| Utilization of carbon dioxide (CO2) is an important research direction of green chemistry. The key issue for CO2chemical transformation is its activation. Because CO2is at the highest oxidation state of carbon, it is so thermodynamically stable. Although CO2is apolar, it contains two polar C-O bonds. The center carbon atom of CO2as an electrophile is easily attacked by strong nucleophiles, which causes the linear structure of CO2into bent geometry, making CO2under activated state. N-heterocyclic olefins (NHOs) and alkoxide-functionalized imidazolium betaines (AFIBs) have strong nucleophilicity, so they possess great potentiality in the application of CO2activation. However, there are no reports about activation and catalytic transformation of CO2. As a result, the above two systems were detailedly researched to activate and transform CO2to selectively give a-alkylidene cyclic carbonates in this dissertation.(1) A series of NHOS-CO2adducts were successfully designed and synthesized and crystal data revealed the bent geometry of the binding CO2in the NHOS-CO2adducts with an O-C-O angle of127.7～129.9°, which clearly indicated CO2under high activation state in these adducts. The thermal stability of NHOS-CO2adducts were systematically investigated by means of in situ FTIR method with monitoring the v(CO2) region of infrared spectra. The result showed that thermal stability of NHOS-CO2adducts obviously were decreased with the increasing temperature. The length of the CNHO-CCO2bond is in the range of1.55～1.57A, significantly longer than that of the CNHC-CCO2bond (1.52～1.53A) of the previously reported NHCS-CO2adducts. This demonstrated that the decarboxylation of NHOS-CO2adducts is easier than that of the corresponding NHCS-CO2adducts and NHOS-CO2adducts have higher activity. Taking the carboxylative cyclization of2-methyl-4-phenylbut-3-yn-2-ol with CO2as model reaction, the optimum conditions were established by screening of reaction parameters: NHO-CO2adduct (5mol%), propargylic alcohol (3mmol), CO2(2.0MPa),12h,60℃. NHO-CO2adduct was demonstrated to be an efficient organocatalyst for the carboxylative cyclization of CO2and various propargylic alcohols at mild mild reaction condition, selectively giving α-alkylidene cyclic carbonates in good yields. It was found that NHO-CO2adduct was superior to NHC-CO2adduct in catalyzing the carboxylative cyclization by kinetic studies. The plausible mechanism of the carboxylative cyclization was suggested on the basis of deuterated labeling experiments.(2) Alkoxide-functionalized imidazolium betaines (AFIBs), including an alkoxide anion and an imidazolium cation, were successfully designed and synthesized. AFIBs were able to fast react with CO2or CS2, affording the zwitterionic adducts (AFIBs-CO2or AFIBs-CS2adducts). In the presence of adventitious water, AFIBS-CO2adducts could transform into the corresponding bicarbonate salts. Furthermore, AFIBS-CS2adducts are more stable to moisture in comparison with their CO2adducts. X-ray single crystal analysis revealed the bent geometry of the binding CS2in the AFIBs-CS2adducts with a S-C-S angle of126.6Φ126.9°, which indirectly confirm the structures of the AFIBS-CO2adducts in hand. AFIBS-CO2adducts could effectively function as organocatalysts for the coupling reaction of terminal propargylic alcohols with CO2to selectively synthesize the valuable cyclic carbonates. |
