preparation of symmetric and asymmetric aromatic azo compounds from aromatic amines or nitro compounds using supported gold catalysts. Nat Protoc 5, 429–438 (2010). https://doi.org/10.1038/nprot.2009.242
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DOI: https://doi.org/10.1038/nprot.2009.242
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