Bidentate palladium(II) chelation by the common aldoses

The [Pd(II){(R,R)-chxn}(OH)(2)] reagent (chxn=1,2-diaminocyclohexane) is introduced as a metal probe for the detection of the bidentate chelating sites of a glycose. Two moles of hydroxide per mole palladium support double deprotonation of potentially chelating diol functions at a glycose's backbone. The individual chelating sites are detected using one- and two-dimensional NMR techniques. At equimolar amounts of palladium(II) and aldose, the metal-binding sites include mostly the hydroxy function at the anomeric carbon atom. Chelators are derived from both the pyranose and the furanose isomers. Most pyranose-based chelators form five-membered chelate rings by using their 1,2-diol function. Though 1,2-diolate bonding is also common to the furanoses, the formation of six-membered chelate rings by 1,3-bonding is more significant for them. Metal-excess conditions provoke mostly bis-bidentate 1,2;3,4-chelation but unusual isomers form also: thus d-xylose is dimetallated in its all-axial beta-pyranose form, and erythrose's dimetallation results in the formation of two isomers of a metal derivative of the open-chain hydrate. The spectroscopic results are supported by crystal-structure determinations on [Pd{(R,R)-chxn}(alpha-D-Xylp1,2H(-2)-kappaO(1,2))].H(2)O (Xyl=xylose), [Pd{(R,R)-chxn}(alpha-D-Ribp1,2H(-2)-kappaO(1,2))].2.25H(2)O (Rib=ribose), [Pd{(R,R)-chxn}(alpha-L-Thrf1,3H(-2)-kappaO(1,3))].2H(2)O (Thr=threose) and [Pd{(R,R)-chxn}(alpha-D-Eryf1,3H(-2)-kappaO(1,3))].3H(2)O (Ery=erythrose).