Design, Synthesis, and Application of a Nucleophilic Octahedral Stereogenic-Only-at-Metal Iridium(III) Catalyst
««Main Topic»» The design, synthesis, and application of a versatile chiral nucleophilic iridium(III) catalyst, which features a stereogenic octahedral metal center as its exclusive element of chirality ('stereogenic-only-at-metal'), is presented. The devised catalyst features two custom-...
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The design, synthesis, and application of a versatile chiral nucleophilic iridium(III) catalyst, which features a stereogenic octahedral metal center as its exclusive element of chirality ('stereogenic-only-at-metal'), is presented. The devised catalyst features two custom-tailored bidentate cyclometalated phenylbenzoxazole ligands as well as a bidentate deprotonated 7-chloro-3H-imidazo[4,5-h]quinoline ligand, which serves as the catalyst's nucleophilic catalytically active site.
At first, 28 different stereogenic-only-at-metal iridium(III) complexes were synthesized, evaluated, and the best-performing complex eventually selected. In this regard, it was revealed that the selected catalyst candidate was in fact the protonated precatalytic form of the actual catalyst.
Next, it could be demonstrated that the developed stereogenic-only-at-metal complex serves as an efficient catalyst for the asymmetric Steglich rearrangement of O-acylated azlactones (up to 96% ee, up to 99% yield), for the asymmetric Black rearrangement of O-acylated benzofuranones (up to 94% ee, up to 99% yield), and for the asymmetric addition of 2-cyanopyrrole to aryl alkyl ketenes (up to 95% ee, up to 99% yield).
Insights into the mechanism of the Steglich and the Black rearrangements with the developed stereogenic-only-at-metal catalyst, in particular into the catalyst's manner of enantioinduction, could be gained with a crystal structure of the active catalyst, with a crystal structure of a catalysis intermediate analog in combination with a DFT-assisted active site accessibility analysis, and with the explicit quantum chemical modeling of the stereogenic step of a showcase Black rearrangement with a slightly simplified model of the devised catalyst. The respective theoretical calculations were planned and coordinated in close collaboration with and performed by cooperation partner Michael G. Medvedev.
Secondary topic of the present thesis is the attempted development of a cooperative bifunctional stereogenic-only-at-metal enamine/hydrogen-bonding catalyst, which was intended to catalyze Michael additions of enolizable aldehydes and / or ketones to nitroalkenes and / or nitroacrylates.
A synthetic access to the envisioned catalysts could be successfully established, however, stability problems with the complexes as well as unsatisfactory catalysis results ultimately led to the termination of this project. In spite of that, important knowledge could be attained regarding the synthesis of amine-functionalized bis-cyclometalated iridium(III) complexes and C2-symmetric bis-cyclometalated bispyrazole iridium(III) complexes, which has already contributed to successfully accomplished projects from the Meggers group and which may accordingly be helpful for future projects.|
|Physical Description:||366 Pages|