title: CO2 adducts of N-heterocyclic carbenes: thermal stability and catalytic activity toward the coupling of CO2 with epoxides.
abstract: Thermal stability of CO2 adducts of N-heterocyclic carbenes (NHCs) was studied by means of in situ FTIR method with monitoring of the nu(CO2) region of the infrared spectra under various conditions. 1,3-Bis(2,6-diisopropylphenyl)imidazolinium-2-carboxylate (SIPr-CO2) shows higher thermal stability compared with 1,3-bis(2,6-diisopropylphenyl)imidazolium-2-carboxylate (IPr-CO2). The presence of free CO2 can significantly inhibit the decomposition of NHC-CO2 adducts, while the addition of an epoxide such as propylene oxide has a negative effect on stabilizing these adducts. As zwitterionic compounds, NHC-CO2 adducts were also proved to be effective organic catalysts for the coupling reaction of CO2 and epoxides to afford cyclic carbonates, for which a possible mechanism was proposed. Among these NHC-CO2 adducts, the relatively unstable IPr-CO2 exhibits the highest catalytic activity. The presence of an electrophile such as SalenAlEt could greatly improve the catalytic activity of IPr-CO2 due to intermolecular cooperative catalysis of the binary components.
authors.affiliation: State Key Laboratory of Fine Chemicals, Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, China.
authors.forename: Hui
authors.initials: H
authors.lastname: Zhou
authors.forename: Wen-Zhen
authors.initials: WZ
authors.lastname: Zhang
authors.forename: Cui-Hua
authors.initials: CH
authors.lastname: Liu
authors.forename: Jing-Ping
authors.initials: JP
authors.lastname: Qu
authors.forename: Xiao-Bing
authors.initials: XB
authors.lastname: Lu