Stereoselective synthesis of cobalt(III) anionic complexes with chiral pyruvylaminoacid oximes and metal–ligand interactions in aqueous solution

A series of cobalt(III) anionic complexes with hydroxyimino analogs of dipeptides – oximes of pyruvylglycine, pyruvyl-l-alanine, pyruvyl-l-methionine and pyruvyl-l-phenylalanine (H2pamaco, amac = amino acid) of composition Cat3[Co(H−1pamaco)2] · nH2O (Cat – a monovalent metal cation) has been synthesized and investigated by UV–Vis, CD and 1H NMR spectroscopy. It was established that for the two latter ligands, non-racemic mixtures of two diastereomeric complexes were formed as a result of synthesis. In comparison with the complex containing pyruvyl-l-alanine, in which the relative content of R (C2) and S (C2) isomers is approximately equal, the presence of bulky substituents in the molecules of Met- and Phe-containing ligands results in a significant discrimination in formation of the S (C2) isomer. This conclusion was drawn upon analysis of the CD and NMR spectral data as well as from molecular modeling. The relative content of the diastereomers was evaluated on the basis of NMR data. The protonation constants of the ligands have been determined in aqueous solution by glass electrode potentiometry, whereas their interactions with Co(II) were studied both under argon and oxygen atmosphere.

A series of Co(III) anionic complexes with hydroxyiminoanalogs of dipeptides has been synthesized and investigated. For Met and Phe a significant discrimination of the S(C2) isomer is observed in comparison with R(C2). Equilibria in solution have been studied.Figure optionsDownload as PowerPoint slide