Rh2[MTPA]4, a dirhodium complex as NMR auxiliary for chiral recognition.

The dirhodium complex Rh(+II)2[(+)-MTPA]4 is introduced as an NMR auxiliary for enantiodifferentiation. As a soft Lewis acid, it is particularly useful for the recognition of soft Lewis base molecules. The donor properties of ligands can be divided into several categories and are exemplified by various classes of phosphorus-containing functionalities. In addition, the thermodynamics of the underlying ligand-complex equilibria are discussed and stereochemical implications demonstrated. The extension of the dirhodium method to absolute configuration determination is evaluated. The outlook for future applications with further non-phosphorus ligands is presented.