A robust ionic liquid as reaction medium and efficient organocatalyst for carbon dioxide fixation

摘要

The increasing production ofcarbon dioxide from industrial activities has caused a variety ofenv ironmental problems such as global warming and acid rain over the past few decades. Such environmental consequences have called for the development ofan effective approach in recent years to reducing this harmful greenhouse gas. Transformation of carbon dioxide into industrially valued chemicals is generally regarded as an excellent route in this respect because ofits significant values in both environmental preservation and resource utilization.[1] To this end, a variety ofsynthetic approaches that utilize CO2 as the feedstock for producing organic carbonates have been developed.[2, 3] In particular, addition reaction of CO2 with epoxides to give cyclic carbonates with 100% atom efficiency (Scheme 1) is regarded as an efficient environmentally benign way for fixation of abundant CO2.[4] The resulting cyclic carbonates are valuable fine chemicals which are widely used as solvents, electrolytes, starting materials in polycarbonate production, and intermediates in the manufacture of pharmaceuticals and agricultural chemicals.[5] The addition reaction of CO 2 with epoxides is known to be catalyzed by organic bases,[6] ammonium and phosphonium salts,[7] metal halides,[8] metal oxides[9] and some transitionmetal complexes.[10] Recent studies have shown that a number ofimida zolium ionic liquids can serve as an effective medium for the transformation of CO2 to organic carbonates with certain metal halides as the catalyst/co- catalyst.[4e,f,11] Despite this encouraging observation, this catalytic system is less robust and requires the use ofheavy metals in the synthetic process, thus reducing its applicability and environmental benignancy. As such, we are interested in developing an efficient and environmentally benign catalytic system that offers the advantages ofhigh reactivity and selectivity, easy separation and recycling, and does not require the use ofmetals and organic solvents. This 'green' catalytic system is generally thought to be promising in the production ofcarbo nates.[12] Herein, we report the construction ofa novel and robust ionic liquid 1Br3 that can function both as an efficient organocatalyst and a reaction medium in the synthesis ofcyclic carbonates with atmospheric carbon dioxide under low temperature and pressure conditions (Scheme 2).