Transition metal-catalyzed C-H functionalizations of indoles

Over the last two decades, transition-metal catalyzed C-H functionalization of indoles has emerged as an area of extensive research and tremendous progress has been made in this regard. Early developments led to the positional-selective C-H functionalization at the C3 and C2-positions of indoles, based on the inherent reactivity of indoles and the application of Lewis-basic directing groups. Despite the challenges associated with lower accessibility of the C-H bonds on the benzenoid ring, considerable efforts have also been made in the development of elegant methodologies to enable site-selective C-H functionalization on the benzenoid moiety, at C4-C7 positions. This review summarises a wide range of useful transformations, including C-H arylation, alkenylation, alkynylation, acylation, nitration, borylation, annulations, and amidation with the aid of various transition-metal catalysts.