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->W₇O₂₄^6->Mo₇O₂₄^6- > HCO₃^->WO₄^2->MoO₄^2-

Catalysis by W₇O₂₄^6- and Mo₇O₂₄^6- is competitive but that by WO₄^2- and MoO₄^2- 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.