Visible Light Photoredox Catalysts
The photoreaction undergoes a special molecular transformation to form a product that is not obtained through a thermal reaction. In recent years, the light reaction induced by visible light has been further developed. The light reaction in visible light does not require high-energy ultraviolet light to excite, and the reaction can occur under mild conditions to avoid the influence of high temperatures to obtain unexpected byproducts.
A photocatalyst that can perform both single-electron oxidation and reduction under visible light irradiation, the so-called "visible light photoredox catalyst", has received widespread attention because it is a potential catalyst for solar redox.1 Compared with the thermal reaction in the presence of oxidizing or reducing reagents, the process is simpler, and there are no unpredictable side reactions due to high temperatures or pressures. The reaction cycle using photoredox catalysts involves both oxidation and reduction pathways, so it shows a mechanism of "REDOX cancellation" in general.
Photoredox catalysis via oxidation and reduction pathways
The formation of free radicals usually needs to be activated by light irradiation, REDOX reaction, or a heat source that provides high energy. The development of the field of photocatalysis has promoted the discovery of transition metal complex catalysts and organic catalysts, and the scientific community has continued to explore how to form free radicals under mild conditions, such as only visible light irradiation.
Some ruthenium (II) polypyridine complexes and iridium (III) phenylpyridine complexes can be used as photoredox catalysts under visible light irradiation.2 These transition metal complexes are very useful photocatalysts because they can form three-excited states that are stable for a relatively long time under light irradiation. Chemical modifications of ligands to control the REDOX potential of transition metal complexes, and metal-free organic catalysts have also been developed.3 Some acridine compounds with donor-acceptor structures can be photoredox catalysts because the excited state can be exposed to visible light for a long time charge separation.4 In addition, it has been reported that eosin and xanthine dyes can also be used as photoredox catalysts.5
Transition metal photoredox catalysts
• D155319
• T113540
Transition metal-free photoredox catalysts
• M157877
• E110818
Reaction examples mediated by visible light photoredox catalysts
1) Trifluoromethylation6
2) Carbonylation7
3) Direct acylation to C-H bond8
4) Direct amination to C-H bond9
5) Formation of iminyl radical10
6) Synthesis of quinoxaline derivative11
7) Formation of oxazole by [3 + 2] cycloaddition12
8) Formation of hydrazonyl radical13
References
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3. Review: J. W. Tucker, C. R. J. Stephenson, J. Org. Chem. 2012, 77, 1617. https://doi.org/10.1021/jo202538x
4. Review: S. Fukuzumi, K. Ohkubo, Org. Biomol. Chem. 2014, 12, 6059. https://doi.org/10.1039/C4OB00843J
5. Review: D. P. Hari, B. Koenig, Chem. Commun. 2014, 50, 6688. https://doi.org/10.1039/C4CC00751D
6. R. Tomita, T. Koike, M. Akita, Angew. Chem. Int. Ed. 2015, 54, 12923. https://doi.org/10.1002/ange.201505550
7. M. Majek, A. Jacobi von Wangelin, Angew. Chem. Int. Ed. 2015, 54, 2270. https://doi.org/10.1002/anie.201408516
8. C. L. Joe, A. G. Doyle, Angew. Chem. Int. Ed. 2016, 55, 4040. https://doi.org/10.1002/anie.201508404
9. G. Pandey, R. Laha, Angew. Chem. Int. Ed. 2015, 54, 14875. https://doi.org/10.1002/anie.201506990
10. H. Jiang, X. An, K. Tong, T. Zheng, Y. Zhang, S. Yu, Angew. Chem. Int. Ed. 2015,
54, 4055. https://doi.org/10.1002/anie.201411342
11. Z. He, M. Bae, J. Wu, T. F. Jamison, Angew. Chem. Int. Ed. 2014, 53, 14451. https://doi.org/10.1002/anie.201408522
12. T.-T. Zeng, J. Xuan, W. Ding, K. Wang, L.-Q. Lu, W.-J. Xiao, Org. Lett. 2015, 17,
4070. https://doi.org/10.1021/acs.orglett.5b01994
13. X.-Q. Hu, J.-R. Chen, Q. Wei, F.-L. Liu, Q.-H. Deng, A. M. Beauchemin, W.-J.
Xiao, Angew. Chem. Int. Ed. 2014, 53, 12163. https://doi.org/10.1002/anie.201406491