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Cerium-Catalyzed C–H Functionalizations of Alkanes Utilizing Alcohols as Hydrogen Atom Transfer Agents

An, Qing, Wang, Ziyu, Chen, Yuegang, Wang, Xin, Zhang, Kaining, Pan, Hui, Liu, Weimin, Zuo, Zhiwei
Journal of the American Chemical Society 2020 v.142 no.13 pp. 6216-6226
alcohols, alkanes, alkylation, amination, carbon-hydrogen bond activation, catalysts, catalytic activity, cerium, energy, free radicals, homolytic cleavage, hydrogen, redox reactions, spectroscopy
Modern photoredox catalysis has traditionally relied upon metal-to-ligand charge-transfer (MLCT) excitation of metal polypyridyl complexes for the utilization of light energy for the activation of organic substrates. Here, we demonstrate the catalytic application of ligand-to-metal charge-transfer (LMCT) excitation of cerium alkoxide complexes for the facile activation of alkanes utilizing abundant and inexpensive cerium trichloride as the catalyst. As demonstrated by cerium-catalyzed C–H amination and the alkylation of hydrocarbons, this reaction manifold has enabled the facile use of abundant alcohols as practical and selective hydrogen atom transfer (HAT) agents via the direct access of energetically challenging alkoxy radicals. Furthermore, the LMCT excitation event has been investigated through a series of spectroscopic experiments, revealing a rapid bond homolysis process and an effective production of alkoxy radicals, collectively ruling out the LMCT/homolysis event as the rate-determining step of this C–H functionalization.