Modification Of NHOs-CO₂ Adduct And Application In Carbon Dioxide Activation And Transformation

Carbon dioxide (CO2) is one of the primary greenhouse gases. Accumulation of large amounts of CO2 in the atmosphere leads to the ecological imbalance. CO2 is an abundant Cl feedstock, so its activation and chemical transformations have become a hot research topic.In this thesis, nine modified.N-heterocyclic olefins-CO2 adducts (NHOs-CO2) have been successfully synthesized and identified by HRMS,’H NMR,13C NMR and IR. The molecular structures of adducts 3a and 3f were confirmed by single-crystal X-ray crystallography. These crystal data showed that the O-C-O of NHOa-CO2 adduct angles are 127.5°, which indicates that CO2 was effectively activated by NHO through the nucleophilic attack process. Furthermore, the stability of NHO-CO2 toward D2O and CD3OD (both of them contained trace H2O) was also investigated. The results showed that the structural modificated of NHOs-CO2 adducts have good thermal stability in these polar protic solvents. The thermal stability of different NHOs-CO2 adducts were studied by in situ FT-IR. The alkylated NHOs-CO2 displayed the higher thermal stability. The differences of thermal stability of NHOs-CO2 consist with the crystal data, which means that NHOs-CO2 with the longer CNHO-CCO2 bond show the poorer thermal stability.Moreover, functionalized NHOs-CO2 were employed as organocatalysts for the carboxylation cyclization of propargylic alcohols with CO2. Under optimized reaction conditions (NHOs-CO2 (5 mol%), CO2 (2.0 MPa),3 h,60°), a series of α-alkylidene cyclic carbonates were synthesized in good yields with excellent selectivity. In situ FT-IR experiments showed that the CO2 moiety of products come from free carbon dioxide and NHOs-CO2 adducts served as a nucleophile didn’t decompose in the catalytic process.