Wang Yongtao, Guan Jun, Mei Bingbao, Fan Mengtian, Lu Rui, Du Renfeng, Chen Kaizhou, Yao Jia, Jiang Zheng, Li Haoran
Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China.
Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P. R. China.
Inorg Chem. 2020 Mar 16;59(6):3562-3569. doi: 10.1021/acs.inorgchem.9b02422. Epub 2020 Feb 21.
Phenoxyl radical was generally suggested as the intermediate during copper-catalyzed aerobic oxygenation of phenols. However, the substrate-dependent selectivity has not been well interpreted, due to insufficient characterization of the radical intermediate under reaction conditions. When studying the CuCl-LiCl-catalyzed aerobic phenol oxidation, we obtained EPR spectra of phenoxyl radicals generated by oxidizing phenols with the preactivated catalyst. Upon correlation to the selectivity of benzoquinone, the hyperfine coupling constant of -site proton (a) was found to be better than the Hammett constant. The catalysis mechanism was studied based on EPR detection and the reaction results of phenoxyl radicals under N or O atmosphere. It appeared that the chemoselectivity depended on the attack of activated dioxygen on phenoxyl radicals, and the activation of dioxygen by [CuCl] ( = 1, 2, 3) was suggested as the rate-determining step. Understanding of the substrate-dependent selectivity contributed to predicting the chemoselectivity in the aerobic oxidation of phenols.
苯氧基自由基通常被认为是铜催化酚类有氧氧化过程中的中间体。然而，由于在反应条件下对自由基中间体的表征不足，底物依赖性选择性尚未得到很好的解释。在研究CuCl-LiCl催化的有氧酚氧化反应时，我们获得了用预活化催化剂氧化酚类生成的苯氧基自由基的电子顺磁共振（EPR）光谱。与苯醌的选择性相关联后，发现对位质子的超精细偶合常数（a）比哈米特常数更能体现选择性。基于EPR检测以及在氮气或氧气气氛下苯氧基自由基的反应结果研究了催化机理。结果表明，化学选择性取决于活化的双氧对苯氧基自由基的进攻，并且认为[CuCl]（n = 1, 2, 3）对双氧的活化是速率决定步骤。对底物依赖性选择性的理解有助于预测酚类有氧氧化中的化学选择性。
