The Carboxylative Cyclization System Of Propargyl Alcohol And CO₂ Without The Participation Of Precious Metals

The amount of carbon dioxide accompanying the combustion of fossil fuels in the atmosphere has increased year by year,causing global warming.But carbon dioxide is a rich and cheap carbon resource that is cheap and readily available.The rational development of different systems to convert carbon dioxide is a problem that needs to be solved urgently,and the effective utilization of carbon dioxide resources has also become a focus of attention.The combination of carbon dioxide and propargyl alcohol to obtainα-alkylene cyclic carbonate is an effective and environmentally friendly strategy for converting carbon dioxide resources.However,the strategies for converting carbon dioxide toα-alkylene cyclic carbonates mostly focus on transition metal salt systems.Developing a green and efficient system without the transition metals is a reasonable requirement in carbon dioxide conversion.In this paper,R-Cu Mg Al-LDHs system and ROCO₂Na/ROCO₂H buffer solution system were developed and applied to CO₂ conversion.Among them,for the R-Cu Mg Al-LDHs system,after a series of condition screening and control experiments,it is found that the coordinated catalysis of LDHs and Cu⁰/Cu⁺species in the R-Cu Mg Al-LDHs efficiently convert CO₂ and propargyl alcohol substrates into cyclic carbonates.In the ROCO₂Na/ROCO₂H buffer solution system,the introduction of CO₂ into the propargyl alcohol/sodium tert-amyloxide system can obtain a ROCO₂Na/ROCO₂H buffer solution with basic and acidic substances.The presence of the basic ROCO₂Na and the acidic substance ROCO₂H in the buffer solution can simultaneously promote the formation of the carbonate anion intermediate and the interaction between the alkyne portion of the propargyl alcohol and the H⁺proton,thereby promoting the efficient carboxylative cyclization reaction.1.The reaction system of R-Cu Mg Al-LDHsBy screening the amount of catalyst in the system,temperature and time,the optimal conditions for the CO₂ conversion of the system are obtained.5 mol%of R-Cu Mg Al-LDHs and 1.0 equivalent of DBU can efficiently catalyze the conversion of CO₂ at 80℃and 5 h.Moreover,under the optimal reaction conditions,it has good substrate compatibility for propargyl alcohol substrates containing different functional groups.In addition,the control experiment also provides strong evidence for the reaction mechanism.The large specific surface area of the layered structure of LDH in the catalyst and the loaded Cu⁰/Cu⁺species can synergistically and efficiently catalyze the conversion of CO₂.And the catalyst can be recycled for more than 4 times with good stability and recyclability.2.The reaction system of ROCO₂Na/ROCO₂H buffer solutionBased on the optimization of reaction conditions,the optimal conditions of the buffer solution system for CO₂ conversion were obtained.The cyclization reaction of non-alkyne propargyl alcohol is closely related to the p H of the buffer solution system（the species and amount of base,CO₂ pressure）and the additives.The best reaction conditions were illustrated as followed:propargyl alcohol（1.0 mmol）,Na Ot P（2.4 mmol）,TBAB（2.4 mmol）in 1,4-dioxane for 24 h at 80 ^oC and 2.0 MPa CO₂.Moreover,non-terminal propargyl alcohols containing alkyl,methoxy,halogen,and aryl substituents show excellent compatibility in this buffer solution system at 80 ^oC.Variousα-alkylene cyclic carbonates with different functional groups are obtained in a moderate to good yields.Based on the results of control experiments,the reaction mechanism of the buffer solution was demonstrated.The alkyl carbonate in the buffer solution system provided a basic environment for this reaction,while the acidic alkyl carbonate could provide H⁺activate the alkyne triple bond.Moreover,the bulky（n-C₄H₉）₄N⁺cation in TBAB as an additive plays a vital role in enhancing the nucleophilicity of the carbonate anion intermediate.