Ternary complexes consisting of chiral rhodium(II) tetracarboxylates, derivatives of amino acid and triphenylphosphine: The 31P NMR study

Thirteen ternary complexes of optically pure chiral rhodium(II) tetracarboxylates with chiral ligands and triphenylphosphine, formed in situ in CDCl3 solution, were examined by nuclear magnetic resonance spectroscopy. As chiral substrates, two dimeric rhodium tetracarboxylates, derivatives of Mosher's acid and N-phthaloyl tert-leucine, were used. Two chiral amines, N,N-dimethyl-butan-2-amine and N,N-dimethyl-1-phenylethanamine, and some derivatives of methionine, selenomethionine, S-methylcysteine, leucine and proline, were applied as chiral ligands, both optically pure and racemic. An achiral component of ternary complexes, triphenylphosphine, acted as a 31P NMR probe. Ternary complexes containing optically pure ligands revealed 31P NMR signals (233 K, CDCl3) within the range from ca. −24 to −36 ppm, appearing as doublets due to 1J(103Rh,31P) coupling constants of ca. 90 Hz. For some combinations of racemic ligand and dirhodium tetracarboxylate, the signals appeared as quartets due to the presence of two diastereomeric ternary complexes. In these cases, diastereomeric dispersion varied from 14 to 88 Hz. Such signal splitting was not observed for the corresponding complexes of optically pure ligands. The presence of bulky substituents in rhodium tetracarboxylate increases diastereomeric dispersion. These results suggested that ternary complexes in question, in combination with 31P NMR technique, can serve, in some cases, as a means of the optical purity analysis of chiral ligands.