Enantiomer Recognition of Amides by Dirhodium(II) Tetrakis[methyl 2-oxopyrrolidine-5(S)-carboxylate]

Association constants of the chiral dirhodium(H) carboxamidate Rh-2(5S-MEPY)(4) with Lewis bases including acetonitrile and amides have been determined by UV-vis titration experiments. With chiral lactams and acyclic acetamides in their R- and S-configurations equilibrium constants with chiral dirhodium carboxamidates are measures of chiral differentiation, and equilibrium constant ratios as high as three have been determined. From equilibrium associations with acetamide, N-methylacetamide, and N,N-dimethylacetamide, as well as equilibrium constants for lactams and acyclic amides, higher values occur when both the amide carbonyl oxygen and N-H are bound to Rh-2(5S-MEPY)(4). This cooperative bonding mode is confirmed by NMR measurements that show a distinctive shift of a N-H absorption, as well as perturbation of the ligands on dirhodium compound, and they suggest N-H association with a ligated oxygen of Rh-2(5S-MEPY)(4). Measurements were made on the dirhodium(H) compound from which protective axial ligands have been removed to enhance their reliability.