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Novel Metal Amido-Complexes – Syntheses, Reactivity and Asymmetric Catalysis
(2010)
- In the context of this thesis two classes of novel imidazo[1,5-b]pyridazine-substituted amines 2 were developed. Imidazo[1,5-b]pyridazine-substituted amines can be synthesized in high purity and good yields via the nucleophilic ring transformation of oxadiazolium halides 1 and N-nucleophiles, followed by deacetylation and cyclocondensation with 1,3-diketones. The deprotonated amines can act as monoanionic amido-ligands. Previous results regarding diamine-bridged imidazo[1,5-b]pyridazines have shown, that the deprotonated compounds are suitable for the stabilization of early as well as late transition metal complexes. Since only dinuclear group 9 metal complexes could be obtained, one objective of this work was to enable the synthesis of mononuclear amido-complexes by means of a novel ligand structure. Thus, a series of imidazo[1,5-b]pyridazine-substituted (pyridylmethyl)amines was synthesized via a one-pot approach. Salt metathesis or alcohol elimination route were chosen for the synthesis of the iridium amido-complexes. The (2-pyridylmethyl)amine-derived complexes exhibited an unusual reactivity in solution. An intermolecular C-C coupling reaction of the mononuclear complexes was observed, yielding a dimeric species. Based on mechanistic and kinetic investigations, it was postulated that the coupling reaction is due to tautomerization yielding an enamido hydrido complex, which subsequently undergoes an intermolecular attack. This gives rise to the dimeric species with iridium mediated hydrogen evolution. Because of the modular ligand design, optically active imidazo[1,5-b]pyridazine-substituted amines can easily be obtained via the utilization of chiral N-nucleophiles such as amino alcohols. Motivated by previous results regarding chiral imidazo[1,5-b]pyridazine-stabilized iridium amido-complexes, which exhibit high selectivities and good activities in the asymmetric hydrogenation of ketones, the development of amido-complex catalysts for the enantioselective hydrogenation of imines represents a major focus of this work. A library of novel amines 4 was synthesized by deprotonation of the hydroxyl function of 3 with nBuLi followed by the addition of chlorophosphines or chlorophosphite. Alcohol elimination reaction of 4 with 0.5 equiv. of [MOCH3(cod)]2 (M= Ir, Rh) gave rise to transition metal amido-complexes, which were applied to the asymmetric hydrogenation of N-aryl imines. Upon activation with KOtBu moderate initial selectivities and good activities were obtained for rhodium amido-complexes. Following the optimization of the reaction conditions (temperature, pressure, base) a ligand screening was performed. The highest activities and selectivities in the asymmetric hydrogenation of various imines were obtained by combining electron donating P-substituents (iPr) and amino alcohols (iBu). Additionally, the catalyst loading was reduced from 1 mol%, which represents the common usage, to only 0.1-0.2 mol%. Thus, a novel ligand motif, based on chiral imidazo[1,5-b]pyridazines, was established for the efficient rhodium-catalyzed asymmetric hydrogenation of N-aryl imines. In the third section of this thesis a novel potassium-mediated synthesis of 6-aminofulvenes from N-aryl imines is introduced. During the hydrogenation experiments regarding the optimization of the added base, the formation of a by-product was observed, if potassium hydride was utilized as a base. The by-product could be identified as novel 6-aminofulvene, namely [(2,4-diphenyl-cyclopenta-2,4-dienylidene)-phenyl-methyl]-phenyl-amine. Upon this exciting discovery, the reaction conditions leading to fulvene formation were explored by means of reaction stoichiometry and added base. The resulting novel synthesis route was applied to various N-aryl imines. Mechanistic as well as kinetic investigations indicated, that the reaction is based on the formation of the metalated enamine, which nucleophilicly attacks the imine-carbon. The herein reported new synthesis method provides an easy access to novel 6-aminofulvenes.
