A Brønsted Organic Acid Forming Solid State Salts with Chemically Stable Cationic Species

1,2-Benzenedisulfonic imide (1) is a strong Brønsted organic acid and is used for acid-catalyzed organic synthesis.^1-3) For instance, 1 catalyzed acylations of alcohols with acid anhydrides can be performed under solvent-free conditions.³⁾ After this reaction, the catalyst 1 can be recovered and reused. On the other hand, 1 can form ion-pairs with chemically stable cationic species. Barbero et al. successfully isolated cationic intermediates as solid state salts consisting of a conjugate base of 1, by the condensation reaction of aromatic aldehydes with benzimidazole derivatives under the presence of 1 to give the cationic intermediates.⁴⁾ Isolated salt crystals are determined by X-ray analysis to clarify the structure of cationic intermediate.