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Experimental and Theoretical Studies on Ru(II)-Catalyzed Oxidative C–H/C–H Coupling of Phenols with Aromatic Amides Using Air as Oxidant: Scope, Synthetic Applications, and Mechanistic Insights

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posted on 06.08.2018 by Luoqiang Zhang, Lei Zhu, Yuming Zhang, Yudong Yang, Yimin Wu, Weixin Ma, Yu Lan, Jingsong You
We herein illustrate the dual chelation-assisted strategy for a Ru­(II)-catalyzed oxidative ortho-C–H/C–H cross-coupling of phenols with (hetero)­aromatic amides with the aid of Zn­(OTf)2, which enables to rapidly assemble a rich library of 2′-hydroxybiphenyl-2-carboxylic acid derivatives. This protocol features broad substrate scope, excellent functional group tolerance, air as the terminal oxidant, low molar ratio of coupling partners, and scale-up synthesis. Particularly, this methodology is tolerant of more complex natural product derivatives, thus providing an opportunity for late-stage functionalization. This protocol is also used as a key step for the concise synthesis of Palomid 529, a drug in development for the treatment of glioblastoma and neovascular age-related macular degeneration. With a combination of experimental and theoretical methods, we get more insight into the essential issues of strategy determining the reaction process. The stronger coordinating ability of 2-aryloxypyridine and the less steric hindrance of amide are pivotal to the high chemoselectivity of cross-coupling over homocoupling. The first C–H bond activation step takes place at the amide substrate, and the following C–H bond activation at 2-aryloxypyridine is involved in the rate-determined step.