Mn is one of the most common metal elements, and its reserves rank the third among all transition metals. Mn catalyst is widely used in a series of organic reactions, including C-H bond oxidation, halogenation, amination, hydrosilylation, cross coupling and C-H bond activation. However, the use of Mn as an atom transfer catalyst for the development of radical tandem reactions has received little attention, which is a research area worthy of further exploration. The Mn Mn bond of binuclear Mn Complex (such as Mn2 (CO) 10) has weak bond dissociation energy (BDE of Mn Mn bond is 15.0 kcal / mol), which is prone to light induced Mn Mn bond splitting and Mn free radicals. The Mn radicals formed have a strong affinity for halogen, which can grab the halogen in alkyl halides and produce alkyl radicals. Giese, Parsons, Friestad, Ryu, Alexanian, fadeyi, Nagib, Wang Qingmin and other research groups have successively developed a series of light promoted Mn catalyzed / promoted free radical series reactions, such as free radical addition reaction, free radical carbonylation reaction and Minisci reaction, etc. (Fig. 1, a).
Zhang Bo, associate professor of China Pharmaceutical University, used Mn mediated halogen atom transfer strategy to develop visible light promoted Mn catalyzed atom transfer radical cyclization of non activated alkyl iodides (org. Lett., 2019, 21, 5586) and visible light promoted Mn catalyzed atom transfer radical addition of fluoroalkyl iodides with alkenes and alkynes (adv. synth. Catalyst., 2020), 362, 1131）。 Compared with carbon centered radicals, heteroatom centered radicals are more useful intermediates, which have very important application value in synthetic chemistry. Whether Mn mediated atom transfer reaction can be used to generate heteroatom center radicals in a moderate and efficient way is a very worthy research topic in this field. Recently, inspired by the work of Wang CONGYANG's research group (angel. Chem. Int. ed., 2018, 57, 923), they used Mn as a hydrogen atom transfer catalyst and visible light as the driving force, and successfully realized the activation of hydrosilanes with high oxidation potential at room temperature, and developed the visible light promoted hydrosilylation of alkynes (org. Lett., 2019, 21, 2750) (Fig. 1, b). Inspired by the above work, they concluded that Mn mediated atom transfer reaction can generate nitrogen centered radicals (Fig. 1, c). Nitrogen centered radicals are very important intermediates in free radical chemistry, which can be obtained by many methods. However, using simple and mild reaction conditions to produce nitrogen centered radicals is still a challenge.
Fig. 1. Carbon, silicon, nitrogen radicals and their related transformations are produced by Mn mediated atom transfer reaction. Image source: angelw. Chem. Int. ed
Recently, Professor Zhang Bo, Li Ping, associate professor of China Pharmaceutical University, and Professor Qu Shuanglin, Hunan University, jointly developed a new method to generate nitrogen center free radicals from N-H bond through on-site chlorination and continuous chlorine atom transfer process. In this method, tbuocl is used as chlorination reagent, Mn2 (CO) 10 is used as atom transfer catalyst, and visible light is used as driving force. Under very simple and mild conditions, the production of nitrogen centered free radicals is realized (Fig. 2). The importance of this work includes: (1) it is a new method to generate nitrogen center free radicals from amines; (2) based on this new strategy, the author has developed Mn catalyzed long-range non activated C (SP3) - H bond chlorination of aliphatic amines and intermolecular and intramolecular ketamination of alkenes for the first time; (3) the reaction conditions are simple and mild, and can well respond to It provides a new idea for the structural modification of drugs and bioactive molecules.
Figure 2. Mn mediated atom transfer reaction to produce nitrogen centered radicals and its application. Image source: angelw. Chem. Int. ed
The different reaction substrates were investigated in detail. The results show that the developed reaction has the advantages of a wide range of substrates, good functional group compatibility and large-scale preparation. Then, the mechanism of the reaction was studied, including control experiment, free radical capture experiment and free radical clock experiment. These experiments confirmed that the reaction was carried out by free radical mechanism. Finally, the author suggests that Mn2 (CO) 10 not only acts as a free radical initiator, but also as an atom transfer catalyst.
Fig. 3. A) possible reaction mechanism, b) DFT calculation energy diagram. Image source: angelw. Chem. Int. ed
The results of this study were published in the recent issue of angew. Chem. Int. ed. Liu Runzhou, a postgraduate, is the first author of the paper, and Zhang Bo, associate professor of China Pharmaceutical University, Li Ping and Qu Shuanglin, Professor of Hunan University, are the corresponding authors of the paper. This work has been supported by NSFC, Jiangsu Province's "mass entrepreneurship and innovation team" and China Pharmaceutical University's "double first-class" funds.
Generation and reactivity of amidyl radicals: Manganese mediated atom transfer reaction run Zhou Liu, Jinxia Li, Jun sun, Xian Guan Liu, xuanlin Qu, Ping Li, and Bo zhangangew. Chem. Int. ed., 2020, 59, 4428-4433, DOI: 10.1002/anie.201913042
Tutor introduction: Zhang Bo https://www.x-mol.com/university/faculty/82460 Qu Shuanglin https://www.x-mol.com/university/faculty/50207
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