Personal Account DOI: 10.1002/tcr.201402050
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Towards meso–meso-Linked Porphyrin Arrays and meso-Aryl Expanded Porphyrins Atsuhiro Osuka Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail: [email protected]
Received: May 30, 2014 Published online: ■■
ABSTRACT: meso–meso-Linked porphyrin arrays and meso-aryl-substituted expanded porphyrins have continuously fueled my imagination for many years. In this account, my expertise in chemical research is retrospectively summarized with a particular focus on how these two novel categories of porphyrinoids were found by our group. As part of our photosynthetic model studies in collaboration with Prof. N. Mataga, the energy-gap dependence of intramolecular charge separation was examined by exploring the photoexcited dynamics of 1,4-phenylene-bridged hybrid porphyrin dimers. This study required electron-deficient porphyrins in the dimers that could serve as an electron-accepting unit towards an octaalkyl-substituted Zn(II) porphyrin donor. To this end, we employed meso-nitrated porphyrins and meso-pentafluorophenyl porphyrins. Efforts to prepare these electron-deficient porphyrins allowed us to serendipitously find both a meso–meso-linked porphyrin dimer and a series of meso-pentafluorophenyl-substituted expanded porphyrins. The meso–meso-linked Zn(II) porphyrin dimer was found as a byproduct in the nitration of 5,10-diaryl Zn(II) porphyrin with AgNO2 but became a major product in the reaction with AgPF6. This finding opened up a new path to directly linked porphyrin oligomers. The series of mesopentafluorophenyl-substituted expanded porphyrins were prepared via BF3·OEt2-catalyzed condensation of pyrrole and pentafluorobenzaldehyde when the reaction was run at tenfold-higher substrate concentrations, as compared to the optimal conditions for the synthesis of 5,10,15,20tetrakis(pentafluorophenyl)porphyrin. These expanded porphyrins have been shown to have attractive attributes such as flexible structures, versatile electronic states, multi-metal coordination, anion sensing, and large nonlinear optical properties. While these studies were mostly curiosity-driven, some of our work covers rather more general interests: how linearly connected molecules can be rationally synthesized and isolated in a pure and discrete form, how large π-conjugation can be realized to allow for very low energy electronic transitions, and how easily Möbius aromatic and antiaromatic molecules can be prepared. Keywords: aromaticity, expanded porphyrins, macrocycles, nanostructures, porphyrinoids
1. Introduction It is my pleasure to write this Personal Account tracing my journey in organic chemistry, with a particular focus on how I entered into the field of novel porphyrinoids and how we
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found meso–meso-linked porphyrin arrays and meso-aryl expanded porphyrins. In the second year of my doctoral course at Kyoto University (1979), I was offered a position as
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Assistant Professor in the group of Prof. H. Suzuki in the Department of Chemistry, the Faculty of Science, Ehime University. Prof. Suzuki was kind to allow me to continue my research on the photochemical reactions of epoxynaphthoquinones, which had been assigned as the theme of my doctoral degree by Prof. K. Maruyama of Kyoto
University. Before joining Prof. Suzuki’s laboratory, I had reported that irradiation of 2,3-dimethyl-2,3-epoxy-1,4naphthoquinone (1) led to cleavage of the oxirane ring to generate carbonyl ylide 2 or 1,3-diradical intermediate 3, which was trapped by the carbonyl group of aldehydes or ketones to give 1,3-cycloaddition adducts 4 (Eq. 1).[1,2]
(1) As an extension of these studies, I examined the photoirradiation of 2-methyl-3-(2-methyl)propyl-2,3-epoxy1,4-naphthoquinone (5). Photoirradiation of 5 in a benzene solution gave rise to a preferential intramolecular hydrogenabstraction reaction (Norrish type II photoreaction) over the oxirane cleavage, affording cyclobutanol product 7 as a cycliAtsuhiro Osuka was born in Gamagori, Aichi, Japan, in 1954. He received his Ph.D. degree on the photochemistry of epoxyquinones from Kyoto University in 1982. In 1979, he started his academic career at the Department of Chemistry of Ehime University as an Assistant Professor. In 1984, he moved to the Department of Chemistry of Kyoto University, where he became a Professor of Chemistry in 1996. He was awarded the CSJS Award for Young Chemists in 1988, the Japanese Photochemistry Association Award in 1999, the NOZOE Memorial Lectureship Award at the 13th ISNA conference in 2009, and the Chemical Society of Japan Award in 2010. He was selected as a project leader of Core Research for Evolutional Science and Technology (CREST) of JST during 2001–2006. His research interests cover many aspects of synthetic approaches toward artificial photosynthesis and the development of porphyrin-related compounds with novel structures, electronic systems, and functions.
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zed product and phthiocol 9 as an elimination product via 1,4-biradical 6 (Scheme 1). The cyclobutanol 7 underwent secondary photoreactions to give rearranged products 11 and 12, probably via biradical 10 formed by Norrish type I α-cleavage.[3,4] Under similar photoirradiation conditions, 2-methyl-3-(2-methyl)propyl-2,3-methano-1,4naphthoquinone (13) gave cyclobutanol product 15 as a major product via a Norrish type II 1,4-biradical 14 (Scheme 2). It was found that 15 underwent a secondary photoinduced cleavage to give the same 1,4-biradical 14. Prolonged irradiation of a mixture of 13 and 15 led to formation of γ,δ-unsaturated alcohol 16 as a rare photoproduct, possibly via slow disproportionation of the 1,4-biradical 14.[5,6] I obtained my doctoral degree on the basis of these studies on the photochemistry of epoxynaphthoquinones in 1982. Alongside these studies, I started synthetic organic chemistry using tellurium compounds, which was one of the main research themes in the research group of Prof. Suzuki. Sodium hydrogentelluride (NaTeH) was used for dehalogenation of α-halo carbonyl compounds (Eq. 2),[7] chemoselective reduction of α,β-epoxy ketones to β-hydroxy ketones (Eq. 3),[8] and reductive removal of aliphatic nitro groups (Eq. 4).[9] Reactions of telluronium ylides with carbonyl compounds gave olefination products or epoxidation products depending upon the stability of the telluronium ylide. Namely, the reactions of dialkyltelluronium carbethoxymethylide with aldehydes and
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Scheme 1. Norrish type-II photoreaction of en epoxynaphthoquinone.
Scheme 2. Norrish type-II photoreaction of a methanonaphthoquinone.
ketones gave α,β-unsaturated esters with trans stereoselectivity (Eq. 5),[10] but the reactions of dialkyltelluronium allylide with aldehydes provided vinyl epoxides with cis selectivity (Eq. 6).[11] These reactivities are different from the established features of the corresponding reactions of sulfonium and selenonium ylides with aldehydes.[12] These are, to the best of my knowledge, the first reports on the use of telluronium species for C–C bond-forming reactions.
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Fig. 1. Tyrosine-linked and tryptophan-linked porphyrins.
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(6) In 1984, I moved to the Department of Chemistry in the Faculty of Science at Kyoto University as an Assistant Professor in the group of Prof. Maruyama. At that time, Prof. Maruyama was interested in a chemical approach to photosynthetic reaction centers and naturally he requested me to pursue a research project along such a direction. I was forced to abandon the chemistries pursued at Ehime University and started research related to artificial photosynthesis by preparing a series of octaalkyl-substituted mesoporphyrins bearing amino acids through an ester or an amide linkage. Disappointingly, however, such appended amino acid side chains did not display any meaningful interactions with the porphyrin chromophore in the ground state. Fluorescence emissions of these porphyrins are not quenched, indicating almost no interaction in the singlet excited states of porphyrins with the amino acid residues. However, in the meantime, we noticed strong chemically induced dynamic nuclear polarization (CIDNP) signals during irradiation (λ > 590 nm) of a benzene solution of tyrosinelinked porphyrin 29[13–16] and tryptophan-linked porphyrin 30[17] in the presence of 1,4-benzoquinone (Figure 1).
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Prolonged irradiation of a solution of 29 and 1,4benzoquinone gave quinone-linked porphyrin 32 via oxidation of hydroquinone-linked porphyrin 31 with an excess amount of the quinone (Scheme 3). On the basis of extensive and detailed studies, the formation of 32 has been determined to occur by the mechanism shown in Scheme 4. Electron-transfer quenching of the triplet excited state of the porphyrin by 1,4-benzoquinone gives a porphyrin cation radical and a quinone anion radical. Subsequent proton transfer from the tyrosine residue to the quinone anion radical generates a phenolate anion and a neutral semiquinone radical, which are converted to a neutral radical pair of a semiquinone radical and a phenoxy radical via intramolecular electron transfer from the phenolate to the porphyrin cation radical. Coupling of this radical pair results in the formation of hydroquinone-linked porphyrin, which is finally oxidized to form quinone-linked porphyrin upon photoirradiation in the presence of an excess amount of quinone. The observed strong CIDNP signals were thus ascribed to competitive disproportionation of the neutral radical pair to regenerate the starting state and recombination
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Scheme 3. Photo-induced coupling reaction of 1,4-benzoquione to tyrosine-lined porphyrin.
Scheme 4. Reaction mechanism of photo-induced coupling reaction of 1,4-benzoquinone to tyrosine-linked porphyrin.
to give the hydroquinone-linked porphyrin with concomitant spin polarization. This photoinduced coupling of quinones to phenol-linked porphyrin has been extended to the synthesis of various quinone-linked and quinone-capped porphyrins.[15,16] During these studies, we recognized the appearance of the crystal structure of a bacterial photosynthetic reaction center as an epoch-making event.[18] The structure of the reaction center strongly suggested that the spatial arrangements of the chromophores, such as the special pair, bacteriochlorophylls,
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bacteriopheophytins, and quinones, are critically important for the photoinduced events such as excitation-energy transfer and electron transfer. We thus prepared quinone-capped porphyrins 33, 34, and 35 with different orientations by this photoinduced coupling reaction with the intention to examine the orientation dependence of intramolecular electron transfer from the porphyrin to the quinone (Figure 2).[15] Unfortunately, these quinone-capped porphyrins were conformationally very flexible and unsuitable for detailed investigation of the
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Fig. 2. Quinone-capped porphyrins.
Fig. 3. Naphthalene-bridged porphyrin dimers.
orientation dependence of the intramolecular electron-transfer reactions. In addition, the electron-transfer reactions of the quinone-capped porphyrins were too fast to allow determination of the precise rates in those days. We then turned our attention to the synthesis of covalently linked porphyrin dimers that are conformationally restricted at varying orientations. As a structural motif, we chose a peripherally octaalkyl-substituted porphyrin that is linked directly at its meso positions to a naphthalene bridge (36, 37, and 38, Figure 3). In these diporphyrins, the naphthalene bridge is held nearly perpendicularly to the porphyrin
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plane due to the steric congestion of the flanking alkyl groups. This conformationally restricted feature is suitable for studies on the geometry dependence of excitation-transfer and energy-transfer reactions. Zn(II) complexes of these dimers exhibit remarkable exciton coupling in their Soret bands, reflecting the respective orientation.[19] The observed split Soret bands of these dimers were analyzed on the basis of the exciton coupling theory developed by Kasha.[20] These porphyrin dimers were used to examine the geometry dependencies of intramolecular excitation-energy transfer and electron-transfer reactions.[21,22] As an extension of these
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Fig. 4. Conformationally restricted porphyrin oligomers.
studies, we developed an efficient method that allowed the synthesis of conformationally restricted porphyrin oligomers such as 39 and 40 (Figure 4).[23] With this synthetic method in hand, we explored covalently linked donor–acceptor porphyrin compounds as models of the photosynthetic reaction center, with the ultimate goal of mimicking the whole events of the excitation-energy transfer and electron transfer in the photosynthetic reaction centers within a single molecular entity.[24–31] As part of these studies, we were interested in the energygap dependence of intramolecular electron-transfer reactions in 1,4-phenylene-bridged diporphyrins that bear an octaalkylsubstituted Zn(II) porphyrin as an electron donor and various electron-deficient porphyrins as an electron acceptor. Using diporphyrins 41–48 (Figure 5), we revealed a bell-shaped energy-gap dependence for the rates of charge separation and charge recombination.[32] More importantly, this study allowed us to find two important reactions. In the course of our study to prepare meso-nitrated diporphyrins 46 and 48, we found a direct meso–meso coupling reaction of 5,15-diaryl-substituted Zn(II) porphyrins, and our attempt to prepare mesopentafluorophenyl-substituted diporphyrin 45 led to the discovery of an effective synthesis for a series of meso-arylsubstituted expanded porphyrins.
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Fig. 5. 1,4-Phenylene-bridged porphyrin dimers.
2. meso–meso-Linked Porphyrin Arrays In the above-mentioned study on the energy-gap dependence of the intramolecular electron-transfer reaction of 1,4phenylene-bridged diporphyrins, we chose meso-nitrated porphyrins as the electron-deficient porphyrin part. By following Baldwin’s protocol,[33] 5,15-diaryl Zn(II) porphyrin 49 was nitrated with AgNO2 and I2 in CHCl3 (Scheme 5). The
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Scheme 5. Formation of meso-meso linked porphyrin dimer and trimer.
nitration proceeded nicely to give meso-nitrated porphyrin 50 in 90% yield, but my student did not miss a side product that eluted on a silica gel TLC plate essentially in the same manner as 49. This side product was soon identified as meso–mesolinked Zn(II) diporphyrin 51. The formation of 51 was thought to be initiated by one-electron oxidation of 49 with Ag(I) ion to generate the cation radical of 49, followed by nucleophilic attack by another neutral molecule of 49. We thus chose AgPF6 as an oxidant, since it has a non-nucleophilic anion that has no chance to react with the cation radical of 49. The reaction of 49 with AgPF6 in CHCl3 gave 51 and 52 in 27% and 4% yields, respectively, along with recovery of 49 (47%) (Scheme 5).[34] Synthesis of an extremely long discrete molecule was indeed a challenge. We decided to synthesize long meso–meso-linked Zn(II) porphyrin arrays, since such molecules would be promising as photonic wires and the orthogonal arrangement of neighboring porphyrins is favorable for a high degree of solubility. However, we soon faced a serious solubility problem at the stage of the octamer. Thus, we designed a more soluble substrate, Z1, which bears two 3,5dioctyloxyphenyl substituents. The established standard coupling protocol is very simple and involves treatment of Z1 or its oligomers Zn with AgPF6 in CHCl3 at room temperature for several hours (Scheme 6).[35] It is important to monitor the progress of the coupling reaction by gel permeation chromatography (GPC) HPLC or 1H NMR spectroscopy, and to quench the reaction at 30–40% conversion, since further reaction leads to production of larger porphyrin arrays, which makes separation of the products more tedious. Usually the coupling products were separated from the starting substrate through GPC-HPLC columns. In each iterative coupling reaction, the rigorous purification of coupled products is critically
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important for the separation of their further coupled products. This coupling reaction has proved very effective, particularly for large porphyrin substrates.[35] It is amazing that extremely long porphyrin arrays (even Z128 and Z256) smoothly undergo the coupling reaction to provide Z1024,[36] despite the extremely long molecular shapes with only two edge free meso positions available for the coupling. This meso–meso coupling reaction of porphyrins allowed us to prepare threedimensionally extending windmill porphyrin arrays,[37–39] dihedral-angle-controlled meso–meso-linked diporphyrins,[40,41] large porphyrin wheels,[42–44] helical porphyrin arrays held by intermolecular hydrogen-bonding interactions,[45] and directly linked porphyrin rings (Figure 6).[46] The meso–meso-linked diporphyrin motifs are suitable for supramolecular assembly. By this strategy, three-dimensional porphyrin boxes and other interesting architectures have been constructed through rigorous self-sorting assembly of pyridine-appended or cinchomeronimide-appended meso–meso-linked Zn(II) diporphyrins (Scheme 7).[47–50] In the meantime, we explored an effective oxidative fusion– dimerization reaction of Ni(II) porphyrins to give meso, β doubly linked porphyrin dimers (Eq. 7) and a ring-closure reaction of meso–meso-linked diporphyrins to β, meso, β triply linked diporphyrins, initially with tris(4bromophenyl)aminium hexachloroantimonate (BAHA) (Eq. 8)[51–53] and later with the combined use of 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) and Sc(OTf )3 (Eq. 9).[54] The latter method is superior to the former because of the absence of serious halogenation side products, and allow the conversion of long meso–meso-linked Zn(II) porphyrin arrays to the corresponding meso–meso, β–β, β–β triply linked porphyrin arrays that are called porphyrin tapes. On the basis of this
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Scheme 6. Iterative doubling elongation of meso-meso linked porphyrin oligomers.
oxidation, we have succeeded in the synthesis of π-conjugated porphyrin tapes that display remarkably red-shifted and enhanced absorptions reaching deep into the infrared region (Scheme 8).[54–57] We have also explored an antiaromatic tetrameric porphyrin sheet, which exhibits a strong paratropic ring current above the central plane of the cyclooctatetraene core,[58– [61] The 60] and two-dimensionally extending porphyrin tapes. bay-area selective cycloaddition reactions of porphyrin tapes proceeded nicely with an o-xylylene[62] and an azomethine ylide.[63] The chemistry of meso–meso-linked porphyrin arrays and porphyrin tapes has been reviewed elsewhere.[64–70]
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Fig. 6. Various meso-meso linked porphyrin oligomers.
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Scheme 7. Self-assembled formation of porphyrin boxes.
Scheme 8. Synthesis of porphyrin tape 12-mer.
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Scheme 9. Synthesis of meso-aryl substituted expanded porphyrins.
3. meso-Aryl-Substituted Expanded Porphyrins In recent years, the chemistry of expanded porphyrins has been actively explored in light of their favorable attributes, such as rich coordination chemistry, anion sensing, large two-photon absorption cross-sections, and extended π-electronic systems.[71– 79] Sessler, an important pioneer of this field, developed the synthesis of a pentadentate texaphyrin in 1988[80] and a rational synthesis of sapphyrins in 1990,[81] improvisation that led to an exciting boom of novel expanded porphyrins. meso-Aryl expanded porphyrins can be regarded as legitimate expanded porphyrins in terms of their regular and alternating arrangements of pyrroles and aryl-substituted meso carbons. We serendipitously found the formation of these meso-aryl expanded porphyrins during the synthesis of tetrakis (pentafluorophenyl)porphyrin by the Lindsey method[82] using pentafluorobenzaldehyde and pyrrole. We made the fortunate mistake to run the reaction at a substrate concentration of 67
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(9) mM, which is approximately tenfold higher than the optimized concentration for porphyrin synthesis. Under these conditions, a series of meso-aryl expanded porphyrins including N-fused pentaphyrins, hexaphyrin, heptaphyrin, octaphyrin, nonaphyrin, decaphyrin, undecaphyrin, and dodecaphyrin were formed in a surprisingly effective manner (Scheme 9).[83,84] At this time, we knew that a communication by Cavaleiro et al. had appeared, which reported the isolation of [26]hexaphyrin and [28]hexaphyrin in low yields.[85] We were very shocked by this paper but soon realized that our synthetic protocol was superior to Cavaleiro’s method in regard to the formation of a series of expanded porphyrins and better yields. Initially, the separation of meso-aryl expanded porphyrins was not easy and needed repeated chromatographic separation, but this separation difficulty has been somewhat mitigated by size-selective synthesis of the expanded porphyrins using a dipyrromethane and a tripyrromethane as precursors.[86,87] Use of high concentrations of the substrates led to better yields of larger expanded
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porphyrins.[88] These meso-aryl expanded porphyrins have proved to be attractive platforms for rich coordination chemistry,[89–95] versatile aromatic compounds such as strongly Hückel aromatic[90,91,96] and antiaromatic molecules,[90,91] and stable organic radical species.[96–98] Unprecedented rearrangements are triggered by transannular electronic interactions aided by the conformational flexibility of expanded porphyrins.[99,100] Besides these, the meso-aryl expanded porphyrins are quite interesting from the viewpoint of mutual chemical interconversions (metamorphosis).[101,102] The most remarkable example of this is the efficient and quantitative splitting reaction of bis-Cu(II) [36]octaphyrin complex 66 into two molecules of Cu(II) porphyrins 67 upon heating (Eq. 10).[103] This splitting reaction also proceeded quantitatively in a Co(II)–
Cu(II) hybrid octaphyrin complex.[104] In a heptaphyrin system, treatment of heptaphyrin Cu(II) complex 68 with BBr3 in the presence of a sterically hindered amine led to the formation of subporphyrin 69 and Cu(II) porphyrin 70 in 36% and 13% yields, respectively (Eq. 11).[105] Similar treatment of meso-trifluoromethyl-substituted heptaphyrin Cu(II) complex 71 with BBr3 gave meso-trifluoromethyl subporphyrin 72 and Cu(II) porphyrin 73 (Eq. 12).[106] Subporphyrins are my other favorite porphyrinoids, which were first synthesized in my group in 2006.[107] The chemistry of subporphyrins are reviewed elsewhere.[108,109] It is worthwhile to note that subporphyrins 69 and 72 cannot be synthesized by the usual condensation methods and thus these splitting reactions are important from a synthetic viewpoint.
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(12) These splitting reactions require the rapture and formation of two carbon–carbon double bonds in a metathesis manner. We thought that the transannular electronic interaction would be enhanced at the hinge position of the figure-eight conformation of the expanded porphyrins upon metalation and, thus, this trend would be more general. In order to examine the scope of this unique metal-induced splitting reaction, we examined the complexation of expanded porphyrins with various transition metal ions such as Rh(I), Ni(II), Pd(II), and Pt(II). In 2006, we reported [28]hexaphyrin Ni(II), Pd(II), and Pt(II) complexes (74, 75, and 76, Figure 7).[110] Although we collected all the data for these complexes including their crystal structures and noted
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that the inner β-protons appeared at high field, we did not notice that the high-field shifts were caused by the diatropic ring currents of 74–76, which are all Möbius aromatic molecules. We also reported the formation of Rh(I) complexes 77 and 78 from N-fused pentaphyrin 60 (Scheme 10).[111] The 1H NMR spectrum of 78 shows a signal due to the inner β-proton in the high-field region at 0.10 ppm. At the time of publication, we could not explain this particular high-field shift. In 2008, we reported that meso-aryl expanded porphyrins such as octaphyrin, heptaphyrin, and hexaphyrin can form Möbius aromatic complexes almost spontaneously upon metalation with Pd(II) ions.[112] In this paper, we revealed that the com-
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Fig. 7. Structures of Möbius aromatic hexaphyrin Ni(II)-, Pd(II)-, and Pt(II) complexes.
Scheme 10. Rh(I) metalation of N-fused [22]pentaphyrin.
plexes 79 and 80 (Figure 8) along with 74–76 are all Möbius aromatic molecules. Soon after, we reported that the complex 78 is also a Möbius aromatic molecule.[113] Following these studies, we have explored many expanded porphyrins, which display versatile electronic states such as Hückel aromatic, Hückel antiaromatic, Möbius aromatic, Möbius antiaromatic,[114,115] and stable radical states.[96,98] Such molecules are outside the scope of this account and are reviewed elsewhere.[116,117]
4. Summary As described above, my research career has been guided by no means by my idea but simply led by many fortunate unex-
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pected encounters with the assistance of many students and postdocs. Throughout my career in the Department of Chemistry, the Faculty of Science, Ehime University, and in the Department of Chemistry, the Graduate School of Science, Kyoto University, I have been very fortunate to have had the opportunity to serve as a mentor for many talented students. To be honest, my laboratory has not been well organized. In other words, I have tried to keep my students and postdocs as free as possible. They have to think and explore novel chemistry by themselves. Most of them have demonstrated themselves to be quite motivated in synthesizing new porphyrinoids and exploring new chemistry of such molecules and have now become eligible staff members of universities, institutes, and companies. They are indeed my pride. Therefore, it is obvious
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Fig. 8. Structures of Möbius aromatic heptaphyrin Pd(II) complex and octaphyrin bis-Pd(II) complex.
that I should thank them for their outstanding devotion to the chemistry of novel porphyrinoids.
Acknowledgements This work was partly supported by Grants-in-Aid (No. 25220802 (S)) from MEXT. The author thanks Dr. T. Tanaka for making the schemes, equations, and figures.
REFERENCES [1] K. Maruyama, A. Osuka, Chem. Lett. 1979, 8, 77. [2] K. Maruyama, A. Osuka, J. Org. Chem. 1980, 45, 1898. [3] K. Maruyama, A. Osuka, H. Suzuki, J. Chem. Soc., Chem. Commun. 1980, 324. [4] A. Osuka, J. Org. Chem. 1982, 47, 3131. [5] A. Osuka, M. H. Chiba, H. Shimizu, H. Suzuki, J. Chem. Soc., Chem. Commun. 1980, 919. [6] A. Osuka, H. Shimizu, M. H. Chiba, H. Suzuki, K. Maruyama, J. Chem. Soc., Perkin Trans. 1 1983, 2037. [7] A. Osuka, H. Suzuki, Chem. Lett. 1983, 12, 119. [8] A. Osuka, K. Taka-oka, H. Suzuki, Chem. Lett. 1984, 13, 271. [9] H. Suzuki, K. Takaoka, A. Osuka, Bull. Chem. Soc. Jpn. 1985, 58, 1067. [10] A. Osuka, Y. Mori, H. Shimizu, H. Suzuki, Tetrahedron Lett. 1983, 24, 2599. [11] A. Osuka, H. Suzuki, Tetrahedron Lett. 1983, 24, 5109. [12] W. Dumont, P. Bayet, A. Kreif, Angew. Chem. Int. Ed. Engl. 1974, 13, 274.
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[13] K. Maruyama, H. Furuta, A. Osuka, Chem. Lett. 1986, 15, 475. [14] K. Maruyama, H. Furuta, A. Osuka, Tetrahedron 1986, 42, 6149. [15] A. Osuka, H. Furuta, K. Maruyama, Chem. Lett. 1986, 15, 479. [16] A. Osuka, K. Maruyama, J. Chem. Res., Synop. 1987, 2401; J. Chem. Res., Miniprint 1987, 2428. [17] A. Osuka, K. Maruyama, Chem. Lett. 1986, 15, 1853. [18] J. Deisenhofer, O. Epp, K. Miki, R. Huber, H. Michel, J. Mol. Biol. 1984, 80, 385. [19] A. Osuka, K. Maruyama, J. Am. Chem. Soc. 1988, 110, 4454. [20] M. Kasha, Radiat. Res. 1963, 20, 55. [21] A. Osuka, K. Maruyama, I. Yamazaki, N. Tamai, Chem. Phys. Lett. 1990, 165, 392. [22] A. Osuka, K. Maruyama, N. Mataga, T. Asahi, I. Yamazaki, N. Tamai, J. Am. Chem. Soc. 1990, 112, 4958. [23] T. Nagata, A. Osuka, K. Maruyama, J. Am. Chem. Soc. 1990, 112, 3054. [24] K. Maruyama, A. Osuka, Pure Appl. Chem. 1990, 62, 1511. [25] K. Maruyama, A. Osuka, N. Mataga, Pure Appl. Chem. 1994, 66, 867. [26] A. Osuka, N. Mataga, T. Okada, Pure Appl. Chem. 1997, 69, 797. [27] A. Osuka, S. Nakajima, K. Maruyama, N. Mataga, T. Asahi, I. Yamazaki, Y. Nishimura, T. Ohno, K. Nozaki, J. Am. Chem. Soc. 1993, 115, 4577. [28] M. Ohkohchi, A. Takahashi, N. Mataga, T. Okada, A. Osuka, H. Yamada, K. Maruyama, J. Am. Chem. Soc. 1993, 115, 12137. [29] A. Osuka, H. Yamada, K. Maruyama, N. Mataga, T. Asahi, M. Ohkouchi, I. Yamazaki, Y. Nishimura, J. Am. Chem. Soc. 1993, 115, 9439.
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[30] A. Osuka, S. Nakajima, T. Okada, S. Taniguchi, K. Nozaki, T. Ohno, I. Yamazaki, Y. Nishimura, N. Mataga, Angew. Chem. Int. Ed. Engl. 1996, 35, 92. [31] A. Osuka, S. Marumo, N. Mataga, S. Taniguchi, T. Okada, I. Yamazaki, Y. Nishimura, T. Ohno, K. Nozaki, J. Am. Chem. Soc. 1996, 118, 155. [32] A. Osuka, G. Noya, S. Taniguchi, T. Okada, Y. Nishimura, I. Yamazaki, N. Mataga, Chem. Eur. J. 2000, 6, 33. [33] J. E. Baldwin, M. J. Crossley, J. DeBernardis, Tetrahedron 1982, 38, 685. [34] A. Osuka, H. Shimidzu, Angew. Chem. Int. Ed. Engl. 1997, 36, 135. [35] N. Aratani, A. Osuka, Y. H. Kim, D. H. Jeong, D. Kim, Angew. Chem. Int. Ed. 2000, 39, 1458. [36] N. Aratani, A. Takagi, Y. Yanagawa, T. Matsumoto, T. Kawai, Z. S. Yoon, D. Kim, A. Osuka, Chem. Eur. J. 2005, 11, 3389. [37] A. Nakano, A. Osuka, I. Yamazaki, T. Yamazaki, Y. Nishimura, Angew. Chem. Int. Ed. 1998, 37, 3023. [38] A. Nakano, T. Yamazaki, Y. Nishimura, I. Yamazaki, A. Osuka, Chem. Eur. J. 2000, 6, 3254. [39] A. Nakano, A. Osuka, T. Yamazaki, Y. Nishimura, S. Akimoto, I. Yamazaki, A. Itaya, M. Murakami, H. Miyasaka, Chem. Eur. J. 2001, 7, 3134. [40] N. Yoshida, A. Osuka, Org. Lett. 2000, 2, 2963. [41] N. Yoshida, T. Ishizuka, A. Osuka, D. H. Jeong, H. S. Cho, D. Kim, Y. Matsuzaki, A. Nogami, K. Tanaka, Chem. Eur. J. 2003, 9, 58. [42] X. Peng, N. Aratani, A. Takagi, T. Matsumoto, T. Kawai, I.-W. Hwang, T. K. Ahn, D. Kim, A. Osuka, J. Am. Chem. Soc. 2004, 126, 4468. [43] T. Hori, N. Aratani, A. Takagi, T. Matsumoto, T. Kawai, M.-C. Yoon, Z. S. Yoon, S. Cho, D. Kim, A. Osuka, Chem. Eur. J. 2006, 12, 1319. [44] T. Hori, X. Peng, N. Aratani, A. Takagi, T. Matsumoto, T. Kawai, Z. S. Yoon, M.-C. Yoon, J. Yang, D. Kim, A. Osuka, Chem. Eur. J. 2008, 14, 582. [45] C. Ikeda, Z. S. Yoon, M. Park, H. Inoue, D. Kim, A. Osuka, J. Am. Chem. Soc. 2005, 127, 534. [46] Y. Nakamura, I.-W. Hwang, N. Aratani, T. K. Ahn, D. M. Ko, A. Takagi, T. Kawai, T. Matsumoto, D. Kim, A. Osuka, J. Am. Chem. Soc. 2005, 127, 236. [47] A. Tsuda, T. Nakamura, S. Sakamoto, K. Yamaguchi, A. Osuka, Angew. Chem. Int. Ed. 2002, 41, 2817. [48] I.-W. Hwang, T. Kamada, T. K. Ahn, D. M. Ko, T. Nakamura, A. Tsuda, A. Osuka, D. Kim, J. Am. Chem. Soc. 2004, 126, 16187. [49] T. Kamada, N. Aratani, T. Ikeda, N. Shibata, Y. Higuchi, A. Wakamiya, S. Yamaguchi, K. S. Kim, Z. S. Yoon, D. Kim, A. Osuka, J. Am. Chem. Soc. 2006, 128, 7670. [50] C. Maeda, T. Kamada, N. Aratani, T. Sasamori, N. Tokitoh, A. Osuka, Chem. Eur. J. 2009, 15, 9681. [51] A. Tsuda, A. Nakano, H. Furuta, H. Yamochi, A. Osuka, Angew. Chem. Int. Ed. 2000, 39, 558. [52] A. Tsuda, H. Furuta, A. Osuka, Angew. Chem. Int. Ed. 2000, 39, 2549.
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[53] A. Tsuda, H. Furuta, A. Osuka, J. Am. Chem. Soc. 2001, 123, 10304. [54] A. Tsuda, A. Osuka, Science 2001, 293, 79. [55] T. Ikeda, A. Tsuda, N. Aratani, A. Osuka, Chem. Lett. 2006, 35, 946. [56] T. Ikeda, J. M. Lintuluoto, N. Aratani, Z. S. Yoon, D. Kim, A. Osuka, Eur. J. Org. Chem. 2006, 3193. [57] T. Ikeda, N. Aratani, A. Osuka, Chem. Asian J. 2009, 4, 1248. [58] Y. Nakamura, N. Aratani, H. Shinokubo, A. Takagi, T. Kawai, T. Matsumoto, Z. S. Yoon, D. Y. Kim, T. K. Ahn, D. Kim, A. Muranaka, N. Kobayashi, A. Osuka, J. Am. Chem. Soc. 2006, 128, 4119. [59] Y. Nakamura, N. Aratani, A. Osuka, Chem. Asian J. 2007, 2, 860. [60] Y. Nakamura, N. Aratani, K. Furukawa, A. Osuka, Tetrahedron 2008, 64, 11433. [61] Y. Nakamura, S. Y. Jang, T. Tanaka, N. Aratani, J. M. Lim, K. S. Kim, D. Kim, A. Osuka, Chem. Eur. J. 2008, 14, 8279. [62] T. Tanaka, Y. Nakamura, A. Osuka, Chem. Eur. J. 2008, 14, 204. [63] T. Tanaka, Y. Nakamura, N. Aratani, A. Osuka, Tetrahedron Lett. 2008, 49, 3308. [64] N. Aratani, A. Osuka, Bull. Chem. Soc. Jpn. 2001, 74, 1361. [65] N. Aratani, A. Tsuda, A. Osuka, Synlett. 2001, 1663. [66] D. Kim, A. Osuka, J. Phys. Chem. A 2003, 107, 8791. [67] N. Aratani, A. Osuka, Chem. Rec. 2003, 3, 225. [68] D. Kim, A. Osuka, Acc. Chem. Res. 2004, 37, 735. [69] Y. Nakamura, N. Aratani, A. Osuka, Chem. Soc. Rev. 2007, 36, 831. [70] N. Aratani, D. Kim, A. Osuka, Acc. Chem. Res. 2009, 42, 1922. [71] A. Jasat, D. Dolphin, Chem. Rev. 1997, 97, 2267. [72] J. L. Sessler, S. J. Weghorn, Expanded, Contracted and Isomeric Porphyrins (Tetrahedron Organic Chemistry Series Vol. 15), Pergamon Press, Oxford, 1997. [73] J. L. Sessler, D. Seidel, Angew. Chem. Int. Ed. 2003, 42, 5134. [74] T. D. Lash, Angew. Chem. Int. Ed. 2000, 39, 1763. [75] T. K. Chandrashekar, S. Venkatraman, Acc. Chem. Res. 2003, 36, 676. [76] R. Misra, T. K. Chandrashekar, Acc. Chem. Res. 2008, 41, 265. [77] S. Shimizu, A. Osuka, Eur. J. Inorg. Chem. 2006, 1319. [78] M. Ste˛pien´, N. Sprutta, L. Latos-Graz˙yn´ski, Angew. Chem. Int. Ed. 2011, 50, 4288. [79] S. Saito, A. Osuka, Angew. Chem. Int. Ed. 2011, 50, 4342. [80] J. L. Sessler, T. Murai, V. Linch, M. Cyr, J. Am. Chem. Soc. 1988, 110, 5586. [81] J. L. Sessler, M. J. Cyr, V. Linch, E. McGhee, J. A. Ibers, J. Am. Chem. Soc. 1990, 112, 2810. [82] J. S. Lindsey, I. C. Schreiman, H. C. Hsu, P. C. Kearney, A. M. Marguerettaz, J. Org. Chem. 1987, 52, 827. [83] J.-Y. Shin, H. Furuta, A. Osuka, Angew. Chem. Int. Ed. 2001, 40, 619. [84] J.-Y. Shin, H. Furuta, K. Yoza, S. Igarashi, A. Osuka, J. Am. Chem. Soc. 2001, 123, 7190. [85] M. G. P. M. S. Neves, R. M. Martins, A. C. Tomé, A. J. D. Silvestre, A. M. S. Silva, V. Félix, M. G. B. Drew, J. A. S. Cavaleiro, Chem. Commun. 1999, 385.
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[86] R. Taniguchi, S. Shimizu, M. Suzuki, J.-Y. Shin, H. Furuta, A. Osuka, Tetrahedron Lett. 2003, 44, 2505. [87] Y. Kamimura, S. Shimizu, A. Osuka, Chem. Eur. J. 2007, 13, 1620. [88] Y. Tanaka, J.-Y. Shin, A. Osuka, Eur. J. Org. Chem. 2008, 1341. [89] S. Shimizu, V. G. Anand, R. Taniguchi, K. Furukawa, T. Kato, T. Yokoyama, A. Osuka, J. Am. Chem. Soc. 2004, 126, 12280. [90] S. Mori, A. Osuka, J. Am. Chem. Soc. 2005, 127, 8030. [91] S. Mori, K. S. Kim, Z. S. Yoon, S. B. Noh, D. Kim, A. Osuka, J. Am. Chem. Soc. 2007, 129, 11344. [92] S. Saito, K. Furukawa, A. Osuka, Angew. Chem. Int. Ed. 2009, 48, 8086. [93] S. Mori, S. Shimizu, J.-Y. Shin, A. Osuka, Inorg. Chem. 2007, 46, 4374. [94] S. Mori, A. Osuka, Inorg. Chem. 2008, 47, 3937. [95] H. Rath, N. Aratani, J. M. Lim, J. S. Lee, D. Kim, H. Shinokubo, A. Osuka, Chem. Commun. 2009, 3762. [96] T. Koide, G. Kashiwazaki, M. Suzuki, K. Furukawa, M.-C. Yoon, S. Cho, D. Kim, A. Osuka, Angew. Chem. Int. Ed. 2008, 47, 9661. [97] H. Rath, S. Tokuji, N. Aratani, K. Furukawa, J. M. Lim, D. Kim, H. Shinokubo, A. Osuka, Angew. Chem. Int. Ed. 2010, 49, 1531. [98] T. Koide, K. Furukawa, H. Shinokubo, J.-Y. Shin, K. S. Kim, D. Kim, A. Osuka, J. Am. Chem. Soc. 2010, 132, 7246. [99] M. Suzuki, A. Osuka, Angew. Chem. Int. Ed. 2007, 46, 5171. [100] M. Suzuki, A. Osuka, J. Am. Chem. Soc. 2007, 129, 464. [101] L. Latos-Graz˙yn´ski, Angew. Chem. Int. Ed. 2004, 43, 5124. [102] M. O. Senge, N. Sergeeva, Angew. Chem. Int. Ed. 2006, 45, 7492.
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Towards meso–meso-Linked Porphyrin Arrays and meso-Aryl Expanded Porphyrins Atsuhiro Osuka Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan) E-mail: [email protected]
Received: May 30, 2014 Published online: ■■
ABSTRACT: meso–meso-Linked porphyrin arrays and meso-aryl-substituted expanded porphyrins have continuously fueled my imagination for many years. In this account, my expertise in chemical research is retrospectively summarized with a particular focus on how these two novel categories of porphyrinoids were found by our group. As part of our photosynthetic model studies in collaboration with Prof. N. Mataga, the energy-gap dependence of intramolecular charge separation was examined by exploring the photoexcited dynamics of 1,4-phenylene-bridged hybrid porphyrin dimers. This study required electron-deficient porphyrins in the dimers that could serve as an electron-accepting unit towards an octaalkyl-substituted Zn(II) porphyrin donor. To this end, we employed meso-nitrated porphyrins and meso-pentafluorophenyl porphyrins. Efforts to prepare these electron-deficient porphyrins allowed us to serendipitously find both a meso–meso-linked porphyrin dimer and a series of meso-pentafluorophenyl-substituted expanded porphyrins. The meso–meso-linked Zn(II) porphyrin dimer was found as a byproduct in the nitration of 5,10-diaryl Zn(II) porphyrin with AgNO2 but became a major product in the reaction with AgPF6. This finding opened up a new path to directly linked porphyrin oligomers. The series of mesopentafluorophenyl-substituted expanded porphyrins were prepared via BF3·OEt2-catalyzed condensation of pyrrole and pentafluorobenzaldehyde when the reaction was run at tenfold-higher substrate concentrations, as compared to the optimal conditions for the synthesis of 5,10,15,20tetrakis(pentafluorophenyl)porphyrin. These expanded porphyrins have been shown to have attractive attributes such as flexible structures, versatile electronic states, multi-metal coordination, anion sensing, and large nonlinear optical properties. While these studies were mostly curiosity-driven, some of our work covers rather more general interests: how linearly connected molecules can be rationally synthesized and isolated in a pure and discrete form, how large π-conjugation can be realized to allow for very low energy electronic transitions, and how easily Möbius aromatic and antiaromatic molecules can be prepared. Keywords: aromaticity, expanded porphyrins, macrocycles, nanostructures, porphyrinoids
1. Introduction It is my pleasure to write this Personal Account tracing my journey in organic chemistry, with a particular focus on how I entered into the field of novel porphyrinoids and how we
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found meso–meso-linked porphyrin arrays and meso-aryl expanded porphyrins. In the second year of my doctoral course at Kyoto University (1979), I was offered a position as
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Assistant Professor in the group of Prof. H. Suzuki in the Department of Chemistry, the Faculty of Science, Ehime University. Prof. Suzuki was kind to allow me to continue my research on the photochemical reactions of epoxynaphthoquinones, which had been assigned as the theme of my doctoral degree by Prof. K. Maruyama of Kyoto
University. Before joining Prof. Suzuki’s laboratory, I had reported that irradiation of 2,3-dimethyl-2,3-epoxy-1,4naphthoquinone (1) led to cleavage of the oxirane ring to generate carbonyl ylide 2 or 1,3-diradical intermediate 3, which was trapped by the carbonyl group of aldehydes or ketones to give 1,3-cycloaddition adducts 4 (Eq. 1).[1,2]
(1) As an extension of these studies, I examined the photoirradiation of 2-methyl-3-(2-methyl)propyl-2,3-epoxy1,4-naphthoquinone (5). Photoirradiation of 5 in a benzene solution gave rise to a preferential intramolecular hydrogenabstraction reaction (Norrish type II photoreaction) over the oxirane cleavage, affording cyclobutanol product 7 as a cycliAtsuhiro Osuka was born in Gamagori, Aichi, Japan, in 1954. He received his Ph.D. degree on the photochemistry of epoxyquinones from Kyoto University in 1982. In 1979, he started his academic career at the Department of Chemistry of Ehime University as an Assistant Professor. In 1984, he moved to the Department of Chemistry of Kyoto University, where he became a Professor of Chemistry in 1996. He was awarded the CSJS Award for Young Chemists in 1988, the Japanese Photochemistry Association Award in 1999, the NOZOE Memorial Lectureship Award at the 13th ISNA conference in 2009, and the Chemical Society of Japan Award in 2010. He was selected as a project leader of Core Research for Evolutional Science and Technology (CREST) of JST during 2001–2006. His research interests cover many aspects of synthetic approaches toward artificial photosynthesis and the development of porphyrin-related compounds with novel structures, electronic systems, and functions.
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zed product and phthiocol 9 as an elimination product via 1,4-biradical 6 (Scheme 1). The cyclobutanol 7 underwent secondary photoreactions to give rearranged products 11 and 12, probably via biradical 10 formed by Norrish type I α-cleavage.[3,4] Under similar photoirradiation conditions, 2-methyl-3-(2-methyl)propyl-2,3-methano-1,4naphthoquinone (13) gave cyclobutanol product 15 as a major product via a Norrish type II 1,4-biradical 14 (Scheme 2). It was found that 15 underwent a secondary photoinduced cleavage to give the same 1,4-biradical 14. Prolonged irradiation of a mixture of 13 and 15 led to formation of γ,δ-unsaturated alcohol 16 as a rare photoproduct, possibly via slow disproportionation of the 1,4-biradical 14.[5,6] I obtained my doctoral degree on the basis of these studies on the photochemistry of epoxynaphthoquinones in 1982. Alongside these studies, I started synthetic organic chemistry using tellurium compounds, which was one of the main research themes in the research group of Prof. Suzuki. Sodium hydrogentelluride (NaTeH) was used for dehalogenation of α-halo carbonyl compounds (Eq. 2),[7] chemoselective reduction of α,β-epoxy ketones to β-hydroxy ketones (Eq. 3),[8] and reductive removal of aliphatic nitro groups (Eq. 4).[9] Reactions of telluronium ylides with carbonyl compounds gave olefination products or epoxidation products depending upon the stability of the telluronium ylide. Namely, the reactions of dialkyltelluronium carbethoxymethylide with aldehydes and
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Scheme 1. Norrish type-II photoreaction of en epoxynaphthoquinone.
Scheme 2. Norrish type-II photoreaction of a methanonaphthoquinone.
ketones gave α,β-unsaturated esters with trans stereoselectivity (Eq. 5),[10] but the reactions of dialkyltelluronium allylide with aldehydes provided vinyl epoxides with cis selectivity (Eq. 6).[11] These reactivities are different from the established features of the corresponding reactions of sulfonium and selenonium ylides with aldehydes.[12] These are, to the best of my knowledge, the first reports on the use of telluronium species for C–C bond-forming reactions.
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Fig. 1. Tyrosine-linked and tryptophan-linked porphyrins.
(4)
(3)
(5)
(6) In 1984, I moved to the Department of Chemistry in the Faculty of Science at Kyoto University as an Assistant Professor in the group of Prof. Maruyama. At that time, Prof. Maruyama was interested in a chemical approach to photosynthetic reaction centers and naturally he requested me to pursue a research project along such a direction. I was forced to abandon the chemistries pursued at Ehime University and started research related to artificial photosynthesis by preparing a series of octaalkyl-substituted mesoporphyrins bearing amino acids through an ester or an amide linkage. Disappointingly, however, such appended amino acid side chains did not display any meaningful interactions with the porphyrin chromophore in the ground state. Fluorescence emissions of these porphyrins are not quenched, indicating almost no interaction in the singlet excited states of porphyrins with the amino acid residues. However, in the meantime, we noticed strong chemically induced dynamic nuclear polarization (CIDNP) signals during irradiation (λ > 590 nm) of a benzene solution of tyrosinelinked porphyrin 29[13–16] and tryptophan-linked porphyrin 30[17] in the presence of 1,4-benzoquinone (Figure 1).
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Prolonged irradiation of a solution of 29 and 1,4benzoquinone gave quinone-linked porphyrin 32 via oxidation of hydroquinone-linked porphyrin 31 with an excess amount of the quinone (Scheme 3). On the basis of extensive and detailed studies, the formation of 32 has been determined to occur by the mechanism shown in Scheme 4. Electron-transfer quenching of the triplet excited state of the porphyrin by 1,4-benzoquinone gives a porphyrin cation radical and a quinone anion radical. Subsequent proton transfer from the tyrosine residue to the quinone anion radical generates a phenolate anion and a neutral semiquinone radical, which are converted to a neutral radical pair of a semiquinone radical and a phenoxy radical via intramolecular electron transfer from the phenolate to the porphyrin cation radical. Coupling of this radical pair results in the formation of hydroquinone-linked porphyrin, which is finally oxidized to form quinone-linked porphyrin upon photoirradiation in the presence of an excess amount of quinone. The observed strong CIDNP signals were thus ascribed to competitive disproportionation of the neutral radical pair to regenerate the starting state and recombination
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Scheme 3. Photo-induced coupling reaction of 1,4-benzoquione to tyrosine-lined porphyrin.
Scheme 4. Reaction mechanism of photo-induced coupling reaction of 1,4-benzoquinone to tyrosine-linked porphyrin.
to give the hydroquinone-linked porphyrin with concomitant spin polarization. This photoinduced coupling of quinones to phenol-linked porphyrin has been extended to the synthesis of various quinone-linked and quinone-capped porphyrins.[15,16] During these studies, we recognized the appearance of the crystal structure of a bacterial photosynthetic reaction center as an epoch-making event.[18] The structure of the reaction center strongly suggested that the spatial arrangements of the chromophores, such as the special pair, bacteriochlorophylls,
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bacteriopheophytins, and quinones, are critically important for the photoinduced events such as excitation-energy transfer and electron transfer. We thus prepared quinone-capped porphyrins 33, 34, and 35 with different orientations by this photoinduced coupling reaction with the intention to examine the orientation dependence of intramolecular electron transfer from the porphyrin to the quinone (Figure 2).[15] Unfortunately, these quinone-capped porphyrins were conformationally very flexible and unsuitable for detailed investigation of the
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Fig. 2. Quinone-capped porphyrins.
Fig. 3. Naphthalene-bridged porphyrin dimers.
orientation dependence of the intramolecular electron-transfer reactions. In addition, the electron-transfer reactions of the quinone-capped porphyrins were too fast to allow determination of the precise rates in those days. We then turned our attention to the synthesis of covalently linked porphyrin dimers that are conformationally restricted at varying orientations. As a structural motif, we chose a peripherally octaalkyl-substituted porphyrin that is linked directly at its meso positions to a naphthalene bridge (36, 37, and 38, Figure 3). In these diporphyrins, the naphthalene bridge is held nearly perpendicularly to the porphyrin
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plane due to the steric congestion of the flanking alkyl groups. This conformationally restricted feature is suitable for studies on the geometry dependence of excitation-transfer and energy-transfer reactions. Zn(II) complexes of these dimers exhibit remarkable exciton coupling in their Soret bands, reflecting the respective orientation.[19] The observed split Soret bands of these dimers were analyzed on the basis of the exciton coupling theory developed by Kasha.[20] These porphyrin dimers were used to examine the geometry dependencies of intramolecular excitation-energy transfer and electron-transfer reactions.[21,22] As an extension of these
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Fig. 4. Conformationally restricted porphyrin oligomers.
studies, we developed an efficient method that allowed the synthesis of conformationally restricted porphyrin oligomers such as 39 and 40 (Figure 4).[23] With this synthetic method in hand, we explored covalently linked donor–acceptor porphyrin compounds as models of the photosynthetic reaction center, with the ultimate goal of mimicking the whole events of the excitation-energy transfer and electron transfer in the photosynthetic reaction centers within a single molecular entity.[24–31] As part of these studies, we were interested in the energygap dependence of intramolecular electron-transfer reactions in 1,4-phenylene-bridged diporphyrins that bear an octaalkylsubstituted Zn(II) porphyrin as an electron donor and various electron-deficient porphyrins as an electron acceptor. Using diporphyrins 41–48 (Figure 5), we revealed a bell-shaped energy-gap dependence for the rates of charge separation and charge recombination.[32] More importantly, this study allowed us to find two important reactions. In the course of our study to prepare meso-nitrated diporphyrins 46 and 48, we found a direct meso–meso coupling reaction of 5,15-diaryl-substituted Zn(II) porphyrins, and our attempt to prepare mesopentafluorophenyl-substituted diporphyrin 45 led to the discovery of an effective synthesis for a series of meso-arylsubstituted expanded porphyrins.
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Fig. 5. 1,4-Phenylene-bridged porphyrin dimers.
2. meso–meso-Linked Porphyrin Arrays In the above-mentioned study on the energy-gap dependence of the intramolecular electron-transfer reaction of 1,4phenylene-bridged diporphyrins, we chose meso-nitrated porphyrins as the electron-deficient porphyrin part. By following Baldwin’s protocol,[33] 5,15-diaryl Zn(II) porphyrin 49 was nitrated with AgNO2 and I2 in CHCl3 (Scheme 5). The
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Scheme 5. Formation of meso-meso linked porphyrin dimer and trimer.
nitration proceeded nicely to give meso-nitrated porphyrin 50 in 90% yield, but my student did not miss a side product that eluted on a silica gel TLC plate essentially in the same manner as 49. This side product was soon identified as meso–mesolinked Zn(II) diporphyrin 51. The formation of 51 was thought to be initiated by one-electron oxidation of 49 with Ag(I) ion to generate the cation radical of 49, followed by nucleophilic attack by another neutral molecule of 49. We thus chose AgPF6 as an oxidant, since it has a non-nucleophilic anion that has no chance to react with the cation radical of 49. The reaction of 49 with AgPF6 in CHCl3 gave 51 and 52 in 27% and 4% yields, respectively, along with recovery of 49 (47%) (Scheme 5).[34] Synthesis of an extremely long discrete molecule was indeed a challenge. We decided to synthesize long meso–meso-linked Zn(II) porphyrin arrays, since such molecules would be promising as photonic wires and the orthogonal arrangement of neighboring porphyrins is favorable for a high degree of solubility. However, we soon faced a serious solubility problem at the stage of the octamer. Thus, we designed a more soluble substrate, Z1, which bears two 3,5dioctyloxyphenyl substituents. The established standard coupling protocol is very simple and involves treatment of Z1 or its oligomers Zn with AgPF6 in CHCl3 at room temperature for several hours (Scheme 6).[35] It is important to monitor the progress of the coupling reaction by gel permeation chromatography (GPC) HPLC or 1H NMR spectroscopy, and to quench the reaction at 30–40% conversion, since further reaction leads to production of larger porphyrin arrays, which makes separation of the products more tedious. Usually the coupling products were separated from the starting substrate through GPC-HPLC columns. In each iterative coupling reaction, the rigorous purification of coupled products is critically
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important for the separation of their further coupled products. This coupling reaction has proved very effective, particularly for large porphyrin substrates.[35] It is amazing that extremely long porphyrin arrays (even Z128 and Z256) smoothly undergo the coupling reaction to provide Z1024,[36] despite the extremely long molecular shapes with only two edge free meso positions available for the coupling. This meso–meso coupling reaction of porphyrins allowed us to prepare threedimensionally extending windmill porphyrin arrays,[37–39] dihedral-angle-controlled meso–meso-linked diporphyrins,[40,41] large porphyrin wheels,[42–44] helical porphyrin arrays held by intermolecular hydrogen-bonding interactions,[45] and directly linked porphyrin rings (Figure 6).[46] The meso–meso-linked diporphyrin motifs are suitable for supramolecular assembly. By this strategy, three-dimensional porphyrin boxes and other interesting architectures have been constructed through rigorous self-sorting assembly of pyridine-appended or cinchomeronimide-appended meso–meso-linked Zn(II) diporphyrins (Scheme 7).[47–50] In the meantime, we explored an effective oxidative fusion– dimerization reaction of Ni(II) porphyrins to give meso, β doubly linked porphyrin dimers (Eq. 7) and a ring-closure reaction of meso–meso-linked diporphyrins to β, meso, β triply linked diporphyrins, initially with tris(4bromophenyl)aminium hexachloroantimonate (BAHA) (Eq. 8)[51–53] and later with the combined use of 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ) and Sc(OTf )3 (Eq. 9).[54] The latter method is superior to the former because of the absence of serious halogenation side products, and allow the conversion of long meso–meso-linked Zn(II) porphyrin arrays to the corresponding meso–meso, β–β, β–β triply linked porphyrin arrays that are called porphyrin tapes. On the basis of this
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Scheme 6. Iterative doubling elongation of meso-meso linked porphyrin oligomers.
oxidation, we have succeeded in the synthesis of π-conjugated porphyrin tapes that display remarkably red-shifted and enhanced absorptions reaching deep into the infrared region (Scheme 8).[54–57] We have also explored an antiaromatic tetrameric porphyrin sheet, which exhibits a strong paratropic ring current above the central plane of the cyclooctatetraene core,[58– [61] The 60] and two-dimensionally extending porphyrin tapes. bay-area selective cycloaddition reactions of porphyrin tapes proceeded nicely with an o-xylylene[62] and an azomethine ylide.[63] The chemistry of meso–meso-linked porphyrin arrays and porphyrin tapes has been reviewed elsewhere.[64–70]
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Fig. 6. Various meso-meso linked porphyrin oligomers.
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Scheme 7. Self-assembled formation of porphyrin boxes.
Scheme 8. Synthesis of porphyrin tape 12-mer.
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Scheme 9. Synthesis of meso-aryl substituted expanded porphyrins.
3. meso-Aryl-Substituted Expanded Porphyrins In recent years, the chemistry of expanded porphyrins has been actively explored in light of their favorable attributes, such as rich coordination chemistry, anion sensing, large two-photon absorption cross-sections, and extended π-electronic systems.[71– 79] Sessler, an important pioneer of this field, developed the synthesis of a pentadentate texaphyrin in 1988[80] and a rational synthesis of sapphyrins in 1990,[81] improvisation that led to an exciting boom of novel expanded porphyrins. meso-Aryl expanded porphyrins can be regarded as legitimate expanded porphyrins in terms of their regular and alternating arrangements of pyrroles and aryl-substituted meso carbons. We serendipitously found the formation of these meso-aryl expanded porphyrins during the synthesis of tetrakis (pentafluorophenyl)porphyrin by the Lindsey method[82] using pentafluorobenzaldehyde and pyrrole. We made the fortunate mistake to run the reaction at a substrate concentration of 67
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(9) mM, which is approximately tenfold higher than the optimized concentration for porphyrin synthesis. Under these conditions, a series of meso-aryl expanded porphyrins including N-fused pentaphyrins, hexaphyrin, heptaphyrin, octaphyrin, nonaphyrin, decaphyrin, undecaphyrin, and dodecaphyrin were formed in a surprisingly effective manner (Scheme 9).[83,84] At this time, we knew that a communication by Cavaleiro et al. had appeared, which reported the isolation of [26]hexaphyrin and [28]hexaphyrin in low yields.[85] We were very shocked by this paper but soon realized that our synthetic protocol was superior to Cavaleiro’s method in regard to the formation of a series of expanded porphyrins and better yields. Initially, the separation of meso-aryl expanded porphyrins was not easy and needed repeated chromatographic separation, but this separation difficulty has been somewhat mitigated by size-selective synthesis of the expanded porphyrins using a dipyrromethane and a tripyrromethane as precursors.[86,87] Use of high concentrations of the substrates led to better yields of larger expanded
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porphyrins.[88] These meso-aryl expanded porphyrins have proved to be attractive platforms for rich coordination chemistry,[89–95] versatile aromatic compounds such as strongly Hückel aromatic[90,91,96] and antiaromatic molecules,[90,91] and stable organic radical species.[96–98] Unprecedented rearrangements are triggered by transannular electronic interactions aided by the conformational flexibility of expanded porphyrins.[99,100] Besides these, the meso-aryl expanded porphyrins are quite interesting from the viewpoint of mutual chemical interconversions (metamorphosis).[101,102] The most remarkable example of this is the efficient and quantitative splitting reaction of bis-Cu(II) [36]octaphyrin complex 66 into two molecules of Cu(II) porphyrins 67 upon heating (Eq. 10).[103] This splitting reaction also proceeded quantitatively in a Co(II)–
Cu(II) hybrid octaphyrin complex.[104] In a heptaphyrin system, treatment of heptaphyrin Cu(II) complex 68 with BBr3 in the presence of a sterically hindered amine led to the formation of subporphyrin 69 and Cu(II) porphyrin 70 in 36% and 13% yields, respectively (Eq. 11).[105] Similar treatment of meso-trifluoromethyl-substituted heptaphyrin Cu(II) complex 71 with BBr3 gave meso-trifluoromethyl subporphyrin 72 and Cu(II) porphyrin 73 (Eq. 12).[106] Subporphyrins are my other favorite porphyrinoids, which were first synthesized in my group in 2006.[107] The chemistry of subporphyrins are reviewed elsewhere.[108,109] It is worthwhile to note that subporphyrins 69 and 72 cannot be synthesized by the usual condensation methods and thus these splitting reactions are important from a synthetic viewpoint.
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(12) These splitting reactions require the rapture and formation of two carbon–carbon double bonds in a metathesis manner. We thought that the transannular electronic interaction would be enhanced at the hinge position of the figure-eight conformation of the expanded porphyrins upon metalation and, thus, this trend would be more general. In order to examine the scope of this unique metal-induced splitting reaction, we examined the complexation of expanded porphyrins with various transition metal ions such as Rh(I), Ni(II), Pd(II), and Pt(II). In 2006, we reported [28]hexaphyrin Ni(II), Pd(II), and Pt(II) complexes (74, 75, and 76, Figure 7).[110] Although we collected all the data for these complexes including their crystal structures and noted
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that the inner β-protons appeared at high field, we did not notice that the high-field shifts were caused by the diatropic ring currents of 74–76, which are all Möbius aromatic molecules. We also reported the formation of Rh(I) complexes 77 and 78 from N-fused pentaphyrin 60 (Scheme 10).[111] The 1H NMR spectrum of 78 shows a signal due to the inner β-proton in the high-field region at 0.10 ppm. At the time of publication, we could not explain this particular high-field shift. In 2008, we reported that meso-aryl expanded porphyrins such as octaphyrin, heptaphyrin, and hexaphyrin can form Möbius aromatic complexes almost spontaneously upon metalation with Pd(II) ions.[112] In this paper, we revealed that the com-
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Fig. 7. Structures of Möbius aromatic hexaphyrin Ni(II)-, Pd(II)-, and Pt(II) complexes.
Scheme 10. Rh(I) metalation of N-fused [22]pentaphyrin.
plexes 79 and 80 (Figure 8) along with 74–76 are all Möbius aromatic molecules. Soon after, we reported that the complex 78 is also a Möbius aromatic molecule.[113] Following these studies, we have explored many expanded porphyrins, which display versatile electronic states such as Hückel aromatic, Hückel antiaromatic, Möbius aromatic, Möbius antiaromatic,[114,115] and stable radical states.[96,98] Such molecules are outside the scope of this account and are reviewed elsewhere.[116,117]
4. Summary As described above, my research career has been guided by no means by my idea but simply led by many fortunate unex-
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pected encounters with the assistance of many students and postdocs. Throughout my career in the Department of Chemistry, the Faculty of Science, Ehime University, and in the Department of Chemistry, the Graduate School of Science, Kyoto University, I have been very fortunate to have had the opportunity to serve as a mentor for many talented students. To be honest, my laboratory has not been well organized. In other words, I have tried to keep my students and postdocs as free as possible. They have to think and explore novel chemistry by themselves. Most of them have demonstrated themselves to be quite motivated in synthesizing new porphyrinoids and exploring new chemistry of such molecules and have now become eligible staff members of universities, institutes, and companies. They are indeed my pride. Therefore, it is obvious
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Fig. 8. Structures of Möbius aromatic heptaphyrin Pd(II) complex and octaphyrin bis-Pd(II) complex.
that I should thank them for their outstanding devotion to the chemistry of novel porphyrinoids.
Acknowledgements This work was partly supported by Grants-in-Aid (No. 25220802 (S)) from MEXT. The author thanks Dr. T. Tanaka for making the schemes, equations, and figures.
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