Transition metal complexes as auxiliary chromophores in chiroptical studies on carbohydrates

Circular dichroism is obtained only within absorption bands. Carbohydrates, but also alcohols, amines, ethers etc, are transparent in the UV-VIS regio n of commercially CD spectropolarimeters. In order to see Cotton effects of such compounds, new chromophors have to be introduced in their molecules t o form so-called cottonogenic derivatives. One procedure for this is the in situ formation of chiral complexes by mixing the optically active but nona bsorbing substance with a solution of an achiral transition metal complex. By examination of the CD bands of these chiral complexes, configurational a nd conformational features of the ligands can be determined. Some of our in vestigations exemplify the application of this approach to carbohydrates: a ) Pyranose sugars, bearing free 1,2,3-hydroxyl groups, react with acidic mo lybdate solutions and form stable Mo-sugar complexes in the pH-range 5-6. T hey exibit, depending on the sugar structure, two kinds of typical CD curve s, whose parameters are correlated, according to a chirality rule, with the sugar configurations. b) Cuprammonium complexes were used first by Reeves to determine configurations and conformations of sugars. Cotton effects of 1,2-diol and 1,2,3-triol pyranoside sugar-cuprammonium complexes are closel y examined; for these compounds a helicity rule is verified and proposed. c ) Dinuclear transition metal acylates have been widely used recently as chr omophores for cottonogenic derivatives. CD spectra of 1,2- and 1,3-diols (i ncluding sugar diols) with dimolybdenum tetraacetate as well as the corresp onding chirality and sector rules are presented in detail and discussed. d) It is shown that dimolybdenum tetraacetate does not form chiral complexes with monohydroxyalcohols. Dirhodium acetate has more affinity to such ligat ion. Some complexes of dirhodium tetracis(trifluoroacetate) with monohydrox yalcohols of sugar series have been prepared in situ. Finally the possibili ty is discussed applying a bulkiness rule to correlate the alcohol geometry with the sign of one of the CD bands of the corresponding rhodium-alcohol- complex.