Rhodium-catalyzed annulation for the construction of indole core: An update

Rhodium-catalyzed C–H activation involves mild reaction conditions, high efficiency, versatility, and regioselectivity. Consequently, this strategy is gradually becoming a powerful tool for constructing diverse heterocyclic scaffolds. In fact, N-heterocycles, especially indoles, are ubiquitous in natural products and pharmaceuticals. The present review summarizes recent advances (2019–2023) in Rh-catalyzed annulation of various substrates for the synthesis of indole skeletons. Annulations involving substrates, such as, anilines, anilides, amidines, N-nitrosoanilines, ortho-alkynylanilines, aryl hydrazines, nitrones/nitroarenes, 1-phenylpyrazolidinones etc. with suitable reaction partner efficiently afforded functionalized indoles. The reactions discussed in this review employed readily available starting materials that are environmentally benign and easy to handle. Moreover, mechanistic rationale for most of the reactions was highlighted for clear understanding of the readers.