Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 242

After the slow Au(III) reduction by thioether, the process involves
conversion of the oxidized sulfur center to the sulfoxide. The first
intermediate of this conversion is a chlorosulfonium ion (eq 9, Scheme 1).
These species are known to hydrolyze rapidly to yield sulfoxides. 50 The
study with isotope labeled is entirely consistent with this mechanism

(see below).
As mentioned above, the reaction slows down with time due to inhibition
by CEESO products. CEESO replaces or in the active complex 1
and gives an inactive complex analogous to 2 . This inhibition has been
modeled by DMSO, a molecule structurally and electronically similar to the
CEESO product, and is addressed below (section 13). A full evaluation and
nonlinear fitting of the kinetics of self-inhibition by CEESO is discussed in a
recent paper. 23 The result of this fitting is shown as a solid line in Figure 1,
and the anticipated CEES consumption (an exponential decay with
if there were no inhibition by CEESO is given as a dotted line (see

below).

9. MECHANISMS RULED OUT

Several other possible mechanisms for the rate-limiting redox step are
discussed below and ruled out. The first one assumes that a slow formation
of the required Au(III) complex (for example, 1 ) is the rate limiting step, and
redox transformation is fast. However, this mechanism is not supported by
the literature studies on stoichiometric Au(III) reduction by thioethers, nor
by the kinetic data obtained in this research. Clearly the reduction of
Au(III) is the slow step, but this in turn may proceed through different
possible mechanisms.
One mechanism for Au(III) reduction by thioethers involves the
intramolecular reductive elimination of 1 and 2 to form a thioxonium salt (eq
9 in Scheme 1) and a Au(I) complex, eqs 22 and 23.