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Supramolecular Asymmetric Catalysis with Pseudo-Peptide containing L2M complexes

Rational design and optimization of new catalysts is one of the most important goals in supramolecular asymmetric catalysis dealing with weak interactions, e.g. hydrogen bonding and π∙∙∙π stacking.[1, 2] For this type of catalysis, amino acids and peptides are a convenient source of chirality with the capacity of forming diverse hydrogen bonding structures (helixes, sheets and turns). The mechanism of chirality transmission from the catalyst to the substrate is not well understood in many systems. Implying that, C2-symmetric ligands have received considerable attention. In this work, pseudo-peptide ligands L1–L14 with phosphorus or nitrogen donor atoms were prepared. Supramolecular metal complexes with 1:2 metal to ligand stoichiometry can be obtained by binding the ligands to a transition metal (Rh(I), L1–L10) or a rare earth metal (Yb(III), L11–L14). In the ML2 species, intra-complex hydrogen bonding of the amino acid side chains consequently defines a pseudo-C2-symmetric spatial arrangement around the metal center. Longer side chains allow the formation of more inter-strand hydrogen bonds making the metal center more rigid and therefore provide higher enantioselectivity. The prepared metal complexes were characterized by various spectroscopic techniques and tested as catalysts in model enantioselective reactions. [1] P. W. N. M. van Leeuwen (Ed.), Supramolecular Catalysis, Wiley-VCH, Weinheim, 2008. [2] P. J. Walsh and M. C. Kozlowski, Fundamentals of Asymmetric Catalysis, University Science Books, Sausalito, 2009.

amino acids; asymmetric hydrogenation; enantioselective catalysis; metallated bioconjugates; Rh(I) complexes

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