A Homogeneous Catalyst for Selective O2 Oxidation at Ambient Temperature. Diversity-Based Discovery and Mechanistic Investigation of Thioether Oxidation by the Au(III)Cl2NO3(thioether)/O2 System

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Abstract
A library of inorganic complexes with reversible redox chemistry and/or the ability to catalyze homogeneous oxidations by peroxides, including but not limited to combinations of polyoxometalate anions and redox-active cations, was constructed. Evaluation of library members for the ability to catalyze aerobic sulfoxidation (O2 oxidation of the thioether, 2-chloroethyl ethyl sulfide, CEES) led to the discovery that a combination of HAuCl4 and AgNO3 forms a catalyst that is orders of magnitude faster than the previously most reactive such catalysts (Ru(II) and Ce(IV) complexes) and one effective at ambient temperature and 1 atm air or O2. If no O2 but high concentrations of thioether are present, the catalyst is inactivated by an irreversible formation of colloidal Au(0). However, this inactivation is minimal in the presence of O2. The stoichiometry is R2S + 1/2O2 → R2S(O), a 100% atom efficient oxygenation, and not oxidative dehydrogenation. However, isotope labeling studies with H218O indicate that H2O and not O2 or H2O2 is the source of oxygen in the sulfoxide product; H2O is consumed and subsequently regenerated in the mechanism. The rate law evaluated for every species present in solution, including the products, and other kinetics data, indicate that the dominant active catalyst is Au(III)Cl2NO3(thioether) (1); the rate-limiting step involves oxidation of the substrate thioether (CEES) by Au(III); reoxidation of the resulting Au(I) to Au(III) by O2 is a fast subsequent step. The rate of sulfoxidation as Cl is replaced by Br, the solvent kinetic isotope effect (kH2O/kD2O = 1.0), and multiparameter fitting of the kinetic data establish that the mechanism of the rate-limiting step involves a bimolecular attack of CEES on a Au(III)-bound halide and it does not involve H2O. The reaction is mildly inhibited by H2O and the CEESO product because these molecules compete with those needed for turnover (Cl-, NO3-) as ligands for the active Au(III). Kinetic studies using DMSO as a model for CEESO enabled inhibition by CEESO to be assessed.