Chromium-Catalyzed Cross-Coupling Reactions by Selective Activation of Chemically Inert Aromatic C-O, C-N, and C-H Bonds

Transition-metal-catalyzed cross-coupling has emerged as one of the most powerful and useful tools for the formation of C-C and C-heteroatom bonds. Given the shortage of resources of precious metals on Earth, the use of Earth-abundant metals as catalysts in developing cost-effective strategies for cross-coupling is a current trend in synthetic chemistry. Compared with the achievements made using first-row nickel, iron, cobalt, and even manganese catalysts, the group 6 metal chromium has rarely been used to promote cross-coupling. This perspective covers recent advances in chromium-catalyzed cross-coupling reactions in transformations of chemically inert C(aryl)-O, C(aryl)-N, and C(aryl)-H bonds, offering selective strategies for molecule construction. The ability of low-valent Cr with a high-spin state to participate in two-electron oxidative addition is highlighted; this is different from the mechanism involving single-electron transfer that is usually assigned to chromium-mediated transformations. 1 Introduction 2 Chromium-Catalyzed Kumada Coupling of Nonactivated C(aryl)-O and C(aryl)-N Bonds 3 Chromium-Catalyzed Reductive Cross-Coupling of Two Nonactivated C(aryl)-Heteroatom Bonds 4 Chromium-Catalyzed Functionalization of Nonactivated C(aryl)-H Bonds 5 Conclusions and Outlook