ABSTRACT
The long-term stability of chirally modified platinum catalysts used in the enantioselective hydrogenation of ethyl pyruvate was investigated. Using chirally modified platinum colloids, the washing out of the modifier from the catalyst's surface could be avoided. For the retention of the quasi-homogeneous catalyst, a novel heterogenization method has been developed and applied to the model reaction. This new heterogenization method is based on the three-dimensional entrapment of the Pt-colloid by electrostatic attraction between a polyelectrolyte and the oppositely charged catalyst. Different polyanions have been found to be suitable for entrapment of the chirally stabilized Pt-colloid. Using alginate for heterogenization, a constant enantioselectivity and a good activity during 25 hydrogenation cycles were obtained if cyclohexane was chosen as solvent in the enantioselective hydrogenation of ethyl pyruvate. In this solvent the long-term stability of the chiral catalyst could be improved considerably since the destructive hydrogenation of the modifier taking place in acetic acid and causing a drop in enantioselectivity could be suppressed.
