Efficient Aluminum Catalysts for the Chemical Conversion of CO2 into Cyclic Carbonates at Room Temperature and Atmospheric CO2 Pressure

A series of dimeric aluminum compounds [Al(OCMe2CH2N(R)CH2X)](2) [X=pyridin-2-yl, R=H (Pyr(H)); X= pyridin-2-yl, R=Me (Pyr(Me)); X=furan-2-yl, R=H (Fur(H)); X= furan-2-yl, R=Me (Fur(Me)); X=thiophen-2-yl, R=H (Thio(H)); X= thiophen-2-yl, R=Me (Thio(Me))] containing heterocyclic pendant group attached to the nitrogen catalyze the coupling of CO2 with epoxides under ambient conditions. In a comparison of their catalytic activities with those of aluminum complexes without pendant groups at N [X=H, R=H (H-H); X=H, R=Me (H-Me)] or with non-heterocyclic pendant groups [X=CH2CH2OMe, R=H (OMeH); X=CH2CH2NMe2, R=H (NMe2(H)); X=CH2CH2NMe2, R=Me (NMe2(Me))], complexes containing heterocycles, in conjunction with (nBu)(4)NBr as a cocatalyst, show higher catalytic activities for the synthesis of cyclic carbonates under the same ambient conditions. The best catalyst system for this reaction is Pyr(H)/(nBu)(4)NBr system, which gives a turnover number of 99 and a turnover frequency of 4.1 h(-1), making it 14- and 20-times more effective than H-H/(nBu)(4)NBr and H-Me/(nBu)(4)NBr, respectively. Although there are no direct interactions between the aluminum and the heteroatoms in the heterocyclic pendants, electronic effects combined with the increased local concentration of CO2 around the active centers influences the catalytic activity in the coupling of CO2 with epoxides. In addition, Pyr(H)/(nBu)(4)NBr shows broad epoxide substrate scope and seven terminal epoxides and two internal epoxides undergo the designed reaction.