Alcohols and 1,2-diols oxidation by
o-iodoxybenzoic acid (IBX) has been examined by
1H-NMR
spectroscopy. Reversible formation of reactive intermediates,
iodic esters 5, has been observed,
and their structures in DMSO-d
6 solution have
been defined as 10-I-4 axial alkoxyiodinane oxides
by comparison of the chemical shift difference data with those obtained
for Dess−Martin periodinane
(DMP)−alcoholate and −diolate adducts. The dichotomous
behavior exhibited by IBX and DMP
with 1,2-diols can be explained in terms of the different architecture
of the reactive intermediates
involved in the oxidation. With aliphatic alcohols, kinetic
evidences support a two-step reaction
mechanism involving a fast pre-equilibrium step leading to
5, followed by a rate-determining
disproportionation step. With electronically activated benzyl
alcohol, the attainment of pre-equilibrium is largely dependent on initial water concentration as a
consequence of a particularly
high k
2 value. The influence of the alcohol
structure on measured equilibrium (K
eq) and
rate
constants (k
2) and the effect of water on the
overall reaction rate are discussed.
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