Iron-catalyzed continuous-flow synthesis of sulfoxide with hydrogen peroxide

A continuous-flow synthesis of sulfoxides from sulfides was achieved <i>via</i> oxidation with hydrogen peroxide (H₂O₂) in the presence of the catalyst generated <i>in situ</i> by simply packing a reactor column with a mixture of iron(III) chloride (FeCl₃) and activated carbon followed by the pre-activation step in which an H₂O₂-acetonitrile solution was passed into the column. This activation step appeared to convert FeCl₃ to iron oxide species, thereby enhancing catalytic efficiency. Sulfoxides were continuously produced in >90% yield for up to 150 h. Although over-oxidation to sulfones occurred, the selectivity for some sulfoxides (defined as sulfoxide/(sulfoxide + sulfone)) reached up to 98%, with no residual sulfides, by fine-tuning the flow rate of the reaction solution.