Dirhodium tetraacetate as an auxiliary chromophore in a circular dichroic study on vic-amino alcohols

Abstract

The dirhodium method proposed herein, involves the in situ formation of chiral complexes of optically active vic-amino alcohols with the achiral dirhodium tetraacetate [Rh 2(OAc) 4] acting as an auxiliary chromophore. The resulting CD spectra are suitable for stereochemical studies, since the observed sign of Cotton effects arising within the d–d absorption bands of the metal core is determined by the chirality of the amino alcohol ligand. It has been demonstrated that in acetonitrile and chloroform solutions, chiral complexes are formed by axial ligation of the ligand to the metal atom(s) of the dirhodium core. This axial binding occurs through the nitrogen atom of the 1,2-amino alcohol unit independently of the ligand-to-metal molar ratio, as shown from the 15N and low-temperature NMR experiments. In agreement with NMR results, ESI MS experiments indicate that in solution, a mixture of chiral complexes with ligand-to-metal molar ratios 1:1 and 2:1 is present. An empirically based rule correlating the sign of Cotton effect occurring above 600 nm with the stereochemistry of ligand has been formulated.