Formation of Cyclic Carbonates from Carbon Dioxide and Epoxides Coupling Reactions Efficiently Catalyzed by Robust, Recyclable One-Component Aluminum-Salen Complexes
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Aluminum salen complexes bearing appended quaternary ammonium salt substituents have been synthesized and shown to be effective catalysts for the coupling of epoxides and carbon dioxide to generate cyclic carbonates. 27Al NMR spectra have demonstrated that these exist as both five- and six-coordinate Al(III) species in dimethylsulfoxide (DMSO) solution, whereas only a five-coordinate Al(III) species was detected in the (salen)AlCl analogue in the presence of an external onium salt. The onium salt group tethered on the salen ligand was found to play an important role in enhancing the catalytic activity. The effects of reaction variables such as temperature, time, pressure, molar ratio of epoxide to catalyst on the catalytic performance were systematically investigated. These bifunctional catalysts were found to be highly stable to moisture and oxygen, resistant to impurities, and recyclable with only minor losses in catalytic activity. 2012 American Chemical Society.