Harnessing the Unquenched Reactivity of Novel Frustrated Lewis Pairs for Transition Metal-Free Hydrogenation Catalysis
Final Report Abstract
The frustrated Lewis pair methodology is based on the tailored interplay of a Lewis acid with a Lewis base. The electronic properties and the steric environment of both partners are the basic design principals and vital parameters to avoid the Lewis acid/base adduct formation. In this project novel FLP catalysts derived from (1R)-(+)-camphor were synthesized and tailored for the application in asymmetric hydrogenation, hydrosilylation and hydroboration reactions. The reactivity of these catalysts was tested in different asymmetric transformations, with a special focus on the role of both FLP partners in the activation of the respective reagents and the enantioselectivity. In the asymmetric hydrogenation, the Lewis base had minor influence on the enantiomeric excess obtained in the transformation and enantioselectivity up to 83 %ee could be obtained. Moreover, the developed chiral intramolecular FLP catalyst showed a significant increase in stability and enabled efficient catalyst recycling. However, in the asymmetric hydrosilylation and hydroboration, the Lewis base has a vital effect on the rate of the reaction and the enantiomeric excess obtained. Consequently, combinations with different Lewis bases were tested and the best enantioselectivity was obtained using t-Bu3P in combination with the chiral Lewis acid. Further modifications in the perfluorinated ring of the Lewis acid catalyst showed a significant influence on reactivity and enantioselectivity, resulting in the phospholane derived chiral catalyst with an enantiomeric excess of 88 %ee in the asymmetric hydrosilylation of imines. In the hydroboration reaction compounds new FLP catalysts could be established as effective catalysts, resulting in an enantioselectivity up to 80 % ee. Based on detailed mechanistic investigations the influence of the two FLP components in the enantioselective hydroboration was investigated, resulting in a detailed reaction mechanism and guiding principles for the development of new catalyst systems for this transformation. Furthermore, new bifunctional perfluorinated borane compounds were synthesized and fully characterized. The different arrangement of the two Lewis acids in these compounds showed a profound influence on reactivity and enantioselectivity, and catalysts 7 could be established as novel effective catalysts system, paving the way for future FLP catalyst development. As perfluorinated boranes represent the critical part in FLPs chemistry, a new synthetic pathway for the important hydroborating reagents, HB(C6F5)2 and HB(p-C6F4H)2 could be developed, facilitating the synthesis of novel metal-free catalysts in the future.
Publications
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„Frustrated Lewis Pair-Catalysts hydrogenation, hydrosilylation and hydroboration” (12/2015)
Ghazi Ghattas
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„Entwicklung multifunktioneller Ruthenium/Lewis- Säure Komplexe zur Aktivierung kleiner Moleküle“ (03/2018)
Christoph Malbertz
