Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 238

It is worth mentioning that thioethers are catalytically oxidized to
sulfoxides in nitromethane-aqueous nitric acid in the presence of
with the rate-limiting step being the reoxidation of Au(I) to Au(III). 28 This
involves the use of an excess of relative to Au; however, in our systems
and not is the terminal oxidant. This follows from the fact that only
one equivalent of is used in our reactions, but 200 equivalents of
CEESO product per equivalent of can be obtained.

8. PROPOSED REACTION MECHANISM

As mentioned above, Figure 2 indicates that the most catalytically active
species contains in a 1 : 2 : 1 ratio. Dimeric or oligomeric d 8
square planar Au(III) complexes are very unlikely, 43 therefore a monomeric
Au(III) complex with 2 ligands and 1 ligand has the capacity to
bind one more ligand strongly, most likely a CEES ligand. Five-coordinate
Au(III) complexes are very rare. The only 5-coordinate Au(III) complexes
with appreciable stability contain chelating ligands such as
bromodicyano(l,10-phenanthroline)gold(III) isolated from dimethylform-
amide. 44 If there is an interaction between a positive Au(III) center and a
negative axial ligand counterion, this ligand would likely be because
associates very weakly with Au(III) (and not at all with Au(I)) and
probably can be ruled out in our case. A proposed mechanism that is
compatible with the stoichiometric thioether-Au(III) reactions in the
literature and all our data is given in Scheme 1.
Literature data on the stoichiometric Au(III) reduction by thioethers
provide evidence for the rapid exchange of all ligands on Au(III) prior to
thioether oxidation. 26,27,45-47 In our mechanism these pre-equilibria are
summarized by eqs 7 and 8 in Scheme 1. Mixed Au(III) complexes are
formed with and CEES: a complex with one CEES ligand (1), with
formation constant and a complex with two CEES ligands (2), with
formation constant These ligands drive eq 7 to the right, while and
CEESO (or DMSO, data in Table 2), drive it to the left. Additionally,
or other solvent molecules may also bind to Au(III) or shift both
equilbria, eqs 7 and 8.
Au(III) complexes with one thioether ligand, analogous to 1, are
considerably more abundant in solution than complexes with two thioether
ligands, such as 2 Au(III) complex with three thioether
ligands are not considered because there is no data indicating these species
form in solutions