Sugar rearrangements mediated by Lewis acidic molecular sieves in liquid media

Abstract

Monosaccharide and disaccharide isomerization and epimerization reactions are catalyzed by Lewis acidic sites (M = Sn, Ti, Zr) isolated in silica-based heterogeneous solids, with selectivity and stereochem. specificity that are inaccessible to sugar rearrangements involving enolate intermediates catalyzed by bases. Such selectivity and specificity reflect the ability of electrophilic Lewis acid centers to coordinate with oxygenated sugar functional groups and to subsequently mediate intramol. rearrangements of specific H and C atoms, the mechanistic details of which are probed using D and ^(13)C isotopically-labeled reactants. Here, we discuss recent progress in understanding the influence of: (i) heteroatom and solvent identity on active site structures and reaction mechanisms and (ii) the surrounding silicate environments on the turnover rates of sugar reactions. For example, glucose isomerization to fructose is mediated by Lewis acidic Ti sites isolated within hydrophobic and hydrophilic environments provided by cryst. mol. sieves with the Beta topol. and by amorphous silica. Measured first-order rate consts. (per total Ti atom; 373 K), detd. from batch reactor studies under conditions of strict kinetic control, are an order of magnitude higher in liq. water for hydrophobic than for hydrophilic solids. Mechanistic interpretation of these rate consts. indicates that the environments surrounding Ti centers influence free energy differences between isomerization transition states and two bound solvent mols. that adsorb competitively at Lewis sites and are most abundant surface intermediates during catalysis. Similar roles of hydrophobic pockets confining Lewis centers are found in heterogeneous, homogeneous and enzymic active sites that mediate sugar reactions. These findings help clarify the mechanistic details and site requirements for sugar conversion on heterogeneous Lewis acid solids, and also provide guidance for active site structural modification to promote desired reaction pathways and thus to develop new heterogeneous catalysts for selective sugar conversion in liq. media.