ORGANIC LETTERS
An Efficient Synthetic Approach to Highly Conjugated Porphyrin-Based Assemblies Containing a Bipyridine Moiety†
2000 Vol. 2, No. 2 131-133
Fabrice Odobel,* Franck Suzenet, Errol Blart, and Jean-Paul Quintard Laboratoire de Synthe` se Organique, UMR CNRS 6513, Faculte´ des Sciences et des Techniques de NANTES, 2 Rue de la Houssinie` re, BP 92208, 44322 Nantes Cedex 03, France
[email protected] Received November 12, 1999
ABSTRACT
An efficient and potential stepwise strategy involving the mixed sequence of Stille and Wittig−Horner reactions was used for the preparation of a polyene-substituted bis-porphyrin incorporating a bipyridine moiety.
The preparation of molecular-level photonic devices is a rapidly growing field and an important research area in view of both fundamental interest, such as detailed understanding of the photosynthesis process,1 and of practical applications in molecular electronics and conversion of solar energy into electricity or chemical energy.2 In this context, porphyrins are appealing units since they offer a wide variety of photochemical and redox properties which can be easily † The present work was presented in the 10th IUPAC Symposium on Organometallic Chemistry directed toward Organic Synthesis, OMCOS 10, Versailles, July 1999 (selected for oral presentation). (1) Wasielewski, M. R. Chem. ReV. 1992, 92, 435-461. Chambron, J.C.; Sauvage, J.-P. Chem. Eur. J. 1998, 4, 1363-1366. Chambron, J.-C.; Chardon-Noblat, S.; Harriman, A.; Heitz, V.; Sauvage, J.-P. Pure Appl. Chem. 1993, 65, 2343-2349. Gust, D.; Moore, T. A.; Moore, A. L.; Lee, S. J.; Bittersmann, E.; Luttrull, D. K.; Rehms, A. A.; DeGraziano, J. M.; Belford, R. E.; Trier, T. T. Science 1990, 248, 199-201. (2) Argazzi, R.; Bignozzi, C. A. J. Am. Chem. Soc. 1995, 117, 1181511822. Collin, J. P.; Gavina, P.; Heitz, V.; Sauvage, J.-P. Eur. J. Inorg. Chem. 1998, 1, 1-14. Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: England, 1991, and references therein.
10.1021/ol990356j CCC: $19.00 Published on Web 12/18/1999
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tuned by peripheral substituent modifications or metal complexations. One promising route to such materials involves the preparation of multicomponent arrays in which highly cooperative interactions between each unit provide fast and efficient photoinduced electron or energy transfers.3 Although many multiporphyrin systems have been reported in the literature, there are few examples in which a fully delocalized π-conjugated system is directly linked to the porphyrin core. Usually, the spacer links porphyrin units through one of the meso-aryl substituents but rarely directly on the meso position of the porphyrin core.4 However due (3) Davis, W. B.; Svec, W. A.; Ratner, M. A.; Wasielewski, M. R. Nature 1998, 396, 60-63. (4) Wagner, R. W.; Johnson, T. E.; Lindsey, J. S. J. Am. Chem. Soc. 1996, 118, 11666-11180. Mongin, O.; Papamicae¨l, C.; Hoyler, N.; Gossauer, A. J. Org. Chem. 1998, 63, 5568-5580. Kajanus, J.; van Berlekom, S. B.; Albinsson, B.; Martensson, J. Synthesis 1999, 1155-1162. Kuciauskas, D.; Liddell, P. A.; Lin, S.; Johnson, T. E.; Weghorn, S. J.; Lindsey, J. S.; Moore, A. L.; Gust, D. J. Am. Chem. Soc. 1999, 121, 86048614.
to steric interactions, meso aryl groups are known to be perpendicularly oriented to the flat porphyrin core, preventing high electronic communication between the bonded units.4 Recently, several groups have prepared fascinating molecular architectures using polyacetylene5 or polyvinylene6 chains directly bound to the porphyrin macrocycle which present large electronic coupling. New methodologies to functionalize porphyrins at the meso position with substituents other than aromatic groups are of particular interest for preparation of highly coupled systems. Herein we describe the synthesis of porphyrin dyads 13 and 14 (cf. abstract) constructed around a polyene-based bipyridine spacer directly connected to a porphyrin meso position and report preliminary results on its photochemical properties. Because of its low steric hindrance, it was anticipated that the double bond pattern will lie in the plane of the porphyrin in contrast to meso-arylporphyrins. Our approach for the construction of the dyad is based for the first time on a sequence involving the Stille crosscoupling of porphyrin 6 with the recently described (E,E) dienyltin acetal 77 combined with a Wittig-Horner reaction since these reactions offer high yields in very mild experimental conditions. Our interest in the preparation of a mesobound porphyrin system calls for a preparative route to the meso iodo-porphyrin 68 which is also a valuable key synthon for many types of cross-coupling reactions (Stille, Suzuki, Heck, or Sonogashira), allowing an easy functionalization with various kinds of connectors. The key porphyrin precursor 3 was prepared by condensation of pyrrole, unsubstituted dipyrrylmethane 1, and di-tert-butylbenzaldehyde 2 according to Lindsey’s conditions recently published.9 In this way, a one-batch preparation of about 600 mg of porphyrin 3 is routinely obtained. Iodination of 3 according to Dolphin’s procedure10 using a bis(trifluoroacetoxy)iodobenzene-iodine mixture followed by zinc metalation afforded 6 in 85% overall yield (cf. Scheme 1).
At this stage, grafting of the polyenyl chain on the porphyrin core was preferred over initial modification of the dienyl unit before the coupling reaction as is often done with β-tributylstannylacrolein acetals.11 A palladium-catalyzed Stille reaction12 between zinc iodoporphyrin 6 and the dienyltin 77 in the experimental conditions used for β-tributylstannylacrolein acetals13 gave the expected coupled product 8 in 86% yield (cf. Scheme 2). Selective demetalation of the zinc porphyrin affording 9
Scheme 2. Synthetic Route to the Bis-Porphyrin 13a
a Conditions: (i) Pd(PPh ) , DMF, 80 °C, 86%; (ii) TFA, CH Cl , 3 4 2 2 94%; (iii) SiO2, HCl, CH2Cl2, 92%; (iv) AcOH, HCl, H2O, CH2Cl2, 90%; (v) tBuOK, THF, 72%.
Scheme 1. Preparation of the meso Iodo-Porphyrins 5 and 6a
without hydrolysis of the acetal group can also be accomplished in 93% yield by treatment of 8 with anhydrous
a Conditions: (i) BF ‚Et O, NaCl, CH Cl , then DDQ, 12%; (ii) 3 2 2 2 I2, (CF3COO)2IPh, CHCl3, 87%; (iii) Zn(OAC)2‚2H2O, MeOH/ CH2Cl2 > 95%.
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(5) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. Nature 1994, 264, 11051111. Taylor, P. N.; Wylie, A. P.; Huuskonen, J.; Anderson, H. L. Angew. Chem., Int. Ed. 1998, 37, 986-989. Mak, C. C.; Pomeranc, D.; Montalti, M.; Prodi, L.; Sanders, J. K. M. Chem. Commun. 1999, 1083-1084. Arnold, D. P.; Heath, G. A. J. Am. Chem. Soc. 1993, 115, 12197-12198. Arnold, D. P.; James, D. A. J. Org. Chem. 1997, 62, 3460-3469. Wytko, J.; Berl, V.; McLaughlin, M.; Tykwinski, R. R.; Schreiber, M.; Diedrich, F.; Boudon, C.; Gisselbrecht, J. P.; Gross, M. HelV. Chim. Acta 1998, 81, 1964-1977. (6) Burrell, K. A.; Officer, D. L. Synlett 1998, 1297-1307; Tetrahedron 1999, 55, 2401-2418. Effenberger, F.; Schlosser, H.; Bau¨erle, P.; Maier, S.; Port, H.; Wolf, H. C. Angew. Chem., Int. Ed. Engl 1988, 27, 281-284. Higuchi, H.; Takeuchi, M.; Ojima, J. Chem. Lett. 1996, 593-594. (7) Suzenet, F.; Blart, E.; Quintard, J.-P. Synlett 1998, 879-881. (8) Nakano, A.; Shimizu, H.; Osuka, A. Tetrahedron Lett. 1998, 39, 9489-9492. (9) Li, F.; Yang, K.; Tyhonas, J. S.; MacCrum, K. A.; Lindsey, J. S. Tetrahedron 1997, 53, 12339-12360. (10) Boyle, R. W.; Johnson, C. K.; Dolphin, D. J. Chem. Soc., Chem. Commun. 1995, 527-528. (11) Lipschutz, B. H.; Lindsley, C. J. Am. Chem. Soc. 1997, 119, 45554556. Oswald, R.; Chavant, P. Y.; Stadtmu¨ller, H.; Knochel, P. J. Org. Chem. 1994, 59, 4143-4153. Kim, Y.; Singer, R. A.; Carreira, E. M. Angew. Chem., Int. Ed. . 1998, 37, 1261-1263. Org. Lett., Vol. 2, No. 2, 2000
trifluoroacetic acid in dichloromethane. This allows for insertion of various metals inside the porphyrin core before deprotection of the reactive aldehyde functionality. Simultaneous removal of the zinc and deprotection of the aldehyde group was achieved using wet acetic acid (94% yield in 11), while hydrolysis of the acetal group occurs with no demetalation when performed with wet acidic silica gel (92% yield in 10). At this stage, it is worth noting that after Stille coupling and deprotection of the acetal group, accompanied or not by demetalation, the retention of the all-trans double bonds pattern was fully conserved in molecules 8, 9, 10, and 11 as evidenced by the NMR spectra. Finally, the readily available bipyridine bis-phosphonate 1214 was deprotonated in THF with tert-BuOK and reacted with the pentadienal-substituted porphyrin 11 to give dyad 13 in 72% yield (cf. Scheme 2). Wittig-Horner reaction does not require the utilization of n-BuLi or LDA and generally proceeds with higher yields and better stereoselectivities than a Wittig reaction. Indeed, during this step, the diastereoselectivity is sufficiently high to generate dyad 13 with all double bonds in a trans configuration within the limits of the NMR detection. Furthermore, our approach is compatible with a stepwise incorporation of each porphyrin unit with a possible modification of the porphyrin core, allowing thus an unambiguous preparation of hetero-bimetallic systems. The utilization of the free base porphyrin 11 in WittigHorner reactions leads to the coupled compound in a nice 72% yield, whereas the analogous reaction run with zinc porphyrin 10 resulted surprisingly in a low yield (17%). This unexpected behavior with zinc porphyrin most probably arises from the coordination of the phosphonate group above the zinc porphyrin. We have indeed clearly shown using proton and phosphorus NMR and mass spectrometry that the phosphonate group binds as an axial ligand to zinc porphyrins.15 Molecular devices such as 13 or 14, incorporating a bipyridine moiety, are very attractive because their ionoactive module offers unparalleled entry into photoactive metal complexes.16 It is obviously expected that molecules 13 and 14 can be the basis for the construction of light (12) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-516. Mitchell, T. N. Synthesis 1992, 803-815. For cross-coupling involving porphyrins, see: DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. Org. Chem. 1993, 58, 5983-5993; J. Am. Chem. Soc. 1993, 115, 2513-2515. (13) Parrain, J.-L.; Ducheˆne, A.; Quintard, J.-P. J. Chem. Soc., Perkin Trans. 1 1990, 187-189. Parrain, J.-L.; Beaudet, I.; Ducheˆne, A.; Watrelot, S.; Quintard, J.-P. Tetrahedron Lett. 1993, 34, 5445-5448. Launay, V.; Beaudet, I.; Quintard, J. P Synlett 1997, 821-823. Suzenet, F.; Parrain, J.-L.; Quintard, J.-P. Eur. J. Org. Chem. 1999, 2957-2963. (14) Peng, Z.; Gharavi, A. R.; Yu, L. J. Am. Chem. Soc. 1997, 119, 4622-4632. (15) Large shielding of 1H and 31P NMR signals for the PO3Et2 group were observed with a similar zinc porphyrin substituted with a phosphonic acid. (16) Balzani, V.; Juris, A.; Ventury, M.; Campagna, S.; Serroni, S. Chem. ReV. 1996, 96, 759-776.
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harvesting systems if the bipyridine is coordinated with an appropriate transition metal. Attachment of a polyene moiety to the porphyrin skeleton imparts a significant red shift in the absorption and emission bands due to an overall energy decrease of the delocalized π-conjugated orbitals (cf. Figure 1). This indicates that the
Figure 1. Room-temperature absorption and emission (inset) spectra of porphyrin 4 (solid line) and dyad 14 (dashed line) measured in dichloromethane. The excitation wavelength was 540 nm for both molecules.
π-conjugation of the porphyrin moiety is extended into the polyene spacer, allowing electronic communication between the two neighboring units in the ground and excited states. In summary we have shown that the dyads 13 and 14 can be efficiently prepared through a new modular approach involving a Stille cross-coupling and a Wittig-Horner reaction. This strategy is compatible with a sequential grafting of porphyrin units along the assembly. The dyads exhibit suitable properties for potential uses as components for the construction of valuable photonic molecular-scale devices, such as light harvesting antennae or molecular switches. This work is underway in our laboratory, and further results focusing both on synthesis and physical properties of these targets will be reported in due course. Acknowledgment. This work was financially supported by CNRS and AGISMED. The authors express their gratitude to Dr. D. Maume for mass spectrometry (LDH in Ecole Ve´te´rinaire of Nantes) and Pr. C. Tellier for fluorimetry (Nantes University). Supporting Information Available: Text giving detailed experimental procedures for the synthesis and the characterizations of all new compounds, including meaningful region of 1H NMR spectra. This material is available free of charge from Internet at http://pubs.acs.org. OL990356J
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