Phthalimide-N-sulfonic acid and Isatin-N-sulfonic acid as highly efficient catalysts for the synthesis of bis-coumarins

Two highly efficient protocols for the production of bis-coumarins under solvent-free conditions have been introduced. The reaction of 4-hydroxycoumarin (2 eq.) with arylaldehydes (1 eq.) using phthalimide-N-sulfonic acid (PhtSA) or isatin-N-sulfonic acid (IsSA), as solid-acid catalysts, afforded the mentioned compounds with good to excellent yields in short times.Briefly, we have introduced two new SO3H-bearing solid-acid catalysts, entitled phthalimide-N-sulfonic acid and isatin-N-sulfonic acid, for the preparation of bis-coumarins. The advantages of the presented protocols include: high effectuality and generality of the catalysts, high to excellent yields, short reaction times, simple synthesis of the catalysts from available and inexpensive starting materials, easy work-up and purification of the products, relatively mild conditions, application of solvent-free technique, and good compliance with the green chemistry protocols.