MECHANISM OF METAL-ION CATALYZED CARBOHYDRATE TRANSFORMATIONS - STOICHIOMETRY OF ALDOPENTOSE REARRANGEMENT PROCESSES CATALYZED BY A MACROCYCLIC CHROMIUM(III) COMPLEX

The detailed stoichiometry of the reaction between the aldopentoses, ribose, arabinose, lyxose and xylose, and chromium(III) complexes of the optically active macrocyclic 5,5,7,12,12, 14-hexamethyl-1,4,8,11-tetraazacyclotetradecane tetraamine ligand, has been studied by a combination of H-1- and C-13-NMR, of ion-exchange chromatography, and of deuterium and carbon-13 labelling. The reaction gives complexes of coordinated 2-hydroxycarboxylic acids by an intramolecular redox reaction, and the same reaction product ligand mixtures are obtained from all the pentoses, but with relative amounts which vary as a function of the temperature and the combination of chirality of chromium(III) complex and chirality of the carbon-4 atom of the aldopentose. The suggested mechanism of this transformation process involves bidentate substrate coordination followed by protonation, dehydration, carbocation formation, deprotonation and formation of an enolic intermediate. The further reactions involve protonation followed by intramolecular 1,2-hydride shift, and deprotonation to give the coordinated deprotonated 2-hydroxycarboxylic acids. A parallel protonation of either the 4-hydroxy group or the carbon-3 atom of the enolic intermediate leads to the unsaturated 2,5-dihydroxypent-3-enoic acid or a diastereomeric mixture of the saturated 2,4,5-trihydroxypentanoic acids, respectively. The mechanism established here for the chromium(III) catalyzed rearrangement process is discussed in relation to the molybdenum(VI) and nickel(II)-amine catalyzed epimerization processes at the carbon-2 atom of aldoses.