Rh(II) metallopeptides for asymmetric catalysis
Ball, Zachary T.; Engel, Paul S.
Doctor of Philosophy
The development of peptides as chiral ligands for asymmetric Rh(II) catalysis is discussed in this work. Mother Nature’s solution to chiral ligand design is to make use of the naturally available chiral building blocks – amino acids. Polypeptides, built from amino acids, provide a diverse, modular and functional-group-rich framework for the development of selective transition- metal catalysts and enable facile ligand screening. The dirhodium core has a paddlewheel structure that can ligate readily to the side chain carboxylate of aspartate or glutamate in a peptide chain. When a peptide containing aspartates or glutamates in the i and i+4 position is complexed to dirhodium, the transition metal complex thus formed has a defined peptide secondary structure with retention of catalytic activity at the metal center. The first-generation approach used a solution-phase library of dirhodium bis-peptide catalysts for the enantioselective Si–H bond insertion and cyclopropanation. The parallel and anti-parallel orientation of bis-peptide catalysts was determined by pyrene fluorescence. Subsequently, parallel on-bead screening of catalyst libraries allowed significantly higher throughput by obviating the need for purification and isolation of individual catalysts. This enables the synthesis and screening of catalysts in 96-well plate format within a few days. More recently, a mono-peptide catalyst with a tridentate eq-eq-ax peptide ligand has been identified which exhibits differential behavior in solution and on bead. New insights into the development of immobilized homogeneous catalysts for stereoselective catalysis are discussed.
metallopeptide; asymmetric catalysis; rhodium catalyst