Unexpected CO2 Splitting Reactions To Form CO with N-Heterocyclic Carbenes as Organocatalysts and Aromatic Aldehydes as Oxygen Acceptors

摘要

Carbon dioxide, as the main cause of global warming, attractsnmuch attention around the world. The anthropogenic source ofnatmospheric carbon dioxide is mainly from the burning of fossilnfuel (>60%).n1nAn important pathway to cut global carbon dioxidenemission is its reduction to carbon monoxide.n2nCarbon monoxidencould easily convert water to hydrogen via the industrial well-nestablished “water gas shift reaction”.n3nCarbon dioxide may thusnbe utilized as a “green” renewable source for making fuel andnchemically reduced to carbon monoxide in an economical way.n4nDue to the high stability of carbon dioxide, to split the OdC(O)nbond and generate carbon monoxide requires large energy input.n5nThe current carbon dioxide splitting methods could be classifiedninto four categories: (1) Enzyme carbon monoxide dehydrogenaseacetyl-CoA synthase (CODH/ACS);n6n(2) Photoreduction;n7n(3)nElectrochemical reduction;n8n(4) Use of metal complexes or metalnoxides to abstract the oxygen from carbon dioxide to form carbonnmonoxide in low turnover.n9nRecently, Sadighi group developednorganocopper(I) complexes supported by N-heterocyclic carbenen(NHC) ligands in the reduction of carbon dioxide to carbonnmonoxide with diboron reagent.n10nMetals play an important rolenin all of these catalytic systems. But reduction of carbon dioxidenwith organocatalysts remained widely undeveloped until our groupnreported the first hydrosilylation of carbon dioxide using NHCncatalystn11 13nunder mild conditions recently.n14nIn our efforts to looknfor cheaper and more accessible reductants for CO2 reduction, thennew reaction for carbon dioxide splitting into carbon monoxide withnaromatic aldehydes as oxygen acceptors was successfully developedn(Scheme 1). To our best knowledge, this is the first case in thenreduction of carbon dioxide to form carbon monoxide usingnorganocatalysts.