Mutarotation of D(+)-glucose. III. Evidence for cooperative and competitive nucleophilic catalysis by molybdenum(VI) and tungsten(VI) anions

Abstract

The rate of mutarotation of α-D(+)-glucose is subject to general base catalysis by a variety of oxo anions. The relative magnitudes of the second-order rate constants are B4O72->W7O246->Mo7O246- > HCO3->WO42->MoO42- Catalysis by W7O246- and Mo7O246- is competitive but that by WO42- and MoO42- is cooperative. A mechanism for catalysis by the polymeric anions is proposed which involves ion-pair formation between the anion and the conjugate acid of glucose. Specifically oriented aquation sheaths about the charged ions cause them to act as enhanced nucleophiles in the rate-determining step to form the aldehyde intermediate. Activation parameters support this model.