Enhanced enantioselectivity of chiral hydrogenation catalysts after immobilisation in thin films of ionic liquid

Chiral organometallic complexes were immobilized in silica supported thin films of ionic liquid. The heterogenized catalysts were tested in the hydrogenation of acetophenone, which was chosen as test reaction for the enantioselective reduction of prochiral ketones. High enantioselectivities (up to 74% ee) were achieved in supported ionic liquids using a catalyst/substrate pair, which showed no enantioselectivity in methanol. This is attributed to the unique solvent properties of ionic liquids including the formation of solvent cages of ionic liquid molecules around the complexes. Within these cages, transition states are modified and chemical transformations follow alternative reaction pathways.

Chiral complexes immobilized in silica supported thin films of ionic liquid provided considerable enantioselectivity in the hydrogenation of acetophenone. In contrast, no enantioselectivity was observed in the corresponding homogeneous catalysis. As explanation for this phenomenon, the formation of solvent cages of ionic liquid molecules around the organometallic complexes is proposed, which leads to enhanced substrate–catalyst interactions. Figure optionsDownload as PowerPoint slide