Fast CO₂ Sequestration, Activation, and Catalytic Transformation Using <i>N</i>‑Heterocyclic Olefins

Abstract

<i>N</i>-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO–CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO–CO₂ adducts with an O–C–O angle of 127.7–129.9°, dependent on the substitute groups of <i>N</i>-heterocyclic ring. The length of the C_carboxylate–C_NHO bond is in the range of 1.55–1.57 Å, significantly longer than that of the C_carboxylate–C_NHC bond (1.52–1.53 Å) of the previously reported NHC–CO₂ adducts. The FTIR study by monitoring the ν­(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO–CO₂ adducts is easier than that of the corresponding NHC–CO₂ adducts. Notably, the NHO–CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10–200 times that of the corresponding NHC–CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO–CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle