Current mechanistic understanding of Cobalt-Catalyzed C–H functionalization

Arguably, one of the biggest advancements in synthetic chemistry over recent decades, has been the development of Pd cross-coupling procedures. The application of Pd-catalyzed cross-coupling reactions is nowadays a powerful and widely applied tool during the preparation of a wide range of pharmaceuticals, agrochemicals, and synthetic intermediates. Recently, the use of cheaper, more abundant, and less toxic first-row transition metals to replace more expensive Pd has started to attract significant attention. While at the same time, direct C–H functionalization to replace the necessity of halogenated precursors is also a topic of interest in order to develop cleaner and more environmentally friendly procedures. In this context, cobalt-catalyzed C–H functionalization has provided a platform to address these desires. The mechanistic diversity of newly developed protocols using cobalt is quite extraordinary and more varied than when applying the corresponding second and third row analogs. This overview seeks to exemplify and highlight the potential of cobalt as the basis for C–H functionalization protocols, focusing on the wide range of mechanisms available arising from the rich redox chemistry of this metal.