Lewis basic ionic liquids-catalyzed conversion of carbon dioxide to cyclic carbonates

A series of easily prepared Lewis basic ionic liquids were developed for cyclic carbonate synthesis from epoxide and carbon dioxide at low pressure without utilization of any organic solvents or additives. Notably, quantitative yields together with excellent selectivity were attained when 1,8-diazabicyclo[5.4.0]undec-7-enium chloride ([HDBU]Cl) was used as a catalyst. Furthermore, the catalyst could be recycled over five times without appreciable loss of catalytic activity. The effects of the catalyst structure and various reaction parameters on the catalytic performance were investigated in detail. This protocol was found to be applicable to a variety of epoxides producing the corresponding cyclic carbonates in high yields and selectivity. Therefore, this solvent-free process thus represents an environmentally friendly example for the catalytic conversion of carbon dioxide into value-added chemicals by employing Lewis basic ionic liquids as catalyst. A possible catalytic cycle for the hydrogen bond-assisted ring-opening of epoxide and activation of carbon dioxide induced by the nucleophilic tertiary nitrogen of the ionic liquid was also proposed.