Frustrated Lewis pair catalysts for asymmetric hydrogenation, hydrosilylation and hydroboration reactions
- Frustrierte Lewis-Paar Katalysatoren für die asymmetrische Hydrierung, Hydrosilylierung und Hydroborierungsreaktionen
Ghattas, Ghazi; Klankermayer, Jürgen (Thesis advisor); Leitner, Walter (Thesis advisor); Niggemann, Meike (Thesis advisor)
Dissertation / PhD Thesis
Dissertation, RWTH Aachen, 2016
Asymmetric catalysis using molecular transition metal complexes represents an important synthetic tool in basic research and industrial chemistry. The recent development of frustrated Lewis-pair catalysts (FLP) enabled the possibility to perform these transformations with metal-free systems. This thesis describes the synthesis of novel chiral FLP catalysts and their application in selected asymmetric hydrogenation, hydrosilylation and hydroboration reactions. In the first part of the thesis, the development of new chiral boranes derived from (R)-Camphor is presented, with a detailed focus on the stability and recyclability of the respective FLP catalysts. Based on these catalysts the enantioselective hydrogenation of various imines to the corresponding amines could be achieved with an enantiomeric excess up to 83 % ee. Moreover, the novel FLP structures allowed reusing the catalysts in consecutive reactions at constant enantioselectivity. In the second part of the thesis, metal-free asymmetric hydrosilylation and hydroboration reactions were investigated. In the hydrosilylation of imines an enantiomeric excess up to 88 % ee could be achieved using novel tailored FLP catalysts. The subsequent application of these FLP catalysts in the asymmetric hydroboration was challenging and additional information on the respective reaction mechanisms had to be obtained, to guide the catalytic experiments and the catalyst optimization. Consequently, detailed NMR spectroscopic investigations paved the way towards an efficient catalysts system and up to 80 % ee could be achieved in the hydroboration of selected imines. However, the future applicability of these metal-free catalysts strongly depends on the synthetic accessibility of versatile borane reagents, the important part of the FLP system. Therefore, a novel synthetic pathway to HB(C6F5)2 and HB(p-C6F4H)2 starting from low cost and non-toxic starting materials could be established, avoiding laborious preparation steps and enabling the multigram scale preparation of the respective important boranes.