Oxygen Versus Sulfur Stabilization of Carbenium Ions

Oxygen Versus Sulfur Stabilization of Carbenium Ions 

The general meehanistie features of the aeid-eatalyzed hydrolysis of glyeo-pyranosides have been reeognized for about 30 years. It is generally aeeepted that the hydrolysis oeeurs via speeifie aeid eatalysis. The rate-determining step involves exoeyelie C-O bond eleavage and an oxaearbenium ion is formed (Seheme 13.1). 

Does the hydrolysis mechanism involve the endocyclic C-X bond cleavage in the slow step of the reaction Are oxa- or thiacarbenium ions formed in the slow step of the hydrolysis process  

To answer these questions, comment the results obtained after a complete Kinetic Isotope Effect (KIE) study on the hydrolysis reactions of methyl D-xylopyra-nosides 1 and methyl D-thioxylopyranosides 2 (both anomers in each case). 

Comment the different types of kinetic isotope data and explain how their values can be used to obtain information about the mechanism of the reaction. 

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Oxygen Versus Sulfur Stabilization of Carbenium Ions 89... 

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