Catalysts for selective aerobic oxidation under ambient conditions

Abstract

Diversity-based methods for catalyst discovery coupled with the knowledge of lead systems for the catalysis of O2-based organic oxidation reactions has led to the development of new species that actually catalyze rapid and selective (non-radical-chain), reductant-free, O2 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: R2S (thioether)+1/2 O2→R2S(O) (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 Au(III)Cl2NO3(thioether) (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 O2 is a fast subsequent step. The solvent kinetic isotope effect (KH 2 O/KD 2 O=1,0) 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 H2O Isotope labeling studies with H2 18O indicate that H2O and not O2 or H2O2 is the source of oxygen in the sulfoxide product. Interestingly, H2O 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 Cu(II) concentrations. Co-catalysis by the d-block ions also results in elimination of the induction period in some cases.