Comparing Various Chiral Dirhodium Tetracarboxylates in the Dirhodium Method

Three enantiopure dirhodium tetracarboxylates are compared in their NMR properties to differentiate chiral ligands of various kinds (dirhodium method). ne complex with four (S)-2-methoxy-2-(1-naphthyl) propionate (M alpha NP) 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. Chirality 21:843-849, 2009. (C) 2009 Wiley-Liss. Inc.