Transition Metal-Catalyzed Direct C(sp³)-H Functionalization Reactions of Aliphatic Carboxylic Acids

The selective functionalization of C-H bonds is a cornerstone of contemporary organic synthesis. Carboxylic acids, which range among the most abundant functional groups in nature and manmade molecules, find widespread applications in multiple disciplines. Consequently, substantial research has been devoted to using the weakly coordinating carboxylate group to direct the selective functionalization of C-H bonds. In this context, the activation of C-H bonds has emerged as a powerful strategy to achieve the site-selective functionalization of unactivated C(sp³)-H bonds with diverse coupling partners. Alternatively, radical-mediated approaches have delivered complementary mode of reactivity. This review provides an overview of transition metal-catalyzed C(sp³)-H functionalization reactions, enabled by the carboxylic acid group, spanning contributions from 1991 to 2025. It thoroughly examines the distinct mechanistic manifolds for the C-H functionalization of carboxylic acids, systematically comparing these modes of reactivity and their respective challenges/opportunities. In addition, the discussion encompasses reaction design, substrate scopes, mechanistic features, and a critical assessment of the current methodologies, providing readers with valuable insights to guide future research efforts.