Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 243

This mechanism has been proposed for some thioether oxidations by
Au(III) 26,45,47 and the conversion of 2 to and CEESO can be
assisted by eq 24. 26 However, this mechanism is also very unlikely in
our catalytic system; analysis of the reaction kinetics unambiguously rules it
out. If eqs 22 and 23 were rate limiting, then the overall rate would be eq
25.
Combining eqs 16, 17 and 25 gives eqs 26 and 27 for the reaction rate.
This rate law is incompatable with our experimental kinetic data. If
then the reaction rate should increase quadratically with
which is in agreement with Figure 3, while the reaction rate should
decrease with increasing [CEES], which is inconsistent with our data (Figure
4). then the rate should increase with both
and [CEES] finally reaching saturation with an observed reaction rate order
< 1. This is in disagreement with the data presented in Figure 3. Thus, only
one of the rate dependences, either that for [CEES] or that for can
be explained by this mechanism, but not both simultaneously. Incorporation
of eq 24 in the above analysis leads to the same expression as eq 27, where
is replaced by
Thus, the addition of eq 24 into the reaction mechanism does not
affect the theoretical reaction rate law, eq 27. Additional evidence against
the unimolecular collapse of 2 , assisted by a water molecule, being the rate
limiting step (eq 24) derives from a nonexistent kinetic isotopic effect
when was used instead of The kinetics data are
in accord with functioning as an inhibitor competing with or
CEES for a coordination site in the Au(III) complexes and thus driving eq 7,
Scheme 1, to the left.
In principal, a radical-chain thioether oxidation initiated by Au(III)
complexes may also take place. Such a mechanism has been proposed by
Riley et al. 25 for aerobic thioether oxidation catalyzed by Ce salts. This
sulfoxidation reaction proceeds in at elevated temperatures (> 70
°C). The proposed mechanism includes the reduction of Ce(IV) by thioether
to form the radical cation which is efficiently trapped by to give
The latter radical, being a strong oxidant, reoxidizes Ce(III) to