Mechanism of lactic acid formation catalyzed by tetraamine rhodium(III) complexes

The transformation of methylglyoxal and of 1,3-dihydroxyacetone and glyceraldehyde into lactic acid can be catalyzed by cis-tetraaminediaquarhodium(III) complexes of ethane-1,2-diamine and of the macrocyclic racemic 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane ligand. The detailed stoichiometry of this process has been investigated by isotopic labelling studies and H-1 and C-13-n.m.r. spectroscopy. The suggested mechanism of the methylglyoxal transformation process involves bidentate substrate coordination, followed by an intramolecular 1,2-hydride shift in a resonance stabilized carbocation. The transformations of 1,3-dihydroxyacetone and glyceraldehyde are stoichiometrically more complicated, and rhodium(III) catalyzed conversion of 1,3-dihydroxyacetone into glyceraldehyde is observed. Ultimately both substrates are converted into coordinated lactate in which one hydrogen atom in the methyl group originates from the solvent water.