Chirale ionische Flüssigkeiten in der homogenen Katalyse (2009)

The aim of asymmetric synthesis is to produce enantiomerically enriched compounds, which are for example important for pharmaceutical or agrochemical commodities. Different methods can be applied to achieve this: the separation of racemic compounds, the use of chiral substrates or the application of chiral reagents (such as catalysts) in the reaction of prochiral substrates. In this work, the use of chiral Ionic Liquids (IL = ionic liquid) as chiral reagent was investigated. The first part therefore addresses the synthesis and characterization of chiral ILs. The intention hereby was the efficient synthesis of pure salts and the expansion of the synthesis to new, structurally related ILs. In doing so two different categories of chiral ILs were examined. Starting with L-(-)-malic acid an anionchiral IL with a borate structure was synthesized. The optimization of the synthesis of these borates clearly showed the impact of concentration and temperature on the anion exchange reaction. Through extraction and drying impurities were removed from the compound. The resulting IL with higher purity possessed a higher glas transition temperature and a much higher viscosity. The second class of IL wears the chirality in the cation and was synthesized from L-(-)-proline. Via variation of the nitrogen substituent (R = H, Me, Bn) new structures were generated and the properties were characterized. The N-methylated compound with [NTf2]- as counterion is liquid at room temperature and thermically stable up to 300 °C. In the N-benzylated salt crystallization occured after a few days. In the second part of the thesis the impact of the chiral ILs on homogeneous catalysis was investigated. For this, the influence of the anionchiral IL on the organocatalyzed Aza-Baylis-Hillman reaction was analyzed. Addition of the chiral IL led to a loss in selectivity and increased formation of a byproduct: through the hydrolysis of the substrate the corresponding aldehyde was formed. With low concentrations of the ioniq liquid an increase in selectivity was achieved. Moreover, very good conversions and selectivities were obtained in the Aza-Henry reaction of aromatic imines using hydroquinidine or DABCO as catalyst in conventional solvents. Upon changing the solvent to achiral ILs the activity of the catalysts was sustained. This work shows the first example of a successful Aza-Henry reaction in ILs. In the anionchiral IL moderate selectivities up to 49% were achieved. Representing another categorie of reactions the rhodiumcatalyzed hydrogenation of dimethylitaconate with chiral Bisphosphineligands was investigated. An asymmetric induction can not only be reached through chiral ligands, but also very successfully through the method of asymmetric (de-)activation of racemic ligands. When using the racemic BINAP-ligand in combination with the cationchiral IL on the basis of proline enantioselectivities up to 69% were reached. Using kinetic and spectroscopic measurements, the impact of the chiral IL as chiral deactivator of one enantiomere of the catalyst was analyzed and the impact of the reaction condition determined. By 31P-NMR-measurements it was shown, that the (R)-enantiomer of the racemic catalyst is partially deactivated through the chiral IL. The degree of deactivation and therefore the ee of the catalyst depends on the ability of the IL to selectively build a complex with the catalyst. A sterically demanding IL was not able to deactivate the catalyst precursor, however, an increasing amount of IL led to higher enantioselectivities in the hydrogenation and suggests a higher degree of deactivation. Nevertheless, the partial deactivation of one catalyst enantiomer led to the same ee in the hydrogenation as the enantiopure ligand.