Rhodium(III)-Catalyzed C-H Activation and Indole Synthesis With Hydrazone as an Auto-Formed and Auto-Cleavable Directing Group

An efficient, practical, and external-oxidant-free indole synthesis from readily available aryl hydrazines was developed, by using hydrazone as a directing group for Rh-III-catalyzed CH activation and alkyne annulation. The hydrazone group was formed by in situ condensation of hydrazines and CO source, whereas its NN bond was served as an internal oxidant, for which we termed it as an auto-formed and auto-cleavable directing group (DG(auto)). This method needs no step for pre-installation and post-cleavage of the directing group, making it a quite easily scalable approach to access unprotected indoles with high step economy. The DG(auto) strategy was also applicable for isoquinoline synthesis. In addition, synthetic utilities of this chemistry for rapid assembly of -extended nitrogen-doped polyheterocycles and bioactive molecules were demonstrated.