Strategic Formation of Indoles Using Phenyl Hydrazine as a Key Reagent

Indole formation from phenylhydrazine represents one of the most versatile and mechanistically rich domains in heterocyclic synthesis, bridging classical Fischer indolisation with emerging transition‐metal‐catalyzed and tandem methodologies. Transition‐metal catalysis, such as Rh(III), Ru(II), and Co(III) complexes, has been used for C─H activation, oxidative annulation, and redox‐neutral coupling that yield high selectivity and diverse functional group‐containing indoles. Moreover, hydroformylation‐assisted Fischer reactions, ligand‐controlled rearrangements, and acid‐promoted condensations have demonstrated controllable C2/C3‐substitution and regioselectivity across alkenes, aldehydes, and cyclic precursors. Collectively, these advancements establish phenyl hydrazine as a robust scaffold for the synthesis of multifaceted indoles. This review encompasses alkyne, alcohol, ester, ketone, carboxamide acid, alkene, cyclic ether, and aldehyde routes to diversely substituted indoles.