Asymmetric Catalysis with Octahedral Chiral-at-Metal Iridium and Rhodium Complexes
This thesis details the applications of a class of chiral-at-metal iridium(III) and rhodium(III) complexes for asymmetric catalysis. A rhodium-based asymmetric catalyst Δ-RhO is introduced which derives its optical activity from octahedral centrochirality. Besides serving as the exclusive source of...
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|This thesis details the applications of a class of chiral-at-metal iridium(III) and rhodium(III) complexes for asymmetric catalysis.
A rhodium-based asymmetric catalyst Δ-RhO is introduced which derives its optical activity from octahedral centrochirality. Besides serving as the exclusive source of chirality, the rhodium center functions as a Lewis acid to activate α,β-unsaturated 2-acyl imidazoles by two point binding and thereby catalyzes the asymmetric Michael addition of CH-acidic β-dicarbonyl compounds, for which the rhodium catalyst is found to be superior to its iridium congener (chapter 3.1). Due to its straightforward proline-mediated synthesis, high catalytic activity, and tolerance towards moisture and air, this chiral-at-rhodium complex has been used as chiral Lewis acid catalyst for many other asymmetric transformations in the Meggers group.
The chiral-at-metal complexes Δ-IrO and Δ-IrS are investigated as highly efficient dual function photoredox/chiral Lewis acid catalysts in asymmetric photoactivated reactions. A simple chiral iridium complex Δ-IrO is capable of catalyzing the visible light activated α-aminoalkylation of 2-acyl-1-phenyl imidazoles, thereby serving as a “2-in-1” catalyst by combining photoinduced oxidation with asymmetric alkylation (chapter 3.2). Moreover, its derivative Δ-IrS is successfully utilized to the catalytic enantio- and diastereoselective redox coupling of trifluoromethyl ketones with tertiary amines to form 1,2-diamino alcohols (chapter 3.3). This single catalyst strategy provides new avenues for the synthesis of non-racemic molecules.
An alternative strategy of merging the chiral Lewis acid Δ-RhS with photoredox catalyst fac-[Ir(ppy)3] is well applied to the asymmetric photoredox-mediated C(sp3)-H functionalization. This synthetic strategy exploits a radical translocation (1,5-hydrogen transfer) from an oxygen-centered to a carbon-centered radical with a subsequent stereocontrolled radical addition, affording C-C bond formation products with high enantioselectivities (up to 97% ee). Notably, the previously developed dual function catalyst Δ-IrS is not applicable for this asymmetric transformation (chapter 3.4).