Zwitterionische Iridiumkomplexe als Katalysatoren für enantioselektive Hydrierungen

In this thesis different variants of zwitterionic iridium precatalysts for the enantioselective
hydrogenation of unfunctionalised alkenes were developed. First, systems based on imidazolines
and oxazolines were synthesised, whose anionic moiety is located in the periphery of the
respective P,N-ligands (figure 49).
These betaines were evaluated in direct comparison with their cationic structural analogues
regarding their efficiency in the asymmetric reduction of selected substrates. With respect to
the borate-functionalised compounds, which were studied in more detail, it was shown that
the new zwitterionic and the established charged precatalysts yield similar results when the
solvent is dichloromethane. In contrast to that the zwitterions give higher conversions in
aliphatic hydrocarbons. Furthermore, it was observed in the case of the oxazolines that the
location of the anchoring point between the ligand backbone and the counterion exerts a
pronounced influence on the characteristics of the complexes. The betaines with more coordinating
aryltrifluoroborate anions show even in dichloromethane an up to fourfold reactivity
compared to their cationic reference systems.
For the synthesis of the species in figure 49 several borate building
blocks for the anionic modification of neutral molecules were developed
(figure 50), which can be introduced by both nucleophilic and
electrophilic reactions. These molecules may also be of interest in
areas other than asymmetric catalysis, for example mass spectrometry
or Ziegler-Natta polymerisation.
Two indenide-based iridium betaines were synthesised as exemplary representatives of the
second class of zwitterionic coordination compounds, which bears the negative charge directly within the ligand backbone (figure 51). These showed only low efficiency
in enantioselective hydrogenation reactions though.
Employing a series of dimeric,
monocationic iridium complexes
with different free counterions
(figure 52), further evidence was
found that the reduced activity of
the established precatalysts with more coordinating
anions is caused by an acceleration of the decomposition
of the active species relative to the productive
steps of the hydrogenation.
Finally, the concept of using negatively charged ligands
was successfully transferred to the rhodium-catalysed
reduction of functionalised olefins with the help of monodentate
phosphorus ligands (figure 53) and promising
preliminary results were obtained there.
In addition to the application of zwitterionic coordination
compounds in enantioselective reactions the influence of
the substituent at the aminic nitrogen atom in imidazoline-
based precatalysts on their chiral induction was
studied (figure 54). It was shown by a combination of
X-ray crystallographic studies and asymmetric hydrogenation
experiments that this is steric rather than electronic
in nature.
Last but not least, a new class of iridium complexes was synthesised
starting from the cheap amino acid serine (figure 55).
Interestingly its representatives are, in contrast to the established
phosphinooxazoline derivatives, deactivated to iridium dimers.
Apart from that, these precatalysts are significantly more
efficient in the enantioselective reduction of unfunctionalised
alkenes than structurally related systems, which are retrosynthetically
derived from tert-leucine.