Application of Transition Metal Acylates for Determination of the Absolute Configuration of vic-Amino Alcohols
by Circular Dichroism Spectroscopy
Jadwiga Frelek
Amino alcohols represent an important group of organic compounds due to their significant biological activity and wide application in synthesis. For those reasons the determination of their absolute configuration is very important. To enable the circular dichroic studies on amino alcohols that are transparent in an accessible spectral range, a suitable chromophoric system needs to be introduced into the molecule. One possible solution is based on the methodology that consists of making chiral complexes in situ with achiral transition metal complex displaying adequate absorption characteristics.
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The aim of this work is to establish whether the transition metal acylates of general formula [M2(O2CR3)4] with M = Mo, Rh can be used as appropriate auxiliary chromophores for reliable, rapid and effective determination of the absolute configuration of vic-amino alcohols. As model compounds a variety of amino alcohols of both ephedrine and adrenaline types has been chosen. During the present study the dependence of the |
CD spectra on the ligand to stock complex ratio, on a solvent used as well as on time will be tested. For this purpose, the electronic absorption spectroscopy will be used in addition to the circular dichroism. In the case of dirhodium tetraacetate to explain the possible mechanism of complexation of amino alcohols to the Rh2-core, i.e., chelating or bridged type of bidentate ligation or unidentate ligation, the 1H and 13C NMR spectroscopy will be also applied.
The resulting CD spectra are suitable for the assignment of absolute configuration, since the observed signs of Cotton effects arising within the d-d absorption bands of the metal cluster depend solely upon the chirality of the amino alcohol ligand. Therefore, a straightforward and versatile method for the determination of the absolute configuration of vic-amino alcohols can be proposed. In the case of dimolybdenum tetraacetate an empirically based rule correlating a positive/negative helicity expressed by the O-C-C-N torsional angle with the sign of Cotton effects occurring in the 400 – 260 nm spectral range has been formulated. In the case of dirhodium tetaracetate a correlation between structure of an amino alcohol and sign of the CD band arising around 600 nm has been found.
This work is supported by the CSR (Committee of Scientific Research) grant No. 7 T0 9A 004 21.
