Koenigs–Knorr reactions. Part II. A mechanistic study of mercury(II) cyanide-promoted reactions of 2,3,4,6-tetra-O-methyl-α-D-glucopyranosyl bromide with cyclohexanol in benzene–nitromethane
The kinetics and products of mercury(II) cyanide-promoted reactions of 2,3,4,6-tetra-O-methyl-α-D-glucopyranosyl bromide with cyclohexanol in benzene–nitromethane (1 : 1 v/v) at 2–20 °C were investigated by polarimetry and quantitative g.l.c. The reactions exhibited a first-order kinetic dependence on the glucosyl bromide and mercury(II) cyanide concentrations, but the rates were independent of the cyclohexanol concentration. Under the conditions employed, cyclohexyl 2,3,4,6-tetra-O-methyl-β-D-glucopyranoside was the main product (ca. 60–80%), but in all reactions the α-glucoside was also formed. The stereoselectivity of the reactions for β-glucoside formation increased when the alcohol concentration was increased and decreased when the reaction temperature was increased. The initial reaction is believed to involve rate-determining, mercury(II) cyanide-assisted heterolysis of the carbonbromine bond to form the glucopyranosyl carbocation. The stereochemical course of the reaction is dependent on the rate of dissociation of the carbocation and the attendant anion relative to the rate of reaction of the alcohol with the carbocation. Reasons for the observed autocatalysis in the reaction are discussed.