Transition metal catalyzed meta-C-H functionalization of aromatic compounds.

Direct functionalization of C-H bonds represents a powerful strategy for the synthesis of complex organic compounds due to its inherent efficiency. Among various approaches, transition metal catalyzed direct activation of unreactive C-H bonds is particularly effective for this purpose. However, the development of practical methods for transition metal catalyzed direct C-H functionalization has been challenging. Apart from identifying the reaction conditions that allow the activation of relatively unreactive C-H bonds, these reactions need to be selective, allowing one C-H bond to be differentiated from the rest of the ubiquitous C-H bonds of the compound. Whereas directing group guided, transition metal catalyzed ortho-C-H functionalization of aromatic compounds has seen significant growth in the past few decades, methods for meta-C-H functionalization of arenes have also emerged. This review summarizes approaches for directing group guided, transition metal catalyzed meta-C-H functionalization of aromatic compounds. Some steric-controlled, transition metal catalyzed formal meta-C-H functionalization reactions without coordinating directing groups are also discussed.