Study On NHC-CO₂ For The Coupling Of Carbon Dioxide With Epoxides

In this paper,a series of CO₂ adducts of N-heterocyclic carbenes are designed and synthesized.Then,they are employed in the coupling reactions of carbon dioxide with epoxides as the catalysts.And the effects of molecular structure on the catalytic activity are observed.The results indicate that the catalytic activity is obviously affected by the thermodynamics stability of NHC-CO₂.In contrast with others,IPr-CO₂(3b) shows the highest catalytic activity on the same reaction conditions.It can effectively catalyze the coupling reaction of propylene oxide with CO₂ in 100%yields of propylene carbonate with 100%selectivity at 120℃with 2.0 Mpa CO₂ in 24 hours.A binary system of IPr-CO₂ and SalenAlEt is employed in the coupling reactions of epoxides with CO₂.The result indicates that binary IPr-CO₂/SalenAlEt system could remarkably enhance its catalytic activity in comparison to the IPr-CO₂ adduct alone as catalyst.It can catalyze the coupling reaction of propylene oxide with CO₂ in 100%yields of propylene carbonate with 100%selectivity at 120℃with 2.0 Mpa CO₂ only in 8 hours.It is suggested that there is an intermolecular Lewis acid-base cooperative catalysis in the binary components,which significantly increases the catalytic activity.On the basis of above-mentioned view,possible catalysis paths are studied by employed IPr-CO₂/SalenAlEt to catalyze the coupling reactions oftrans-1-deuterio-1,2-hexene oxide with CO₂.In order to increase the stability of NHC-CO₂ adducts,neutral and acidic MCM-41-supported CO₂ adducts of N-heterocyclic carbene are respectively synthesized and employed in the coupling reaction of CO₂ with epoxides.And the effects of molecular structure on the catalytic activity as well as the recycle feasibility are investigated.The result shows that in comparison to Si-MCM-41-NHC-CO₂ adduct,A1-MCM-41-NHC-CO₂ adduct exhibits higher catalytic activity that possibly due to the cooperative effect in the catalyst between the Lewis-acidic site A1 and the Lewis-basic site NHC-CO₂ adduct or free NHC.The recycle feasibility of MCM-41-supported NHC-CO₂ adducts are investigated by taken A1-MCM-41-NHC-CO₂ adduct as catalyst.The result indicates it could be recycled based on no obvious decrease in the yields of propylene carbonate after three times of reaction.