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Einfluss elektronischer Faktoren auf die katalytische Aktivität von chiral-at-metal Ruthenium Komplexen
Transition metal complexes with metal centered chirality, also called chiral-at-metal complexes, provide a useful and expandable toolbox for asymmetric catalysis. This thesis focuses on the synthesis of chiralat- metal ruthenium complexes with previously unused substituents to explore the electron...
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| Format: | Doctoral Thesis |
| Language: | English |
| Published: |
Philipps-Universität Marburg
2022
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| Online Access: | PDF Full Text |
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| Summary: | Transition metal complexes with metal centered chirality, also called chiral-at-metal complexes, provide
a useful and expandable toolbox for asymmetric catalysis. This thesis focuses on the synthesis of chiralat-
metal ruthenium complexes with previously unused substituents to explore the electronic influence
concerning reactivity and enantioselectivity.
Within the first part of the thesis (Chapter 3.1) the synthesis of bis(pyridinyl-NHC) ruthenium complexes
with 5-bromo and 4-dimethyl amino substitution at the pyridine moiety is presented. During the investigation
of structural isomers, the formation of a mixed normal/abnormal NHC complex was observed
and further investigated. By lowering the temperature and extending the reaction time, a ratio of 9.8:1
in favor of the mixed nNHC/aNHC complex was achieved. Formation of this mixed coordination was
only observed for 5-CF3 (62%) and 5-bromo (53%) substituted complexes, while trimethylsilyl as well
as 4-dimethyl amino substitution showed selective formation of the nNHC complex.
The separation of diastereomeric complexes was realized by application of (tolylsulfonyl)benzamides
as chiral auxiliaries. Due to the non–C2–symmetry of the mixed complexes, four diastereomers can
be formed. Two of the resulting complexes were stable against silica gel and were found to consist
of the same metal-centered chirality. Brùnsted acid induced labilization of the auxiliary ligand under
retention of configuration created the corresponding complexes with up to > 99% ee. The same protocol
was applied to di-isopropylphenyl substituted complexes, providing access to sterically more demanding
complexes.
The second part of this thesis (Chapter 3.2) focuses on the application of the obtained complexes in
asymmetric catalysis. In detail, the ring contraction of isoxazoles to 2H-azirines was investigated. Here,
the 5-bromo substituted complex allowed a faster conversion at lower temperatures compared to the previously
reported rhodium complexes. After a broad screening of reaction conditions, a maximum of
67% ee was obtained. During the investigation, non-linear effects were observed as well as a light induced
racemisation of the product catalyzed by the applied ruthenium complexes. For the asymmetric
C(sp3)-H amination reaction, the newly obtained mixed nNHC/aNHC complex was found to provide an
160-fold increased turnover frequency compared to the analogus nNHC complex. This allowed a reduction
of the reaction time down to 10 min. For the alkynylation of trifluoroacetophenone, the differences
between the nNHC and mixed nNHC/aNHC complexes were negligible. For a propargylic substitution,
the di-isopropylphenyl substituted complexes showed higher enantioselectivity (up to 55% ee), the
mixed nNHC/aNHC complex provided low enantioselectivity (11% ee) while the bis-nNHC complex induced
no enantioselectivity. Initial UV-Vis studies with acyl imidazoles indicated a broad absorption
around 550 nm. Among the tested complexes, the 4-dimethyl amino substitution showed a maximum at
640 nm, followed by the mixed nNHC/aNHC complexes, while the bis-nNHC complexes showed maxima at 520 nm. This could be used in further studies concerning the potential application of ruthenium
complexes in asymmetric photocatalysis. |
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| DOI: | 10.17192/z2022.0121 |