Enantioselective catalysis. Part 102 . Epimerization of glucose and mannose in the presence of nickel(II) complexes of optically active ligands

Abstract

The epimerization reaction glucose ⇌ mannose, catalyzed by nickel(II) complexes, was investigated. The achiral ligand N,N,N′,N′-tetramethyl-1,2-diaminoethane 1 and the chiral ligands ( R,R)- and (S,S)-N,N,N′,N′-tetramethyl-1,2-diamino-1,2-diphenylethane 2 and 3 were used. The standard epimerization of glucose and mannose, respectively, was carried out in methanol at 60°C using a stoichiometric amount of nickel(II) complex. The influence of substoichiometric and overstoichiometric amounts of the catalyst and the temperature and time dependence were studied. Some of the kinetic and thermodynamic parameters turned out to be dependent on whether the ( R,R)- or ( S,S)-ligand 2 or 3 was used in the nickel(II) complex. The data were subjected to a Michaelis-Menten and Eyring analysis.