Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 227

Chapter 5
Catalysts for selective aerobic oxidation under
ambient conditions
Thioether sulfoxidation catalyzed by gold complexes
Eric Boring, Yurii V. Geletii and Craig L. Hill
Department of Chemistry, Emory University, Atlanta, Georgia 30322
Abstract: Diversity-based methods for catalyst discovery coupled with the knowledge of
lead systems for the catalysis of organic oxidation reactions has led to the
development of new species that actually catalyze rapid and selective (non-radical-chain),
reductant-free, oxidation under ambient conditions (room temperature and 1.0 atmosphere
of air). The first process of focus is selective sulfoxidation of thioethers (organic sulfides).
The principal work reviewed here involves homogeneous catalysis, but highly reactive
heterogeneous formulations have already been identified. The stoichiometry is that
characteristic of dioxygenase enzymes: (sulfoxide).
Oxidative dehydrogenation, a less desirable net process, is not seen. Studies have primarily
been conducted with 2-chloroethyl ethyl sulfide (CEES), which is both notoriously unreactive
and a useful simulant for mustard. Extensive kinetics and product studies have identified the
active catalyst, at least in acetonitrile solution, to be (1) , and the rate
limiting step to be reaction of 1 with another molecule of the thioether substrate. Reoxidation
of the resulting Au(I) to Au(III) by is a fast subsequent step. The solvent kinetic isotope
effect rate of sulfoxidation when Cl is replaced by Br, and multiparameter
fitting of the kinetic data establish that the mechanism of the rate-limiting step itself involves
a bimolecular attack of CEES on a Au(III)-bound halide and it does not involve Isotope
labeling studies with indicate that and not or is the source of oxygen in
the sulfoxide product. Interestingly, is consumed and subsequently regenerated in the
mechanism. Despite the impressive (unique) reactivity attributes above, these recently
developed catalytic systems have some limitations that include an induction period and
inhibition by sulfoxide product. However, these two difficulties are eliminated in other
solvents or in nontoxic developmentally attractive perfluoropolyether (PFPE) media. Another
potential problem, is catalyst inactivation by precipitation of the Au as colloidal Au(0), but
this can be largely avoided by use of appropriate reaction conditions. Finally, these Au-
catalyzed aerobic sulfoxidation reactions can be co-catalyzed by some d-block ions. Cu(II) is
particularly effective in this context resulting in substantial increases in reaction rate at low