Bimolecular Photoreduction of Aromatic Sulfoxides Jerry W. Cubbage, Troy A. Tetzlaff, Heather Groundwater, Ryan D. McCulla, Mrinmoy Nag, and William S. Jenks* Department of Chemistry, Iowa State University, Ames, Iowa 50011-3111 wsjenks@iastate.edu Received September 21, 2001 Photolysis of aromatic sulfoxides in the presence of alkoxides in alcoholic solvents provides a photochemical route to the corresponding sulfides. Other electron donors also give sulfide with various degrees of success. The reaction could also be carried out using carbazoles as sensitizers, and quantitative yields could be obtained using N-methylcarbazole in methanol. Evidence points toward a hydroxysulfuranyl radical as the key intermediate, and solvent effects point to heterolysis, rather than homolysis, as the step that breaks the S-O bond. Introduction The reduction of sulfoxides to sulfides may be ac- complished with a variety of reagents. 1-5 The photo- chemical reduction of sulfoxides, however, has not re- ceived a great deal of attention. In this paper, we report a study of this reaction, which is as notable for being a member of the rare class of bimolecular photochemical reactions of sulfoxides as for its potential synthetic utility. Unimolecular photochemical deoxygenation of sulfox- ides is a known process that occurs with low quantum yield. 6-10 The sulfide is usually only a major product if R-cleavage chemistry is suppressed by structural factors. Good chemical yields have been obtained in a few instances. 6 Bimolecular photochemical reduction of sulf- oxides was mentioned by Kropp in a study designed to investigate whether certain cleavage reactions of aro- matic sulfides were homo- or heterolytic. 11 In the pres- ence of 200 mM sodium methoxide in methanol, nor- bornyl phenyl sulfide was produced in yields up to 64% on photolysis of the corresponding sulfoxide. In the absence of methoxide, only a trace of sulfide was pro- duced. The mechanism proposed by Kropp involved formation of a 9-S-3 hydroxysulfuranyl radical, followed by homolysis to give the sulfide. In this paper, we report several experiments designed to test this mechanism and expand the scope of the reaction. Results Most of the experiments described below involve pho- tolysis of solutions containing a sulfoxide and an additive or sensitizer, followed by analysis of the reaction mixture. Unless otherwise noted, solutions were approximately 5 mL samples, photolyzed through quartz after having been sealed under a septum and flushed with Ar to remove molecular oxygen. Samples were analyzed either by HPLC or by GC. p-Xylene and dodecane were used as internal standards, respectively. Alkoxide/Alcohol Solvent Systems. To replicate the results of Kropp, where an alkyl phenyl sulfoxide was studied, methyl phenyl sulfoxide 1 was chosen as a model starting material. Solutions of 10 mM 1 and the desired concentration of NaOCH 3 were prepared in methanol. Solutions were photolyzed using the broadly emitting 300 nm fluorescent bulbs of a Rayonet minireactor, and yields were deter- mined by HPLC (Figure 1). Control experiments without methoxide produced only trace quantities of thioanisole (2). The low concentration methoxide runs showed that methoxide was not consumed stoichiometrically. The downward curvature in yields is due to secondary pho- tolysis of 2. One conceivable decomposition pathway of a hydroxy- sulfuranyl radical is by loss of the elements of water. Thus, it was next determined if there was a requirement for a hydrogen adjacent to the sulfinyl group by use of diphenyl sulfoxide (3) as a substrate. In experiments otherwise identical to those reported in Figure 1, the (1) Drabowicz, J.; Numata, T.; Oae, S. Org. Prep. Proc. Int. 1977, 9, 63-83. (2) Drabowicz, J.; Togo, H.; Mikolajczyk, M.; Oae, S. Org. Prep. Proc. Int. 1984, 16, 171-98. (3) Madesclaire, M. Tetrahedron 1988, 44, 6537-80. (4) Kukushkin, V. Y. Coord. Chem. Rev. 1995, 139, 375-407. (5) Kukushkin, V. Y. Rus. Chem. Rev. 1990, 59, 844-52. (6) Gurria, G. M.; Posner, G. H. J. Org. Chem. 1973, 38, 2419-20. (7) Shelton, J. R.; Davis, K. E. Int. J. Sulfur Chem. 1973, 8, 217- 28. (8) Jenks, W. S.; Gregory, D. D.; Guo, Y.; Lee, W.; Tetzlaff, T. In Organic Photochemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New York, 1997; Vol. 1, pp 1-56. (9) Still, I. W. J. In The Chemistry of Sulfones and Sulfoxides; Patai, S., Rappaport, Z., Stirling, C. J. M., Eds.; John Wiley & Sons Ltd.: New York, 1988; pp 873-87. (10) Gregory, D. D.; Wan, Z.; Jenks, W. S. J. Am. Chem. Soc. 1997, 119, 94-102. (11) Kropp, P. J.; Fryxell, G. E.; Tubergen, M. W.; Hager, M. W.; Harris Jr., G. D.; McDermott Jr., T. P.; Tornero-Velez, R. J. Am. Chem. Soc. 1991, 113, 7300-10. 8621 J. Org. Chem. 2001, 66, 8621-8628 10.1021/jo016134s CCC: $20.00 © 2001 American Chemical Society Published on Web 11/17/2001