Synthesis of Cyclic Carbonates from CO2 and Epoxides

A series of catalytic cycles are examined to test the catalyst recycling. As a result, the catalyst (BrBu3PPEG6000PBu3Br) keeps high catalytic activity and selectivity after five cycles. Hence, immobilization of a phosphonium salt on a soluble polymer (e.g. PEG) could provide an alternative pathway for realizing homogeneous catalyst recycling. 

The high efficiency is presumably due to activation of epoxide assisted by hydrogen bonding and activation of C02 by the oxygen atoms of the ether linkages of PEG [16, 17], or the presence of the secondary amino group in the cation, which has the potential to activate C02 molecule [18-24], on the basis of in situ FT-IR investigation under C02 pressure. 

The scope of the substrates is further explored. A series of epoxides are chosen to be tested for the synthesis of the corresponding carbonates under the reaction conditions (BrTBDPEG150TBDBr 1 mol %, 120 °C, 0.1 and 1 MPa, 3-20 h). The catalyst can be applicable to a variety of terminal epoxides, providing the corresponding cyclic carbonates in high yields ( 93 %) and excellent selectivities ( 99 %). 

To gain a deeper insight into the reaction mechanism, in situ FT-IR spectroscopy under C02 pressure is employed to identify the possible intermediates during the reaction proceeding. The in situ FT-IR spectrum of BrTBDPEG150TBDBr before and after reaction with C02 (at 3 MPa, 120 °C) is shown in Fig. 5.1a. There 

Finally, the catalyst is readily recovered after separation from the reaction mixture and then used for the next run without further purification. The results indicate that the yield of PC is almost constant after five successive recycles (Fig. 5.2). 

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A much more convenient method of obtaining five-membered cyclic carbonates is the insertion of gaseous carbon dioxide into the appropriate oxirane ring (Scheme 7). The catalyst development and the reaction mechanisms over alkali metal halides, " organic bases, " metal oxides, zeolite,titanosilicates, " and metal com-plexes " have been reported for the synthesis of cyclic carbonates from CO2 and epoxides. 

Cyclic carbonates are important chemical products and have wide utility as the electrolyte of batteries and capacitors, as polar organic solvents, and as intermediates for the synthesis of different chemical products. Synthesis of cyclic carbonates from CO2 and epoxides is one of the successful examples of CO2 utilization (Scheme 13.14).Various catalysts have been developed for the cycloaddition of CO2 with epoxides. 

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