New Catalytic Properties of Chiral-at-Metal Complexes and a Cyclometalated Ru Complex
Asymmetric transition-metal catalysis constitutes one of the most powerful strategies to construct non-racemic chiral molecules. This thesis deals with enantioselective catalysis of chiral-at-metal iridium and ruthenium complexes as well as a chiral mono-cyclometalated ruthenium complex. 1) Kinetic...
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|Summary:||Asymmetric transition-metal catalysis constitutes one of the most powerful strategies to construct non-racemic chiral molecules. This thesis deals with enantioselective catalysis of chiral-at-metal iridium and ruthenium complexes as well as a chiral mono-cyclometalated ruthenium complex.
1) Kinetic resolution of racemic epoxides with CO2 catalyzed by a chiral-at-metal bis-cyclometalated iridium complex was accomplished, and s-factors between 6.4 and 16.6 were obtained for overall 21 monosubstituted epoxides containing diverse functional side chains. Notably, all reactions were performed at room temperature, and no copolymerization side reaction which occurred often in other catalytic systems was observed (Chapter 3.1).
2) Enantioselective intramolecular benzylic C-H amination of primary aliphatic azides was achieved by using a chiral-at-metal bis(pyridyl-NHC) ruthenium complex in combination with tris(p-fluorophenyl)phosphine (both 1 mol%) to provide a variety of chiral -aryl pyrrolidines with enantioselectivities of up to 99% ee. In this unique case, the phosphine serves as a crucial nitrene transfer co-catalyst and activates the organic azide through the formation of an intermediate iminophosphorane. This methodology offers direct access to non-racemic -aryl pyrrolidines which are very important structural motifs in many bioactive compounds. (Chapter 3.2).
3) A chiral cyclometalated ruthenium catalyst enabled direct enantioselective and highly diastereoselective oxidative homocoupling of 2-acyl imidazoles in the presence of one equivalent BrCCl3 to provide chiral symmetric 1,4-dicarbonyl compounds in 38-75% yield with 57-95% ee. Only one diastereomer was obtained for all the investigated substrates. Mechanistic experiments support a unique ruthenium catalyzed two-steps mechanism. The first step is a ruthenium catalyzed bromination of 2-acyl imidazole generating a brominated intermediate, followed by a ruthenium catalyzed stereo-controlled radical-enolate reaction providing the final product (Chapter 3.3).|
|Physical Description:||256 Pages|