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, re spectively, in aqueous HClO4 (mu = 1.0 M, NaClO4) at 80 degreesC: alpha -D, 1.128 +/- 0.004, 1.098 +/- 0.005; beta -D, 1.088 +/- 0.008, 1.042 +/- 0.00 4; gamma -D-2, (C5) 0.986 +/- 0.001, 0.967 +/- 0.003; leaving-group O-18, 1 .023 +/- 0.002, 1.023 +/- 0.003; ring O-18, 0.983 +/- 0.001, 0.978 +/- 0.00 1; anomeric C-13, 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 alp ha- and beta -glucopyranosides, it is possible to conclude that at the tran sition state for xylopyranoside hydrolysis resonance stabilization of the d eveloping 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 depar ture. In the analogous hydrolysis reactions of methyl 5-thioxylopyranosides , the measured KIEs in aqueous HClO4 (mu = 1.0 M, NaClO4) at 80 degreesC fo r the alpha- and beta -anomers were, respectively, alpha -D, 1.142 +/- 0.01 0, 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 O-18, 1.027 +/- 0.00 1, 1.035 +/- 0.001; anomeric C-13, 1.031 +/- 0.002, 1.028 +/- 0.002; solven t, 0.423 +/- 0.015, 0.380 +/- 0.014. The acid-catalyzed hydrolyses of methy l 5-thio-alpha- and beta -xylopyranosides, which occur faster than methyl a lpha- and beta -xylopyranosides by factors of 13.6 and 18.5, respectively, proceed via reversibly formed O-protonated conjugate acids that undergo slo w, rate-determining exocyclic C-O bond cleavage. These hydrolysis reactions do not have a nucleophilic solvent component as a feature of the thiacarbe nium ion-like transition states.