C4–H indole functionalisation: precedent and prospects
C4-decorated indoles feature in a plethora of bioactive and functional compounds of importance to natural product synthesis, material sciences, as well as crop protection and pharmaceutical industries. Traditionally, their syntheses largely involved harsh stoichiometric metalations and radical reactions. However, transition metal catalysed C–H activation has recently evolved into a powerful strategy for the late-stage diversification of indoles at the C4–H position. Modern photoredox, enzymatic and precious transition metal catalysis represent the key stimuli for developing challenging C–C and C–Het bond forming transformations under mild reaction conditions. Herein, we discuss the evolution and application of these methods for the step-economical transformations of otherwise inert C4–H bonds up to December 2017.