N-Heterocyclic Carbene as Catalyst for CO₂ Fixation

Significance

Ikariya and colleagues report the use of N-heterocyclic carbenes as catalysts for ­incorporating CO₂ into electrophilic acceptors. Their previous work revealed that phosphane CO₂ adducts formed in supercritical CO₂ gener­ated carbonates 3 from propargylic alcohols 2. Now, the authors report that the same transformation can be catalyzed by NHC compounds under milder conditions. Optimizing the structure of the NHC, the authors could obtain carbonates 3 in good to excellent yields, and extended the methodology to epoxides 4. The reaction is proposed to be initiated by the nucleophilic addition of im­idazolium-2-carboxylate to the electrophile followed by an intramolecular cyclization of the alkoxide intermediate which regenerates the NHC catalyst.