Scalable Electrochemical Aerobic Oxygenation of Indoles to Isatins without Electron Transfer Mediators by Merging with an Oxygen Reduction Reaction
Weihui Zhuang, Jiaqi Zhang, Cheng Ma, James S. Wright, Xiaofeng Zhang*, Shao-Fei Ni* and Qiufeng Huang*
*Fujian Normal University, Fuzhou, Fujian 350007; Shantou University, Shantou, Guangdong 515063, P. R. China, Email: xfz_fz163.com, sfnistu.edu.cn, qiufenghuangfjnu.edu.cn
W. Zhuang, J. Zhang, C. Ma, J. S. Wright, X. Zhang, S.-F. Ni, Q. Huang, Org. Lett., 2022, 24, 4229-4233.
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An electrochemical oxygenation of indoles provides isatins by merging with a complementary cathode oxygen reduction reaction. This green protocol uses molecular oxygen as the sole oxidant, is free of an an electron transfer mediator, and enables gram-scale preparation.
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General Procedure for the electrochemial oxidative 2,3-difunctionalization of indoles
To a 10 mL three-necked bottle with a stir bar were successively added the N-substituted indole derivatives (0.2 mmol), KPF6 (0.2 mmol, 1.0 equiv., 36.8 mg), NaOEt (0.1 mmol, 0.5 equiv, 6.8 mg) , AdCOOH (1.0 mmol, 5.0 equiv., 0.2 mg) and 6 mL of DMF. The electrolysis was carried out at room temperature using a constant current of 2 mA, a reticulated vitreous carbon (RVC, 10 mm × 10 mm×6 mm) anode and a Fe plate (10 mm×10 mm×0.2mm) cathode with stirring for 18 h under air atmosphere. The reaction mixture was extracted with water and ethyl acetate. The combined organic layers were washed with water and brine, dried over anhydrous MgSO4 and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel to give the desired oxidative products.