Oxygen Versus Sulfur Stabilization of Carbenium Ions 95
We discussed previously that the slow step of the hydrolysis mechanism of
pyranosides 1 and 2 was the formation of a carbenium ion. In the case of 5 we can
suggest that the departure of the fluoride ion is rate-limiting and hence the capture
of the cation 9 is the fast step of the reaction. An increase in the azide concentration results in an increase of the yield of the substitution products, in detriment of
f
the hydrolysis product 7 obtained by direct attack of the water. As expected, the
f
more stable anomer E
r -6 is obtained with preference to the D- anomer.
A Ad dd di it ti io on na al l C Co om mm me en nt ts s
This problem is based on the work by Indurugalla D, Bennet AJ (2001) J. Am.
Chem. Soc. 123:10889-10898 and on the work by Johnston BD, Indurugalla D,
Pinto BM, Bennet AJ (2001) J. Am. Chem. Soc. 123:12698-12699.
S Su ub bj je ec ct ts s o of f R Re ev vi is si io on n
Kinetic isotope effects: primary and secondary deuterium kinetic isotope effects.
Heavy atom isotope effects. Solvent isotope effects. S N 1 and S N 2 mechanisms.