title: Einfluss elektronischer Faktoren auf die katalytische Aktivität von chiral-at-metal Ruthenium Komplexen
title: Influence of electronic factors on the catalytic activity of chiral-at-metal ruthenium complexes
titleType: AlternativeTitle
Creators: name: Winterling, Erik
Publisher: name: Philipps-Universität Marburg
Year: 2022
Subjects: Asymmetrisch
Katalyse
Chemistry &amp; allied sciences
Chemie
Ruthenium

description: 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 &gt; 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.