Isolable CO2 Adducts of Polarized Alkenes: High Thermal Stability and Catalytic Activity for CO2 Chemical Transformation

Various CO2 adducts of tetra-hydropyrimidin-2-ylidene (THPE) derived from the commercially available 1, 5-diazabicyclo[4.3.0]non-5-ene (DBN) were firstly synthesized. X-ray single crystal analysis revealed the bent geometry of the binding CO2 having an O-C-O angle of 127.50 similar to 129.51 degrees for THPE-CO2 adducts. In situ FTIR experiments demonstrated that THPE-CO2 adducts had unprecedented thermal stability in DMSO, even at 100 degrees C without decomposition. It was found that the THPE-CO2 adducts were highly active in catalyzing the carboxylative cyclization of CO2 with propargylic alcohols under mild conditions, significantly higher than the previously reported organocatalysts. Various internal and terminal functionalized propargylic alcohols were tolerated in these processes to afford the corresponding alpha-alkylidene cyclic carbonates in moderate to good yields with complete (Z)-stereoselectivity. Isotope labeling, in combination with in-situ FTIR and stoichiometric experiments, reveal that the catalytic reaction tends to proceed via the THPE-CO2-mediated basic ionic pair mechanism.