Thirteen enantiopure paddlewheel-shaped dirhodium(II) tetrakiscarboxylate complexes have been checked for their efficiency in the dirhodium method (differentiation of enantiomers by NMR spectroscopy); six of them are new. Their diastereomeric dispersion effects were studied and compared via so-called key numbers KN. Adducts of each complex were tested with five different test ligands representing all relevant donor properties from strong (phosphane) to very weak (ether). Only one of them, the dirhodium complex with four axial (S)-N-2,3-naphthalenedicarboxyl-tert-leucinate groups (N23tL), showed results significantly better for all ligands than the conventional complex Rh* [Rh(II) 2 [(R)-(+)-MTPA] 4 ; MTPA = methoxytrifluoromethylphenylacetate]. On the basis of 1 H{ 1 H} NOE spectroscopy and X-ray diffraction, a combination of favourable anisotropic group orientation and conformational flexibility is held responsible for the high efficiency of N23tL in enantiodifferentiation. Both complexes, Rh* and N23tL, are recommended as chiral auxiliaries for the dirhodium experiment.

Complexation of the oxygen atom in 2-butylphenylethers and sulfur in 2-butylphenylthioethers to a rhodium atom in dirhodium tetracarboxylate Rh((II)) (2)[(R)-(+)-MTPA](4) is compared. Oxygen atoms complex via electrostatic attraction exclusively leading to an increase in alpha effects on C-2 complexation shifts in the sequence OCH(3) > F > Br > NO(2). However, that trend is opposite in thioethers. This can be rationalized by an additional highest occupied molecular orbital (HOMO)-LUMO interaction and the response of this interaction upon complex formation shifts. Thereby, an experimental evidence was found for the existence of the HOMO-LUMO binding mechanism which has been proposed previously based on theoretical considerations and indirect spectroscopic evidence. Sulfones hardly bind to Rh((II)) (2)[(R)-(+)-MTPA](4). Diastereomeric dispersion effects at (13)C and (1)H signals can be observed for all compounds indicating that enantiodifferentiation is easy in all classes of functionalities.

Three enantiopure dirhodium tetracarboxylates are compared in their NMR properties to differentiate chiral ligands of various kinds (dirhodium method). The complex with four (S)-2-methoxy-2-(1-naphthyl) propionate (MalphaNP) residues (Rh2) is slightly better for strong donors than the complex with four Mosher acid anions (Rh1), but it is inferior for weak donors. On the other hand, the dirhodium tetracarboxylate complex with four (S)-N-phthaloyl-(S)-tert.-leucinate residues (Rh3) is generally more effective than Rh1. These results are explained by the estimated conformational behavior of the substituents within the equatorial acid residues and the anisotropy (ring-current) effect of aryl groups.

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