Chirale mono- und bedentate Phosphorliganden für die enantioselektive Metallkatalysierte Hydrierung (English shop)

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In this work, novel ligands for the metal-catalysed enantioselective hydrogenation of olefins, imines and acetophenone were developed. In the first part, neutral and anionic monodentate phosphorus ligands were initially prepared for the rhodium-catalysed asymmetric hydrogenation of functionalised olefins. In contrast to the neutral phosphorus ligands, the anionic systems bear a negative charge on their borate moiety.

With regard to asymmetric induction, these anionic monodentate ligands are often more efficient than their neutral analogues. With monodentate chiral phosphorus ligands, very good enantioselectivities of up to 99 % ee were achieved. Furthermore, ligand mixtures consisting of two different monodentate ligands were also tested. NMR studies showed that by using one anionic and one neutral ligand the proportion of the heterocombination can be increased at the expense of the two homocombinations. In most cases, the use of such a mixture led to a further increase in activity and/or selectivity compared with two uncharged ligands.

By choosing a suitable monodentate coordination partner, the proportion of the heterocomplex was increased further. Diol esters of phosphonic acid proved to be particularly efficient partners in this respect. Owing to secondary interactions, these systems in some cases gave a quantitative proportion of the heterocombination. With such systems, the enantiomeric excess was in some cases increased from 76 % to up to 97 % compared with the corresponding homocombination.

In the second part of this work, secondary phosphine oxides (SPO) were investigated both as P,P- and as P,N-ligands for the iridium-catalysed hydrogenation of imines. In particular, SPO ligands with a ferrocene backbone showed promising results with up to 95 % ee.

In the copper-catalysed hydrogenation of acetophenone, ligands forming a larger chelate ring with the metal catalysts proved to be suitable. Therefore, xanthene-based P,P- and P,N-SPO systems were developed and additionally complemented by a phosphinooxazoline-analogous ligand with a xanthene backbone.

These xanthene ligands were tested in the hydrogenation of olefins and acetophenone with various metals. However, so far they have not proven to be successful ligands for metal-catalysed hydrogenation.