Multifaceted Role of Oxalic Acid in Organic Synthesis: A Sustainable C1 and Hydrogen Surrogate for Catalytic Transformations.

Transition-metal-catalyzed carbonylation and carboxylation reactions represents one of the dominant fields of synthetic chemistry, enabling the construction of structurally diverse and value-added carbonyl compounds. Although, different C1 surrogates have been developed to replace toxic carbon monoxide (CO), many of these require harsh acidic/basic conditions, metal catalysts, additives, or complex precursors, thereby generating substantial waste with lower atom economy. Herein, oxalic acid emanates as powerful multifaceted C1 synthon, capable of delivering CO, CO2, and H2 through clean, additive-free thermal decomposition. The decomposition profile not only enables in situ gas generation but also uniquely positions oxalic acid as both a carbonyl source and reducing agent. Herein, this account highlights the development of oxalic acid from a traditional reagent to strategically important platform in sustainable synthesis. Special emphasis is placed on development of oxalic acid-mediated protocols for functionalized carboxylic acids, ketones, alkynones, thioesters, bis(indolyl)methanes, formamides, heterocycles, and carbocycles synthesis. Furthermore, innovative reaction system designs including single- and dual-vial systems that employed oxalic acid as sustainable in situ and ex situ gaseous surrogate have also been highlighted. Overall, this article underscores the practical utility of oxalic acid as a bench-stable, cost-effective, and environmentally benign trifunctional reagent, paving way for next-generation carbonylation chemistry aligned with green synthetic principles.