Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 231

the homogeneous oxidation of thioethers, namely
and The Ru-based catalyst oxidizes
tetrahydrothiophene (THT) to tetrahydrothiophene oxide (THTO) at elevated
temperature and ~8 atm of with a rate of only 3.4 turnovers (at 110
°C). The latter catalyst is more active, producing 17.6 turnovers of THTO
after 30 min, but the conditions are at elevated temperature and pressure (60
°C and 14 bar of Both these systems are completely inactive under
ambient conditions. In contrast, our first unoptimized system
was producing nearly comparable turnovers to the Ce-
based system (35.4 equivalents of CEESO per Au(III) after 4 hr) under
ambient conditions.

s worth mentioning that CEES is significantly harder
to oxidize than THT, both thermodynamically and kinetically, since it is less
reducing and less nucleophilic. 16 The selectivity (CEESO formed based on
consumed CEES) was also noteworthy.16 CEESO was the only product
formed in the reaction, and further oxidation to did not occur. The
simultaneous measurement of dioxygen and CEES consumption, as well as
CEESO formation revealed the following reaction stoichiometry:

It '

Given the unprecedented reactivity and selectivity of the discovered
system (mixture of Au(III) and Ag(I) salts), we chose it to investigate the
reaction kinetics in detail. The principal system of focus comprises 1, 0.75,
and 1.25 equivalents of the and precursors
respectively. The general features of this catalytic system, the complex rate
law and other kinetic features of this reaction, and the reaction mechanism
are reported below.

3. STOICHIOMETRIC Au(III) REDUCTION BY
THIOETHERS

Stoichiometric thioether oxidation by Au(III) complexes has been
extensively studied in the literature. The major features of this reaction are
the following: (1) is not reactive towards thioethers under ambient
conditions; (2) halide and thioether ligands exchange in rapid pre-equilibria;
(3) the rate-limiting redox step involves thioether reduction of Au(III)
forming Au(I) and sulfoxide; (4) the formation constant for the Au(III)
complex with 1 thioether ligand > the formation constant for the Au(III)
complex with 2 thioether ligands (5) Au(III) complexes with 3
thioether ligands are not known; and (6) the lability of the transient Au(III)
complexes renders them nonisolable. 26-30 Our Au(III)-based catalytic