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.