Controlled Scrambling in Porphyrin Synthesis—Selective Synthesis of 5,10-Disubstituted Porphyrins
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TETRAHEDRON LETTERS Pergamon Tetrahedron Letters 42 (2001) 355–358 Controlled scrambling in porphyrin synthesis—selective synthesis of 5,10-disubstituted porphyrins Andrew N. Cammidge* and Orhan O8ztu¨rk School of Chemical Sciences, University of East Anglia, Norwich NR47TJ, UK Received 30 October 2000; accepted 7 November 2000 Abstract—Condensation of phenyldipyrromethane with bisnaphthaldehyde 3 leads to unexpected formation of the scrambled 5,10-bridged porphyrin 5; this unprecedented selectivity suggests that an alternative scrambling mechanism to dipyrromethane acidolysis is operating. © 2000 Elsevier Science Ltd. All rights reserved. The synthesis of porphyrins has been widely studied In nature, elaborate porphyrin derivatives are built up since the first synthesis of haemin by Fisher and Zeile via stepwise biosynthesis to give tetrapyrrolic interme- in 1929.1 The interest, for the main part, stems from diates (bilanes) which are then cyclised to the por- the wide occurrence of porphyrin derivatives in nature phyrinogen (which is subsequently oxidised to and the ability of natural and synthetic derivatives to porphyrin). Much work has gone into elaborating act as models for biological processes (light harvest- the biosynthetic pathway for their synthesis. In nature ing, oxygen transport, catalysis, molecular recogni- there is often quite remarkable control and this is 2 tion). Synthesis of porphyrin derivatives typically exemplified by the biosynthesis of vitamin B12 involves condensation of pyrroles with aldehydes or during which a bilane is cyclised with concomitant their equivalent.2,3 Such one-pot procedures are only inversion of one of the pyrrole units. Such ‘scram- generally applicable to symmetrical, tetrasubstituted bling’ of pyrroles in chemical synthesis is well docu- derivatives. More elaborate approaches are required mented but is usually an unwanted and uncontrolled in the synthesis of unsymmetrically substituted por- side reaction.2,5 Here, we report the first example of phyrins. A widely used approach uses preformed controlled scrambling in porphyrin synthesis leading dipyrromethanes. Their condensation with aldehyde to selective synthesis of a 5,10-disubstituted por- gives access to derivatives with reduced symmetry.2,4 phyrin. OH Ph HO Ph N HN NH N OR RO NH HN Ph 1 CHO OHC OH Ph NH HN N HN NH N Ph Ph HO 2 Scheme 1. * Corresponding author. 0040-4039/01/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0040-4039(00)01965-1 356 A. N. Cammidge, O. O8ztu¨rk / Tetrahedron Letters 42 (2001) 355–358 As part of a study aimed at the synthesis of efficient tive mechanism is operating (Scheme 3). Normal elec- molecular recognition hosts based on the metallopor- trophilic aromatic substitution at the 5-position of the phyrin platform, we targeted derivatives such as 1 dipyrromethane by naphthaldehyde gives intermediate which incorporate 8-hydroxyl-substituted naphth-1-yl 6. Electrophilic substitution at the 5%-position of the substituents at the 5- and 15-porphyrin meso positions second pyrrole of the dipyrromethane with the second to produce a tight cleft directly above the porphyrin naphthaldehyde of the dimer (path A) would lead to plane.6 Naphth-1-yl porphyrins are stable to isomerism the (expected) 5,15-bridged porphyrin. However, it (atropisomerism) because of sterically restricted rota- would appear that attack at the 2-position of the substi- tion.6 A statistical synthesis (using a dipyrromethanes tuted pyrrole is preferred (path B) to give intermediate and 8-substituted naphth-1-aldehyde) would therefore 7. Fragmentation of 7 can occur to give cyclic 2,5-dis- be expected to yield two isomers (Scheme 1). ubstituted pyrrole 8 and pyrrolic carbocation 9. Com- pound 8 can react further with another equivalent of In order to overcome this problem we adopted a strat- dipyrromethane and 9 (directly or sequentially) with egy which involved presynthesis of a bis-naphthalde- cyclisation to give the porphyrinogen which, on oxida- hyde to force production of a single atropisomer. tion, gives the observed 5,10-bridged isomer. Produc- p-Xylyl was chosen as the linking7 group because a tion of a small quantity of TPP is easily explained by simple molecular model indicated that it could be ac- this mechanism (tetramerisation of 9). commodated across the porphyrin with minimal strain and it was envisaged that it could be easily removed to It is plausible that this mechanism may operate quite reveal the single atropisomer dihydroxide. Compound 3 generally in porphyrin syntheses. Noting that it impli- was therefore synthesised (by reaction of 2 equivalents cates carbocation 9, we undertook a further study to of 8-hydroxynaphth-1-aldehyde8 with a,a%-dibromo-p- seek conditions designed to suppress this pathway and xylene) and condensed with phenyldipyrromethane9 us- thereby demonstrate control over the fate of species 6. ing BF3·OEt2 as catalyst under standard conditions for We reasoned that destabilisation of the carbocation porphyrin synthesis. Quite remarkably, however, con- would disfavour this fragmentation and promote for- densation of the dialdehyde with phenyldipyrromethane mation of the ‘normal’ 5,15-disubstituted product. This did not yield the expected 5,15-bridged porphyrin 4 but was indeed found to be the case. Repetition of the gave the 5,10 bridged isomer 510 (Scheme 2). The reaction using (pentafluorophenyl)-dipyrromethane structure of 5 was elucidated initially by analysis of its leads to total elimination of the fragmentation/scram- 1H NMR spectrum, which was significantly more com- bling pathway and exclusive isolation of the 5,15- plex than that expected for 4. Two doublets and a bridged porphyrin 10 in 12% yield (Scheme 3). broad resonance (which does not sharpen significantly at 80°C) are observed for the a-protons and phenyl The original target, 4, was eventually synthesised ac- protons, respectively, of the xylyl bridge. The structure cording to Scheme 4. Bisnaphthaldehyde 3 was con- was confirmed by X-ray crystallography.11 Subsequent densed with an excess of pyrrole to yield careful examination of the reaction mixture allowed bis(dipyrromethane) 11. Compound 11 was further con- isolation of a trace (<1%) of the expected 5,15-bridged densed with benzaldehyde in the presence of zinc ace- porphyrin 4. tate using propionic acid in dichloromethane12 to give 4 as its zinc complex (4Zn).10 It is particularly notewor- Scrambling in related dipyrromethane/aldehyde synthe- thy that 5 is not formed when 11 is treated with ses is usually interpreted as arising from breakdown of benzaldehyde under identical conditions to those used the dipyrromethane under the acidic reaction condi- for its formation from 3 and phenyldipyrromethane. tions, eventually giving a complex mixture of all possi- This important observation lends further support for ble porphyrins.5d From the observation that 5 and the alternative mechanism because any acidolysis of the tetraphenylporphyrin (TPP) are the main porphyrin (bis)dipyrromethane would lead to rapid formation of products from this reaction it is likely that an alterna- the 5,10-bridged product. O Ph O N HN NH N Ph O O X Ph 4 NH HN CHO OHC NH HN O 3 26% Ph O Ph N HN NH N Ph 5 Scheme 2. A. N. Cammidge, O. O8ztu¨rk / Tetrahedron Letters 42 (2001) 355–358 357 O O O O path A O C F O A 6 5 O OHC OH A O N HN R =C F path A 6 5 NH N NH HN NH HN C6F5 10 R 6 R =C6H5 R path B path B O O O OH HO O O O H N OH AO N Ph HN H 8 NH N N Ph NH Ph Ph H 5 7 HN HN N Ph HN Ph 9 Scheme 3. Alternative mechanism for scrambling leading to 5. O O O O N PhCHO O Ph O H N N CHO OHC H N N excess H Zn(OAc)2 Zn NH N N 56% HN 37% 11 Ph 4Zn Scheme 4. Further work is in progress to determine the generality Kearney, P. C.; Marguerettaz, A. M. J. Org. Chem. 1987, of this process and to deprotect and elaborate the novel 52, 827. porphyrins for investigation as molecular recognition 4. Ogoshi, H.; Sugimoto, H.; Nishiguchi, T.; Watanabe, T.; hosts. Matsuda, Y.; Yoshida, Z. I. Chem. Lett. 1978, 29. 5. (a) Shanmugathasan, S.; Edwards, C.; Boyle, R. W. Tetrahedron 2000, 56, 1025. (b) Littler, B. J.; Ciringh, Y.; Acknowledgements Lindsey, J. S. J. Org. Chem. 1999, 64, 2864. (c) Davoust, E.; Granet, R.; Krausz, P.; Carre´, V.; Guilloton, M. Tetrahedron Lett. 1999, 40, 2513. (d) Wallace, D. M.; We thank the EPSRC mass spectrometry service Leung, S.; Senge, M. O.; Smith, K. M. J. Org. Chem. (Swansea, UK) for FAB and accurate mass 1993, 58, 7245. measurements. 6. Meso-naphth-1-yl porphyrins bearing hydroxyl sub- stituents at the 2- and/or 7-positions have been reported. (a) Mizutani, T.; Kurahashi, T.; Murakami, T.; Matsumi, References N.; Ogoshi, H. J. Am. Chem. Soc. 1997, 119, 8991. (b) Hayashi, T.; Miyahara, T.; Koide, N.; Kato, Y.; Masuda, 1. (a) Fischer, H.; Zeile, K. Liebigs Ann. Chem. 1929, 468, 98. (b) Fischer, H.; Orth, H. Die Chemie des pyrrols; H.; Ogoshi, H. J. Am. Chem. Soc. 1997, 119, 7281. (c) Akademische Verlag: Leipzig, 1934; Vol. 1. (c) Fischer, Mizutani, T.; Murakami, T.; Ogoshi, H. Tetrahedron H.; Orth, H.; Die Chemie des pyrrols; Akademische Ver- Lett. 1996, 37, 5369. (d) Hayashi, T.; Miyahara, T.; lag: Leipzig, 1937; Vol. 2, part 1. (d) Fischer, H.; Stern, Aoyama, Y.; Nonoguchi, M.; Ogoshi, H. Chem. Lett. A. Die Chemie des pyrrols; Akademische Verlag: Leipzig, 1994, 1749. (e) Hayashi, T.; Miyahara, T.; Hashizumi, N. 1940; Vol. 2, part 2. Ogoshi, H. J. Am. Chem. Soc. 1993, 115, 2049. (f) 2. The Porphyrin Handbook; Kadish, K. M.; Smith, K.