10579-10-9Relevant academic research and scientific papers
A kinetic isotope effect study on the hydrolysis reactions of methyl xylopyranosides and methyl 5-thioxylopyranosides: Oxygen versus sulfur stabilization of carbenium ions
Indurugalla,Bennet
, p. 10889 - 10898 (2007/10/03)
The following kinetic isotope effects, KIEs (klight/kheavy), have been measured for the hydrolyses of methyl α- and β-xylopyranosides, respectively, in aqueous HClO4 (μ = 1.0 M, NaClO4) at 80 °C: α-D, 1.128 ± 0.004, 1.098 ± 0.005; β-D, 1.088 ± 0.008, 1.042 ± 0.004; γ-D2, (C5) 0.986 ± 0.001, 0.967 ± 0.003; leaving-group 18O, 1.023 ± 0.002, 1.023 ± 0.003; ring 18O, 0.983 ± 0.001, 0.978 ± 0.001; anomeric 13C, 1.006 ± 0.001, 1.006 ± 0.003; and solvent, 0.434 ± 0.017, 0.446 ± 0.012. In conjunction with the reported (J. Am. Chem. Soc. 1986, 108, 7287-7294) KIEs for the acid-catalyzed hydrolysis of methyl α- and β-glucopyranosides, it is possible to conclude that at the transition state for xylopyranoside hydrolysis resonance stabilization of the developing carbenium ion by the ring oxygen atom is coupled to exocyclic C-O bond cleavage, and the corresponding methyl glucopyranosides hydrolyze via transition states in which charge delocalization lags behind aglycon departure. In the analogous hydrolysis reactions of methyl 5-thioxylopyranosides, the measured KIEs in aqueous HClO4 (μ = 1.0 M, NaClO4) at 80 °C for the α- and β-anomers were, respectively, α-D, 1.142 ± 0.010, 1.094 ± 0.002; β-D 1.061 ± 0.003, 1.0185 ± 0.001; γ-D2, (C5) 0.999 ± 0.001, 0.986 ± 0.002; leaving-group 18O, 1.027 ± 0.001, 1.035 ± 0.001; anomeric 13C, 1.031 ± 0.002, 1.028 ± 0.002; solvent, 0.423 ± 0.015, 0.380 ± 0.014. The acid-catalyzed hydrolyses of methyl 5-thio-α- and β-xylopyranosides, which occur faster than methyl α- and β-xylopyranosides by factors of 13.6 and 18.5, respectively, proceed via reversibly formed O-protonated conjugate acids that undergo slow, rate-determining exocyclic C-O bond cleavage. These hydrolysis reactions do not have a nucleophilic solvent component as a feature of the thiacarbenium ion-like transition states.
