Phosphine-Rh2[(R)-MTPA]4 Adducts in Solution: Characterization by NMR Spectroscopy and Chiral Discrimination

The adducts of dirhodium tetraacylates and phosphines are characterized in solution by 1H and 31P NMR spectroscopy at room temperature. A differentiation of the enantiomers of chiral phosphine ligands is easily performed by NMR signal integration after adduct formation with the enantiopure Rh2[(R)-(+)-MTPA]4 complex (Rh*). Stereochemical aspects are discussed in terms of chiral discrimination and adduct diastereomerism. A second type of chiral recognition was discovered, namely, that between two ligand molecules in 2:1 adducts across the Rh2[(R)-(+)-MTPA]4 building block. Conditions for optimizing the experiment for the determination of enantiomeric composition of chiral phosphines by the “dirhodium method” are presented. The possibility of determining absolute configurations of chiral phosphines is briefly discussed.