A kinetic isotope effect study on the hydrolysis reactions of methyl xylopyranosides and methyl 5-thioxylopyranosides: oxygen versus sulfur stabilization of carbenium ions.
The following kinetic isotope effects, KIEs (k(light)/k(heavy)), have been measured for the hydrolyses of methyl alpha- and beta-xylopyranosides, respectively, in aqueous HClO(4) (mu = 1.0 M, NaClO(4)) at 80 degrees C: alpha-D, 1.128 +/- 0.004, 1.098 +/- 0.005; beta-D, 1.088 +/- 0.008, 1.042 +/- 0.004; gamma-D(2), (C5) 0.986 +/- 0.001, 0.967 +/- 0.003; leaving-group (18)O, 1.023 +/- 0.002, 1.023 +/- 0.003; ring (18)O, 0.983 +/- 0.001, 0.978 +/- 0.001; anomeric (13)C, 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 alpha- and beta-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 HClO(4) (mu = 1.0 M, NaClO(4)) at 80 degrees C for the alpha- and beta-anomers were, respectively, alpha-D, 1.142 +/- 0.010, 1.094 +/- 0.002; beta-D 1.061 +/- 0.003, 1.018(5) +/- 0.001; gamma-D(2), (C5) 0.999 +/- 0.001, 0.986 +/- 0.002; leaving-group (18)O, 1.027 +/- 0.001, 1.035 +/- 0.001; anomeric (13)C, 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-alpha- and beta-xylopyranosides, which occur faster than methyl alpha- and beta-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.
Indurugalla
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Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada.
Bennet
A J
AJ
D003903:Deuterium; D006027:Glycosides / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D010100:Oxygen / Q000737:chemistry*; D010103:Oxygen Isotopes; D011714:Pyrans / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry*
D006027:Glycosides; D010103:Oxygen Isotopes; D011714:Pyrans; D013455:Sulfur; D003903:Deuterium; D010100:Oxygen