Tuning radical reactivity for selective radical/radical cross-coupling

Radical/radical cross-coupling represents a straightforward way for the construction of new chemical bonds in theory. Until now, it is challenging to achieve the selective bond formation between two transient radicals since selective radical cross-coupling usually occurs between persistent radical and transient radical. A recent study proposes a novel strategy "tuning radical reactivity" that could tune transient radical into persistent radical.

The paper was reported in Science Bulletin (2018, 63(15): 1006-1009), entitled "Tuning radical reactivity for selective radical/radical cross-coupling" by Aiwen Lei et al from Wuhan University. The authors introduced four methods to achieve "tuning radical reactivity".

Radical chemistry laid the foundation for the development of organic synthesis, biological processes and polymerization. Generally, radicals with a single electron have a strong tendency to form chemical bonds. Due to the quick reaction rate and low activation energy, radical/radical coupling is an efficient way to construct new chemical compounds. However, radicals are present only under special and limited conditions. It still remains a great scientific challenge to achieve highly selective radical/radical cross-coupling for further application.

In 1936, the persistent radical effect (PRE) provides the theoretical basis for the realization of radical/radical cross-coupling between persistent radical (* Rp) and transient radical (* Rt). Recently, a few radical/radical cross-coupling reactions have been realized between R-H and R-X (X= CN, F, Cl, Br, I, etc.) via photocatalysis. Although, the photo-induced radical generation became a useful method, most studies were still limited in the reaction between * Rp and * Rt. Therefore, it is still a great challenge to achieve the selective bond formation between two transient radicals that may be more commonly used in chemical transformations. It is reasonable to assume that if methods could be developed to stabilize one of reactive radicals, it will provide a solution for the direct cross-coupling between two transient radicals. In this paper, the authors introduced the concept of "tuning radical reactivity" and its applications in oxidative cross-coupling.

Based on previous researches in XAFS, the authors found that the equilibrium between organocuprates and radicals could serve as persistent radical. This reveals that transient radical could be translated into persistent radical in special conditions. Inspired by that, the authors proposed "tuning radical reactivity": through transforming * Rt into a more stable intermediate, radical/radical cross-coupling could be realized more easily. This novel strategy has been achieved by the following ways: method a) by tuning * Rt into R-Mn+1 via oxidative addition, method b) by tuning * Rt into * R-Mn via metal chelation, method c) by tuning * Rt into C-X bond (X = I, Br, Cl) and method d) by tuning * Rt into * (R-Arene) intermediates.

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This work was supported by the National Natural Science Foundation of China (21390402, 21520102003) and the Hubei Province Natural Science Foundation of China (2017CFA010). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.

See the article:

Shengchun Wang, Shan Tang, Aiwen Lei. Tuning radical reactivity for selective radical/radical cross-coupling. Science Bulletin, 2018, 63(15):1006-1009

https://www.sciencedirect.com/science/article/pii/S2095927318302767

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