MTO/H2O2/Pyrazole-Mediated N-Oxidation of meso-Tetraarylporphyrins and -chlorins, and S-Oxidation of a meso-Tetraaryldithiaporphyrin and -chlorin Subhadeep Banerjee,† Matthias Zeller,‡ and Christian Bru¨ckner*,† Department of Chemistry, UniVersity of Connecticut, Unit 3060, Storrs, Connecticut 06269-3060, and Department of Chemistry, Youngstown State UniVersity, One UniVersity Plaza, Youngstown, Ohio 44555-3663
[email protected] ReceiVed March 11, 2009
The methyltrioxorhenium (MTO)/pyrazole-mediated H2O2 oxidation of octaethylporphyrin and a number of meso-tetraarylporphyrins offers simple and good yielding access to the corresponding N-oxides, only few of which were prepared before. The crystal structure of a free base tetraarylporphyrin N-oxide demonstrates the degree to which the oxygenated pyrrole moiety is slanted with respect to the rest of the otherwise nearly planar macrocycle. The method is also suitable to the preparation of hitherto unknown chlorin N-oxides. Oxidation of meso-tetraphenyldithiaporphyrin and meso-tetraphenyldithiachlorin furnishes the corresponding novel S-oxides. The optical properties of the novel chromophores are described and rationalized.
Introduction Porphyrin N-oxides, such as octaethylporphyrin (OEP) Noxide 1, were first described in 1978 by Bonnett and Ridge.1 Subsequently, it was shown that they could form metal complexes with Ni(II), Cu(II), Zn(II), Fe(III), and Tl(III) in which the oxygen bridges one N-M bond.2-6 OEP N-oxide, in its free base or Ni(II) complex form, can be rearranged into * To whom correspondence should be addressed: Fax: (+1) 860 486-2981. Phone: (+1) 860 486-2743. † University of Connecticut. ‡ Youngstown State University. (1) Bonnett, R.; Ridge, R. J.; Appleton, E. H. J. Chem. Soc., Chem. Commun. 1978, 310–311. (2) Andrews, L. E.; Bonnett, R.; Ridge, R. J.; Appelman, E. H. J. Chem. Soc., Perkin Trans. 1 1983, 103–107. (3) Balch, A. L.; Chan, Y. W.; Olmstead, M.; Renner, M. W. J. Am. Chem. Soc. 1985, 107, 2393–2398. (4) Balch, A. L.; Chan, Y.-W.; Olmstead, M. M. J. Am. Chem. Soc. 1985, 107, 6510–6514. (5) Yang, F.-A.; Cho, K.-Y.; Chen, J.-H.; Wang, S.-S.; Tung, J.-Y.; Hsieh, H.-Y.; Liao, F.-L.; Lee, G.-H.; Hwang, L.-P.; Elango, S. Polyhedron 2004, 25, 2207–2214. (6) Yang, F.-A.; Guo, C.-W.; Chen, Y.-J.; Chen, J.-H.; Wang, S.-S.; Tung, J.-Y.; Hwang, L.-P.; Elango, S. Inorg. Chem. 2007, 46, 578–585.
10.1021/jo9005443 CCC: $40.75 2009 American Chemical Society Published on Web 05/07/2009
a β-oxochlorin or its metal complex, respectively.2,7 A tetraarylporphyrin N-oxide Fe(III) complex has been the source of an unusual ring-oxidized Fe(III) porphyrin complex.8 The significance of the porphyrin N-oxide Fe(III) complexes with respect to possible heme degradation products or P-450 suicide reactions was also discussed.1,3,9-12 N-Oxide 1, or its meso-tetraarylporphyrin analogues,9,13 can be synthesized from their corresponding porphyrins by oxidation using either hypofluorous acid or organic acid peroxides, such as peracetic, permaleic acid, or m-CPBA.2,3,14 Hypofluorous acid needs to be prepared fresh from gaseous fluorine and is po(7) Balch, A. L.; Chan, Y. W. Inorg. Chim. Acta 1986, 115, L45–L46. (8) Tsurumaki, H.; Watanabe, Y.; Morishima, I. J. Am. Chem. Soc. 1993, 115, 11784–11788. (9) Groves, J. T.; Watanabe, Y. J. Am. Chem. Soc. 1986, 108, 7836–7837. (10) Groves, J. T.; Watanabe, Y. J. Am. Chem. Soc. 1988, 110, 8443–8452. (11) Mizutani, Y.; Watanabe, Y.; Kitagawa, T. J. Am. Chem. Soc. 1994, 116, 3439–3441. (12) Rachlewicz, K.; Latos-Grazynski, L. Inorg. Chem. 1996, 35, 1136–1147. (13) Arasasingham, R. D.; Balch, A. L.; Olmstead, M. M.; Renner, M. W. Inorg. Chem. 1987, 26, 3562–3568. (14) For an alterative, nongeneral photochemical pathway toward the TidO complex of a porphyrin N-oxide, see: (a) Hoshino, M.; Yamamoto, K.; Lillis, J. P.; Chijimatsu, T.; Uzawa, J. Inorg. Chem. 1993, 32, 5002–5003.
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Banerjee et al. SCHEME 1. MTO/H2O2 Oxidation of mesoTetraarylporphyrins
tentially explosive.15 The peracid oxidation of meso-tetraarylporphyrins requires in some instances a large stoichiometric excess of peracids9,13 and yet gives low yields for some derivatives (∼5% for meso-4-pyridylporphyrins).16 The best yielding peracid, permaleic acid, needs to be freshly prepared. We are interested in the β,β′-bond manipulation of porphyrins. More generally, we search for novel methods to converting porphyrins to chlorins or pyrrole-modified porphyrins. As an entry point into the manipulation of the pyrrole moieties, we have largely relied on the stoichiometric OsO4-mediated dihydroxylation of meso-tetraarylporphyrins,17-20 but we are also searching for substitutes for the highly toxic and costly OsO4. Over the past decade, the methyltrioxorhenium (MTO)/H2O2/ base system has established itself as a superb epoxidation and dihydroxylation method.21 Thus, we wanted to test whether MTO/H2O2/base could be used for the β,β′-epoxidation or dihydroxylation of porphyrins. We describe here the results of this study. In short, MTO/ H2O2/pyrazole proved unsuitable for the manipulation of a porphyrin β,β′-bond. Instead, we found these reagents to convert OEP and meso-tetraarylporphyrins to their N-oxides in good yields. Moreover, we successfully applied this reaction to mesotetraphenyl-2,3-dimethoxychlorin, meso-tetraphenylporpholactone, and meso-tetraphenyldithiaporphyrin, generating the novel N- and S-oxides, respectively. We report here also on the effects N-oxidation has on the UV-vis spectra of the neutral and protonated chromophores. Results and Discussion When we reacted OEP with the MTO/H2O2/pyrazole system under conditions reported to be optimized for olefin epoxidations (as opposed to dihydroxylation), that is, 25 mol % of MTO, a 20-fold stoichiometric excess of H2O2 (as a 30% aq solution), in the presence of pyrazole as base/ligand in CH2Cl2 at ambient temperature,22 we observed its clean conversion into a product we identified as the corresponding N-oxide 1.2 Even over the course of reactions running up to 24 h, we observed no sign of (15) W. Poll, W.; Pawelke, G.; Mootz, D.; Appelman, E. H. Angew. Chem., Int. Ed. Engl. 1988, 27, 392–393. (16) Posakony, J. J.; Pratt, R. C.; Rettig, S. J.; James, B. R.; Skov, K. A. Can. J. Chem. 1999, 77, 182–198. (17) (a) Bru¨ckner, C.; Sternberg, E. D.; MacAlpine, J. K.; Rettig, S. J.; Dolphin, D. J. Am. Chem. Soc. 1999, 121, 2609–2610. (b) Campbell, C. J.; Rusling, J. F.; Bru¨ckner, C. J. Am. Chem. Soc. 2000, 122, 6679–6685. (c) Daniell, H. W.; Bru¨ckner, C. Angew. Chem., Int. Ed. 2004, 43, 1688–1691. (d) McCarthy, J. R.; Hyland, M. A.; Bru¨ckner, C. Org. Biomol. Chem. 2004, 2, 1484–1491. (18) Bru¨ckner, C.; Rettig, S. J.; Dolphin, D. J. Org. Chem. 1998, 63, 2094– 2098. (19) McCarthy, J. R.; Jenkins, H. A.; Bru¨ckner, C. Org. Lett. 2003, 5, 19– 22. (20) Lara, K. K.; Rinaldo, C. K.; Bru¨ckner, C. Tetrahedron 2005, 61, 2529– 2539. (21) (a) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 1638–1641. (b) Ku¨hn, F. E.; Scherbaum, A.; Herrmann, W. A. J. Organomet. Chem. 2004, 689, 4149–4164. (c) Espenson, J. H. Chem. Commun. 1999, 479–488. (22) Herrmann, W. A.; Kratzer, R. M.; Ding, H.; Thiel, W. R.; Glas, H. J. Organomet. Chem. 1998, 555, 293–295.
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any β,β′-bond reactivity. The yields using this method are around 57% (not taking into consideration that nearly 40% of the starting material that can be recovered). This compares to the reported yields using peracids which range, depending on the peracids used, from 26 to 68%.2 Overall, we perceive the greatest advantage of the MTO/H2O2/pyrazole method toward the preparation of OEP N-oxide to be the use of readily available reagents, combined with its procedural simplicity. Like the peracid method, the MTO/H2O2/pyrazole reaction conditions could also be applied toward the N-oxidation of mesotetraarylporphyrins (Scheme 1).23 Thus, meso-tetraphenylporphyrin (2a) was converted in good average 61% yields (typical yields ranged from 50 to 80%) to the corresponding N-oxide 3a, with ∼20% of the starting material recovered. This compares very well to the reported 14.6% yield using permaleic acid.23 A range of electron-rich (2c and 2d) or electron-poor tetraarylporphyrins (2b, 2e) are susceptible to these reaction conditions (Scheme 1). All products possessed the spectroscopic and analytical properties supporting their assigned structures. In all cases, we observed that very long reaction times or the use of much larger stoichiometric ratios of oxidant increased the formation of smaller amounts (
