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

The transformation of methylglyoxal and of 1,3-dihydroxyacetone and glycera ldehyde into lactic acid can be catalyzed by cis-tetraaminediaquarhodium(II I) 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 sto ichiometry of this process has been investigated by isotopic labelling stud ies and H-1 and C-13-n.m.r. spectroscopy. The suggested mechanism of the methylglyoxal transformation process involve s bidentate substrate coordination, followed by an intramolecular 1,2-hydri de 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 glycer aldehyde is observed. Ultimately both substrates are converted into coordin ated lactate in which one hydrogen atom in the methyl group originates from the solvent water.