Synthesis of Propiolic and Butynedioic Acids via Carboxylation of CaC₂ by CO₂ under Mild Conditions

Carbon dioxide (CO₂) is a greenhouse gas, and its resource use is vital for carbon reduction and neutrality. Herein, the nucleophilic addition reaction of calcium carbide (CaC₂) to CO₂ was studied for the first time to synthesize propiolic and butynedioic acids by using CuI or AgNO₃ as catalyst, Na₂CO₃ as additive, and triphenylphosphine as ligand in the presence/absence of a hydrogen donor. The effects of the experimental conditions and intensification approach on the reaction were investigated. The reactivity of CaC₂ is closely associated with its synergistic activation by the catalysts, solvent, and external intensification, such as the ultrasound and mechanical force. Ultrasound helps to promote the reaction by enhancing the interfacial mass transfer of CaC₂ particulates. Mechanochemistry can effectively promote the reaction, yielding 29.8% of butynedioic acid and 74.8% of propiolic acid after 2 h ball milling at 150 rpm, arising from the effective micronization and interfacial renewal of calcium carbide. The present study sheds a light on the high-value uses of CO₂ and CaC₂ and is of reference significance for the nucleophilic reaction of CaC₂ with other carbonyl compounds.