Highly enantioselective Ru-catalyzed asymmetric hydrogenation of β-keto ester in ionic liquid/methanol mixtures

The enantioselective hydrogenation of methyl acetoacetate (MAA) was studied in detail using a ruthenium-monodentate binaphthophosphepine complex in a homogeneous solution formed by different ionic liquids and methanol. Remarkably, ionic liquid additives did not only open an attractive way for catalyst recycling in repetitive batch-mode but also led to significantly increased catalytic activity compared to pure methanol. Enantioselectivities up to 95% have been achieved in mixed ionic liquid/methanol systems which are comparable to the values obtained in pure methanol.

Graphical abstractA group of ionic liquids, which increase the activity of a ruthenium-phenyl-4,5-dihydro-3H-dinaphtho[2,1-c;1′2′-e]phosphepine complex, was identified, synthesized and applied as co-solvent in the asymmetric hydrogenation of MAA in methanol. In these systems the enantioselectivities were found to be in the same range like for pure methanol reaching typical values of 90–96%.Figure optionsDownload full-size imageDownload as PowerPoint slide