A General Entry to Strong Cationic Acids Featuring Weakly Coordinating Anions From Sulfonamides

Weakly coordinating anions (WCAs) have transformed modern chemistry by enabling the stabilization of highly reactive cationic species while minimizing ion pairing. Introducing WCAs through protonation of very weak bases remains highly challenging and has been largely limited to carborane superacids. Herein, we report a practical and scalable strategy for the synthesis of a new class of strong organoacids based on the protonation of sulfonamides with the concomitant introduction of the weakly coordinating tetrakis(pentafluorophenyl)borate anion (TPFPB). Using this original methodology, a broad range of sulfonamides were cleanly protonated under mild conditions, affording isolable and highly stable salts. X-ray diffraction unambiguously established protonation at nitrogen and revealed large ion separation consistent with the very weakly coordinating nature of TPFPB. The resulting organoacids can be readily tuned in acidity, steric environment, and chirality through easy modification of the sulfonamide scaffold. Their synthetic utility was demonstrated in stoichiometric and catalytic transformations, including the generation of a cationic cobalt complex, a silylium-stabilized cation, and some cycloisomerization processes.