Notes Bull. Korean Chem. Soc. 2003, Vol. 24, No. 7 1009 An Efficient Method for the Sulfonylation of Aromatic Rings with Arene/Alkane Sulfonic Acid by Using PzOs/SiO? under Heterogeneous Conditions§ BiBi Fatemeh Mirjalili,* Mohamad Ali Zolfig ,' Abdolhamid Bamoniri,* and Leila Khazdooz Department of Chemistry, College ofScience, Yazd University, Yazd 89195-741, Iran 'Department of Chemistry, College ofScience, Bu-Ali Sina University, P.O. Box 4135, Hamadan 65174, Iran ^Department of Chemistry, College ofScience, Kashan University, Kashan 51167, Iran Received March 19, 2003 Key Words : Sulfonylation, Sulfones, Di-phosphorus pentoxide Aryl sulfones are of great use in organic synthesis and industry.1,2 Diaryl sulfones are important drugs active against leishmaniasis, malaria, and infections in patients with AIDS discoid lupus erythematosus.3,4 Diaryl, aryl/alkyl sulfones can be synthesized by conven- tional Friedel-Crafts type sulfonylation of aromatics by sulfonyl halides in the presence of Lewis acids such as AlCL, BF3, triflic acid/BiCl3, Zn-exchanged zeolites, Fe(III)-exchanged montmorillonite clay, scandium and lanthanide (III) salts,5 Cu(OTf)2 or Sn(OTf)26 and etc. Recently, Olah and coworkers have reported the sulfonylation of arenes by sulfonic acids in the presence of Nafion-H.7 Very recently, we among many others have demonstrated that heterogeneous reagent systems have many advantages such as simple experimental proce- dures, mild reaction conditions and minimization of chemi- cal wastes as compared to the homogeneous counterparts.8 In this article, we report an efficient preparation of aromatic sulfones directly from arene or alkane sulfonic acid with PzOs/SiOz system. Literature survey shows that phosphorus pentoxide (phosphoric anhydride) can be used as a dehy- drating agent for the formation of anhydride from two molecules of an ordinary acid, ketenimines from amides, vinyl ethers from acetals, nitriles from amides,9 amides from oximes,10 phenolic esters from carboxylic acids11 and so on. On the basis of the above mentioned ability of P2O5 we decided to use it embedded on silica-gel for the production of sulfones via direct sulfonylation of aromatic rings. Preparation of symmetric and unsymmetric aromatic sulfones was achieved in moderate to good yields (50-95%) via Friedel-Crafts type sulfonylation of arenes with various sulfonic acids (Scheme 1). R1-SO3H + Ar-H P2O5/SiO2 (w/w 75%) reflux R1SO2Ar Ri = alkyl or aryl Scheme 1 The sulfonylation reactions were carried out by refluxing a stirred mixture of the corresponding arene or alkane sulfonic §Dedicated to our teacher, professor Abdolhossein Rustaiyan on the occasion of his seventyfifth birthday Corresponding author. E-mail: fmiijalili@yazduni.ac.ir acid and excess dried arene in the presence of P2O5-SiO2 (w/w 75%). The arene acts as both substrate and solvent. Activated arenes such as anisole and mesitylene afforded the corresponding sulfones in excellent yields (90-95%). The yield of the corresponding sulfones from benzene was low (50-52%) due to its lower boiling point. The present method provides an easy and fast approach to sulfones without any by-product. Sulfonylations of anisole, cumene, toluene and bromobenzene afforded para substituted sulfones as the major product indicating that the reaction belongs to electrophilic aromatic substitutions (Table 1). In contrast to the reported method by Olah et al.,7 the present method does not require a Dean-Stark trap for removing azeotropic removal of water. In conclusion, the cheapness and the availability of the reagents, and good yields would make this method attractive for the large-scale operations. We believe that our new process would be an important addition to the existing methods. Experiment Section General. chemicals such as various arenes, p-toluene sulfonic acid, benzene sulfonic acid, methane sulfonic acid, silica gel and di-phosphorus pentoxide were purchased from Fluka, Merck and Aldrich Chemical Companies. The sulfonylation products were characterized by comparison of their spectral (IR, 1H-NMR), TLC and physical data with authentic samples.6 Typic procedure for the preparation of sulfones: preparation of anisyl tolyl sulfone. To a mixture of toluene- 4-sulfonic acid monohydrate (0.6 g, 3.1 mmole) and dry anisole (5 mL) was added P2O5/SiO2 (w/w 75%, 1.2 g). The solution was refluxed continuously with stirring for 30 minutes. The reaction mixture was filtered and washed with 10% NaHCO3 solution to remove unreacted sulfonic acid. After washing with water and drying with anhydrous Na2SO4, excess anisole was distilled off by vacuum and the residue was recrystalized with n-hexane to afford anisyl tolyl sulfone as a white crystalline solid (0.75g, 90%), mp=79-80 °C, Rf= 0.5 (50 : 50 ether/petroleum ether). Acknowledgment. Financial support made by the research affairs, Yazd University, Yazd, Iran and also Bu-Ali Sina University, Hamadan, Iran, is gratefully acknowledged.