Synthesis of Sulfoxides by the Hydrogen Peroxide Induced Oxidation of Sulfides Catalyzed by Iron Tetrakis(pentafluorophenyl)porphyrin: Scope and Chemoselectivity Enrico Baciocchi,* Maria Francesca Gerini, and Andrea Lapi Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy
[email protected] Received January 21, 2004
Abstract: The oxidation of sulfides with H2O2 catalyzed by iron tetrakis(pentafluorophenyl)porphyrin in EtOH is an efficient and chemoselective process. With a catalyst concentration 0.03-0.09% of that of the substrate, sulfoxides are obtained with yields generally around 90-95% of isolated product. With vinyl and allyl sulfides, no epoxidation is observed. With a catalyst concentration between 0.09% and 0.25% of that of the substrate, sulfones are obtained in almost quantitative yield and with the same high chemoselectivity observed in the synthesis of sulfoxides.
The capacity of iron tetraarylporphyrins to catalyze the oxidation of sulfides by H2O2, presumably through the formation of the iron oxo complex P•+Fe(IV)dO as the active species (Scheme 1, P ) tetraarylporphyrin), has been long known.1 However, the pioneering study by Oae and co-workers3 and the few other studies that dealt later with this oxidizing system4 were mainly focused on its mechanistic aspects (e.g., oxygen vs electron-transfer mechanisms) with little or no concern about its possible practical application in organic synthesis. Very recently, Ruasse’s group reported interesting results on the exploitation of this catalytic system for mustard decontamination.5 A number of iron tetraarylporphyrins were examined as catalysts for the H2O2-induced oxidation of mustard models to the corresponding sulfones. A high catalytic efficiency was observed, particularly with iron tetrakis(pentafluorophenyl)porphyrin (F20TPPFe). However, given the aim of the study, very few (specific) substrates were * Address correspondence to this author. Phone: +39-06-49913711. Fax: +39-06-490421.. (1) However, there is a lively debate about the structure of the active species in oxidation catalyzed by iron porphyrins. Another possibility is that, when the oxidant is H2O2, the active species structure would be P-Fe(III)-O-OH.2 (2) Wang, S. H.; Mandimutsira, B. S.; Todd, R.; Ramdhanie, B.; Fox, J. P.; Goldberg, D. P. J. Am. Chem. Soc. 2004, 126, 18-19. (3) Oae, S.; Watanabe, Y.; Fujimori, K. Tetrahedron Lett. 1982, 23, 1189-1192. (4) (a) Baciocchi, E.; Lanzalunga, O.; Marconi, F. Tetrahedron Lett. 1994, 35, 9771-9774. (b) Baciocchi, E.; Lanzalunga, O.; Pirozzi, B. Tetrahedron 1997, 53, 12287-12298. (c) Baciocchi, E.; Gerini, M. F.; Lanzalunga, O.; Lapi, A.; Lo Piparo, M. G. Org. Biomol. Chem. 2003, 1, 422-426. (5) (a) Marques, A.; Di Matteo, M.; Ruasse, M.-F. Can. J. Chem. 1998, 16, 770-775. (b) Marques, A.; Marin, M.; Ruasse, M.-F. J. Org. Chem. 2001, 66, 7588-7595.
SCHEME 1
considered and the actual merit of this system for the synthesis of sulfoxides remains therefore unassessed. In view of the general and continuous interest for the oxidation of sulfides6 and particularly for the development of synthetic methods for the selective conversion of sulfides into sulfoxides7 and the use of H2O2 as an environmental benign oxidant, we have considered it worthwhile to carry out a detailed investigation on the oxidation of sulfides by H2O2 in ethanol catalyzed by F20TPPFe, with the aim of acquiring information on the scope and chemoselectivity of the process. The commercially available F20TPPFe was the iron porphyrin of choice on the basis of the Ruasse’s results5 and also because a recent study by Nam and co-workers8 established that this iron porphyrin is the most effective also in the H2O2-induced epoxidation of alkenes in protic solvents. The results of this investigation are reported herewith. For the synthesis of sulfoxides, sulfoxidations were carried out by using equimolar concentrations of H2O2 (6 mL of a 1 M solution in ethanol) and substrate (6.0 mmol in 20 mL of ethanol) at room temperature. The concentration of F20TPPFe was extremely small ranging from 0.03% to 0.09% with respect to that of the substrate. The H2O2 solution was slowly added to that of the substrate during 1 min with a syringe and the mixture was stirred for 3 min, then sodium dithionite was added. After the solvent was removed, chromatography of the residue on silica gel provided excellent yields of isolated pure sulfoxide which were close to those determined by GC or 1H NMR on the crude reaction mixture. The sulfides investigated and the sulfoxide yields are reported in Table 1. Looking at the results reported in Table 1, the first observation is that the reaction is very efficient affording yields of sulfoxide generally around 90% with a very low catalyst/substrate ratio (from 1/3300 to 1/1100) and with a reaction time as short as 4 min. The selectivity with respect to the formation of sulfone is also very high as the yields of sulfone, determined by GC or 1H NMR, are only between 1% and 4%. More importantly, such small amounts of sulfone did not cause significant problems in preparative experiments, where high yields of pure sulfoxide were obtained. Going more into detail, it can be observed that in the series of aryl methyl sulfides (Table 1, entries 1-4), high yields of the sulfoxide are also found when the strong electron-withdrawing group (EWG) CN is present in the (6) Chu, J.-W.; Trou, B. L. J. Am. Chem. Soc. 2004, 126, 900-908. (7) (a) Choi, S.; Yang, J.-D.; Ji, M.; Choi, H.; Kee, M.; Ahn, K.-H.; Byeon, S.-H.; Baik, W.; Koo, S. J. Org. Chem. 2001, 66, 8192-8198. (b) Gelalcha, F. G.; Schulze, B. J. Org. Chem. 2002, 67, 8400-8406. (c) Matteucci, M.; Bhalay, G.; Bradley, M. Org. Lett. 2003, 5, 235237. (d) Linde´n, A. A.; Kru¨ger, L.; Ba¨ckvall, J.-E. J. Org. Chem. 2003, 68, 5890-5896. (8) Nam, W.; Oh, S.-Y.; Sun, Y. J.; Kim, J.; Kim, W.-K.; Woo, S. K.; Shin, W. J. Org. Chem. 2003, 68, 7903-7906. 10.1021/jo049879h CCC: $27.50 © 2004 American Chemical Society
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Published on Web 04/20/2004
TABLE 1. Isolated Yields of Sulfoxides in the Oxidation of Aromatic and Aliphatic Sulfides by H2O2 (1/1) Catalyzed by F20TPPFe in EtOH, at Room Temperaturea
a Reaction time ) 4 min. b Yields of isolated product referred to the initial amount of substrate. In parentheses are reported the yields determined by GC and 1H NMR analysis on the crude reaction mixture. Average of at least 2 determinations. The error is
