✍ Scribed by Hans-Ulrich Blaser; Hans-Peters Jalett; Felix Spindler
No coin nor oath required. For personal study only.
A case study concerning the development of enantioselective hydrogenation processes for aliphatic a-ketoesters, particularly ethyl 2-oxo-4-phenylbutyrate (1) and ethyl pyruvate (2), using homogeneous Rh-diphosphine and heterogeneous Pt/A1203cinchona catalysts is presented. Important parameters to obtain high enantioselectivities using the homogeneous system were shown to be the ligand, the nature of the Rh complex and the solvent. Unfortunately, the best optical yields (ee 96%) were achieved only at a low substrate/catalyst ratio of s/c = 50. For the heterogeneous system, the structure and concentration of the modifier, the nature and pretreatment of the Pt/AI203 catalyst, the type of solvent and the hydrogen pressure were decisive for high enantioselectivity (ee's up to 95%). A major problem was the effect of the substrate quality on rate and optical yield. A comparison of catalyst costs showed a significant advantage for the heterogeneous catalyst, mainly due to its very high activity and catalyst productivity.
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