Chemists have a few choices when they want to transform aromatic C–H bonds into something more complicated. Brominating and borylating these bonds are popular options, but those reactions usually produce a mixture of isomers. While seeking a more selective strategy, Tobias Ritter and colleagues at the Max Planck Institute for Kohlenforschung discovered that the tetrafluorothianthrene radical cation will react with aromatics almost exclusively at the para position (Nature 2019, DOI:10.1038/s41586-019-0982-0). “We had to develop new chemistry to quantify the selectivity, as it was so high,” Ritter says. Previously, chemists generated related persistent sulfur-based radical cations using moisture-sensitive, explosive thianthrenium perchlorate radical salts. But Ritter’s group developed a new method using a bench-stable, fluorinated, sulfoxide-based thianthrene reagent. They can use this reagent to functionalize aromatic C–H bonds in the presence of many functional groups, including amines, alcohols, esters, and heterocycles. And the resulting thianthrenium salts will participate in many palladium-catalyzed cross couplings as well as several photoredox reactions, making the transformation useful for adding complexity during the final stages of a chemical synthesis (example shown).