The carboxylative cyclization of propargyl alcohol and carbon dioxide catalyzed by organocatalysts

Abstract

Carbon dioxide (CO2), as a cheap, non-toxic, and abundant C1 resource, can be transformed into high value-added chemicals by constructing C−C, C−O, C−N, and C−S bonds, which is in agreement with the requirements of green chemistry and sustainable development. For example, CO2 can not only be transformed into fuel-related formic acid, methanol etc., but also serves as raw materials for the synthesis of fine chemicals such as carbonates, carbamates and salicylic acid. However, CO2 has the disadvantage of thermodynamic stability and chemical inertness, which can hinder its chemical conversion and utilization. It has become one of the research hotspots to develop efficient catalysts of activating CO2 and reducing the reaction energy barrier in the field of CO2 conversion. Under the continuing efforts of chemists, a series of highly efficient catalysts have been developed and applied in the catalytic conversion of CO2. The carboxylative cyclization of propargyl alcohol to prepare α-alkylidene cyclic carbonates is one of desired chemical reaction in CO2 fixation, which presents 100% atomic utilization and meets the requirements of green chemistry. Now, investigations on the carboxylative cyclization of propargyl alcohol and CO2 have been developing rapidly, and a series of novel reaction systems have been reported. In the carboxylative cyclization, transition metals are extensively studied. Various transition-metal catalytic systems, including silver (AgI), copper (CuI), zinc (ZnII), are closely investigated in the carboxylative cyclization. The transition-metal salts can effectively activate the carbon-carbon triple bond in the propargyl alcohol structure, facilitating the subsequent attack of carbon-carbon triple bonds by alkyl carbonate anions to generate the target products. Although the transition metals are efficient catalysts in this cyclization reaction, their disadvantages, such as high cost and transition-metal residues, are not favourable. Recently, transition metal-free organocatalysts have been becoming one hotspot in the catalysis field, and found wide applications in the field of CO2 conversion, burgeoning in the literature. A series of organocatalysts, such as ionic liquids (ILs), organic strong bases, phosphines and CO2 adducts, have been reported in this carboxylation cyclization reaction. Herein, an overview about the research progresses of organocatalyst-catalyzed carboxylative cyclization is provided, with a particular emphasis on the application of ionic liquids, CO2 adducts and phosphines, and their corresponding mechanisms. Rationally designed CO2-reactive site-containing ILs could activate CO2 effectively, in which the anion is the main contributor, while the cations make a lower contribution. By introducing one or more CO2-reactive sites into the anion structure of ILs, linear CO2 molecules can transformed into active carbonate anions and carbamate anions, thereby achieving the effect of activating CO2. In virtue of CO2-reactive sites, ILs-catalyzed carboxylation cyclization is realized under mild conditions. Strong nucleophiles are found to easily react with CO2 molecule to generate CO2 adducts, such as N -heterocyclic carbenes-CO2 adducts (NHC-CO2), N-heterocyclic olefins-CO2 adducts, alkylated N -heterocyclic olefins CO2-adducts (ANHO-CO2), carbodicarbenes-CO2 adducts (CDCs-CO2), alkoxide-functionalized imidazolium betaines CO2-adducts(AFIB-CO2) and so on, which can serve as efficient organocatalysts in the carboxylative cyclization reaction. Phosphines also present strong nucleophilicity, can activate CO2 and propargyl alcohol molecules by nucleophilic addition to CO2 and carbon-carbon triple bonds of propargyl alcohols. Furthermore, chiral phosphines can act as chiral organocatalysts to afford optically active functionalized cyclic carbonates. Taken together, these rationally designed organocatalysts exhibited excellent performance, obtaining a series of α-alkylidene cyclic carbonates in good yields. In this paper, the mechanisms of the carboxylative cyclization promoted by organocatalysts are closely discussed, hoping to pave a way for the design of more efficient and green organocatalysts.