Minimal Artificial Enzymes: Expanding the Application of "Backdoor Induction" through new Substrates and new Asymmetric Reactions
This project aims at the development of chiral transition metal catalysts with particular emphasis on inspiration taken from nature. A beta-turn structure is proposed as key feature of chiral catalysts that resemble artificial enzymes with a minimal but functional outer-coordination sphere. These minimal artificial enzymes could find applications in asymmetric synthesis of drugs or drug intermediates produced by pharmaceutical industry.
In asymmetric catalysis it is well established that the chiral information should be incorporated as close as possible to the coordination sphere of the catalytic metal. We challenge this important principle and use a catalytic metal center that is only prochiral. The chirality is transmitted by "backdoor induction" via hydrogen bonding of distant pendant amino acids that represent the outer-coordination sphere. Preliminary results provide a proof of concept and strongly indicate that simple amino acid bioconjugates of monodentate triphenylphosphane ligands can form supramolecular L2M complexes with Rh(I) that selectively catalyze a model asymmetric hydrogenation reaction with major impact of the distant amino acid sequence on selectivity [Kirin and Kokan, RSC Adv. 2012, Eur. J. Org. Chem. 2013 and Organometallics 2014; ee’s up to 84 %].
Within this project, a significant extension is proposed including (i) the use of new ligand bioconjugates with phosphorous or nitrogen donor atoms, (ii) the use of new substrates and (iii) the use of other transition metals for different asymmetric reactions, namely Ru(II)-catalyzed hydrogenation, Pd(II)-catalyzed alkylation and Ln(III) catalyzed silylcyanation.