HEPTAMOLYBDATE-ION CATALYZED C(2) EPIMERIZATION OF D-MANNOSE TO D-GLUCOSE IN ACETONITRILE-WATER MIXTURES - A KINETIC-STUDY

Binding constant, reaction rate, and equilibrium of the heptamolybdate (HM) ion catalyzed C(2) epimerization Of D-mannose (D-Man) to D-glucose (D-Glc) are strongly influenced by the mole fraction of acetonitrile (MeCN) in water. MeCN enhances the rate of epimerization more than 2-propanol, dioxane or tetrahydrofuran, all used in the 0.58 molar ratio in water. At a mole fraction of 0.58 of MeCN (ca. 80% vol/vol) K(m) for [D-Man.HM] complex is ca. 2.5 times lower than in water. In the same solvent mixture the CD spectrum of the complex exhibits a ca. 2.5 times greater Cotton effect at ca. 260 nm, than in water, indicating stronger perturbation of the bound ligand within the precatalytic species. The initial velocity (upsilon(i)) exhibited non-linear dependence on the solvent composition, and was increased ca. 10-fold at 75-degrees-C, and ca. 5.5-fold at 85-degrees-C, on going from zero to a mole fraction of 0.58 of MeCN in water, whereas the standard free energy difference falls off linearly from 2.10 kJ mol-1 to 0.83 kJ mol-1. Dependence of the kinetic data on the solvent composition can either be explained by the presence of various (poly)molybdate catalytic species, or by strong solvation/desolvation phenomena for both the catalyst and substrate.