Catalytic aerobic oxidation of 2-chloroethyl ethylsulfide, a mustard simulant, under ambient conditions: Effect of solvents, ligands, and transition metals on reactivity

Abstract

The complexes Au(III)(Hal)2(NOx)(L), Hal=Cl− or Br−, NOx=NO3− or NO2−, L=thioether, catalyze the selective aerobic sulfoxidation of thioethers, including the mustard simulant 2-chloroethyl ethyl sulfide (CEES), by dioxygen (stoichiometry: CEES+0.5O2→CEESO) under ambient conditions (25°C, 1atm O2) in both homogeneous solution (acetonitrile, trifluoroethanol, nitromethane, 1,2-dichloroethane) or non-toxic perfluoropolyether (PFPE) suspensions. The reaction rate, induction period, and the extent of product inhibition are dependent on the Au(III) ligands, the solvent, and the presence of additional redox active metals. Catalytic aerobic CEES oxidation is 1.8 times faster when NO3− is replaced by NO2− and 3.3 times faster when Cl− is replaced by Br− in acetonitrile. This reaction in trifluoroethanol exhibits no induction period and is 2.8 times faster than in acetonitrile. Addition of 2equiv. of Cu(II) per Au(III) to the system increases the rate by a factor of 2.7. The Au(III)/(Br−)2/NO3−/Cu(II) system exhibits high rates for the selective aerobic oxidation of CEES to CEESO under ambient conditions (∼68 turnovers per hour), with little if any inhibition by the CEESO product. At low concentrations, sulfoxides reduce the induction period and increase the rate of CEES oxidation in acetonitrile, but at high concentrations they inhibit the reaction. These Au(III) catalysts are extremely efficient for aerobic CEES sulfoxidation when suspended in the PFPE Fomblin® MF-300 (up to 200 turnovers in 10min). This is a significant improvement from the Au system described in the first study, Au(III)(Cl)2(NO3)(CEES) in acetonitrile, which yielded approximately 5 turnovers of CEESO after 10min. The catalytic reactivity of the Au(III)(Cl)2(NO3)(L) in Fomblin® MF-300 for aerobic CEES oxidation was evaluated in the presence of the common amino acids to assess the extent to which the various functional groups in human skin (epidermal polypeptides) might inhibit the catalysis. Some amino acids inhibit the reaction, but the reaction still proceeds even in the presence of 7.5equiv. of the most inhibitory functional group, indole (tryptophan).