Cycloaddition of CO2 and epoxides catalyzed by imidazolium bromides under mild conditions: influence of the cation on catalyst activity
作者:Michael H. Anthofer、Michael E. Wilhelm、Mirza Cokoja、Iulius I. E. Markovits、Alexander Pöthig、János Mink、Wolfgang A. Herrmann、Fritz E. Kühn
DOI:10.1039/c3cy01024d
日期:——
The synthesis and characterization of a library of imidazolium-based compounds of the type [R1R2R3im]Br (R1 = H, CH3, benzyl (Bz), 1-(2,3,4,5,6-pentafluoro)benzyl (BzF5); R2 = H, CH3, C2H5; R3 = n-butyl, n-octyl) are reported. All compounds were characterized by means of 1H and 13C NMR spectroscopy, elemental analysis and single crystal X-ray diffraction in the case of 1-BzF5-3-n-octylimidazolium bromide and examined as catalysts for the cycloaddition of carbon dioxide and propylene oxide (PO) to yield propylene carbonate (PC). The catalyst screening shows the influence of different substituents (steric and ion pairing effects) on the catalytic activity. It is shown that the most active catalyst, 1-BzF5-3-n-octylimidazolium bromide (10), gives very good conversions and carbonate yields at 70 °C and below 5 bar CO2 pressure, which is the lowest temperature for the synthesis of PC from PO and CO2 using metal-free catalysts. Catalyst 10 can be recycled and reused at least 10 times without loss of yield and activity.
报道了一系列咪唑鎓基化合物 [R1R2R3im]Br 的合成与表征(R1 = H, CH3, 苯甲基 (Bz), 1-(2,3,4,5,6-五氟)苯甲基 (BzF5); R2 = H, CH3, C2H5; R3 = n-丁基, n-辛基)。所有化合物通过 1H 和 13C NMR 谱、元素分析以及单晶 X 射线衍射(针对 1-BzF5-3-n-octylimidazolium bromide)进行表征,并作为催化剂用于二氧化碳与环氧丙烷(PO)的环加成反应,以生成碳酸丙烯酯(PC)。催化剂筛选显示不同取代基(立体效应和离子配对效应)对催化活性的影响。结果表明,最活跃的催化剂 1-BzF5-3-n-octylimidazolium bromide (10) 在 70 °C 和低于 5 bar CO2 压力下,能获得非常好的转化率和碳酸盐产率,这是使用无金属催化剂合成 PC 的最低温度。催化剂 10 可以回收再利用至少 10 次而不损失产率和活性。