Silver(Ⅰ)-Catalyzed Three-component Reactions Of Propargyl Alcohols,Carbon Dioxide And O-Nucleophiles

Carbon dioxide is abundant,cheap,easily available,nontoxic and nonflammable.Therefore,utilization of CO₂ as the renewable carbon source to prepare value-added products,such as urea,salicylic acid,methanol and so on,has gained much attention over the past decades.However,thermodynamically feasible processes for CO₂conversion are limited and reactions involving CO₂ are always carried out under harsh conditions such as high temperature and/or high CO₂ pressure because of its thermodynamic stability and kinetic inertness.Therefore,development of thermodynamically favorable reactions including CO₂ will be significant and challenging for efficient CO₂ transformation especially under mild conditions.Organic carbonates,as a kind of heterocyclic compounds with extensive application,are commonly used in agriculture,pharmaceutics and lithium battery industry.Traditional synthetic methods commonly use toxic reagents such as CO and phosgene.In view of Green Chemistry and Sustainable Development,usage of CO₂as a cheap and environmentally friendly raw material to prepare organic carbonates is attractive.This thesis aims at developing thermodynamically favorable CO₂ transformation based on the reaction of nucleophiles withα-alkylidene cyclic carbonate obtained from propargyl alcohols with CO₂.Using CO₂ as raw material to synthesize cyclic carbonates under mild conditions has been achieved via the silver-catalyzed three-component reactions of propargyl alcohols,CO₂ and O-nucleophiles.1)Three-component cascade reactions of propargyl alcohols,CO₂ and monohydric alcohols have been developed to synthesizeβ-oxopropyl carbonates under mild conditions with silver carbonate/triphenylphosphine as the catalytic system.Results of stepwise experiments demonstrate this three-component reaction proceeds through the carboxylative cyclization of propargyl alcohols with CO₂,followed by transesterification ofα-alkylidene cyclic carbonates with monohydric alcohols.Control experiments are carried out to investigate the role of silver species,showing that silver compound could promote both the carboxylative cyclization and transesterification.The current catalytic system exhibits wide substrate scope and various propargyl alcohols and monohydric alcohols could well react with CO₂,giving the correspondingβ-oxopropyl carbonates in 36%to 98%yields and the structure of the product is confirmed by a single-crystal X-ray analysis.In addition,gram-scale experiment indicates this strategy is practical.2)The direct condensation of vicinal diols and CO₂ is an ideal route access to cyclic carbonates.However,this reaction is limited by thermodynamic equilibrium.Using dehydrating agent to remove the formed water in the reaction could promote the equilibrium shifting to the product and therefore effectively improve the yield of cyclic carbonate.Nitriles are commonly used dehydrating agents.However,one of the problems when using nitriles as the dehydrating agents is the lowered selectivity of targeted molecule due to the side reaction induced by hydrolytic products of nitriles with vicinal diol.To this,the silver（I）-promoted three-component cascade reaction is developed for synthesis of cyclic carbonates from propargyl alcohol,CO₂and vicinal diol.Compared with direct condensation of vicinal diol with CO₂,this protocol provides a thermodynamically favorable route to cyclic carbonate without the dehydration step.Experimental results show that cyclic carbonate could be successfully synthesized by using this three-component strategy with silver carbonate and organophosphate as the catalytic system.Under the optimized conditions（1 MPa CO₂,80 ^oC）,various vicinal diols could be transformed into the corresponding cyclic carbonates in 32%to 91%yields.A possible reaction pathway is proposed with the assistance of control experiments:propargyl alcohol initially reacts with CO₂ to giveα-alkylidene cyclic carbonate,subsequent nucleophilic attack of vicinal alcohol onα-alkylidene cyclic carbonate leads to the formation of cyclic carbonate.Through labeling experiments,α-alkylidene cyclic carbonate from CO₂ is confirmed as the unique carbonyl source.