Aromatic sulfonation with sulfur dioxide via Trifluoroacetylsulfuric acid

We report the regioselective sulfonation of aromatic compounds using SO₂ and O₂ with catalytic H₂O₂ in TFAOH/TFAA via a one-pot two-step sequence, affording arylsulfonic acids in good to excellent yields. In the first step, the reactive sulfonating intermediate, TFAOSO₃H (1), is generated in situ, which then efficiently sulfonated the aromatic compounds in the second step. Regioselectivity of the reaction follows general electrophilic aromatic substitution (EAS) trends, where mesomeric and inductive effects of substituents dictate the sulfonation position. Compared to conventional methods in aromatic sulfonation using SO₃ and oleum, this approach provides milder reaction conditions, easier isolation of the sulfonated products, and improved step economy, offering a cost-effective route to access arylsulfonic acids.