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Visible light induced photocatalysis has always been a research hotspot in organic synthesis chemistry. So far, the realization of photocatalytic reaction mainly depends on the addition of photocatalyst in the reaction system, and the conversion of light energy into chemical energy to drive the reaction. However, the commonly used photocatalysts are usually expensive and the preparation process is tedious. Therefore, the development of a photocatalytic reaction without the addition of photocatalyst has become the research goal of organic chemists. Recently, Zhao Binlin, associate professor of Nanjing Forestry University, and Shi Zhuangzhi, Professor of Nanjing University, used the alkyne copper complex formed in situ by terminal alkyne and copper as photosensitizer for catalytic reaction to realize the csp3-csp coupling reaction of copper catalyzed cycloketoxime ester and terminal alkyne (chem. Commun. 2020, DOI: 10.1039 / d0cc00988a).
Photocatalytic reactions based on visible light induction play an important role in green organic synthesis chemistry. By adding photocatalyst into the reaction system, absorption of light energy to drive the reaction is one of the important strategies to achieve the corresponding conversion. However, the commonly used photocatalysts are usually expensive and the preparation process is complex. So more and more organic chemists pay attention to photocatalysis without photocatalyst. In recent years, the coupling reaction of C-C bond construction involving transition metal catalyzed organic halides has been successfully developed (j.am. Chem. SOC., 2015, 137, 13902; angelw. Chem. Int. ed., 2016, 55, 4759), C-N (Science, 2012, 338, 647; science, 2016, 351, 681), C-O (chem. SCI., 2014, 5, 2831), C-S (j.am. Chem. SOC., 2013, 135, 9548). However, the functional reaction of inert bond breaking based on this reaction strategy has not been reported yet. Zhao Binlin, associate professor of Nanjing Forestry University and Professor Shi Zhuangzhi's research group reported a synthesis method of copper catalyzed cycloketoxime ester to produce carbon bond breaking and terminal alkynes to generate internal cyanogens (Fig. 1).
Figure 1. Promoting the coupling reaction of cycloketoxime ester and terminal alkyne with copper catalyst
(source: chem. Commun.)
Figure 2. Substrate range of cyclobutanone oxime ester
(source: chem. Commun.)
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The reaction of 4-trifluoromethylbenzoylcyclobutanone oxime ester and phenylacetylene was studied. In the process of condition selection, the author found that the copper catalyst, the mixed solvent of α, α, α - tripyridine ligand, potassium carbonate, N, N-dimethylformamide and methanol, and the light source are all the necessary conditions to promote the reaction. In addition, the tetrabutylammonium iodide with the amount of catalyst can significantly improve the yield of cyanogen endoyne. In order to further investigate the substrate compatible range of the reaction system, the author tried a series of different kinds of cycloketoxime esters (Fig. 2). The results show that the cyclobutanone oxime esters with different substituents at different positions can react to form corresponding target products under template conditions, and the ether, ester, cyano, amide and olefin functional groups can be compatible with the reaction system. In addition, if the α position of CYCLOKETONE contains substituents, the carbon carbon bond will selectively break from the side containing substituents to form a relatively stable second-order carbon radical, which will eventually transform into the coupling product of csp3-csp. In addition, different kinds of terminal alkynes were studied (Fig. 3). The results showed that both aromatic and aliphatic terminal alkynes could participate in the reaction to obtain the cyanogens with good yield. It is worth noting that this method can also be used in the synthesis and modification of complex molecules and natural product derivatives.
Figure 3. Substrate range of terminal alkynes
(source: chem. Commun.)
Finally, in order to learn more about the mechanism and details of the reaction transformation, the author carried out a series of mechanism experiments (Fig. 4), and carried out UV absorption and fluorescence emission spectroscopic tests on the in-situ formed cupric alkyne complex in the reaction system (Fig. 5). The experimental results show that the reaction is carried out through the free radical path; the in-situ alkyne copper complex in the reaction system is an important intermediate for the reaction, which can be used as a photosensitizer to drive the reaction and convert the light energy into the compound energy. According to the literature reports and experimental results, the author speculates that the reaction mechanism is as shown in Fig. 6: first, terminal alkynes react with copper salt to form alkyne copper complex under alkaline conditions, the complex absorbs light energy to the excited state expansion, and it is easy to obtain the β - carbon break of electron cyclobutanone oxime ester and the alkyne copper complex with reduction ability to generate the alkyl free radical through single electron transfer. At last, it is trapped by the high valence copper alkyne complex to form the corresponding internal cyanogen products and release the low valence copper catalyst to complete the catalytic cycle.
Figure 4. Mechanism experiment
(source: chem. Commun.)
Figure 5. UV absorption and fluorescence emission spectrum
(source: chem. Commun.)
Figure 6. Presumed reaction mechanism
(source: chem. Commun.)
In conclusion, the coupling reaction of csp3-csp without catalyst addition was realized by using the in-situ formed alkyne copper complex of copper salt and terminal alkyne as photosensitizer to drive the photoreaction. This method will provide a new idea and strategy for photocatalysis of inert bond breaking functionalization. This research achievement was recently published on chem. Commun. (chem. Commun. 2020, DOI: 10.1039/d0cc00988a). It was completed by Zhao Binlin, associate professor of Nanjing Forestry University (studied in the research group of Professor Shi Zhuangzhi of Nanjing University during the period of 2015-2019 Ph.D.) and Professor Shi Zhuangzhi of Nanjing University. The research work was supported by the national young thousand talents program and the distinguished professors program of Jiangsu Province.
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