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    Home > Angelw: Ackermann group, University of Gottingen to realize visible light / ruthenium catalyzed C-H alkylation

    Angelw: Ackermann group, University of Gottingen to realize visible light / ruthenium catalyzed C-H alkylation

    • Last Update: 2019-06-18
    • Source: Internet
    • Author: User
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    The development of selective functionalization of C-H bonds has been a key challenge in molecular synthesis At present, chelation assisted by guiding group is a powerful tool for regioselective C-H metallization Therefore, the C-H activation induced by ortho lays a foundation for the functionalization of ortho selective C-H However, the strategies for the construction of meta substituted aromatics are rarely studied In order to overcome the challenge of C-H functionalization, researchers have designed a variety of useful methods (Figure 1) Therefore, it is necessary to develop the substituent model of the substrate itself, but the scope of application of the substrate is limited As a unique alternative, the intermodal C-H functionalization of aromatics activated by σ can be realized by the chelate assisted ortho cyclization Although σ activation has made great progress, it is easy to lead to poor yield and functional group tolerance due to its high temperature conditions On the contrary, metal catalyzed functionalization of m-c-h at room temperature is still difficult to achieve In recent years, photoinduced C-H functionalization has become a powerful tool for molecular synthesis Recently, the Lutz Ackermann group of the University of Gottingen, Germany, designed a method of visible light induced C-H alkylation at room temperature The results were published on angelw Chem Int ed (DOI: 10.1002 / anie 201902258) (picture source: angelw Chem Int ed.) firstly, the model reaction of C-H alkylation between aromatic 1a and tert butyl bromide (2a) was carried out, and the product 3A was successfully obtained The effects of ruthenium catalysts, ligands, bases and solvents were verified by contrast experiments In addition, other transition metal catalysts commonly used for C-H activation, such as Pd (OAC) 2, [Cp * RhCl 2] 2 and [Cp * IRCL 2] 2, have defects The addition of exogenous photosensitizers such as IR (PPy) 3, Ru (bpy) 3cl2, Eosin Y, rhodamine 6G, Rose Bengal, 10-methyl-9-homotrimethylphenylacridine perchlorate had no significant effect (picture source: angelw Chem Int ed.) under the optimized reaction conditions, the author studied the practicability of the photocatalytic C-H alkylation of aromatics 1 (scheme 1) It is found that the third and second-order unactivated alkyl bromides are suitable electrophilic reagents, and the expected product 3 can be obtained Therefore, alkylbromide 2 containing imide, enone, piperidinyl and ester group can tolerate the C-H functionalization (image source: angelw Chem Int ed.) at room temperature, ruthenium catalyzed visible light induced C-H alkylation is not only limited to alkylbromine 2, but also applicable to scheme 2 Therefore, alkyl bromine containing sugar, menthol or steroids can also be substituted in the intermediate position, which has a wide application prospect in the pharmaceutical industry (image source: angelw Chem Int ed.) in view of the unique characteristics of photoinduced / ruthenium catalyzed C-H functionalization, the author attempts to elucidate its mode of action (scheme 3) In the presence of the ligand (PHO) 2p (o) Oh, the author found that not only 6 has the catalytic activity (scheme 3a), but also 7 is effective for the photoinduced distal C-H functionalization, and even 6 and 7 can be almost quantitatively separated to obtain the product 3a In addition, the catalytic activity of 1,1-diphenylethylene (scheme 3b) was significantly inhibited by the free radical scavengers tempo, BHT, galvinoxyl and scheme 3B Finally, the competition experiment between secondary alkyl and tertiary alkyl bromide 2 shows that tertiary alkyl bromide preferentially transforms (scheme 3C) (image source: angelw Chem Int ed.) considering the moderation of photoinduced / ruthenium catalyzed C-H alkylation, the role of visible light in the reaction was studied The absorption spectra of ruthenium complexes 6 and 7 show weak and significant light absorption in the blue region (Figure 2a) In addition, according to stern Volmer equation, the fluorescence quenching was studied, the conversion curve of photocatalytic metal alkylation was monitored, and it was found that in the absence of light, the remote C-H alkylation was completely inhibited (Figure 2b) The above experiments highlight the importance of visible light for the functionalization of C-H at room temperature, and show that the formation of effective products needs constant irradiation Cyclic voltammetry revealed the reversible oxidation process, and the oxidation peak value in DCE was 0.98 V (Figure 2C) (picture source: angelw Chem Int ed.) according to the above research results, the author proposed a reasonable catalytic cycle (scheme 4): first, 1a is ruthenized by C-H with the aid of phosphate ester, the intermediate b * is produced by the Ru complex B metallized by blue light absorption excitation ring, and then the alkylhalide 2 is obtained by set, thus the Ru (III) complex C is produced And alkyl radical D; next, the Antiposition of Ru on the aromatic ring is attacked by radicals to form intermediate e, and then through intramolecular set and aromatization to form G Finally, the expected m-alkylation product 3 was obtained by G demetallization and Ru (II) catalyst a was regenerated (photo source: angelw Chem Int ed.) finally, the author investigated the visible light catalyzed C-H alkylation of heteroaromatics (scheme 5), and successfully realized the intermodal functionalization to obtain products 9 and 11, which can be converted into amino and carboxyl groups In addition, the visible light catalyzed C-H alkylation developed by the authors can also be used for the later structure modification of piperidine (scheme 5C) (photo source: angelw Chem Int ed.) conclusion: Lutz Ackermann team has developed a new strategy for the construction of m-substituted aromatics through visible light induced ruthenium catalyzed remote C-H functionalization The remarkable characteristics of the strategy include: 1) ruthenium catalyzed C-H alkylation; 2) visible light induced metal / photocatalysis for remote C-H functionalization; 3) photocatalysis under non exogenous photosensitizer conditions; 4) extremely mild reaction conditions In addition, the fusion potential of visible light redox catalyst and Ru (II) - mediated C-H functionalization has been demonstrated for the first time.
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