Research team of Professor Li Hongxi and Professor Lang Jianping of Suzhou University: photoinduced reverse hydrogen atom transfer to construct C-S bond

The author: Xu Zeming benzothiazole compounds are widely found in natural products, drug molecules, organic materials and so on Many synthetic methods of these compounds have been reported in the literature, and intramolecular C (SP 2) - H sulfolation is one of the most attractive methods Under the condition of heating, benzothiazole can be synthesized by catalytic activation of C-H bond of thioamide to form C-S bond Li pixu group used Ru (bpy) 3 (PF 6) 2 as photosensitizer and oxygen as oxidant to construct benzothiazole under visible light These compounds were synthesized by dehydrogenation of ruthenium and cobalt complexes There are some disadvantages in the above methods, such as the need of expensive metal catalysts or photosensitizers, the easy deiodination of iodine containing substrate, the easy generation of desulfurization by-products, etc Recently, the research team of Professor Li Hongxi and Professor Lang Jianping of Suzhou University developed a method for the construction of benzothiazole compounds by the reaction of thioamide with 2,2,6,6-tetramethylpiperidine nitrogen oxide (tempo) under visible light (sunlight) without additional photosensitizers, metal catalysts and inorganic bases The method has the advantages of simple reaction conditions, wide application range of substrates, and is suitable for the substrates containing halogen such as iodine, which is convenient for functionalization (Fig 1) Relevant research results were published in org Lett (DOI: 10.1021/acs.orglett 8b03679) Fig 1 C-H sulfolation of n-phenylthioamide (source: org Lett.) the author first optimized the reaction conditions with n-phenylthiobenzamide (1AA) as the substrate, and finally obtained 2-phenylbenzothiazole in 99% yield Then, under the optimized reaction conditions, the author extended the substrate (Fig 2) The results show that n-phenylthioamide with different substituents can react efficiently, and the highest separation yield is 95% The method can also be applied to the substrate containing halogen The reaction does not produce dehalogenated products, which is convenient for later modification Figure 2 Substrate development (source: org Lett.) further, the author has carried out a detailed study on the reaction mechanism By analyzing the data of electrochemical experiments, the author found that the reaction experienced not the electron transfer process, but the visible light induced reverse hydrogen atom transfer process Heating can't make the reaction go on, but visible light can make the reaction go on smoothly, which shows that the reaction is induced by light The kinetic isotope effect experiment shows that the reaction is a first-order reaction, and the formation step of sulfur radical is the decisive step of the reaction (Fig 3) Figure 3 Kinetic isotope effect experiment (source: org Lett.) finally, the author proposed the possible reaction mechanism The substrate 1AA forms the excited state of * 1AA after receiving a photon, which is transferred to tempo by hydrogen atom to obtain sulfur free radical a; a undergoes cyclization reaction to generate aryl free radical C, which is converted to D under the guidance of visible light; D undergoes further hydrogen atom transfer to obtain the target product (Fig 4) Figure 4 The possible reaction mechanism (source: org Lett.) was recently published in org Lett (DOI: 10.1021 / ACS Org lett 8b03679) The first author of this paper is Xu Zeming, a master of Suzhou University, and the corresponding authors are Professor Li Hongxi and Professor Lang Jianping of Suzhou University The above research work has been supported by the National Natural Science Foundation of China (21771131, 21471108, 21531006 and 21773163) and the natural science foundation of Jiangsu province (BK20161276).