Electro- and acid-catalysis in tetrafluoroborate-based ionic liquid: new alternative routes for the oxidation of sulfides with UHP Laura Palombi a, * , Carmen Bocchino a , Tonino Caruso a , Rosaria Villano b , Arrigo Scettri a a Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo 84084 Fisciano (Salerno), Italy b Istituto di Chimica Biomolecolare-CNR, trav. La Crucca 3, Reg. Baldinca, 07040 Li Punti (Sassari), Italy article info Article history: Received 20 May 2008 Revised 4 July 2008 Accepted 7 July 2008 Available online 11 July 2008 Keywords: Sulfoxides Electrochemical oxidation Ionic liquid abstract By means of anodic activation, BF 4 -based ionic liquid was found to play the triple role of electrolysis/reac- tion medium, supporting electrolyte and pre-catalyst versus the oxidation of sulfides with UHP. Galvano- static electrolysis in the presence of substrate and UHP molecules allowed the fast, efficient and selective achievement of sulfoxides. Comparable results have been attained by acid catalysis based on R-CSA. Ó 2008 Elsevier Ltd. All rights reserved. As evidenced by a number of literature reports, room tempera- ture ionic liquids (RTILs) are recently finding large application in organic synthesis both as reaction media and as catalysts or co- catalysts. 1 The versatile use of this class of compounds arises from their unique and tunable properties such as high thermal stability, negligible vapour pressure and high polarity. On the contrary, despite their recognized potential as conduct- ing media, which avoid the employment of supporting electrolytes, the use of RTILs in electrosynthesis remains rare. 2 Furthermore, the few examples concerning electrosynthetic procedures in RTIL mainly refer to cathodic processes, and very little references have addressed attention to the anodic ones. Here we describe the first electro-activation of urea hydrogen peroxide (UHP) adduct in tetrafluoroborate-based ionic liquid at a platinum anode, usefully exploited for the in situ oxidation of sulfides (Scheme 1). In parallel with the electrochemical procedure, we also report a purely chemical method, based on the (1R)-(À)- 10-Camphor sulfonic acid (R-CSA) catalysis in 1-ethyl-3-methyl- imidazolium tetrafluoroborate (EMImBF 4 ). The electrochemical experiments were carried out under galva- nostatic conditions, using a U-two compartment cell equipped with a Pt anode and cathode and a G4-glass diaphragm as separa- tor septum. In a typical experiment, the anolyte was made up of a solution of sulfide 1 and UHP in ionic liquid, while the catholyte consisted in pure ionic liquid. The cell was put in a water bath to avoid the overheating during the electrolysis. As summarized in Table 1, the preliminary investigations pertained to the choice of the suitable ionic liquid and the optimal setting of the electrochemical conditions, focusing on the oxidation of sulfide 1a as a model substrate. Interestingly, all the RTILs explored ensured a good transport of current through the cell. Nevertheless, the presence of the BF 4 À counteranion was strictly required to enable the efficient conversion of the starting material 1a. Concerning the set-up of the optimal operational conditions, we can highlight the following features of the electrochemical procedure: – the reaction requires sub-stoichiometric amount of electricity to proceed (Q = 0.3 F/mol); – the application of a higher current density and quantity (from 50 mA/cm 2 to 150 mA/cm 2 ; from 0.1 F/mol to 0.3 F/mol) mark- edly reduces the reaction time without affecting the chemose- lectivity of the process; 0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2008.07.051 * Corresponding author. E-mail address: lpalombi@unisa.it (L. Palombi). R S Me /IL electrolysis R S Me O IL(A): EMImEtSO 4 IL(B): EMImBF 4 IL(C): BMImBF 4 1 1a: R= p-MeC 6 H 4 1b: R= p-MeOC 6 H 4 1c: R= o-MeOC 6 H 4 1d: R= p-BrC 6 H 4 1e: R= o-BrC 6 H 4 1f: R= p-NO 2 C 6 H 4 1g: R= 2-naphthyl 1h: R= n-C 8 H 17 1i: R= C 6 H 5 CH 2 1j: R= S N UHP 2 Scheme 1. Tetrahedron Letters 49 (2008) 5611–5613 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet