Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 234

easily dehydrogenated substrates including alcohols and aliphatic
hydrocarbons.

However, substrates with oxidizable lone pairs (such as thioethers, RSR)
are usually oxidized through “oxygen atom transfer” mechanism, eq 5:

Peroxides, high-valent transition metal oxo species, or peroxo radicals
are the typical “oxygen donor” reactive species, “O”, in sulfoxidation
reactions. 38-40 A conversion of to peroxo species or other forms of
reduced oxygen requires a consumption of a 2-electron sacrificial reducing
agent, The most frequently used reductants are NADH, NADPH
(particularly in enzyme-catalyzed oxidation processes) or ascorbate and
thiols. An exemplary “monooxygenase” stoichiometry of sulfoxidation by
dioxygen is eq 6:

The "dioxygenase" stoichiometry, eq 1, is optimal because all of the
oxidizing capacity of dioxygen is used and both oxygen atoms are accounted
for in the desired product. For both stoichiometries, eq 1 and 6, the question
arises whether the oxygen atom in the sulfoxide originates from dioxygen or
from water, which is always present in our system in small amounts. The
experiments with unambiguously showed that our Au-based catalytic
oxidation always resulted in the formation of 100% labeled (the

mechanism and details are discussed below).
Determination of the stoichiometry also involves the product selectivity
since sulfoxides can undergo further oxidation to the corresponding sulfones.
This is particularly important in our work since mustard sulfoxide is much
less toxic than sulfone and the sulfoxide is not a vesicant while both the
sulfide and sulfone are. 41 In our case, the selectivity for CEESO is
exceptionally high and the sulfone, has never been
detected. It is worth mentioning that a high selectivity is commonly
observed at low conversions for most oxidation reactions. In acetonitrile,
total CEES conversion was impossible to achieve due to the inhibition of the
reaction by the CEESO product. However, the same catalytic system with
trifluoroethanol as a solvent (see Section 15) can completely oxidized CEES,
while the selectivity for CEESO formation was close to 100%.