Cooperative Activation Strategy On Base Metal-catalyzed Transformation Of Carbon Dioxide Under Atmospheric Pressure

The large consumption of fossil fuel has contributed to the continuous growth of carbon dioxide?CO₂?concentration in the atmosphere,which is regarded as the primary factor resulting in the Global Warming.From the standpoint of green synthesis,CO₂ is a promising C₁ resource with advantages of abundance,easy availability,non-toxicity,non-flammability and renewability.In this context,the development of catalyst system with high efficiency for the transformation of CO₂into value-added chemicals is of significant research value in both sustainable development and organic synthesis.However,due to the highest oxidation state of central carbon atom and the linear molecular structure of CO₂,the CO₂ molecule features the characters of thermodynamic stability and kinetic inertness.Thus,the efficient utilization of CO₂ under mild conditions especially at atmospheric pressure still remains challenging.This paper focuses on the incorporation of CO₂ into five-membered heterocycles i.e.cyclic carbonates and 2-oxazolidinones by cycloaddition of epoxides or carboxylative cyclization of propargylic amines with CO₂,respectively.The cooperative activation strategy has been applied for the construction of heterocycles with CO₂ as a building block under atmospheric pressure.On the development of catalytic system,non-noble and low toxic calcium or zinc Lewis acid catalysts are investigated for the alternatives of precious metal catalysts thus providing an eco-friendly synthetic methodology.?1?Withcalciumbromide?CaBr₂?asLewisacid,1,8-diazabicyclo[5.4.0]undec-7-ene?DBU?as Lewis base,N,N-dimethylformamide?DMF?as solvent,the cycloaddition of epoxides with CO₂ under atmospheric pressure was achieved for the synthesis of cyclic carbonates.A series of calcium halides in cooperation with organic bases were examined for this cycloaddition reaction.Solvent effect and catalyst loading were also investigated.The turnover number?TON?can up to 3300 when 0.01 mol%of calcium catalyst was employed.The kinetic analysis was based on kinetic curve and in-situ infrared spectroscopy?in situ FT-IR?.In situ FT-IR was also applied for monitoring the reaction process and the depolymerization mechanism was ruled out.In addition,the activation role of CaBr₂towards propylene oxide was studied using density functional theory?DFT?calculations.With this catalytic system,aromatic or aliphatic substrates with electron-withdrawing or electron-donating substituents can deliver the corresponding carbonates in 87%⁹9%yields.?2?The zinc chloride?ZnCl₂?and 1,5,7-triazabicyclo[4,4,0]dec-5-ene?TBD?can generate zinc?II?complex ZnCl₂?TBD?₂ in situ to serve as catalytic active species for the carboxylative cyclization of propargylic amines with atmospheric pressure of CO₂to synthesize 2-oxazolidinones.We have investigated the catalytic efficiencies of a series of zinc?II?salts in combination with N-contained organic bases.Such conversion could proceed under solvent-free conditions.Control experiments were carried out for exploring the possible intermediate.A hypothetic intermediate was proposed for reasonable interpretation of control experiments results,in which zinc was assumed to be capable of stabilizing carbamate anion as well as enhancing its O-nucleophilicity.Besides,we also investigated the interaction between zinc?II?and carbon-carbon triple bond of substrate using ¹³C NMR spectroscopy,thus suggesting the activation effect of carbon-carbon triple bond by zinc?II?.