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Photocatalytic C-F, C-H Bifunctionalization - A Novel Catalytic System for C-F Bond Breakage


Fluorinated aromatic compounds are a very important class of compounds, widely present in the fields of pharmaceuticals, pesticides, and materials. Professor Jimmy D. Weaver from near Oklahoma State University published in JACS a simple and efficient photocatalytic C-F, C-H bifunctionalization reaction to prepare multifluorinated aromatic hydrocarbons.

We know that in recent years, transition metal catalyzed C-H bond functionalization has achieved very outstanding achievements, while there are relatively few reports on C-F bond activation. The main reason is that the C-F bond energy is strong, and the binding ability of fluoride anion and transition metal is strong, which leads to the low turnover number of transition metal catalyzed C-F bond activation reaction.

In 2014, Professor Jimmie D. Weaver reported the photocatalytic reaction of C-F bond breaking and Protonation of PAHs, in which cheap amines were used as reducing agents to provide protons. The author believes that the reaction takes place in the following process (Scheme 1): the PAHs remove F-ions under the photocatalyst to generate the PAHs radical [ArFn-1] · intermediate, which digests hydrogen from the base to obtain the defluorinated Protonation products (J. Am. Chem. Soc. 2014, 136, 3002). In 2015, the team utilized the same strategy to obtain multifluoroaromatic radicals [ArFn-1] · under photocatalysis, and then captured them with olefins to extract hydrogen from the base to obtain multifluoroaromatic hydrocarbons with different substituents (Chem. Sci. 2015, 6, 7206). This year, the group expanded the substrate to the aromatic system, where the perfluoroaromatic radical [ArFn-1] · is added to the aromatic hydrocarbon to obtain the dearomatic free radical intermediate int-a. This intermediate undergoes oxidation before dehydrogenation or dehydrogenation before oxidation to obtain aromatic hydrocarbon compounds.

After screening the conditions based on previous work, the author expanded the substrate and synthesized two multi fluorine substituted aromatic compounds, demonstrating the strong application prospects of this method. The author first used methyl Pentafluorobenzoic acid and polyfluoropyridine to obtain 24a and 26a through aromatic Nucleophilic substitution, then used the photocatalysis C-F, C-H functionalization developed by the author to obtain biaromatic hydrocarbon compounds 15a and 14a, and then used the photocatalysis C-F bond removal Protonation reaction developed by the author in 2014 to finally synthesize polysubstituted difluorobenzene Formate 25a and polysubstituted difluoropyridine 27a, However, using other methods to synthesize these two compounds is a very difficult task (Scheme 2).

Summary: The photocatalysis C-F bond functionalization reaction developed by Professor Jimmy D. Weaver generates polyfluoroaromatic free radicals, which can be reduced by cheap and easily available amines, and can also be added to olefins and aromatics to obtain polyfluoroaromatic compounds with diverse skeletons. The combination of this catalytic system and aromatic Nucleophilic substitution will have broad development space. The inadequacy of this catalytic system may be that the substrate type is relatively limited, and the development of photocatalytic C-F bond activation of simple fluorinated aromatic hydrocarbon substrates or C-F bond activation of alkanes will be a major challenge.