Continuous-Flow Asymmetric Hydrogenation of the β-Keto Ester Methyl Propionylacetate in Ionic Liquid-Supercritical Carbon Dioxide Biphasic Systems

A continuous-flow process for the asymmetric hydrogenation of methyl propionylacetate as a prototypical beta-keto ester in a biphasic system of ionic liquid and supercritical carbon dioxide (scCO2) is presented. An established ruthenium/2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) catalyst was immobilised in an imidazolium-based ionic liquid while scCO2 was used as mobile phase transporting reactants in and products out of the reactor. The use of acidic additives led to significantly higher reaction rates and enhanced catalyst stability albeit at slightly reduced enantioselectivity. High single pass conversions (>90%) and good enantioselectivity (8082% ee) were achieved in the first 80 h. The initial catalyst activity was retained to 91% after 100 h and to 69% after 150 h time-on-stream, whereas the enantioselectivity remained practically constant during the entire process. A total turnover number of 21,000 and an averaged space-time yield (STYav) of 149 g?L-1?h-1 were reached in a long-term experiment. No ruthenium and phosphorus contaminants could be detected via inductively coupled plasma optical emission spectrometry (ICP-OES) in the product stream and almost quantitative retention by the analysis of the stationary phase was confirmed. A comparison between batch-wise and continuous-flow operation on the basis of these data is provided.