Oxidizing Directing Groups Enable Efficient and Innovative C-H Activation Reactions

摘要

The development of the concepts of green chemistry has resulted from the desire of chemists to minimize synthetic steps and the amount of toxic waste formed, and, moreover, to find milder and more selective transformations. Transition-metal-catalyzed cross-coupling reactions through C-H activation processes have emerged in recent years because such transformations circumvent the need for prior steps to activate the substrate. As a consequence of the often oxidative character of these coupling reactions, the use of an external oxidant is generally required to regenerate the catalyst. An alternative emerging strategy is the use of oxidizing directing groups (acting as an internal oxidant) in C-H activation processes, which was recently and independently introduced by the research groups of Cui and Wu, Hartwig, Yu, and Guimond and Fagnou. This approach can lead to improved levels of reactivity and selectivity and also to a broader scope compared to the use of external oxidants. The internal oxidant strategy typically involves a covalent bond within the directing group, which oxidizes the metal catalyst (for example, through an oxidative addition), but is distinct from the C-H activated and functionalized position. The result is a multifunctional, oxidative directing group (DG~(ox)) that combines many attractive features [Eq. (1)]. In this Highlight we discuss, compare, and analyze the benefits, drawbacks, and opportunities resulting from this novel concept of oxidizing directing groups.