Dokument

Summary:
This thesis is deals with the development of new types of octahedral chiral-at-ruthenium and chiral- at-iron complexes. The chiral-at-iron complexes were investigated for applications in asymmetric catalysis. In the first section, we introduce a new type of racemic and non-racemic octahedral ruthenium complexes where the metal center is cyclometalated by two 7-alkyl-1,7-phenanthrolium ions, giving chelating pyridylidene remote N-heterocyclic carbene ligands. The octahedral coordination sphere is incorporating two additional acetonitriles and the dicationic complexes are complemented by two hexafluorophosphate counterions. The racemic complexes are synthesized in a C 2 -symmetric geometry. Resolution of racemic complexes with a chiral sulfinamide auxiliary, followed by treating with TFA, results in non-C 2 -symmetric chiral-at-ruthenium complexes. Single crystal X-ray analysis showed that one of the bidentate ligands underwent a reorganization within the ligand sphere. Depending on the helical twist of the bidentate ligands, the metal center can adopt either a Λ or Δ metal-centered configuration. These chiral-at-ruthenium complexes are stable and enantiomeric enriched as confirmed by 1 H NMR and CD spectroscopy. Furthermore, the trans-effect of the pyridylidenes in this family of ruthenium complexes is investigated by crystallographic analysis, indicating the stronger electron-donating ability of pyridylidenes in comparison to pyridyl donors, as well as imidazol-2-ylidenes in related complexes. In the second section, the first example of a chiral-at-iron complex for asymmetric catalysis is described, which is exclusively constructed with achiral bidentate and monodentate ligands and the iron ion serves as the stereogenic center. The cis-geometric coordination consists of two N-(2-pyridyl) imidazol-2-ylidene ligands and two acetonitrile ligands. Two acetonitrile ligands are labilized due to a strong electron donating imidazolylidene ligand in trans-position and hence promotes asymmetric transition metal catalysis. This is demonstrated with an enantioselective Cannizzaro reaction (96% yield, 89% ee) and an asymmetric Nazarov cyclization (89% yield, >20:1 d.r., 83% ee). A highly enantioselective and diastereoselective transformation catalyzed by this new class of chiral-at-iron catalysts is developed by introduction of high steric crowding around the catalytic site. An iron complex bearing bulky 2,6-diisopropylphenyl moieties at the imidazolylidene ligands, proved to be the best catalyst for inverse electron demand hetero-Diels-Alder cycloadditions of b,r-unsaturated a-ketoester with enol ethers, providing 3,4-dihydro-2H-pyrans in high yields with excellent diastereoselectivities (up to 99:1 dr) and excellent enantioselectivities (up to 99:1 er). An electron rich vinyl azide is also cyclized to the corresponding product with excellent diastereoselectivity (95:5 d.r.) and enantioselectivity (95% ee).

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