(661f) Direct carboxylation of thiophene and CO2 in the cesium carbonate system

Based on green chemistry and atomic economy principles, the direct carboxylation of thiophene and CO₂ is an important research direction in the field of CO₂ utilization. The main carboxylate products have been widely applied in the fields of medicine and materials. However, it is difficult for both substrates to achieve the carboxylation reaction under mild conditions due to the relatively stable CO₂ and less active thiophene. The highly active organometallic reagents are generally employed to promote the carboxylation reaction accompanied by the potential environment and safety problems. The direct carboxylation of thiophene is primarily achieved in the batch reactor with Cs₂CO₃ and simple carboxylates (cesium acetate, formate and oxalate and pivalate) as the reaction medium. The synergistic effect of Cs₂CO₃ and simple carboxylates is observed, and the cesium pivalate is the most efficient additive in the carboxylation reaction. The effects of different reaction factors (reaction temperature, Cs₂CO₃ proportion, CO₂ pressure, total salt amount, substrate amount and reaction period) are investigated. Meanwhile, high temperature in-situ X-ray diffraction (in-situ XRD) characterization technique indicates that the direct carboxylation is carried out in the heterogeneous system with Cs₂CO₃-CsOPiv eutectic phase and Cs₂CO₃ solid phase. The carboxylation mechanism is further speculated in this system. The complex [Cs₂CO₃-CsOPiv] is primarliy formed through the interaction of Cs₂CO₃ and CsOPiv, which can activate and further break the C-H bond of thiophene, and the CO₂ insertion is easily conducted to form C-C bond of carboxylate products.