Design and Synthesis of Cyclometalated Chiral-at- Ruthenium Complexes for Asymmetric Catalysis
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Philipps-Universität Marburg
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Cycloruthenated complexes are organometallic compounds useful for many interesting applications in the fields of chemistry, physics, and biology, owing to the strong σ-donating ability of the cyclometalating ligand. In addition, this family of complexes exhibits unique catalytic properties for applications in organic synthesis. This thesis deals with developing new types of octahedral cyclometalated chiral-at-ruthenium complexes and demonstrates applications as chiral catalysts in asymmetric catalysis.
Chapter 2. A method for synthesizing mono-cyclometalated ruthenium complexes containing cyclometalated (C^N) and bidentate nitrogen-containing (N^N) ligands was developed. Preparation of these ruthenium complexes was accomplished by a stepwise protocol that relies on the formation of a stable tetra-MeCN-ruthenium(II) precursor in the first step, and a different second ligand was introduced in the subsequent step. Resolution of the racemic complexes into Ʌ- and Δ-enantiomers was achieved by using a chiral sulfinamide auxiliary. Interestingly, during the formation of the auxiliary complexes, geometrical isomerization of the second N^N ligands occurred. Single crystal X-ray analysis showed that one of the bidentate ligands underwent a reorganization within the ligand sphere. The final nonracemic chiral-at-ruthenium complexes contain a cyclometalated ligand, a bidentate nitrogen ligand, one labile acetonitrile ligand trans to the C, one less labile acetonitrile ligand trans to the N, and a counterion.
The newly prepared cyclometalated chiral-at-ruthenium complex containing a phenylpyrazole ligand catalyzes the conversion of diazoketones to chiral flavanones in up to 99% yield with up to 96% e.e. The competing oxygen attack pathway involving the formation and [1,2]-shift of oxonium ylide intermediates was successfully suppressed and is thus in favor of the catalytic enantioselective ring-closing C(sp3)–H carbene insertion. The new method provides access to a variety of enantioenriched flavanones (27 examples), which are privileged scaffolds in diverse biologically active molecules.
The intermolecular [2,3]-sigmatropic rearrangement of sulfur ylides was achieved by using a cyclometalated chiral-at-ruthenium complex containing a phenylbenzothiazole ligand to provide chiral sulfide products in excellent yields (up to 99%) and with good enantioselectivities (up to 80% e.e.). In this case, the steric hindrance of α-diazo ester substrates plays an essential role in chiral control. Further studies showed that the reaction proceeds through a metal-bound ylide intermediate. The catalytic transformations of diazo esters and sulfides generally proceeded rapidly (reaction completes within 5-10 min in most cases) with excellent results.
Chapter 3. A new cyclometalated tetra-MeCN-ruthenium precursor is described, which contains a cyclometalated N-heterocyclic carbene (NHC) ligand. For increasing the stability of the cyclometalated ruthenium complex, a nitro group was introduced into the phenyl moiety. 8-Mesitylimidazo[1,5-a][1,8]naphthyridin-9(8H)-ylidene was used as the second ligand, which affords the first example of a chiral-at-ruthenium complex that has three carbons bound to the ruthenium center. The chiral-at-ruthenium complex contains two achiral bidentate ligands and two labile acetonitriles. One MeCN is trans to the phenyl unit, and the other is trans to the NHC ligand. The resolution of the racemic ruthenium complex afforded lambda- and delta-Ru enantiomers through a single step. However, this complex is sensitive to air and moisture due to the strong σ-donating ability of both ligands. Nevertheless, the freshly prepared complex is configurationally stable, which was used in a highly enantioselective and high-yielding asymmetric intramolecular cyclopropanation of trans-cinnamyl diazoacetate (89% yield and 93% e.e.).
This ruthenium precursor was also used for coordination with a bidentate 4-mesityl-2-(pyridin-2-yl)thiazole ligand and two acetonitriles to complement the octahedral coordination sphere of a monocationic complex. Tetrafluoroborate served as the counterion. Since all coordinated ligands are achiral, the overall chirality is formally due to the stereogenic metal center generating either a left-handed (lambda) or right-handed (delta) helical topology of the chiral-at-metal complex. Enantiopure lambda- and delta-complexes were synthesized using (R)- and (S)-N-benzoyl-tert-butanesulfinamide as the chiral auxiliary ligand, respectively. The position of the nitro group in the metallated phenyl moiety is crucial for the generation of enantiomerically pure lambda- and delta-complexes. The catalytic activity of the new cycloruthenated chiral-at-metal catalyst was demonstrated for the enantioselective intramolecular cyclopropanation of trans-cinnamyl diazoacetate and an alkenyl diazoketone to generate bicyclic cyclopropanes in high yields (96-97%) and with satisfactory enantioselectivity (93% e.e.).
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Created: 2022Issued: 2025-11-06Updated: 2025-11-06
Faculty
Fachbereich Chemie
Language
eng
Data types
DoctoralThesis
Keywords
asymmetric catalysisCyclometallierungcyclometalationcarbene-insertionasymmetrische KatalyseRuthenium-KatalyseRuthenium-catalysisCarben-Insertionchiral-a-metal
DFG-subjects
DiazoverbindungasymmetrischRutheniumChemieKatalyse
DDC-Numbers
540
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Han, Feng: Design and Synthesis of Cyclometalated Chiral-at- Ruthenium Complexes for Asymmetric Catalysis. : Philipps-Universität Marburg 2025-11-06. DOI: https://doi.org/10.17192/z2022.0248.
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This item has been published with the following license: In Copyright