Complete kinetic isotope effect description of transition states for acid-catalyzed hydrolyses of methyl .alpha.- and .beta.-glucopyranosides

Abstract

The following kinetic isotope effects (k/sub light//k/sub heavy/) have been measured by the isotopic quasi-racemate method for the hydrolyses of methyl ..cap alpha..- and ..beta..-glucopyranosides, respectively in 2.0 M HClO/sub 4/ at 80/sup 0/C (..cap alpha..-D 1.137 +/- 0.007, 1.089 +/- 0.006; ..beta..-D 1.073 +/- 0.003, 1.045 +/- 0.004; ..gamma..-D (C5) 0.987 +/- 0.002, 0.971 +/- 0.003; leaving group d/sub 3/ 1.006 +/- 0.001, 1.015 +/- 0.002; leaving group /sup 18/O 1.026 +/- 0.001, 1.024 +/ +/- 0.002; anomeric /sup 13/C 1.007 +/- 0.001, 1.011 +/- 0.002). In conjunction with studies of the effect of added solutes on the rates of hydrolysis of various aldopyranosyl derivatives, which indicate such reactions are truly unimolecular, and model ring /sup 18/O and ..beta..-deuterium effects, it is possible to locate the dihedral angles about the O5-C1 and C1-C2 bonds at the transition state using these data. If the dihedral angles so derived are used as constraints in calculations using N.L. Allinger's MM2 molecular mechanics program, transition-state structures are obtained. The geometry of these transition states stands in contradiction to the theory of stereoelectronic control.