J. CHEM. SOC. PERKIN TRANS. 1 1990 Efficient Synthetic Methodology for 1,3-Dithiolane 1-Oxides via Singlet Oxidation of 1,3-Dithiolanest Bipin Pandey," Smita Y. Bal, Uday R. Khire and Ashok T. Rao National Chemical Laboratory, Pune 41 1 008, India Singlet oxidation of lr3-dithiolanes furnishes synthetically useful yields of lr3-dithiolane 1-oxides. However, 2-ethyl-2-phenyl-l,3-dithiolane 13gave 2-ethyl-4-hydroxy-2-phenyl-I ,3-dithiolane19as a major product. 1,3-Dithiolane constitutes an important functional group in organic chemistry, but, its photochemistry remains under- explored. Broadly speaking, photolysis of 1,3-dithiolanes, in the absence of oxygen, leads to product(s) derived from initial C-S bond cleavage.'V2 Thus, Berchtold et al.' have rationalized the formation of various photolysis products from dithiolane 1 uia thioketone 2 (Scheme 1).Recently, Takahashi et aL3 have reported a unique and efficient photodethioketalization of 1,3-dithiolanes in the presence of triplet sensitizers and molecular oxygen, but without suggesting any mechanism. For example, photolysis of 1 and benzophenone in an oxygen atmosphere is shown to give ketone 3 (65).3 Since singlet oxidation of thioketones, e.g. 2, are known to yield ketones, e.g. 3,4,s we were curious as to whether lo2,generated by the interaction of either triplet sensitizer or photochemically derived thioketone with molecular oxygen, accomplishes dethioketalization. Also, our recent interest in hydroperoxide-type intermediates e.g. 22 (Scheme 3),' which could also be obtained by singlet oxidation of dithiolanes, prompted us to examine the reaction of *02with 1,3-dithiolanes. As a result, this communication describes a synthetically useful methodology for the preparation of 1,3-dithiolane 1-oxides under neutral and mild conditions and concludes that '0,is certainly not the species involved during Takahashi dethioketali~ation.~ Results and Discussion The various 1,3-dithiolanes (1,4,11-13) and 1,3-dithianes (7,8, 14) were prepared according to the literature procedure' and singlet oxidation was carried out, in MeOH, as per the standard procedure, with slight modification (for details see Experimental t NCL Communication No.4878. $ All I,3-dithiolane l-oxides were characterized by satisfactory IR, 'H NMR and mass spectroscopy.A characteristic feature of the S-oxides was a strong absorption at v 1050 cm-' in the IR and complex multiplets between 2.6 and 4.0 in the 'H NMR for the methylenes in the dithiolane skeleton. Wherever possible, authentic sample of dithiolane S-oxides were prepared by known methods.'" ' and superimposability of IR spectra was confirmed. Selected spectral data for both isomers of 19 are as follows: vmaX(film):3400br/cm-'; G,(CDCI,) 0.75-1.00 (3 H, m), 2.3 (3 H, q), 3.0-3.5 (2 H, m), 5.8 (1 H, br s, D,O exchangeable), 7.2-7.4 (3 H, m) and 7.5-7.7 (2 H, m); S,(CDCl,) 11.14 and 12.19 (q), 38.59 and 39.29 (t), 47.09 and 47.62 (t), 86.35 and 86.59 (d), 126.91, 127.60, 127.78, 127.90 and 128.19 (aromatic);m/z: 226 (M', 5), 197 (40)and 121 (100).n 0 __esos 6-6 1 2 3 Scheme 1. section). Normally 90 of dithiolane or dithiane was consumed during singlet oxidation, although the reaction was fast in the early stages of irradiation. Identical products were obtained with either Rose Bengal or Methylene Blue as sensitizers, ensuring the involvement of lo2.Although many variations in experimental conditions are possible as far as duration of irradiation, degree of 02-saturation/agitation, solvent variation, and concentration of substrate and photosensitizer are concerned, the above reaction conditions furnished synthetically useful yields of 1,3-dithiolane l-oxides and 1,3-dithiane l-oxides as shown in Scheme 24 The stereoselectivity of oxygenation was governed by steric factors.Thus, the major stereoisomers of compounds 16 and 17 contained oxygen 'syn' to the alkyl group, which could be easily ascertained by 'H NMR. Interestingly, no di-S-oxides or sulphone was observed during singlet o~idation,~ and, surprisingly, no dethioketalization occurred either! Subse- quently we have examined the validity of 65 yield of 3 from 1 under reported experimental conditions with a 200 W lamp.3 However, we faced problems in reproducing these results; with repeated trials, a maximum of only 10 dethioketalized product 3 could be observed by GLC.5 Another unusual observation was the isolation of the OH insertion product 19 (50),$ besides the 1,3-dithiolane 1- oxide 17 (32) during the singlet oxidation of 2-ethyl-2-phenyl- 1,3-dithiolane 13.However, such insertion products could not be observed with other substrates, even after repeated trials. The tentative mechanism for this complex oxygenation process is shown in Scheme 3. Presumably, the formation of dithiolane S-oxides and OH insertion products both could be rationalized via the hydroperoxide 22, which could be formed either via a step-wise electron transfer process through a tight 8 Some other side products could be observed by GLC. 1 n=3 5 n =3(67) 4 n=2 6 n =2(78) 7n=3 9 n =3(73) 11 n =2, R=H 8n=2 10 n =2(70) 12 n =2, R=Me 13 n =2, R=Et 14 n=3, R=H 15 n =2, R=H(7W0) 16 n =2, R = Me (75),stereoisomer ratio 7:l) 17 n = 2, R = Et (32).stereoisomer ratio 4:l+ 19 (50) 18 n =3, R = H (72) doH ss PhXC2H5 19 Scheme 2.Reagents and conditions: i, lo2,MeOH. ion-pair 21 or uia a concerted process."." Based on a detailed study by Foote et al." 22 could also be represented as the thiadioxirane 28or H-bonded 29.The reduction of 22/29by 20 could result in the formation of S-oxides, i.e. 25.On the other hand, Pummerer rearrangement l3 of 22 via concerted inter- mediate 26or stepwise intermediate 27could lead to 24,which on further reduction with 20 could furnish OH insertion product 23.14The proposed mechanism explains the need for polar protic solvent (MeOH) and rationalizes the usually fast oxidation in the early stages of oxidation.In summary, 1,3-dithiolane l-oxides constitute important synthetic intermediates and the present unprecedented, efficient singlet oxidation methodology under neutral and mild conditions should complement conventional oxidizing meth- ods.? t In the m-chloroperbenzoic acid procedure,*' more than 50 of dithiolane remains unchanged and under exhaustive conditions, i.e. warming up and longer duration of reaction, side products are formed. J. CHEM. SOC. PERKIN TRANS. 1 1990 Experimental General Procedure for the Preparation of 1,3-Dithiolune (or Dithiane) l-Oxides.-The solution of 1,3-dithiolane (or dithiane) (2-4 x 1W2 mol dm-3) in MeOH (25 ml), with photosensitizer (either Rose Bengal or Methylene Blue) (1-2 x lo-' mol dm-3) was saturated with oxygen for 2amp;30 min.The solution was irradiated in a Pyrex vessel with a 200 W Hanovia lamp for 4-5 h, under continuous oxygen agitation. The ambient temperature was maintained ( f2 "C) by con- tinuous water circulation. The progress of the reaction was monitored by either GLC or 'H NMR. Removal of solvent and purification of the residue by column chromatography on silica elution, hexane-acetone (5 :l) furnished 1,3-dithiolane (or dithiane) l-oxides. In the case of 1,3-dithiolane 13,along with 1,3-dithiolane 1-oxide 17,an OH insertion product 19was also observed. Acknowledgements We thank the Department of Science and Technology, Government of India, for partial instrumentation support, the CSIR and UGC (India) for fellowships (to S.Y. B., U. R. K. and A. T. R.) and Dr. R. A. Mashelkar, Director, NCL, for encouragement and support (to B. P.). References 1 R. Arad-Yellin and D. F. Eaton, J. Am. Chem. SOC.,1982,104,6147. 2 J. D. Willett, J. R.Grunwell and G. A. Berchtold, J.Org. Chem., 1968, 33,2297. 3 T. T. Takahashi, C. Y. Nakamura and S. Y. Satoh, J. Chem. SOC., Chem. Commun., 1977,680. 4 V. J. Rao, K. Muthuramu and V. Ramamurthy, J. Org. Chem., 1982, 47, 127. 5 N. Ramnath, V. Ramesh and V. Ramamurthy, J. Org. Chem., 1983, 48,214. 6 N. J. Turro, Modern Molecular Photochemistry, Benjamin/ Cummings, London, 1978, pp. 587-590. 7 B. Pandey, S. Y. Bal and U. R. Khire, Tetrahedron Lett., 1989,30,4007. 8 B. T. Grobel and D. Seebach, Synthesis, 1977,357.9 Sulphoxides are known to trap perepoxide intermediates to give sulphones during singlet oxidation, see A. P. Schaap, S. G. Recher, G. R. Faler and S.R. Villasenor, J. Am. Chem. Soc., 1983,105, 1691. 10 K. Inoue, T. Matsurra and I. Saito, Tetrahedron, 1985,41,2177. 11 E. L. Clennan and X. Chen, J. Am. Chem. SOC.,1989,111,8212 and references therein. 12 F. Jensen and C. S. Foote, J. Am. Chem. Soc., 1988, 110, 2368 and references therein. 13 S. Oae and T. Numata, in Isotopes in Organic Chemistry; Pummerer and Pumrnerer type of reactions, eds. E. Buncel and C. C. Lee, Elsevier, New York, 1980, ch. 2. 14 We are unable to explain the substrate selectivity (only with 13) of this unprecedented OH insertion during singlet oxidation of 1,3-dithiolanes. To the best of our knowledge, we have encountred only one example of thiazolidine, related to dithiolanes, where OH insertion is reported a to sulphur during singlet oxidation; see, T. Takata, Y. Tamura and W. Ando, Tetrahedron,1985,41,2133. 15 C. H. Chen and B. A. Donatelli, J. Org. Chem., 1976,41, 3053. Paper 0/03720F Received 5th April 1990 Accepted 14th August 1990
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