Silver Catalyzed CO₂ Transformation With Alkynyl Compounds Under Atmospheric Pressure

With the progress of science and the advancement of industrialization,human society is becoming increasingly dependent on fossil fuels.Meanwhile,many environmental and climatic problems caused by the excessive emission of carbon dioxide?CO₂?emerged and became more and more serious in the past few decades.In the perspective of synthetic chemistry,CO₂ is considered as an excellent C1source which is generally available,cheap,nontoxic,renewable and environmentally friendly.Utilizing CO₂ to produce fine chemicals is of great value in industry.However,the thermodynamic and dynamic stability determined by its molecular structure makes it difficult to directly apply CO₂ to organic synthesis.Therefore,the design of effective conversion pathways,the use of catalysts to reduce the energy barriers and the activation of substrates and CO₂ molecules are attracting more and more attentions from the chemists in the area of CO2 chemistry.Alkynyl compounds are a series of highly active substrates bearing the triple bonds.On the other hand,silver is an excellent catalytic activator to these substrates due to its specific d¹⁰ electronic configuration,which makes it easier to coordinate with the?electrons of the triple bonds.Therefore,the transformation of alkynyl compounds and CO₂ into fine chemicals with the catalysis of the silver catalysts has become a hot topic in CO₂ chemistry.Herein,we would like to focus our research on the following aspects:?1?We designed and synthesized a series of N-heterocyclic carbene?NHC?precursors with sulfonated groups.Silver oxide,potassium iodide were employed together with these NHC precursors as the catalytic systems?NHC/Ag systems?for the carboxylation of terminal alkynes and CO₂.The experimental results indicated that these NHC/Ag systems were a series of excellent catalysts for this transformation.A wide range of terminal alkynes could be converted into the target products under mild conditions.Particularly,the silver loading employed for the aromatic terminal alkynes was considered as the lowest loading ever reported.In the mechanistic investigation,the NHC-CO₂ adducts generated during the reaction process were detected by the Nuclear Magnetic Resonance?NMR?for the first time,which proved the feasibility of this CO₂ activation mechanism in this reaction.?2?We continued to employ these NHC/Ag catalytic systems for the cyclization of propargyl alcohol and CO₂ to generate the?-alkylidene cyclic carbonates.The experimental results indicated that these catalytic systems could convert different kinds of substrates into the target products,including the aromatic internal propargyl alcohol.In addition,this catalytic process could be performed effectively under mild conditions without the addition of extra bases.In the mechanistic investigation,we also detected the presence of the NHC-CO₂ adducts generated during the reaction by means of the NMR spectra.?3?In order to simplify the catalytic system,we designed a silver iodide/potassium acetate?AgI/KOAc?catalytic system.This system is simple,cheap,stable and efficient,which could be employed for the transformation of various types of propargyl alcohols and CO₂ under mild conditions.Moreover,the silver loading involved in this catalysis could be reduced to 0.05 mol%,which is the lowest level of all the reported catalytic systems to the best of our knowledge.In the investigation of catalytic mechanism,we verified that the activation of the hydroxyl group in the substrate was accomplished by the synergistic activation of AgI and KOAc.?4?In the previous AgI/KOAc catalytic system,DMF was inevitably used and the catalysts couldn't be recycled.Therefore,the KOAc in this system was replaced by an environmentally friendly ionic liquid which contains the acetate group as the anion.This updated system,namely the silver iodide/1-ethyl-3-methylimidazolium acetate?AgI/[Emim][OAc]?system,is simple,green,stable and efficient,which exhibited excellent catalytic activity and selectivity for the cyclization of numerous propargyl alcohols and CO₂ generating?-alkylidene cyclic carbonates.In addition,this catalytic system could be recycled for at least 20 times without any significant loss of its excellent activity.To the best of our knowledge,this is considered to be the best recycling performance ever reported.Meanwhile,it is lowest loading system among all the recyclable systems that could work under atmospheric pressure.