Native silica nanoparticle catalyzed anti-Markovnikov addition of thiols to inactivated alkenes and alkynes: a new route to linear and vinyl thioethers Subhash Banerjee a , Jayanta Das a,b , Swadeshmukul Santra a,b,c, * a NanoScience Technology Center, University of Central Florida, 12424 Research Parkway, Suite 400, Orlando, FL 32826, USA b Department of Chemistry, University of Central Florida, 4000 Central Florida Blvd., Orlando, FL 32826, USA c Biomolecular Science Center, University of Central Florida, College of Medicine, 4000 Central Florida Blvd., Orlando, FL 32826, USA article info Article history: Received 7 October 2008 Revised 20 October 2008 Accepted 21 October 2008 Available online 25 October 2008 Keywords: Silicananoparticle Catalysis Anti-Markovnikov addition Solvent-free reaction condition Linear and vinyl thioethers Reuse of catalyst abstract A new route for the synthesis of linear and vinyl thioethers has been demonstrated using bare silica nano- particle as catalyst at room temperature under solvent-free conditions. The catalyst can be reused up to six times without loss of catalytic activity. Ó 2008 Elsevier Ltd. All rights reserved. Recently, the use of nanoparticulate materials in catalysis has attracted considerable attention because of their improved effi- ciency under mild and environmentally benign conditions in the context of Green Chemistry. 1,2 Because of enormously large and highly reactive surface area, 3 nanoparticles (NPs) exhibit some un- ique properties in comparison to bulk materials. Among many other NPs, silica-based NPs have been well studied because of the following reasons: (i) silica NPs are easy to syntheses at room temperature, (ii) NP size can be easily tuned, (iii) easy adjustment of synthesis parameters leads to NPs with narrow size distribution (‘monodispersed NPs’), (iv) silica NPs are stable in organic solvents, and (v) they are environmentally friendly materials. Due to these attractive features, silica NPs found wide-spread applications in the synthesis of core-shell hybrid nanomaterial for catalysis of or- ganic reactions. Dominguez-Quintero et al. synthesized nanostruc- tured palladium materials supported on silica for the catalytic hydrogenation of benzene, 2-hexanone, and cyclohexanone. 4 The catalytic performance of silica-coated Pt metal particles for the competitive oxidation of methane and other higher hydrocarbons with gaseous oxygen has been reported by Hori et al. 5 Corma and coworkers demonstrated Pd nanoparticles embedded in a por- ous sponge-like silica as a suitable catalyst for the Suzuki–Miyaura coupling of electron-rich aryl bromides. 6 Chung and co-workers 7 have applied palladium and cobalt nanoparticles immobilized on silica for the Tsuji–Trost reaction. However, the catalytic activity of bare silica NPs in organic transformations has not been studied to the best of our knowledge. Here, we accidentally discovered inherent catalytic activity of native silica NPs. This discovery stimulated us to explore the possibility of using this very old nano- material as catalyst to study a few very basic chemical transforma- tions that would demonstrate its potential. Thioethers play important roles in biological and chemical pro- cesses 8 and also serve as useful building blocks for various organo- sulfur compounds. 9 Therefore, synthesis of thioethers in ‘Green’ and ‘straight-forward’ ways would have tremendous importance. Traditionally, they are synthesized by the addition of thiolate an- ions to olefins. The electrophilic (i.e., ionic process) addition of thiolate anion to olefin promotes Markovnikov addition, 10 whereas free-radical course leads to anti-Markovnikov adduct. 11 Most of the protic acids (e.g., H 2 SO 4 , HClO 4 , and p-TSOH) and Lewis acids (AlCl 3 , BF 3 , TiCl 4 , SnCl 4 , ZnCl 2 ) catalysts are reported for the elec- trophilic additions, only a few reagents such as H-rho-zeolite 12 and benzene-reflux 13 are found to induce anti-Markovnikov prod- ucts. Recently, Ranu et al. 14 have reported water-promoted addi- tion of thiols to inactivated alkenes. Some of these procedures are associated with disadvantages such as unsatisfactory yields, long reaction times, and use of highly carcinogenic and hazardous organic solvent (such as benzene). Moreover, limited reports are available that involves styrenes, possibly due to their facile poly- merization under acidic conditions. 15 The development of robust protocol (i.e., neutral catalyst) for this transformation is thus highly 0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2008.10.110 * Corresponding author. Tel.: +1 407 882 2848; fax: +1 407 882 2819. E-mail address: ssantra@mail.ucf.edu (S. Santra). Tetrahedron Letters 50 (2009) 124–127 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet