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Quantum Chemical Investigations in Homogeneous Catalysis: Dehydroperoxidation and Asymmetric Reductive Amination

Hermsen, Marko

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This thesis describes the investigation of homogeneously catalyzed reactions with quantum chemical methods. Two different reactions were studied in this work: the dehydroperoxidation of alkyl hydroperoxides with both vanadium and chromium catalysts and the direct asymmetric reductive amination of ketones with ruthenium.

In the first part, the dehydroperoxidation of cyclohexyl and 4-heptyl hydroperoxide to the corresponding ketones with a vanadium dipicolinato complex is investigated. It is found that a radical-free mechanism is feasible and that it proceeds through hydrogen abstraction by the vanadium oxo group. The barrier difference for this process for both substrates is in line with higher experimental selectivities for the non-cyclic hydroperoxide. A mechanistic study on the chromium-catalyzed dehydroperoxidation follows, which shows that a similar mechanism is active for Cr. The better selectivity and activity of this catalyst in comparison with the vanadium system is reproduced as the activation energy for dehydroperoxidation is lower with chromium. Finally, we rule out that the reaction proceeds via an intramolecular hydrogen transfer in an alkoxy/alkylperoxo chromium species, which has been suggested in previous research on the topic. The second part of the thesis explores the mechanism of the direct asymmetric reductive amination of ketones with a ruthenium (S,S)-f-binaphane complex. Acetophenone is used as the model ketone for this reaction. The investigations show that the rate-determining step of the reaction is the proton transfer from a σ-dihydrogen complex to liberate the amine. A thorough analysis of possible isomer/conformer combinations turns out to be crucial for a quantitative understanding of the reaction; following such an analysis, the experimental enantioselectivity is accurately reproduced for a series of catalysts with different halide ligands. These results are supplemented by studies on the chemoselectivity of the reaction. The mechanism is then applied to build a simple model for estimating the bite angle dependency of the reaction’s enantioselectivity. It is predicted that larger bite angles should favor higher enantioselectivity. A search in the CCDC database reveals several promising ligand backbone candidates for an optimized binaphane ligand of which a biaryl motif is finally considered as the most promising candidate.

Item Type: Dissertation
Supervisor: Comba, Prof. Dr. Peter
Place of Publication: Heidelberg
Date of thesis defense: 25 October 2018
Date Deposited: 11 Feb 2020 14:58
Date: 2020
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institute of Inorganic Chemistry
Subjects: 540 Chemistry and allied sciences
Controlled Keywords: Computerchemie, Homogene Katalyse
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