Multifaceted Role of Oxalic Acid in Organic Synthesis: A Sustainable C ₁ 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.