Oximes and hydrazones as versatile directing groups: A critical review on C–H activation and functionalization

The selective activation of C–H bonds is a crucial step in modern organic synthesis using a variety of directing groups which allows for efficient access to complex molecular frameworks. In this, oximes and hydrazones have emerged as adaptable directing groups (DGs) in facilitating diverse transformations, including the synthesis of quinolines, carbolines, aza-heterocycles, spirocycles, and annulations with C–N bond formation. Beyond serving as co-ordinating moieties, these groups can act as internal oxidants in specific pathways, enhancing the reaction efficiency and selectivity. Owing to their versatility, this review highlights the recent advancements of C–H activation strategies mediated by oxime and hydrazone as facile directing groups. The review also emphasizes their structural diversity and exploration of mechanistic aspects of the reactions. Key applications includes the construction of nitrogen-rich frameworks with broad functional group tolerance. In additions, challenges such as substrate scope, selectivity and potential difficulties using oximes/hydrazones as directing groups are discussed, including innovative and sustainable approaches.