Chemistry of meso-Aryl-Substituted Expanded Porphyrins: Aromaticity

Review pubs.acs.org/CR

Chemistry of meso-Aryl-Substituted Expanded Porphyrins: Aromaticity and Molecular Twist Takayuki Tanaka and Atsuhiro Osuka* Department of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8501, Japan ABSTRACT: Since the discovery of its facile synthesis in 2001, meso-aryl-substituted expanded porphyrins have been developed as a new class of azaannulenes in light of their facile redox interconversions, conformational flexibilities involving flipping of the constitutional pyrroles, rich metal coordination behaviors, unprecedented chemical reactivities, effective platforms to realize versatile electronic states including Möbius aromatic and antiaromatic species, and abilities to stabilize organic radicals. In this Review, the syntheses, structures, and optical, electronic, and magnetic properties of meso-aryl-substituted expanded porphyrins and their metal complexes have been updated with a particular focus on the relationship between “aromaticity and molecular twist (molecular topology)”. While the importance of the interplay of these two characteristics has been long recognized from the theoretical viewpoint, meso-arylsubstituted expanded porphyrins offered solid experimental evidence to provide Möbius aromatic and antiaromatic molecules with distinct diatropic and paratropic ring currents, respectively. This attribute is not shared with β-alkylated expanded porphyrin counterparts, underlining the importance and uniqueness of meso-aryl-substituted expanded porphyrins.

CONTENTS 1. Introduction 2. Synthesis of Expanded Porphyrins 2.1. Mixed Acid-Catalyzed Condensation Reactions 2.2. Selective Synthesis of Regular Expanded Porphyrins 2.3. Selective Synthesis of Other Expanded Porphyrins 2.4. Synthesis of Peripherally Functionalized Hexaphyrins 2.5. Synthesis of meso-Free Expanded Porphyrins 2.6. Other Examples 2.7. Synthesis of Cyclo[n]pyrroles 3. Metalation Chemistry of Expanded Porphyrins 3.1. Hexaphyrin Metal Complexes 3.1.1. Type-I Hexaphyrin Metal Complexes 3.1.2. Twisted Hexaphyrin Metal Complexes 3.1.3. Type-II Hexaphyrin Metal Complexes 3.1.4. meso-Free Hexaphyrin Metal Complexes 3.2. Metal Complexes of Other Expanded Porphyrins 3.2.1. Heptaphyrin and Octaphyrin Zinc Complexes 3.2.2. Heptaphyrin Copper Complexes 3.2.3. Octaphyrin Cobalt Complexes 3.2.4. Octaphyrin Palladium Complexes 3.2.5. Nonaphyrin Nickel Complexes 3.2.6. Rubyrin Metal Complexes 3.2.7. Siamese-Twin Porphyrin Metal Complexes 3.3. Heterometal Complexes © 2016 American Chemical Society

3.4. Main Group Complexes 3.4.1. Boron Complexes 3.4.2. Phosphorus Complexes 3.5. Metalation-Induced Rearrangements and Ring Cleavages 3.5.1. Splitting Reactions 3.5.2. Rearrangements 3.5.3. Ring Cleavages 3.5.4. Other Reactions 4. Mö bius Aromatic and Antiaromatic Expanded Porphyrins 4.1. Background 4.2. Mö bius Aromaticity of p-Benzihexaphyrin 4.3. Mö bius Aromaticity of Regular Expanded Porphyrins 4.3.1. Metal Complexes 4.3.2. Fusion Reactions 4.3.3. Temperature and Solvent Control 4.3.4. Protonation and Deprotonation 4.4. Mö bius Antiaromaticity 5. Expanded Porphyrin Organic Radicals 5.1. meso-Oxygenated Hexaphyrins 5.2. Hexaphyrin Palladium Complexes 5.3. Decaphyrin Zinc−Copper Heterometal Complex 5.4. Modified Sapphyrins 5.5. Core-Modified Pentaphyrins 5.6. Annulated Rosarins

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins

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Received: June 13, 2016 Published: August 18, 2016 2584

DOI: 10.1021/acs.chemrev.6b00371 Chem. Rev. 2017, 117, 2584−2640

Chemical Reviews 6. Applications of Expanded Porphyrins 6.1. Reactivities 6.2. Expanded Porphyrin Oligomers 6.3. Chirality 6.4. Metal Ion Sensing 7. Conclusion and Outlook Appendix 1: Summary of CCDC Numbers Appendix 2: Synthetic Procedure for Tripyrrane 5-Pentafluorophenyldipyrromethane (2a) 1-Pentafluorobenzoyl-5-pentafluorophenyldipyrromethane (S1) 5,10-Bis(pentafluorophenyl)tripyrrane (3a) Author Information Corresponding Author Notes Biographies Acknowledgments References

Review

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Figure 1. Porphyrin and expanded porphyrins.

Here it is helpful to explain Nonn’s nomenclature of expanded porphyrins,3−6,53,54 which consists of three parts. In the case of [26]hexaphyrin(1.1.1.1.1.1) (Chart 1), the former bracket

1. INTRODUCTION Porphyrins are 18π aromatic azaannulenes consisting of regularly arranged four pyrroles and four methine carbons. Over the years, porphyrins have attracted wide interest because of their vital roles in biological processes and their structural and functional features.1,2 Rich metalation chemistry of porphyrins is also an appealing aspect, as most of the biologically important porphyrins function as metalloporphyrins as seen for hemes or chlorophylls. Encouraged by these features, the synthetic chemistry of porphyrins has been extensively developed, giving access to a plethora of new compounds in fields that range from materials science to biomedical applications. In recent years, porphyrinoids such as contracted porphyrins, expanded porphyrins, isomeric porphyrins, and core-modified porphyrins have emerged as novel π-functional molecules. This Review focuses on the chemistry of expanded porphyrins that possess larger macrocyclic rings than porphyrins, which now constitute a well-recognized group.3−6 In the review of Sessler and Seidel in 2003, the definition of expanded porphyrins was given as “macrocycles that contain pyrrole, furan, thiophene, or other heterocyclic subunits linked together either directly or through one or more spacer atoms in such a manner that the internal ring pathway contains a minimum of 17 atoms”.3 The start of the chemistry of expanded porphyrin can be traced back to R. B. Woodward’s serendipitous discovery of sapphyrin 1a, a pentapyrrolic macrocycle (Figure 1).7,8 Another earlier work was reported by Johnson and co-workers in 1972.9 Later on, expanded porphyrins had been actively studied by LeGoff, Franck, Gossauer, Sessler, and their co-workers.10−20 Among these, Sessler et al. reported the first synthesis of texaphyrin21−36 in 1988 and wrote an important paper on the improved synthesis of sapphyrin 1b in 1990.19 In addition, they demonstrated the potentials of these expanded porphyrins in fields of anion recognition, aromaticity, functional dyes, photodynamic therapy (PDT), and magnetic resonance imaging (MRI).31−33 Especially, they developed water-soluble texaphyrin GdIII complex that was a promising anticancer reagent with clear MRI contrast, called motexafin gadolinium (MGd) in the medicinal field.21−28,37−46 These works stimulated organic chemists and drove them to this field, hence apparently contributing to the renaissance of the chemistry of expanded porphyrins. There are several nice reviews on the chemistry of modern expanded porphyrins.47−52

Chart 1. [26]Hexaphyrin and [28]Hexaphyrin

indicates the number of π-electrons involved in the conjugated circuit, i.e., the [26]hexaphyrin has 26π electrons in the conjugated circuit, which can be increased to 28π electrons by a chemical reduction as seen for [28]hexaphyrin(1.1.1.1.1.1) with concurrent imine−amine transformation to balance the charge. The effective conjugated circuits are shown in the bold lines in Chart 1. The middle part indicates the number of pyrroles that constitute the expanded porphyrins, i.e., “hexaphyrin” indicates that the macrocycle contains six pyrroles. Similarly, pentaphyrin, heptaphyrin, octaphyrin, etc are used for the middle part, showing that the numbers of pyrroles are five, seven, and eight, respectively. The latter round-bracket part shows the number of methine carbons connecting neighboring pyrrole units. The two hexaphyrins in Chart 1 are regularly arranged meso-aryl-substituted macrocycles and should have (1.1.1.1.1.1) for the last part. For simplicity, the latter round-bracket parts will be omitted for the regular meso-aryl-substituted expanded porphyrins in this Review. In the course of our studies on artificial photosynthetic reaction centers,55 we fortunately discovered the one-pot synthesis of a series of meso-pentafluorophenyl-substituted expanded porphyrins in 2001.56−58 The reaction was intended to obtain tetrakis(pentafluorophenyl)porphyrin 4a, but serendipitously we found concurrent formations of expanded porphyrins such as N-fused [22]pentaphyrin (5a′), [26]hexaphyrin (6a), [32]heptaphyrin (7a), [36]octaphyrin (8a), [40]nonaphyrin (9a), and [44]decaphyrin (10a), and even larger analogues when rather high concentrations of the substrates (67 mM) were employed (Scheme 1). Before this synthesis, the main players of expanded porphyrins had been β2585


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Scheme 1. One-Pot Synthesis of meso-Aryl-Substituted Expanded Porphyrins; Pictures Are Solution Colors of 4a−10a in CH2Cl2

chromatography afforded porphyrin 4a as the first fraction that was followed by many fractions of expanded porphyrins. The separation was not easy, especially for large-scale reaction, but expanded porphyrins had different colors (as indicated in Scheme 1), allowing their isolations after repetitive separation. As another convenient method, after the usual workup, the product mixture was dissolved in a minimum amount of CH2Cl2 and was added to an excess amount of MeOH, inducing precipitation of solids that consisted of mainly porphyrin 4a and hexaphyrin 6a. This method enabled rough removal of 4a and 6a from the product mixture. Because the amounts of 4a and 6a corresponded to ca. 20−30% of the whole products, this method made the separation of other expanded porphyrins much easier. Nevertheless, complete separations of heptaphyrin 7a, octaphyrin 8a, nonaphyrin 9a, decaphyrin 10a, and larger analogues were quite tedious. Prior to our report, Cavaleiro and co-workers reported the synthesis of hexaphyrin 6a by a similar Rothemundtype condensation91 in low yield (ca. 1%).92 Gross and coworkers also reported the isolation of 6a under solvent-free conditions.93 Considering the scope of substrates, the reproducibility, and the yields, the synthesis of 6a was best accomplished by our one-pot synthesis. [26]Hexaphyrin 6a represents the benchmark molecule of meso-aryl-substituted expanded porphyrins: the number of papers including the word “hexaphyrin” reported during the period 2001−2015 (181 papers) was 5 times as much as those containing the word “pentaphyrin” (36 papers) or “heptaphyrin” (32 papers), and 4 times as much as “octaphyrin” (47 papers) (Chart 2). It is worthy to note here that meso-aryl-substituted pentaphyrins were obtained as N-fused pentaphyrins like 5a′ and nonfused pentaphyrins were not isolated so far. Facile intramolecular N-fusion reaction of the pentaphyrins may be ascribed to the steric congestion inside of the macrocycle (Scheme 2).94 This reaction also took place in meso-2,6dichlorophenyl-substituted pentaphyrin to give N-fused pentaphyrin 5b′. Heptaphyrin 7a was relatively easily converted to N-

alkyl-substituted congeners, which were mostly synthesized via multistep routes using the MacDonald-type condensation reaction as a key step.3−6,10−12,59−64 Therefore, the one-pot synthesis of meso-aryl-substituted expanded porphyrins has apparent advantages: (i) the reactions can be performed by very simple operation; (ii) the reaction needs only pyrrole and pentafluorobenzaldehyde, which are both commercially available; and (iii) the reaction provides a series of expanded porphyrins in a one-pot reaction, although these molecules have to be separated before their use. In addition, meso-arylsubstituted expanded porphyrins have the bonus of conformational flexibility involving facile inversion of the constitutional pyrrole rings, which is crucially important for the realization of Möbius aromatic molecules. This Review will focus mainly on the syntheses, structures, metalations, and aromaticities of meso-aryl-substituted expanded porphyrins. Schiff-base porphyrinoids65,66 and expanded phthalocyanines67−70 are outside of the scope of this Review. Some important examples of confused expanded porphyrins,71−74 expanded carbaporphyrins,75−83 and core-modified expanded porphyrins84−88 will be discussed in relation to the themed topics, but they will not be fully covered.

2. SYNTHESIS OF EXPANDED PORPHYRINS 2.1. Mixed Acid-Catalyzed Condensation Reactions

In earlier reports, 5,10,15,20-tetraphenylsapphyrin and mesophenyl-substituted pentaphyrin and hexaphyrin were indeed obtained, but their low yields and poor stabilities had hampered their full characterizations.89,90 In the report in 2001, mesopentafluorophenyl-substituted expanded porphyrins were obtained in 5−20% yields as stable compounds in a one-pot reaction sequence including an acid-catalyzed condensation reaction of pyrrole with pentafluorobenzaldehyde and subsequent oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ).56 After being passed through a short column of aluminum oxide several times, separation by silica-gel column 2586


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neighboring meso-pentafluorophenyl substituent. This reaction could be avoided when the meso-aryl substituents did not contain ortho-fluorine atoms as seen for 7b.95 The X-ray crystal structures of the expanded porphyrins 5a′,94 6a,93 7a,95 8a,56 9a,96 and 10a97 are shown in Figure 2. It is important to emphasize that X-ray diffraction analysis is indispensable in the study of expanded porphyrins, because it is usually very difficult to determine the structures without X-ray analysis. Structures of expanded porphyrins were often threedimensional (as seen in 7a, 8a, 9a, and 10a) and sometimes twisted like a Mö bius strip. Expanded porphyrins were structurally quite flexible in solution, and their conformational dynamics occurred comparably or slower as compared with 1H NMR time scale, causing broad 1H NMR spectra. In addition, the crystal structures gave information on the bond lengths and bond angles, which were important to understand the electronic properties of expanded porphyrins. The harmonic oscillator model of aromaticity (HOMA) is an important index to evaluate the degree of aromaticity, in that HOMA = 0 indicates the Kekulé structure of the typical aromatic system and HOMA = 1 means the system with all bonds equal to the optimal value due to full conjugation.98,99 Actually, the HOMA values of 5a′ (0.58) and 6a (0.57) were slightly but distinctly larger than those of 7a (0.48), 8a (0.53), 9a (0.48), and 10a (0.50), differentiating aromatic expanded porphyrins (5a′ and 6a) from nonaromatic ones (7a, 8a, 9a, and 10a). Because of the space limit, all the crystal structures of updated expanded porphyrins are not shown in this Review. Many of the ChemDraw artworks in the figures and schemes have been drawn based on the obtained X-ray crystal structures or as originally reported. Moreover, the crystal information and reference numbers are listed in Appendix 1, for cases where the structures are available in The Cambridge Crystallographic Data Centre (CCDC). Other physical properties, such as NMR, UV/vis absorption, and redox potentials, should be referred to the original papers, because this Review mainly focuses on the synthesis, structure, and aromaticity of expanded porphyrins.

Chart 2. Comparison of the Numbers of Reports Containing the Words “Pentaphyrin” to “Decaphyrin”

Scheme 2. N-Fusion Reactions

fused heptaphyrin 7a′ via a different N-fusion reaction of the pyrrole nitrogen atom with the 2-fluoro-substituent of the

Figure 2. X-ray structures of (a) N-fused [22]pentaphyrin 5a′, (b) [26]hexaphyrin 6a, (c) [32]heptaphyrin 7a, (d) [36]octaphyrin 8a, (e) [40]nonaphyrin 9a, and (f) [44]decaphyrin 10a. 2587


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Scheme 3. One-Pot Synthesis of β-Perfluorinated Expanded Porphyrins

Size-selective syntheses of meso-aryl-substituted expanded porphyrins were achieved by employing dipyrromethane (2)102,103 or tripyrrane (3)104 as starting materials. Here it is also noteworthy to update the best synthetic protocols of 2 and 3, based on the author’s experience. The most effective and actually economic method to prepare dipyrromethane 2 is an acidcatalyzed condensation reaction of pyrrole with pentafluorobenzaldehyde in aqueous media reported by the group of Dehaen and co-workers.102 Accordingly, dipyrromethane 2 is obtained in more than 10 g scale in one shot. With a large quantity of 2 in hand, tripyrrane 3 can be synthesized in three steps from 2 according to the aroylation/condensation sequence reported by Lindsey and co-workers.105 Our modified synthetic protocols of 2 and 3 are now available in Appendix 2. Condensation of 2 and pentafluorobenzaldehyde (33 mM) with the aid of methanesulfonic acid (MSA) catalysis in CH2Cl2 gave increased amounts of porphyrin 4a (10%), hexaphyrin 6a (16%), octaphyrin 8a (8%), and decaphyrin 10a (3%) with significant suppression of the formation of expanded porphyrins bearing odd numbers of pyrrolic units. Moreover, size-selective synthesis of hexaphyrin 6a and nonaphyrin 9a was possible by the reaction of 3 with pentafluorobenzaldehyde, giving 6a (30%) and 9a (15%), together with dodecaphyrin (2%).96,104 Usually, giant expanded porphyrins were quite difficult to synthesize and isolate in their pure forms. However, the reaction of dipyrromethane 2 with pentafluorobenzaldehyde at high concentration (ca. 100 mM) at 0 °C produced [52]dodecaphyrin, [62]tetradecaphyrin, [72]hexadecaphyrin, and [80]octadecaphyrin, which were all isolated and characterized by 1H NMR, MALDITOF MS, and UV/vis absorption spectra.97 It is remarkable that such large expanded porphyrins were formed from the one-pot condensation reaction of simple precursors. Unambiguous structural characterizations of these giant expanded porphyrins by X-ray diffraction analysis will be a challenging issue in the future. After detailed studies, it was thought that only 2,6disubstituted electron-deficient aryl aldehydes gave expanded porphyrins and neither 2-substituted nor 2,6-unsubstituted aryl aldehydes produced expanded porphyrins. Dehaen and coworkers reported the observation of a set of expanded porphyrins 4−8d bearing meso-2,6-dichloropyrimidyl substituents by electrospray-ionization mass-spectrometry (ESI-MS) and actually isolated 4d, 5d′, and 6d. Hexaphyrin 6d underwent a facile N-fusion reaction similarly to 7a.100 Later, Imahori and coworkers found that the condensation of 3,5-bis(trifluoromethyl)benzaldehyde and pyrrole with the aid of trifluoroacetic acid in CH2Cl2 gave [18]porphyrin 4e, N-fused [22]pentaphyrin 5e′, [26]hexaphyrin 6e, and [32]heptaphyrin 7e, along with [22]sapphyrin.101 Probably, the strongly electron-withdrawing nature of the 3,5-bis(trifluoromethyl)phenyl group played an important role to stabilize the expanded porphyrins. Curiously, the use of BF3·OEt2 only led to the production of 4e. β-Perfluorinated meso-aryl-substituted expanded porphyrins were synthesized by the condensation of pentafluorobenzaldehyde with 3,4-difluoropyrrole (11a) using BF3·OEt2 as an acid catalyst (Scheme 3).106 A set of β-perfluorinated expanded porphyrins were isolated in low yields. β-Perfluorinated [26]and [28]hexaphyrins 14 and 19 were interconvertible via redox reactions. Both hexaphyrins took similar figure-eight conformations, while the electronic properties were contrasting; 14 was aromatic and 19 was nonaromatic. Similarly, β-perfluorinated [36]- and [38]octaphyrins 16 and 20 were interconvertible in a quantitative manner.

A modified synthesis of meso-alkynyl-substituted, β-ethylexpanded porphyrins was reported by Anderson and co-workers (Scheme 4).107 BF3·OEt2-catalyzed condensation reaction of 3,4Scheme 4. One-Pot Synthesis of β-Ethyl-meso-alkynylSubstituted Expanded Porphyrins

diethylpyrrole (21) with triisopropylsilyl (TIPS) propynal followed by oxidation with DDQ afforded a range of cyclic and noncyclic products including [18]porphyrin 22 (15%), [24]pentaphyrin 23 (2.4%), [28]hexaphyrin 24 (1.4%), [18]corrole 25 (6.5%), and two linear tripyrromethenes 26 (2.5%) and 27 (7.8%). [18]Porphyrin 22 exhibited a severely ruffled structure, and [28]hexaphyrin 24 showed a figure-eight conformation. Such distorted structures may arise from steric congestion at the periphery. When trifluoroacetic acid (TFA) was used as an acid, a tripyrrolic macrocycle became a main product rather than expanded analogues. meso-Trifluoromethyl-substituted expanded porphyrins were synthesized by one-pot condensation reaction of 2-(2,2,2trifluoro-1-hydroxyethyl)pyrrole (28) followed by oxidation with DDQ (Scheme 5).108 The condensation reaction was catalyzed by an equivalent amount of HCl (1.0 M in diethyl ether) for 12 h, to provide N-fused [24]pentaphyrin 30 (5− 2588


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Scheme 5. One-Pot Synthesis of meso-TrifluoromethylSubstituted Expanded Porphyrins

2.2. Selective Synthesis of Regular Expanded Porphyrins

Under reaction conditions of the one-pot synthesis of meso-arylsubstituted expanded porphyrins, neither ortho-unsubstituted arylaldehydes nor 2-halobenzaldehydes gave only the corresponding porphyrins, suggesting the importance of steric congestion around the formyl group. It was thought that the electron-deficient nature of the meso-aryl substituents was also important for the formation of meso-aryl-substituted expanded porphyrins. For instance, the condensation of pyrrole with 2,6dimethoxybenzaldehyde followed by oxidation with DDQ gave only tetrakis(2,6-dimethoxyphenyl)porphyrin, while the similar reaction of 5-(pentafluorophenyl)dipyrromethane 2a and electron-rich 2,6-dimethoxybenzaldehyde afforded A3B3-type [26]hexaphyrin 39f in 9% yield.110,111 Other ortho-substituted arylaldehydes were also applicable to this cross-condensation reaction to yield corresponding A3B3-type [26]hexaphyrins (Scheme 7, route A). In cases of severely hindered 5-arylScheme 7. A3B3-Type Hexaphyrin Synthesis

10%), [28]hexaphyrin 31 (3−4%), [32]heptaphyrin 32 (4−6%), [46]decaphyrin 33 (∼1%), and [56]dodecaphyrin 34 (∼1%). The structures of 33 and 34 were determined by X-ray analysis as rare cases of giant expanded porphyrins. These expanded porphyrins constitute sole examples bearing sp3-hybridized meso-substituents. The solid-state structures of the decaphyrin 33 and dodecaphyrin 34 were revealed to be a crescent and a two-pitch-helical conformation, respectively. In addition to these expanded porphyrins, calix[5]phyrin analogues 35a,b were also isolated, both of which took roughly planar conformations with an inverted pyrrole opposite to the sp3-hybridized meso-carbon atom. A unique synthesis of expanded porphyrins was achieved by Hiroto, Shinokubo, and Osuka.109 Namely, Sc(OTf)3-catalyzed reaction of pyrrole with pentafluorobenzaldehyde in an aqueous micellar system provided [30]heptaphyrin(1.1.1.1.0.0.0) 36, [30]heptaphyrin(1.1.1.1.1.1.0) 37, and [34]octaphyrin(1.1.1.1.1.1.1.0) 38 (Scheme 6). Electrospray ionization time-of-flight (ESI-TOF) mass analysis of the reaction mixture prior to oxidation showed a series of mass peaks assignable to linear oligopyrroles. These results underlined the importance of the reaction media for the formation of expanded porphyrins. substituents such as 2,4,6-trimethylphenyl and 9-anthryl groups, a similar but reversed cross reaction of aryldipyrromethane with pentafluorobenzaldehyde gave corresponding [26]hexaphyrins 39d and 39l in 15% and 10% yields, respectively (route B). Similarly, tri-meso-alkynyl-tri-meso-anthryl-[28]hexaphyrin 41 was obtained in 37% yield as a stable molecule.112 Cross-condensation reaction often produced a mixture of expanded porphyrins, which were not easy to separate. To circumvent this problem, designed ring-size selective syntheses of expanded porphyrins were developed. As a simple case, Nfused [22]pentaphyrin 5a′ was synthesized by [2 + 3]-type acidcatalyzed condensation of tripyrrane 3a with dipyrromethane dicarbinol 42 followed by oxidation with DDQ in 28% yield almost exclusively (Scheme 8).113 An advantage of this method was easy chromatographic separation of 5a′ from the reaction mixture. In a similar manner, [32]heptaphyrin 7a was prepared by [3 + 4]-type condensation of tripyrrane dicarbinol 43 with

Scheme 6. Expanded Porphyrin Synthesis in an Aqueous Micellar System (SDS = Sodium Dodecyl Sulfate)

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Scheme 8. [2 + 3]-Type Synthesis of N-Fused [22]Pentaphyrin

analogues of these expanded porphyrins will be introduced in the following section.

tetrapyrrane 44 in 39% yield (Scheme 9).95 This method was useful to prepare non-N-fused heptaphyrin 7a, as 7a underwent a facile N-fusion reaction to give 7a′ during a time-consuming separation (Scheme 2). Scheme 9. [3 + 4]-Type Synthesis of [32]Heptaphyrin Figure 3. Structures and conventional names of [22]sapphyrin, [22]smaragdyrin, [24]amethyrin, [22]rosarin, [26]rubyrin, and [40]turcasarin.

Although the synthesis of β-alkylated sapphyrin was significantly improved by Sessler and co-workers in 1990,19 meso-aryl-substituted sapphyrin analogues had not been reported until 1995.90,123 Latos-Grażyński and co-workers reported the first isolation of meso-tetraphenylsapphyrin 47a. They synthesized 47a by BF3·OEt2-catalyzed condensation of benzaldehyde with an excess amount of pyrrole (ca. 3 equiv) followed by oxidation with p-chloranil, but the yield was very low (ca. 1.1%). Rational synthesis of sapphyrin was thought to require the preorganization of linear oligopyrrolic precursors followed by intramolecular pyrrole−pyrrole oxidative coupling. Following this consideration, Gross and co-workers isolated an open-chain pentapyrromethene in the course of their efforts to optimize the synthesis of meso-triarylcorroles.124−126 Then, intramolecular oxidative coupling of the pentapyrromethene with iodine or molecular oxygen provided tetrakis(pentafluorophenyl)sapphyrin 47b almost quantitatively (Scheme 11). In a similar

Tetrapyrrane 44 is also a useful building block for size-selective synthesis of [36]octaphyrins. Actually, octaphyrin 8a was synthesized from 44 and pentafluorobenzaldehyde in 25% yield without much difficulty in separation procedure (Scheme 10).114 This [4 + 4]-type synthetic protocol was applied to the Scheme 10. [4 + 4]-Type Synthesis of A2B6-Type [36]Octaphyrins

Scheme 11. Synthesis of meso-Aryl-Substituted Sapphyrins

syntheses of A2B6-type [36]octaphyrins 45a−d in moderate yields. In the solid-state structure of 45a, less-hindered 2,4,6trifluorophenyl substituents occupied the sterically congested hinge positions of the figure-eight conformation. 2.3. Selective Synthesis of Other Expanded Porphyrins

Expanded porphyrins containing one or more pyrrole−pyrrole direct linkages such as sapphyrin 1a−c were rarely formed in the one-pot synthesis as shown in Scheme 1, because the ratio of pyrrole and arylaldehyde was set in a 1:1 manner. However, various expanded porphyrins possessing 2,2′-bipyrrole segments were identified. Some of those are called conventional names in analogy to the nomenclature of sapphyrin, for which the solution color resembles a jewel “sapphire”. Thereafter, [22]pentaphyrin(1.1.0.1.0) (smaragdyrin), [24]hexaphyrin(1.0.0.1.0.0) (amethyrin), [22]hexaphyrin(1.0.1.0.1.0) (rosarin), [26]hexaphyrin(1.1.0.1.1.0) (rubyrin), and [40]dodecaphyrin(1.0.1.0.0.1.0.1.0.0) (turcasarin) have been studied in the last two decades (Figure 3).115−122 The syntheses and structures of the meso-aryl-substituted

manner, Kadish and co-workers synthesized several meso-arylsubstituted sapphyrins 47c−g and revealed their electrochemical properties.127,128 The tetraarylsapphyrin was also obtained by the condensation of dipyrromethane precursors under appropriate acid-catalyzed conditions.129 An isomer of smaragdyrin, i.e., [20]pentaphyrin(1.1.1.0.0), was synthesized from diformyldipyrromethane 48 and terpyrrole 49 (Scheme 12).130 A mixture of 48 and 49 in CHCl3 was reacted with an excess amount of HCl, and the products 50a,b were obtained as HCl salts in good yields. Curiously, a combination of 2590


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Scheme 12. [2 + 3]-Type Synthesis of [20]Pentaphyrin(1.1.1.0.0)

Scheme 15. [4 + 4]-Type Synthesis of [32]Octaphyrin(1.0.0.0.1.0.0.0)

diformylated terpyrrole 52 and dipyrromethane did not afford 50a. Diformylated terpyrrole 52 was useful to synthesize macrocycles such as 53, namely, [26]isorubyrin.131 Condensation of 52 with tripyrromethene 51 under appropriate conditions followed by neutralization gave 53 in 46% yield (Scheme 13).

Setsune and co-workers reported a series of giant expanded porphyrins by employing 2,2′-bipyrrole 59a as a key building block.135−137 Acid-catalyzed condensation of 59a with benzaldehyde followed by oxidation and neutralization resulted in the formation of rosarin 60a and octaphyrin(1.0.1.0.1.0.1.0) 61a in 60% and 7% yields, respectively (Scheme 16). Although β-

Scheme 13. [3 + 3]-Type Synthesis of [26]Hexaphyrin(1.1.1.1.0.0)

Scheme 16. One-Pot Synthesis of Giant Expanded Porphyrins from 2,2′-Bipyrrole and Arylaldehydes

By extending the same strategy, Sessler and co-workers employed tetrapyrrane 54 prepared by cross condensation of benzaldehyde with an excess amount of pyrrole.132 Then, [30]heptaphyrin(1.1.1.1.1.0.0) 55 was obtained in 16% yield from the reaction of 52 with 54 (Scheme 14).133 X-ray diffraction analysis revealed a figure-eight twisted structure, while the absorption and 1H NMR spectra of 55 suggested a more symmetric structure as shown in Scheme 14. Recently, Sánchez-Garciá and co-workers synthesized [32]octaphyrin(1.0.0.0.1.0.0.0) 58a in 33% yield via the similar reaction sequence using quaterpyrrole 56a and its diformylated derivative 57a (Scheme 15).134 The quaterpyrrole 56a was easily prepared in two steps from 2,2-bipyrroles and thus regarded as a useful building block for the synthesis of novel macrocycles. Scheme 14. [4 + 3]-Type Synthesis of [30]Heptaphyrin(1.1.1.1.1.0.0)

alkylated derivatives of these rosarin and octaphyrin were synthesized earlier by Sessler, Vogel, and co-workers,138−140 the improved yields suggested the importance of steric bulkiness that induced macrocyclic deformation and drove intermediates to form large expanded porphyrins. Along this line, Setsune and coworkers employed bulkier 2,6-dichlorobenzaldehyde in the same reaction, which actually produced 61b in a better yield of 19% 2591


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and [48]dodecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0) 62b in 5% yield. Use of Zn(OAc)2·2H2O as a catalyst instead of TFA led to the production of more giant expanded porphyrin [64]hexadecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0) 63b in 9% yield. Remarkably, the structures of 62b and 63a were revealed by Xray structural analysis. Both 62b and 63a took zigzag-shaped folding conformations, and the bulky β-ethyl groups help increase the steric crowding, making the bipyrrole units twisted. Further, the same group developed an azafluvene-type morereactive precursor and then isolated [80]icosaphyrin 64 and [96]tetracosaphyrin 65 in 3% and 2% yields, respectively, along with 62a (3%) and 63a (2%). meso-Aryl-substituted rubyrin and its higher homologues were synthesized by TFA-catalyzed coupling of tripyrrane 3a followed by oxidation with p-chloranil (Scheme 17).141,142 This reaction

Scheme 18. Protonation of [26]Rubyrin

Scheme 17. Oxidative Synthesis of Giant Expanded Porphyrins from Tripyrrane

heavily dependent upon these peripheral substituents. Throughout these studies, it has been now established that the conformations of hexaphyrins depend on the following factors: (i) the intrinsic structural constraints that arise from the requirement to form a conjugated cyclic structure; (ii) peripheral substituents at the β- and/or meso-positions; (iii) intra/ intermolecular hydrogen bonding; (iv) solvent polarity and hydrogen-bond-donating and -accepting properties of the solvent; (v) the temperature; and (vi) the aromatic versus antiaromatic characters of the electronic π system. Because the electronic, optical, and coordination properties of hexaphyrins can be changed with conformational control, it is crucially important to reveal the structure−property relationships in hexaphyrins. Here, conformational preferences of [26]hexaphyrins are shown (Figure 4). As discussed above, the parent [26]-

procedure was similar to that used for the synthesis of coremodified rubyrins.143,144 After repetitive separations, [26]rubyrin 66 (24%), [38]nonaphyrin(1.1.0.1.1.0.1.1.0) 67 (9%), [52]dodecaphyrin(1.1.0.1.1.0.1.1.0.1.1.0) 68 (2.7%), and [62]pentadecaphyrin(1.1.0.1.1.0.1.1.0.1.1.0.1.1.0) 69 (1.5%) were obtained and fully characterized by HR-MS, 1H NMR, UV/vis, and X-ray diffraction analysis. [26]Rubyrin 66 was proved to be a planar aromatic molecule with a distinct diatropic ring current. Protonation of 66 with HCl led to formation of diprotonated species 71 with all the pyrroles pointing inward, while protonation with TFA produced 70 with only two pyrroles pointing outward (Scheme 18).141 Interestingly, 66, 70, and 71 displayed respectively different absorption spectra. Nonaphyrin 67 showed a distorted figure-eight structure. Larger expanded porphyrins, dodecaphyrin 68 and pentadecaphyrin 69, showed significantly distorted structures due to multiple intramolecular hydrogen-bonding interactions. Pentadecaphyrin 69 displayed a coiled helix-like structure.

Figure 4. Conformations of [26]hexaphyrins.

hexaphyrin 6a takes a planar rectangular shape, which we named “Type-II conformation”. In the Type-II conformer, the two pyrrole rings in the central long side are inverted (pointing outward), while the remaining four pyrrole rings are pointing inward to adjoin the effective intramolecular hydrogen bonding. On the other hand, a dumbbell-shaped conformation, which we named “Type-I conformation”, is apparently preferred over Type-II conformation from a thermodynamic viewpoint because of the four hydrogen-bonding interactions but is disfavored because of the steric repulsion between the inward-pointing meso-substituents. A figure-eight conformation is relatively rare for [26]hexaphyrins(1.1.1.1.1.1), probably due to the intrinsic

2.4. Synthesis of Peripherally Functionalized Hexaphyrins

[26]Hexaphyrin(1.1.1.1.1.1) was probably the most studied expanded porphyrin in the past decade (Chart 2). The β- or meso-positions of hexaphyrins were fabricated with various substituents to show that conformations of hexaphyrins were 2592


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the other hand, reduction of 74d with NaBH4 gave [30]hexaphyrin 76 with distinct 30π aromaticity. The electronic system of 76 can be regarded as an expanded isophlorin. oppDibenzo[26]hexaphyrin 79 was synthesized in the same route from 4,7-dihydro-4,7-ethano-2H-isoindole 77 and dipyrromethane dicarbinol 42 to afford [26]hexaphyrin 78 followed by retro-Diels−Alder reaction (Scheme 21).148−151

structural constraints, but is observed for cases in which bulky groups are substituted at the meso- and/or β-positions like βperfluorinated [26]hexaphyrin 14. In the figure-eight [26]hexaphyrins, the weak 26π aromaticity is still retained. As an extremely rare case, monoprotonated meso-phenyl-substituted [26]hexaphyrin 72 adopts a triangular conformation upon protonation with MSA.145 Although the neutral form of 72 was unstable in solution, the C3-symmetric structure of protonated form 72 is significantly stabilized by the MSA anion in a C3symmetric form favorable for hydrogen bonds. 2,3,17,18-Tetrahalogenated hexaphyrins were synthesized from 3,4-dihalopyrroles 11a−c and dipyrromethane dicarbinol 42 (Scheme 19).146 Interestingly, [28]hexaphyrins 74b,c were

Scheme 21. Synthesis of opp-Dibenzo[26]hexaphyrin

Scheme 19. Synthesis of β-Halo- and Ethylsulfanylhexaphyrins

As described above, meso-aryl-substituents play a crucial role to determine preferred conformations of [26]hexaphyrins. A particularly interesting finding was that 5,20-di(2-thienyl)- and 5,20-di(3-thienyl)-substituted hexaphyrins 81a,b preferred a dumbbell conformation (Type-I) due to the relatively small meso-aryl groups located at the inward-pointing 5,20-positions (Scheme 22).152 In contrast, 5,20-diphenyl-, 5,20-di(2,4,6Scheme 22. A2B4-Type [26]Hexaphyrins obtained preferentially against their 26π congeners 73b,c, while oxidized 26π species 73a was obtained as a stable form. Similarly, 2,3,17,18-tetraethylsulfanylhexaphyrin 74d was synthesized and was oxidized with MnO2 to give the corresponding [26]hexaphyrin 73d.147 X-ray diffraction analysis revealed the structures of 74c and 74d to be figure-eight conformers. In contrast, [26]hexaphyrin 73a−d exhibited a fast conformational dynamic equilibrium involving two kinds of rectangular conformers and a figure-eight conformer. The appended βsubstituents exerted a significant steric influence that was crucial in determining the conformational distribution. Reduction of 74b,c with an excess amount of NaBH4 furnished unexpected meso-hydrohexaphyrins 75b,c that can be regarded as a rare example of a phlorin-type expanded porphyrin (Scheme 20). On Scheme 20. Reduction of β-Halo- and Ethylsulfanylhexaphyrins

trifluorophenyl)-, and 5,20-di(3-methyl-2-thienyl)-substituted hexaphyrins 80a−c were revealed to have a rectangular conformer (Type-II) in the solid states. These differences induced by a quite subtle steric effect are intriguing, and, in a particular case, intact 81b as prepared was characterized as a Type-II conformer but soon changed to a Type-I conformer upon heating at 50 °C in CHCl3. Both conformers were separable and indeed revealed by X-ray diffraction analysis (Figure 5). The Type-I conformer of 81b was calculated to be more stable by only 1.2 kcal/mol than the Type-II conformer. Recently, a series of [26]hexaphyrins bearing two α-oligothienyl substituents 81c−e were prepared, all of which have Type-I conformers similarly to 81a.153 Perturbation by the oligothienyl substituents significantly altered the optical properties and excited-state dynamics of [26]hexaphyrins. 2593


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24).155,156 Changing one or two triisopropylsilylethynyl groups to a less-bulky phenylethynyl group led to a decreased activation Scheme 24. Synthesis of Ethylene-Bridged [28]Hexaphyrins

Figure 5. X-ray crystal structures of 81b: (a) Type-I conformer and (b) Type-II conformer.

Another important conformational dynamics often seen in [26]hexaphyrins is so-called “caterpillar-motion-like isomerization”.154 The 1H NMR spectrum of [26]hexaphyrin 80a in CDCl3 indicated a single stable conformer with the two phenyl groups at the short side of its Type-II rectangular shape (Scheme 23). On the other hand, the 1H NMR spectrum of [26]Scheme 23. Caterpillar-Motion-Like Isomerization

energy of this thermal cross-bridging, giving 86b,c directly via 84b,c without a particular thermal activation. On the basis of these results, this reaction was considered to involve (i) thermal conformational change from Type-II conformer to Type-I conformer, (ii) bond formation between the two unsaturated ethynyl groups, and (iii) migration of a bulkier TIPS-ethynyl group. The overall electronic system in 86a−c can be regarded as either [26]hexaphyrin or [16]diazaannuleno[16]diazaannulene, although the latter contribution should be minor owing to the perpendicular arrangement of the central vinylene bridge. These vinylene-bridged hexaphyrins possess rigidly held structures and thus exhibited a facile redox interconversion between 26π aromatic (86a−c) and 28π antiaromatic (87a−c) states. In contrast, [28]hexaphyrin 85a prepared by the reduction of 84a did not show distinct antiaromaticity. Very recently, 5,20bis(5,10,15-tris(pentafluorophenyl)Ni(II)-porphyrinylethynyl)hexaphyrin 84d was prepared, which was also found to undergo the same transannular reaction to give 86d.157 5-Ethynyl-20-phenyl-substituted hexaphyrin 88a,b underwent the transannulation reaction upon refluxing in toluene to give hexaphyrin 89a,b quantitatively (Scheme 25). These products had a spiro sp3 carbon center at the meso-position, by which the cyclic conjugation was disrupted.158 This reaction is remarkable, in that even a simple phenyl group thermally reacted with an acetylenic group to cause a skeletal rearrangement. This kind of transformation is very rare and only possible on [26]hexaphyrin framework to the best of our knowledge. The transannular reaction was rather general, as it was demonstrated to be applicable to other [26]hexaphyrins to provide methoxysubstituted products 90a−c, naphthalene annulated products 90d,e, and thiophene-annulated product 91. The importance of intramolecular hydrogen bonding was demonstrated in several examples. Two 2-pyridyl groups introduced at the 5,20-positions in [28]hexaphyrin 92 served as an effective hydrogen-bonding motif, helping its structure to be a planar dumbbell shape and hence its electronic system to be

hexaphyrin 80b bearing pentafluorophenyl and 2,4,6-trifluorophenyl groups at the meso-positions showed a conformational equilibrium between Type-II conformations in a 2.5:1 ratio. NMethylation reaction offered additional steric congestion inside of the macrocycle.111 Deprotonation of hexaphyrin 6a with tetran-butylammonium fluoride (TBAF) and subsequent reaction with methyl iodide gave doubly N-methylated [26]hexaphyrin 82. In this molecule, the N-methylated pyrroles are located at the diagonal corner. In the reduced [28]hexaphyrin 83, the Nmethylated pyrrole rings were moved to the middle of the long side, indicating the occurrence of a caterpillar-motion-like macrocyclic ring rotation. The conformational preference of 83 can be apparently ascribed to the stabilization due to the intramolecular hydrogen-bonding interactions. This interconversion between 82 and 83 could be repeated in a reversible manner. Conformational flexibilities of [26]hexaphyrins led to interesting intramolecular cross-bridging reactions. 5,20-Diethynylated [26]hexaphyrin 84a took a Type-II conformation just after its preparation. Upon heating, 84a would start the conformational interconversions between Type-II and Type-I conformers. In the latter conformer, the unsaturated ethynylene groups at the 5,20-positions were forced to locate into close contact, to trigger the transannular reaction of the two ethynyl groups to produce vinylene-bridged hexaphyrins 86a (Scheme 2594


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Hexaphyrin 99 carrying two meso-oxacyclohexadienylidenyl groups had a cross-conjugated electronic circuit (Figure 7).162

Scheme 25. Synthesis of Cross-Bridged [28]Hexaphyrins

Figure 7. Cross-conjugated hexaphyrins.

This hexaphyrin was reduced with NaBH4 to produce a [28]hexaphyrin macrocycle possessing two 4-hydroxyphenyl groups. Such electronic-state switching from a cross-conjugated, nonaromatic molecule to a Hückel aromatic molecule was further studied in its di-Rh(I) complexes 100 and 101 with TypeII conformations. Interestingly, oxidation of 101 with an equimolar amount of DDQ afforded 102, which has both oxacyclohexadienylidenyl and 4-hydroxyphenyl groups on the longer side of hexaphyrin and exhibits a moderate diatropic ring current likely due to the contribution of aromatic zwitterionic resonance.

antiaromatic (Figure 6).159 On the other hand, two 5-formyl-2pyrryl groups at the 5,20-positions in [26]hexaphyrin 93 were

2.5. Synthesis of meso-Free Expanded Porphyrins

In the chemistry of meso-aryl-substituted expanded porphyrins, meso-free congeners have been long sought, because such molecules may be used as substrates for elaborate fabrications at the meso-position. Despite this interest, meso-free expanded porphyrins were rarely explored, mainly due to intrinsic high reactivities of the free meso-position, particularly toward oxidizing reagents. Still only two examples, [22]pentaphyrin 104 and [26]hexaphyrin 106, were reported. The pentaphyrin 104 was synthesized as a planar aromatic molecule by [3 + 2]type condensation using tripyrrane 3a and meso-free dipyrromethane dicarbinol 103 followed by oxidation with DDQ (Scheme 26).163 Interestingly, 104 was obtained as a nonfused form, which was a rare case different from the meso-arylScheme 26. [3 + 2]-Type Synthesis of meso-Free [22]Pentaphyrin Figure 6. meso-Pyridyl-, 5-formyl-2-pyrryl-, imidazolylhexaphyrins, and meso-imidazolyloctaphyrin.

hydrogen bonded with the amino-type pyrroles to keep this molecule as a Hückel aromatic molecule with a dumbbell conformation.160 Hexaphyrin 92 was complexed with metal ions to produce di-Pd(II) complexes 94 and 95. Di-Zn(II), di-Cu(II), and di-Pd(II) complexes 96a, 96b, and 96c were formed from 93. meso-2-Imidazolyl-substituted [28]hexaphyrin 97a,b and [36]octaphyrin 98a−c were synthesized from tripyrrane 3a and tetrapyrrane 44, respectively.161 The introduced two meso-2imidazolyl groups stabilized 28π and 36π antiaromatic electronic networks through effective intramolecular hydrogen-bonding interactions. 2595


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Scheme 28. Synthesis of p-Phenylene-Bridged Decaphyrin

substituted [22]pentaphyrins that were all obtained as N-fused forms. The pentaphyrin 104 slowly decomposed under ambient conditions. Di-Rh(I) pentaphyrin complex 105 was prepared by Rh insertion to 104, and its nonfused structure was confirmed by X-ray structural analysis. In complex 105, the free meso-hydrogen was pointing inward and was kinetically protected by the two rhodium ions, which was important for its chemical stability. MSA-catalyzed condensation of tripyrrane 3a with an excess amount of trimethyl orthoformate followed by oxidation with DDQ gave 5,20-unsubstituted [26]hexaphyrin 106 and 5oxygenated hexaphyrin 107 in 17% and 15% yields, respectively (Scheme 27).164 The hexaphyrin 106 was certainly stable but was Scheme 27. [3 + 3]-Type Synthesis of meso-Free [26]Hexaphyrin and meso-Oxygenated Hexaphyrin

Scheme 29. Synthesis of Internally m-Phenylene and Heterole-Bridged [26]Hexaphyrins

slowly oxidized in the air to give 107. Surprisingly, compound 107 was characterized as a stable organic radical. Detailed studies on 107 will be given in section 5. 2.6. Other Examples

Upon an increase in the ring size, the conformations of expanded porphyrins become increasingly flexible, and large expanded porphyrins exist as a dynamic conformational mixture in solution. Conformational control of large expanded porphyrins is important to tune their electronic properties and thus has been attempted in many ways. As a unique approach, internally-1,4phenylene-bridged [46]decaphyrin 109 was synthesized from tripyrrane dicarbinol 43 and 1,4-phenylene-bridged bis(dipyrromethane) 108 in 7% yield (Scheme 28).165 [46]Decaphyrin 109 displayed a nonplanar and C2-symmetric structure. A 1H NMR signal due to the protons of the 1,4phenylene bridge was shifted to upfield, indicating a diatropic ring current and, hence, Hückel aromaticity for 109. [46]Decaphyrin 109 was quantitatively oxidized to nonaromatic [44]decaphyrin 110 with MnO2. Similarly, the synthesis of an internally 1,4-phenylene-bridged octaphyrin was attempted by the condensation of dipyrromethane dicarbinol 42 and 108. Unexpectedly, however, this reaction afforded calix-octaphyrin 112a bearing two sp3-hybridized meso-carbon atoms at the diagonal positions and a p-quinodimethane bridge. For comparison, gem-dimethyl-substituted analogue 112b was prepared, which showed similar properties with increased stability. 5,20-Aromatic-strapped [26]hexaphyrins were explored to demonstrate their characteristic “annuleno(annulene)” characters (Scheme 29).166 1,3-Phenylene-, 2,5-thienylene-, and 2,5pyrrylene strapped [26]hexaphyrins 114, 117, and 118 were prepared by MSA-catalyzed condensation of tripyrrane 3a with 1,3-diformylbenzene 113, 2,5-diformylthiophene 115, and 2,5-

diformylpyrrole 116, in 13%, 20%, and 14% yields, respectively. Detailed spectroscopic investigations revealed that the [26]hexaphyrin network was predominantly important for 114, while active involvements of the 2,5-thienylene and 2,5-pyrrylene straps in the macrocyclic conjugation caused a roughly equal contribution of [26]hexaphyrin and [18]thiaporphyrin for 117 and a predominant contribution of [18]porphyrin for 118. A similar aromatics-bridged system was also reported for coremodified hexaphyrins that showed a dual conjugation of [18]dithiaporphyrin and/or [26]dithiahexaphyrin.167 Recently, Siamese-twin porphyrin 120 has been developed by the group of Meyer as a novel expanded porphyrin variant (Scheme 30).168,169 Pyrrazole−pyrrole hybrid precursor 119 was prepared170−172 and was used for TFA-catalyzed condensation with benzaldehyde to give 120. Severe steric repulsion imposed by the eight ethyl substituents at the β-positions and eight phenyl substituents at the meso- and pyrrazole-positions forced 120 to take a considerably distorted structure, which was revealed by Xray diffraction analysis.173 Macrocycle 120 was complexed with 2596


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care was needed in the optimized synthetic procedure, this method furnished cyclo[8]pyrroles in high yields (up to 80% yield).177 The effect of counteranion was crucially important as smaller cyclo[6]- and cyclo[7]pyrroles were formed from the same precursors by replacing sulfuric acid by hydrogen chloride (Figure 8).178 The X-ray structures of H2SO4 salt 125b, TFA salt

Scheme 30. Synthesis of Siamese-Twin Porphyrin

many transition metal ions at the two tetradentate metal cavities located in close proximity. This metalation chemistry will be discussed in section 3. Shinokubo and co-workers reported a beautiful metaltemplated synthesis of Ni(II) norcorrole 123 by reductive coupling of α,α′-dibromodipyrrin precursor 121 (Scheme 31).

Figure 8. Cyclo[6]pyrrole and cyclo[7]pyrrole.

Scheme 31. Synthesis of [32]Octaphyrin(1.0.1.0.1.0.1.0) from α,α′-Dibromodipyrrin Complex

126, and HCl salt 127 were revealed. In all cases, the counteranions were nicely bound in their cavities with multiple hydrogen-bonding interactions. While macrocycle 125b was quite planar in the solid state, 126 and 127 showed some deviation from planarity depending on the size of the macrocycles. The electrochemical synthesis of 125a was also achieved by using tetrabutylammonium hydrogen sulfate as an electrolyte.179,180 A small amount of cyclo[7]pyrrole was identified by the same electrochemical reaction of 3,4diethylpyrrole. The effect of anion templation was evident also in the electrochemical synthesis. Anion-binding chemistry of cyclo[n]pyrroles was naturally considered, and a range of applications such as selective sulfate-anion extraction from competing anion pool and supramolecular donor−acceptor system were developed.181−183 Other interesting attributes were their redox behaviors and the formation of liquid crystalline cyclo[8]pyrroles 125e−g.184−189 Using quarterpyrroles 56a,b as larger building blocks, [4 + 4]type oxidative coupling was performed by the group of SánchezGarciá to yield cyclo[8]pyrroles 128a and 128b in 38% and 83% yields, respectively, as a rare example of D2h-symmetric cyclo[8]pyrroles (Scheme 33).134

Ni(II) norcorrole 123 is an amazingly stable molecule in spite of its strong antiaromatic nature and small ring size.174 During their study, Shinokubo and co-workers found that [32]octaphyrin(1.0.1.0.1.0.1.0) di-Pd(II) complex 124 was obtained in 35% yield in the similar Ni(0)-mediated reductive coupling reaction of α,α′-dibromodipyrrin Pd(II) complex 122.175 Demetalation of 124 was accomplished by treatment with ptolylmagnesium bromide at 80 °C to give freebase [32]octaphyrin(1.0.1.0.1.0.1.0) in 38% yield.176 2.7. Synthesis of Cyclo[n]pyrroles

Cyclo[n]pyrroles have emerged as a novel class of expanded porphyrins since the first synthesis of a dihydrogen sulfate salt of [30]octaphyrin(0.0.0.0.0.0.0.0), namely, cyclo[8]pyrrole, by Sessler and co-workers in 2002.177 Cyclo[8]pyrroles 125a−d were elegantly obtained by FeCl3-mediated oxidative coupling of bipyrroles 59a−d under biphasic oxidative conditions in the presence of dilute sulfuric acid (Scheme 32). Although special

Scheme 33. [4 + 4]-Type Synthesis of Cyclo[8]pyrrole

Scheme 32. Synthesis of Cyclo[8]pyrrole

Recently, π-extended cyclo[n]pyrrole derivatives were also explored. Cyclo[4]naphthobipyrroles 129a−d were synthesized by FeCl3-mediated oxidative coupling of naphthobipyrrole precursors by the groups of Panda and Sessler, independently (Figure 9).190,191 Naphthobipyrroles can be regarded as πextended building blocks and were demonstrated to also be effective to construct π-extended rubyrin, sapphyrin, and other related macrocycles.192−195 Okujima et al. reported cyclo[8]isoindole 131 via the oxidative coupling of bicyclo[2.2.2]octadiene-fused bipyrrole followed by thermal retro-Diels−Alder 2597


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analysis revealed that both products were conformational isomers with regard to the acenaphthylene orientations. Sessler and co-workers recently developed pyrrole−pyridine hybrid macrocycles, which were named as cyclo[m]pyridine[n]pyrrole (Scheme 35).201,202 Macrocycles 137 and 138 were Scheme 35. Synthesis of Cyclo[m]pyridine[n]pyrroles

Figure 9. Cyclo[4]naphthobipyrroles and cyclo[8]isoindole.

reaction of cyclo[8](bicyclo[2.2.2]octadiene)pyrrole 130.196 These π-extended cyclo[8]pyrroles showed remarkably redshifted absorptions over 1000 nm.197,198 The electronic state, excited-state dynamics, and anion-binding properties of 129b−d were extensively examined. Large structural deviation from planarity was observed in the solid-state structure of 131, which was ascribed to steric congestion at the periphery. This structural distortion was more pronounced for acenaphthylene-fused cyclo[8]pyrroles 132 and 133 that were synthesized under modified reaction conditions using Ce(SO4)2 as an oxidant (Scheme 34).199,200 Atropisomer 132 was also isolated as a kinetically controlled product, which was converted, upon heating at 150 °C, to 133 as a thermodynamically controlled product. X-ray diffraction Scheme 34. Isomerization of Acenaphthylene-Fused Cyclo[8]pyrrole

synthesized by Suzuki−Miyaura coupling of dihalogenated synthons 134 or 135 with diborylated partner 136. While these molecules took relatively planar structures, the overall macrocyclic conjugation was disrupted because of the dominant contribution of the local aromatic segments. However, the overall macrocyclic conjugation was realized upon protonation. Further synthetic elaboration enabled the preparation of cyclo[6]pyridine[6]pyrrole 140 via linear intermediate 139. In its neutral form, 140 was conformationally more dynamic, existing as a mixture of at least two kinds of figure-eight conformers, while the protonated species was revealed to have a ruffled planar structure or a figure-eight shape depending on the extent of protonation and counteranions.

3. METALATION CHEMISTRY OF EXPANDED PORPHYRINS Expanded porphyrins have served as unique and effective metal coordination ligands by virtue of their large cavities and regularly arranged pyrrolic nitrogen atoms. Expanded porphyrins possess structural and electronic flexibilities to respond to metal coordination and to adjust their oxidation states via a pyrrolic imine−amine conversion. In the earlier stage, limited but 2598


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important metal complexes of β-substituted expanded porphyrins were reported by Gossauer, Sessler, Vogel, and coworkers.16,47−52,203,204 Among these, texaphyrin metal complexes developed by Sessler are worthy to note, because they were actually used for medicinal applications such as promising magnetic resonance image contrasting agents and anticancer agents.21−46 Vogel’s works on figure-eight octaphyrin metal complexes were also of significant importance as earlier examples of twisted expanded porphyrins, which will be discussed in section 3.5.2. After the discovery of the synthesis of meso-arylsubstituted expanded porphyrins in 2001, Osuka and co-workers began to explore their metalation chemistry. Metal ions thus incorporated often served to lock in conformations of expanded porphyrins to be planar, figure-eight, bent gable-like, or a twisted Möbius form, depending upon the metal-coordination mode. The metal insertions sometimes triggered unexpected reactions such as skeletal rearrangement, macrocyclic splitting, and bond cleavage, which will be discussed in section 3.5. 3.1. Hexaphyrin Metal Complexes

3.1.1. Type-I Hexaphyrin Metal Complexes. Hexaphyrin is the benchmark molecule, and its metalations have been extensively studied. Metal complexes of hexaphyrins can be mainly classified into three groups: (i) gable-shaped Type-I hexaphyrin with NNN-type coordination, (ii) rectangular TypeII hexaphyrin with NNC(C)-type coordination, and (iii) Möbius-twisted hexaphyrin with NNNC-type coordination. As described in section 2, various functional groups such as pyridyl-, imidazolyl-, pyrrolyl-, and thienyl groups were appended at the meso-positions of hexaphyrins to serve as additional coordination sites for metal ions.152,159−161 Such examples, however, are excluded from the above classification in this section. In 2003, copper metalations of 6a and 19 were performed.205 Reaction of 6a with CuCl2 in the presence of NaOAc gave chloro-bridged di-Cu(II) complex 141 in 84% yield, while the metalation with anhydrous Cu(OAc)2 led to the formation of diCu(II) complex 142 without chloride bridge in 90% yield (Figure 10). The meso-carbons between the two tripyrrane-like moieties changed their hybridizations from sp2 to sp3, which led to interruption of the overall conjugation of the hexaphyrin πnetwork. A similar metalation on 19 gave complexes 143 and 144. Complex 143 exhibited a structural feature similar to that of 142, while complex 144 was revealed to be a dioxo-di-Cu(II) complex with the two Cu(II) ions separately bound to the different tripyrrolic ligand and meso-attached oxygen atom. While complexes 141−143 exhibit magnetic behaviors typical of antiferromagnetically coupled Cu(II) pairs, complex 144 showed the temperature-independent susceptibility χT value (0.4229 emu K mol−1), which corresponded to S = 1/2 state. These data were interpreted in terms of Cu(I)−Cu(II) mixed-valence state. Similar mixed-valence complex 145 was obtained from intramolecularly benzannulated hexaphyrin 89a. Zinc and cadmium metal ions were accommodated by the hexaphyrin cavity to give di-Zn(II) complex 146 and di-Cd(II) complex 147 bearing a chloride bridge and a meso-attached oxygen bridge.206 Zn(II) ions were also complexed at the cavities of benzannulated hexaphyrin 89a and internally vinylene-bridged hexaphyrin 86c in good yields. In 148, one Zn(II) atom was bound to the monoanionic tridentate tripyrrane segment with a chloride anion as a charge balancer, while the other Zn(II) ion was coordinated by the dianionic tridentate tripyrrodimethene segment. In contrast, 149 showed a symmetric structure, in which the two tripyrrane segments served as a monoanionic

Figure 10. Type-I hexaphyrin metal complexes (Ar = pentafluorophenyl).

tridentate ligand for Zn(II) ion, and the resultant charges were balanced by chloride ions. 3.1.2. Twisted Hexaphyrin Metal Complexes. Ni(II), Pd(II), and Pt(II) complexes of [28]hexaphyrin 151−153 were isolated and characterized in 2005 (Figure 11).207 In these

Figure 11. [28]Hexaphyrins and their group 10 metal complexes (Ar = pentafluorophenyl).

complexes, the [28]hexaphyrin ligand commonly took a twisted conformation to provide an NNNC coordinating cavity to accommodate a divalent metal ion. The Möbius aromatic nature of these complexes had been missed at the initial publication, but these complexes were recognized as 28π Möbius aromatic molecules in 2008.208 The group 10 metal ions took a square planar coordination with the NNNC cavity to cause a molecular twist. Different from these complexes, the freebase [28]hexaphyrin 150a−e showed a dynamic conformational equilibrium between a planar rectangular structure with Hückel antiaromatic character and a twisted structure with Möbius 2599


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spectrum of 156, the outer β-protons appeared at 9.51, 9.36, 9.35, and 9.03 ppm, and the inner β-protons and inner NH protons appeared at −2.93 and −2.08 ppm, respectively, indicating a clear diatropic ring-current effect. In sharp contrast, the 1H NMR spectrum of 157 showed a clear paratropic ring-current effect, exhibiting a signal due to the inner β-protons at 19.39 ppm, a signal due to the inner NH proton at 24.57 ppm, signals due to the outer β-protons at 5.02, 4.92, 4.32, and 4.07 ppm, and a signal due to the outer NH protons at 4.10 ppm. Since the structures of 156 and 157 were revealed to be planar, these observations demonstrated that the facile interconversion between Hückel aromatic (26π) and Hückel antiaromatic (28π) states was possible in a reversible manner. Di-Au(III) complexes 158 and 159 displayed essentially the same characteristics as those of 156 and 157. Similar aromatic/antiaromatic interconversion was also achieved in a pair of Rh(I) complexes 160 and 161.212 In this case, [28]hexaphyrin 150a was treated with [RhCl(CO)2]2 in CH2Cl2 under milder conditions to give [28]hexaphyrin diRh(I) complex 161 in 78% yield, which was oxidized with DDQ to [26]hexaphyrin di-Rh(I) complex 160 quantitatively. In both complexes, the two Rh(I) ions were bound to the dipyrrin moieties and two carbonyl ligands in a square planar coordination fashion. As a result, the two Rh(I) ions were displaced by ca. 1.00 Å from the mean plane defined by the six pyrrolic nitrogen atoms. The facile complexation of Rh(I) ions at the hexaphyrins was a useful means to rigidify a planar hexaphyrin macrocycle, which was applied to derivatizations of other functional hexaphyrins such as 78 and 79. More importantly, a set of redox pair 160 and 161 that can be interconvertible to each other was used to reveal the aromaticity reversal in the lowest triplet excited state (T1) as compared with the ground state (S0).213 Judging from the characteristic electronic absorption spectral signatures, the T1-states of aromatic 160 and antiaromatic 161 were indicated to be antiaromatic and aromatic, respectively. In the lowest singlet excited states, the Rh(I) ions played important roles to rigidify the planar hexaphyrin framework and facilitated the intersystem crossing to the T1 state by their heavy atom effect. As another unique metalation route, mono-Pd(II) complex 162 with a Type-II conformation was prepared.214 Because complex 162 was very difficult to synthesize directly from hexaphyrin 6a, the Möbius aromatic [28]hexaphyrin Pd complex 152 was oxidized with tris(4-bromophenyl)aminium hexachloroantimonate to give 162 in 71% yield. It is notable that the Pd(II) coordination changed from NNNC-mode to NNCC-mode and the molecular topology changed from Möbius to Hückel during this oxidative transformation. In addition, further metalation of 162 with Pd(OCOCF3)2 furnished di-Pd(II) complex 163 in 53% yield, which also took on a Type-II conformation and therefore displayed 26π Hückel aromaticity.215 It should be noted here that both complexes 162 and 163 showed rectangular shapes and 26π Hückel aromaticity but their electronic networks were different from that of the parent [26]hexaphyrin 6a. 3.1.4. meso-Free Hexaphyrin Metal Complexes. Zn(II) complex 165 was prepared by metalation of 106 with ZnCl2 in 75% yield, from which demetalation/remetalation protocol under pH control afforded mono-Zn(II) complex 164 (Figure 13).216 Di-Cu(II) complex 166 was obtained in a similar manner. These complexes displayed a dumbbell-shaped planar conformation, in which one of the meso-positions was oxygenated. Different from 141 and 146, the conjugated network of [26]hexaphyrin was preserved due to the lack of meso-

aromatic character (section 4.3.3). Through these studies, the interplay of the number of π-electrons and the topology of the πsurface was recognized to be crucially important to determine the aromaticity of expanded porphyrins. 3.1.3. Type-II Hexaphyrin Metal Complexes. Hg(II) metalation of [26]hexaphyrin 6a was attempted by using Hg(OAc)2 as a mercury source in a mixture of toluene/ MeOH. This attempt led to isolation of mono- and di-Hg(II) complexes, 154 and 155, in 29% and 27% yields, respectively (Figure 12).206 In 154, the methoxy group was attached at the

Figure 12. Type-II hexaphyrin metal complexes (Ar = pentafluorophenyl).

inner β-position and its 1H NMR signal appeared at −4.51 ppm, indicating a strong diatropic ring-current effect. In 154 and 155, the Hg(II) ions took a square planar coordination with the NNC ligand in the Type-II conformation of hexaphyrins. As another important example, Au(III) complexation of 6a was accomplished upon treatment with NaAuCl4 in the presence of NaOAc.209−211 In the initial report, the Au(III) metalation needed a long reaction time and the yields of mono-Au(III) complex 156 and di-Au(III) complex 158 were low. In these complexes, the Au(III) ion took a square planar coordination with the NNCC cavity. The silver salts were considered to activate Au(III) ion by liberating the chloride anion from the Au(III) salts. Later, the yields of 156 and 158 were considerably improved to 50% and 51% by using silver salts Ag3PO4 and Ag2CO3, respectively.211 [26]Hexaphyrin complexes 156 and 158 were reduced almost quantitatively with NaBH4 to the corresponding 28π hexaphyrins 157 and 159, respectively, both as reasonably stable antiaromatic species. In the 1H NMR 2600


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Figure 13. meso-Free [26]Hexaphyrin Metal Complexes (Ar = pentafluorophenyl).

substituents, and thus these complexes exhibited distinct 26π aromaticity. As another attempt, Ni(II) metalation of 106 was conducted with Ni(acac)2, to give meso,meso-dioxygenated diNi(II) complex 167 in 24% yield.217 The two Ni(II) ions were symmetrically accommodated within an NNNO-type cavity with a square planar fashion.

Figure 15. Heptaphyrin Cu complexes (Ar = pentafluorophenyl).

3.2. Metal Complexes of Other Expanded Porphyrins

second Cu ion was bound to the tripyrrolic moiety in a T-shaped manner without other ligands such as anions or coordinating molecules. In addition, the magnetic susceptibility measurement showed the oxidation state of this T-shaped Cu ion to be +2, thus concluding that this was a rare example of a T-shaped, threecoordinate Cu(II) ion that presumably experienced attractive Cu(II)−arene interactions. Complex 171 was spontaneously oxidized with air to give oxygenated complex 173. This transformation was reproduced by addition of H2O2 or metachloroperoxybenzoic acid (mCPBA). In 173, the oxidation state of the Cu ion in the tripyrrolic cavity was +1. This regioselective oxygenation reaction is interesting in view of its relevance to reactions of natural copper proteins and related arene hydroxylation chemistry. Furthermore, mono-Cu(II) complex 175 was synthesized from quadruply N-fused heptaphyrin 174 that was prepared by successive intramolecular N-fusion reaction of heptaphyrin 7a in the presence of sodium hydride. On the basis of the X-ray structural analysis and electron spin resonance (ESR) spectrum, it was suggested that 175 was a genuine Tshaped, planar, three-coordinated Cu(II) complex without any Cu(II)−arene interaction. 3.2.3. Octaphyrin Cobalt Complexes. Setsune and coworkers synthesized mono- and di-Co(II) complexes of figureeight octaphyrin(1.0.1.0.1.0.1.0) with various alkyl substituents at the β-positions (Figure 16).222,223 The first metalation took

As the size of expanded porphyrins became larger, the macrocyclic conformations tended to be increasingly twisted owing to effective intramolecular hydrogen bonding. These situations usually hampered their structural characterization by X-ray diffraction analysis and spectroscopic means. Therefore, fixation of a specific conformer by metal coordination was useful for the structural characterization of such large expanded porphyrins. Along this line, metal complexes of heptaphyrin, octaphyrin, and nonaphyrin were examined, which will be discussed in this section. Decaphyrin metal complexes were also explored that showed unique redox-responsible conformation change including a Mö bius conformer and spontaneous generation of a stable organic radical, as will be discussed in the following section. 3.2.1. Heptaphyrin and Octaphyrin Zinc Complexes. [32]Heptaphyrin 7a and [36]octaphyrin 8a inherently possessed one or two porphyrin-like tetrapyrrolic cavities, with which these expanded porphyrins accommodated one or two Zn(II) ions to form [32]heptaphyrin mono-Zn(II) complex 168218 and [36]octaphyrin di-Zn(II) complex 169219 quantitatively (Figure 14). These complexes showed twisted figure-eight conformations in the solid state.

Figure 14. [32]Heptaphyrin Zn(II) complex and [36]octaphyrin diZn(II) complex (Ar = pentafluorophenyl).

3.2.2. Heptaphyrin Copper Complexes. [32]Heptaphyrin mono-Cu(II) complex 170 was obtained by metalation of 7a with Cu(OAc)2, leaving the tripyrrolic moiety intact (Figure 15).218 Further treatment of 170 with Cu(OAc)2 under basic conditions provided di-Cu(II) complex 171 quantitatively.220,221 Similar treatment of Zn(II) complex 168 with Cu(OAc)2 gave heterometal complex 172 in a quantitative yield. Remarkably, the X-ray diffraction analysis revealed that the

Figure 16. Octaphyrin Co(II) complexes (Ar = pentafluorophenyl). 2601


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place when Co(OAc)2 was added to a solution of the octaphyrins in CHCl3, giving 176a−d in good yields, while the second metalation to give 177a,c required more harsh reaction conditions. Co(II) metalation of the standard octaphyrin 8a afforded mono-Co(II) complex 178, while further metalation under more forcing conditions did not give di-Co(II) complex probably due to the distorted figure-eight structure of 178.219 3.2.4. Octaphyrin Palladium Complexes. Octaphyrins were shown to take various structures other than a figure-eight conformation, responding to the oxidation state and the coordination mode of the transition metals. Indeed, freebase [36]octaphyrin 8a was reduced with NaBH4 to give [38]octaphyrin 179 quantitatively, whose crystal structure was revealed to be also figure-eight but with two inverted pyrrole units at the hinge positions (Figure 17). As an important

Figure 18. Nonaphyrin Ni(II) complexes.

obtained in 26% and 16% yields, respectively. In 185a, the Ni(II) ion was bound to the tetrapyrrolic cavity of [40]nonaphyrin. A figure-eight conformation of 187 was similar to that of 185a but contained a doubly N-fused segment. On the other hand, a similar Ni(II) metalation of [42]nonaphyrin 184 gave two mono-Ni(II) complexes 186a and 188a in 60% and 20% yields, respectively. Ni(II) complex 186a was found to be a reduced form of 185a, while 188a was revealed to be a structural isomer of 186a with regard to the positions of NH atoms and the structure of uncomplexed hexapyrrolic moiety. The isomerization of 186a to 188a was facilitated upon heating in acetonitrile in the presence of TFA. This conformational isomerization was suggested to proceed via a protonationinduced caterpillar-like rotation of the metal-free hexaphyrin-like segment as probed by an observation for A3B6-type nonaphyrins 186b and 188b. 1H NMR spectra and UV/vis absorption spectra indicated that 186a,b, 187, and 188a,b showed moderate aromaticity in line with their 42π circuit and figure-eight Hückel topology. 3.2.6. Rubyrin Metal Complexes. [26]Rubyrin 66 and [24]rubyrin 189 were shown to be interconvertible (Figure 19).142,226 Upon treatment with Zn(OAc)2·2H2O, 66 and 189 gave 26π aromatic di-Zn(II) complex 190 and 24π antiaromatic Zn(II) complex 191, respectively. Metalation chemistry of [26]rubyrin was further explored. Di-Co(II) rubyrin complex 192 was prepared as a 24π antiaromatic molecule from the reaction of 66 with Co(OAc)2·4H2O, and di-Rh(I) rubyrin complex 193 was prepared as a 26 π aromatic molecule from the reaction of 66 with [RhCl(CO)2]2. In the solid state, the diCo(II) complex 192 exhibited antiferromagnetic interaction between the two Co(II) centers bridged by the acetate ligand. Metalation of 66 with [IrCl2Cp*]2 in tetrahydrofuran (THF) in the presence of potassium tert-butoxide afforded Ir(III) complex 194, in which the Ir(III)Cp* was bound to the dipyrrin moiety. When the Ir(III) metalation was conducted in refluxing 1,4dioxane, N-fused Ir(III) complex 195 was obtained instead of 194. Complex 195 had an extra Ir−C bond at the β-position of N-fused pyrrole segment.

Figure 17. Octaphyrin Pd(II) and Ir(I) complexes (Ar = pentafluorophenyl).

example, Pd(II) metalation of 8a was conducted with Pd(OAc)2 under reflux in MeOH to afford di-Pd(II) complexes 180 and 181 in 20% and 51% yields, respectively. Complex 180 was found to be a 36π Möbius aromatic molecule, while the other complex 181 showed 36π weak antiaromaticity arising from its figureeight conformation (Hückel topology).208 When the metalation of 8a with Pd(OAc)2 was conducted in a 9:1 mixture of 2,2,2trifluoroethanol and methanol, a new Pd(II) complex 182 was generated in 21% yield along with 181 (28%).219 The formation of 182 was considered to arise from another complex including unsymmetric Pd(II) ions bound to NNNN- and NNNC-type coordination that immediately caused a C−C bond formation between the meso- and pyrrolic β-carbons at the most crowded position followed by N−H to C−H hydrogen shift to form a new N−C bond. Metalation of 8a with [IrCl(cod)]2 (cod =1,5cyclooctadiene) gave mono-Ir(I) complex 183 in 66% yield, which exhibited a nontwisted, rectangular-like structure and weak Hückel antiaromaticity.224 3.2.5. Nonaphyrin Nickel Complexes. [40]Nonaphyrin 9a was reduced with NaBH4 to [42]nonaphyrin 184 with a concurrent conformational change from a figure-eight conformation to a butterfly one (Figure 18). Ni(II) metalation of 9a was attempted by treatment with Ni(acac)2 in acetonitrile at 60 °C.225 Two mono-Ni(II) complexes 185a and 187 were 2602


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Cu(II) metalation into 196 also demonstrated a unique multiredox behavior. 3.3. Heterometal Complexes

The formation of heterometal complex manifests rich metalation abilities of expanded porphyrins. Although several earlier examples of β-alkylated octaphyrin heterometal complexes had been reported,230,231 two kinds of recent examples of meso-arylsubstituted hexaphyrins are discussed in this section. First, a variety of heterometal complexes of Type-II hexaphyrins were synthesized from mono-Au(III) complex 156 (Figure 21).209

Figure 19. Rubyrin metal complexes (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl).

3.2.7. Siamese-Twin Porphyrin Metal Complexes. Siamese-twin porphyrin 120 had two NNNN-type porphyrinlike coordination sites that served as a divalent metal ligand. In 2014, Meyer and co-workers reported the selective Ni(II) insertion to 120 (Figure 20).227 According to their protocol,

Figure 21. Hexaphyrin heterometal complexes (Ar = pentafluorophenyl).

Au(III)−Ag(III) complex 200, Au(III)−Cu(III) complex 201, and Au(III)−Rh(I) complex 202 were readily synthesized from 156. [26]Hexaphyrin mono-Pd(II) complex 162 also served to provide heterometal complexes.214 Pd(II)−Ag(III) complex 206 and Pd(II)−Cu(III) complex 207 were synthesized, both of which exhibited 26π aromatic character. When a solution of 206 in CH2Cl2/MeOH was heated in the presence of Ag(OTf) and NaOAc, the complex was converted to the β,β-dimethoxy adduct in good yield. [28]Hexaphyrin mono-Au(III) complex 157 was also used for [28]hexaphyrin heterometal complexes, as seen for the synthesis of [28]hexaphyrin Au(III)−Rh(I) complex 203 from 157. Upon heating in pyridine, 202 was converted to Au(III)−Rh(III) complex 204 via double C−H bond activation in 25% yield.232 A similar reaction was performed by metalation of 156 with [IrCl(cod)]2 in refluxing pyridine to give Au(III)− Ir(III) complex 205.233 This kind of Ir(III) complex with Type-II conformation was also synthesized from 6a. [40]Nonaphyrin 9a possessed a porphyrin-like tetrapyrrolic segment and a hexaphyrin-like hexapyrrolic segment. By using these different coordination sites, multiple metal complexation was demonstrated.96 Zn(II) and Cu(II) metalations were achieved to give mono-Zn(II) complex 208 and mono-Cu(II) complex 209 in 75% and 35% yields, respectively, where the metal ion was bound in the porphyrin-like cavity and the figureeight conformation of nonaphyrin was essentially preserved

Figure 20. Siamese-twin porphyrin metal complexes.

metalation with Ni(OAc)2·4H2O in CH2Cl2/MeOH at room temperature yielded mono-Ni(II) complex 196, and the same reaction at refluxing temperature yielded di-Ni(II) complex 197 both in good yields. In contrast, similar selective Cu(II) metalation was unfeasible, and only di-Cu(II) complex 198 was isolated in 29% yield. Interestingly, these complexes exhibited strongly saddled copper-binding pockets and overall helically twisted structures.228 Magnetic susceptibility measurement of 198 revealed that the two Cu(II) centers were ferromagnetically coupled with J = +16.3 cm−1 (g = 2.17) due to the helical structure, in that the two magnetic dx2−y2 metal orbitals were nearly orthogonal to each other, resulting in a minimal overlap integral through the bridging ligand.169 Another intriguing feature of the Siamese-twin porphyrins was their redox behaviors. A detailed investigation on the di-Cu(II) complex 198 in different oxidation states revealed that the first two oxidations were ligand-centered and that the ligand-centered radicals were antiferromagnetically coupled with the proximate Cu(II) ion, thus showing overall diamagnetic character in its dication state.229 Ni(II)−Cu(II) heterometal complex 199 prepared by 2603


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and [32]octaphyrin(1.0.0.0.1.0.0.0), were synthesized (Figure 23). The structures of bis-BF2 complexes 215 and 217 were

(Figure 22). The vacant hexapyrrolic cavity was used to accommodate the second and third metal ions. Along with this

Figure 23. [24]Hexaphyrin and [32]octaphyrin BF2 complexes.

revealed to be slightly bowl-shaped and twisted figure-eight-like structures, respectively, with the boron dipyrromethene moieties being kept essentially flat. Boron complexation of [22]oxasmaragdyrin 218a−g was synthesized by Ravikanth and co-workers (Figure 24).257,258 The Figure 22. Nonaphyrin heterometal complexes (Ar = pentafluorophenyl).

strategy, Cu(II)−Pd(II) heterometal complex 210, Cu(II)− Pd(II)−Pd(II) trimetal complex 211, and Zn(II)−Pd(II)− Pd(II) trimetal complex 212 were obtained and these structures were revealed by X-ray analysis. In the trinuclear complexes, one Pd(II) ion was bound in the NNCC-type cavity via double C−H bond activation, while the other Pd(II) ion was coordinated by the NNC-type cavity presumably with the aid of agostic interaction with the adjacent C−H bond. [42]Nonaphyrin Ni(II) complex 188a also preserved the vacant hexapyrrolic cavity, to which Rh(I) ion was coordinated to provide [42]nonaphyrin Ni(II)−Rh(I) complex 213 that exhibited a strong diatropic ring-current effect.225 Oxidation of 213 afforded [40]nonaphyrin Ni(II)−Rh(I) complex, and this hybrid complex showed a paratropic ring current owing to its 40π electronic state.

Figure 24. [22]Oxasmaragdyrin boron complexes.

3.4. Main Group Complexes

electronic properties of complexes 219a−g were significantly altered from those of 218a−g as revealed by various spectroscopic techniques. The BF2 moiety of 219a was transformed into the boronic acid by reaction with AlCl3 to give 220a. Subsequent addition of MeOH, EtOH, and glycol furnished alkoxylated derivatives 220b, 220c, and 220d, respectively. B(OH)2 derivative 220a showed a selective fluoride ion sensing ability. Recently, Hung and co-workers demonstrated that boron complexes of [22]oxasmaragdyrin were used for dyesensitized solar cells (DSSCs) as rare examples of expanded porphyrin sensitizer.259,260 Cyclo[6]pyridine[6]pyrrole 140 showed dynamic conformational changes in nonpolar solvents. Conversion of 140 to BF2 complex 221 was achieved via deprotonation with NaH followed by treatment with BF3 (Figure 25). Coordination of BF2 to the pyrrole−pyridine segment was rare but restricted the conformation of 221.202 Shinokubo and co-workers developed a different synthetic route to butadiyne-bridged cyclic BODIPY oligomers (Scheme 36). Trimethylsilyl (TMS)-ethynylated BODIPYs 222a,b were prepared and were homocoupled by sila-Glaser coupling with

Porphyrin complexes housing main group elements such as boron, phosphorus, and silicon have been a recent active topic.234−240 Subporphyrins and subphthalocyanines have been isolated only as boron(III) complexes and have been extensively studied in light of their attractive optical and electronic attributes.241−247 The rich chemistry of these porphyrinoids tempted the development of main group complexes of expanded porphyrins. Curiously, the incorporation of boron atom into expanded porphyrins sometimes caused skeletal rearrangements and bond cleavages in an unexpected manner. The incorporation of phosphorus atom led to the formation of unprecedented species such as a Möbius antiaromatic molecule and expanded isophorins. Although the chemistry of boron dipyrromethenes (BODIPYs) is beyond the scope of this Review,248−255 BF2 complexes of expanded porphyrin complexes and related molecules will be discussed below. 3.4.1. Boron Complexes. Sessler and co-workers reported the first example of BF2 complexes of expanded porphyrins in 2004.256 In their report, mono-BF2 complexes 214 and 216, and bis-BF2 complexes 215 and 217 of [24]hexaphyrin(1.0.0.1.0.0) 2604


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pyrroles in a trigonal bipyramidal manner. In the structure of 228, the second PO moiety was bound to the NNC cavity. On the basis of 1H, 31P NMR, and HR-ESI-MS measurements, 225 and 228 were revealed to have 38π and 40π electronic systems, respectively. It is noteworthy that the diphosphorus complex 228 can be regarded as the first example of a stable expanded isophlorin. Complex 225 was also reduced with NaBH4 to complex 226 possessing 40π electronic system, which, however, was smoothly oxidized back to 225. In the opposite manner, 228 was oxidized to aromatic 38π electronic species 227. Reaction of N-fused pentaphyrin 5a′ with POCl3 in the presence of triethylamine provided PO complex 229 in 66% yield (Figure 26).263 The crystal structure of 229 showed that the

Figure 25. Cyclo[6]pyridine[6]pyrrole bis-BF2 complex.

Scheme 36. Synthesis of Butadiyne-Bridged BODIPY Oligomers

Figure 26. [24]Pentaphyrin phosphorus complexes (Ar = pentafluorophenyl).

PO moiety was bound to the two nitrogen atoms and one βcarbon atom. In addition, the N-fusion reaction between the outer-pointing pyrrolic NH and the ortho position of pentafluorophenyl group and interior C−C bond formation between two β-carbon atoms occurred concurrently. The PO moiety of 229 was converted to PS group (230) in 84% yield by the reaction with Lawesson’s reagent and was reduced to phosphine−borane (P−BH3) complex 231 in 54% yield by the reduction with BH3·SMe2.

CuCl in dimethyl sulfoxide (DMSO) to give dimeric products 223a,b and trimeric products 224a,b.261 Interestingly, these novel macrocycles exhibited substantial antiaromatic characters as revealed by their 1H and 19F NMR spectra as well as theoretical calculations. 3.4.2. Phosphorus Complexes. Phosphorus atom incorporation to expanded porphyrins was first achieved by Osuka and co-workers by reaction of [36]octaphyrin 8a with PCl3 in the presence of triethylamine (Scheme 37). This reaction gave

3.5. Metalation-Induced Rearrangements and Ring Cleavages

Figure-eight conformations of expanded porphyrins are often rather dynamic in solution, but such conformational dynamics are considerably restricted by suitable metal coordination. Under such circumstances, the π-electrons of the different halves are held closely and interact strongly at the hinge position. Sometimes, this situation led to unprecedented reactions such as splitting reactions and skeletal rearrangements.264−266 3.5.1. Splitting Reactions. In 2004, Osuka and co-workers discovered a surprising reaction that may be expressed as “molecular mitosis” (Scheme 38).267 Metalation of [36]octaphyrin 8a with Cu(OAc)2 in the presence of NaOAc was monitored by UV/vis absorption spectroscopy in toluene at 50 °C. Monitoring of the reaction progress indicated the formations of mono-Cu(II) complex 236 and di-Cu(II) complex 232. However, prolonged heating of the reaction mixture resulted in a drastic color change from dark green to vivid red after 2 weeks. From this reaction mixture, Cu(II) porphyrin 233 was isolated and was shown to be responsible for the red color. This splitting reaction was accelerated at high temperature and was completed within 2 h under toluene reflux to give 233 in 91% yield. Heating of mono-Cu(II) complex 236 did not cause the splitting reaction. Thus, the coordination of two Cu(II) ions was crucial for the splitting reaction. Because the structure of 232 was revealed to be severely distorted, the relief of the serious strain in 232 was considered to be a main driving force for the splitting reaction. Further investigation showed that the Co(II)−Cu(II) hybrid

Scheme 37. Octaphyrin P(V) Complexes (Ar = Pentafluorophenyl)

monophosphorus complex 225 and diphosphorus complex 228 in 43% and 9% yields, respectively.262 The yield of 228 increased when the reaction was run in the presence of a small amount of water at elevated temperature. The structure of 225 was revealed by X-ray analysis to be a figure-eight structure with the pentacoordinated phosphorus atom, which was bound to the three nitrogen atoms and two β-carbon atoms of the adjacent 2605


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irradiation at 365 nm. As a more successful example, treatment of Cu(II) complex 170 with excess BBr3 in the presence of diisopropylethylamine at room temperature provided subporphyrin 239 in 36% yield along with Cu(II) porphyrin 233 in 13% yield. This unique extrusion reaction provided an important synthetic route to this particular subporphyrin.268−272 In a similar manner, meso-trifluoromethyl-substituted heptaphyrin Cu(II) complex 241 underwent the splitting reaction to afford the otherwise inaccessible meso-trifluoromethyl-substituted subporphyrin 242 in 12% yield.273 3.5.2. Rearrangements. [28]Hexaphyrin 150a was treated with excess BBr3 in the presence of amine under reflux to give boron complex 244 with a rearranged framework in 22% yield (Scheme 40).274 In the meantime, the treatment of 150a with

Scheme 38. Splitting Reaction of [36]Octaphyrin into Two [18]Porphyrins (Ar = Pentafluorophenyl)

Scheme 40. Rearrangements of [28]Hexaphyrin by Boron Incorporation (Ar = Pentafluorophenyl)

complex 234 also underwent a similar splitting reaction to give Co(II) porphyrin 235 and Cu(II) porphyrin 233 in good yields, while neither Co(II)−Zn(II) complex 237 nor di-Zn(II) complex 168 showed such a reactivity. Density functional theory (DFT) calculations suggested that the reaction proceeded through a series of steps in a stepwise manner involving the initial bond reorganization to give a spirocyclobutane-type intermediate followed by a radical reverse cycloaddition reaction to afford two porphyrins.219 A splitting reaction similar to those of [36]octaphyrins was found for a heptaphyrin scaffold (Scheme 39). Boron insertion to Scheme 39. Splitting Reaction of [32]Heptaphyrin into [18]Porphyrin and [14]Subporphyrin

excess BBr3 in the presence of amine at ambient temperature afforded another product in 55% yield after subsequent oxidation with MnO2. The structure of this oxidation product was revealed by X-ray diffraction analysis to be a boron complex of [28]hexaphyrin(2.1.1.0.1.1) 246, which had a figure-eight conformation containing a directly connected bipyrrole unit and an s-trans-1,3-butadienyl linkage at the opposite position. It is interesting to note that the transposition of the meso-carbons was effected in this metalation reaction. Redox interconversion between 246 and its reduced form 245 was accomplished with concurrent switching of boron coordination modes between tetrahedral and trigonal forms. As an important example, Vogel and co-workers found rearrangement reactions of β-alkylated octaphyrin(1.1.1.0.1.1.1.0) to spirodiporphyrins triggered by Ni(II) or Pd(II) metalation (Scheme 41).275,276 Oxidation of [32]octaphyrin(1.1.1.0.1.1.1.0) 247 with selenium dioxide in CHCl3 regioselectively furnished dioxo derivative 249 in 80% yield, which was metalated with Ni(OAc)2 in dimethylformamide (DMF) under reflux to produce di-Ni(II) complex 250 in ∼40% yield and spirodicorrole di-Ni(II) complex 252 in ∼9%. The latter product was presumably formed via extrusion of two carbon dioxide molecules from conformational isomer 251. In addition, spirodiporphyrin di-Pd(II) complex 248 was obtained upon Pd(II) complexation of 247 via transannulation of the two bipyrrole segments at the hinge position. Intriguingly, the transannulation was found to be thermally reversible, but slow evaporation of the equilibrium mixture afforded 248 in its pure form.

doubly N-fused heptaphyrin 174 led to the formation of B(III) hydroxyl complex 238a in 68% yield, which was further converted to alkoxyboron complexes 238b,c by refluxing in the presence of alcohols.218 Similar treatment of [32]heptaphyrin Zn(II) complex 168 with BCl3 or BBr3 under various conditions gave complicated mixtures, in which a trace amount of mesotris(pentafluorophenyl)-substituted subporphyrin 239 was isolated. This product emitted green fluorescence upon photo2606


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Scheme 41. Rearrangements of [36]Octaphyrins into Spirodiporphyrins

Scheme 43. Rearrangements of [32]Heptaphyrin by Pd(II) Metalation

presence of triethylamine in CH2Cl2 to give mono-Pd(II) complex 257b in 88% yield, which was one of the first examples of Möbius aromatic expanded porphyrins (section 4).208 From this reaction mixture, another Pd(II) complex 258b was isolated in a low yield, but its yield was improved to be 23% when the Pd(II) metalation was conducted in acetone at room temperature. Complex 258b had a formal N-confused Pd-porphyrin (Pd-NCP) moiety, across which a tripyrrane unit was attached at the α- and γ-positions of the confused pyrrole unit. This product was thought to be formed via a sequence of complicated rearrangement reactions. However, the similar complex 258a was obtained from pentafluorophenyl-substituted heptaphyrin 7a in 31% yield, suggesting that this quite unique rearrangement might have some generality. Di-Pd(II) complex 259a was formed in 49% yield when the rearrangement was conducted in the presence of an excess amount of Pd(OAc)2. The second Pd(II) ion was bound to the NNNC-type ligand in a square planar manner. Interestingly, a similar transformation from heptaphyrin to NCP also occurred for mono-Zn(II) complex 168. Metalation of 168 with Pd(OAc)2 in the presence of triethylamine gave Pd(II)−Zn(II) hybrid complex 260 in 62% yield, which possesses an NCP Pd(II) complex structure. 3.5.3. Ring Cleavages. Di-Cu(II) complex of perfluorinated [36]octaphyrin 261 underwent hydrolytic ring-opening in one of the pyrrolic units to give 262 (Scheme 44).279 The structure of 262 was confirmed by X-ray crystal analysis. Surprisingly, the octaphyrin framework was restored with concurrent demetalation upon treatment of 262 with TFA and H2SO4. The observed facile hydrolytic cleavage of a pyrrole moiety would be ascribed to the strongly electron-deficient nature of 261. In contrast, metalation of [36]octaphyrin with AgOAc proceeded smoothly without any skeletal rearrangement of the macrocycle to give twisted figure-eight di-Ag(I) complexes 264 and 265. In accordance with their Ag(I) oxidation states, the octaphyrin exhibited a 34π electronic circuit. The 34π electronic species of octaphyrin was also demonstrated in freebase 263. Doubly N-fused benzo[28]hexaphyrin 266 was synthesized through a retro-Diels−Alder reaction 148−150 of a βbicyclo[2.2.2]octadiene-fused hexaphyrin (Scheme 45).280 [28]Hexaphyrin 266 underwent an oxidative rearrangement in the presence of water to give a fluorescent molecule, 267. This

Regular [36]octaphyrin 8a underwent skeletal rearrangements upon metalation with Ni(acac)2 in refluxing acetonitrile to produce various products in low to moderate yields (Scheme 42).277 Product distribution was certainly dependent upon the Scheme 42. Rearrangement of [36]Octaphyrin into Several Rearranged Products (Ar = Pentafluorophenyl)

amount of the Ni(acac)2 reagent used and the presence of water. The structure of di-Ni(II) complex 253 was easily assigned by its 1 H NMR spectrum in comparison to that of di-Pd(II) complex 181. The structures of rearranged mono-Ni(II) octaphyrin 254 and dioxygenated di-Ni(II) octaphyrin 255 were determined by X-ray diffraction analysis. The last one was assigned to be a mesoβ-linked Ni(II) porphyrin dimer 256 by its derivatizations (the inner metal exchange and para-selective nucleophilic aromatic substitution of pentafluorophenyl group with isopropylamine) to a meso-β-linked Zn(II) porphyrin dimer whose structure was determined by X-ray analysis. Metalation of heptaphyrins 7a,b with Pd(OAc)2 caused a unique rearrangement (Scheme 43).278 2,6-Dichlorophenylsubstituted heptaphyrin 7b was metalated with Pd(OAc)2 in the 2607


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Scheme 46. Double Confusion of [26]Hexaphyrin into NConfused [26]Hexaphyrin (Ar = Pentafluorophenyl)

Scheme 44. Cleavage of [36]Octaphyrin (Ar = Pentafluorophenyl)

oxygenation at the internal pyrrolic α-positions occurred concomitantly. With this novel macrocyclic ligand in hand, other metal complexes such as Co(II), Mn(III), and Fe(III) were prepared.282 Fully π-conjugated helix molecules 273 and 274 were synthesized from di-Cu(II) complexes of heptaphyrin 171 and octaphyrin 232, respectively, through the oxidative cleavages effected with Cu(OAc)2 and molecular oxygen in the presence of NaOAc (Figure 27).283 X-ray diffraction analysis revealed that

Scheme 45. Cleavages of Hexaphyrins (Ar = Pentafluorophenyl)

Figure 27. Cleavage of octaphyrin and heptaphyrin into helix molecules (Ar = pentafluorophenyl).

273 exhibited a helix structure winding around the two copper ions with a constant helix pitch of ∼4.0 Å, while the structure of 274 was an octapyrrolic helix in which one of the mesopentafluorophenyl substituents was inserted between the two turns of the helical backbone. The magnetic-susceptibility measurement indicated larger antiferromagnetic interaction in 273 (J/kB = −6.75 K) than that in 274 (J/kB = −0.88 K), reflecting different Cu(II)−Cu(II) distances. These helical molecules exhibited NIR absorption bands as an evidence of full π conjugation.

4. MÖ BIUS AROMATIC AND ANTIAROMATIC EXPANDED PORPHYRINS The concept of Möbius aromaticity predicts the aromatic nature for [4n]annulenes lying on a singly twisted Möbius strip. This important concept was first proposed by Heilbronner in 1964284 and has been stimulating both theoretical and experimental researchers because of its complementary nature to the established Hückel’s aromaticity rule that predicts aromatic nature for [4n+2]π annulenes lying on a normal plane (Figure 28). In spite of several speculations on Möbius aromatic molecules as transition states, reactive intermediates, and conformational isomers, chemically stable and isolable Möbius aromatic molecules had been elusive for a long time. To synthesize the chemically stable Möbius aromatic molecules, two conflicting structural requirements, a twisted Möbius topology and overall cyclic conjugation of [4n]π-electron system, have to be implemented within a single molecule. The twisted nature of the Möbius topology necessitates the use of large annulenic systems to mitigate the distortion associated with molecular twists, but such large systems lead to substantial conformational

product consisted of two planar diindomethene and isoindolo[2,1-a]isoquinolin-5-one subunits. Heating of [26]hexaphyrin Au(III)−Ir(III) hybrid complex 205 in 1,2-dichlorobenzene caused intramolecular N-fusion reaction to give doubly N-fused hexaphyrin syn-isomer 268 in 58% yield along with anti-isomer 270 as a minor product.233 Oxidation of 268 with MnO2 in the presence of water caused cleavage in one of the N-fused pyrrole moieties, giving diketone derivative 269 in a moderate yield, while the minor product 270 was easily oxidized and rearranged under ambient conditions to form 271 bearing a six-membered lactam ring. 3.5.4. Other Reactions. Treatment of [26]hexaphyrin 6a with CuCl in pyridine in the presence of molecular oxygen furnished the di-Cu(II) complex of doubly N-confused [26]hexaphyrin 272 in a moderate yield, probably via double pyrrolic rearrangement (Scheme 46).281 Along this rearrangement, 2608


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Figure 29. Herges’s [16]annulenes.

the diatropic ring currents of these molecules were only modest, indicating that the aromatic characters were weak.331,332 More recently, a practical design for triply twisted Möbius [24]dehydroannulene was proposed on the basis of a binaphthalene precursor.333 These annulenes are essentially chiral molecules, suggesting a high potential for a number of applications in optoelectronics.

Figure 28. Hückel and Möbius topology.

flexibilities, which often make the fixation of a twisted conformation very difficult. In the past decade, the chemistry of Möbius aromatic molecules became a hot topic in the chemistry of π-conjugated molecules. The seminal work was reported on the first Möbius aromatic molecules by Herges and co-workers in 2003.285,286 Later in 2007, the group of Latos-Grażyński reported a temperaturedependent conformational switch between Hückel and Möbius topologies in di-p-benzi[28]hexaphyrins.287 Soon after, Osuka and co-workers reported the metal complexes of regular expanded porphyrins such as [28]hexaphyrin, [32]heptaphyrin, and [36]octaphyrin, all of which displayed distinct [4n]π aromatic characters with smoothly twisted Möbius topology.208 These findings encouraged further studies to explore Möbius aromatic systems. In this section, expanded porphyrins with distinct Möbius aromatic and antiaromatic characters will be discussed. The chemical shift difference (Δδ value) between the most shielded and the most deshielded protons at the periphery of a macrocycle has been used to evaluate the degree of aromatic and antiaromatic character. The nucleus-independent chemical shifts (NICS) value is also a valuable theoretical index to evaluate the aromaticity.47−49,52,288−297

4.2. Möbius Aromaticity of p-Benzihexaphyrin

In 2007, Latos-Grażyński and co-workers reported the possibility to employ an expanded porphyrin skeleton to implement Möbius aromatic nature.287 They prepared A,D-di-p-benzi[28]hexaphyrin 278 (here, A and D denote the location of the 1,4phenylene rings in the macrocyclic structure) by the acidcatalyzed condensation and subsequent oxidation (Figure 30).334

4.1. Background

In 1964, Heilbronner suggested that Möbius twisted [n]annulenes with n > 20 could be formed without serious strain.284 In 1966, Zimmerman proposed an alternative version of the Woodward−Hoffman rules based on the Hückel−Möbius aromaticity of transition states.298,299 Up to the present, a number of hypothetical Möbius annulenes have been investigated theoretically by Schleyer, Herges, Rzepa, Castro, Karney, and their co-workers.300−329 Although the details of such computational works are outside of this Review, it has been concluded that stabilization by Möbius aromaticity cannot overcome destabilization due to torsional strain or reduced overlap of neighboring π-orbitals in pure annulenes if the ring size is small (n < 20). In 2003, a milestone toward achieving real Möbius-twisted molecules was reported by Herges and co-workers, who employed a sophisticated synthetic strategy of combining a usual planar conjugated system (i.e., cyclooctatetraene) with a beltlike conjugated system (i.e., bianthraquinodimethane).285 Three kinds of [16]annulene molecules 275−277 were actually synthesized and fully characterized by 1H NMR, X-ray diffraction analysis, and UV/vis absorption (Figure 29).330 Compounds 275 and 276 had Möbius topology with C1 and C2 symmetries, respectively, whereas compound 277 displayed Hückel topology with Cs symmetry. Bond-length equalizations as confirmed by the HOMA index in combination with the theoretical calculations were indicative of π-orbital conjugation. However,

Figure 30. Conformational change in p-benzihexaphyrin (Ar = 2,4,6trimethylphenyl).

X-ray diffraction analysis revealed the figure-eight conformation for 278 with the two 1,4-phenylene rings located at the intersection. In the solid state, this was recognized as a Möbius twisted form (278-M) as the π surface lay a single-sided strap, while the 1,4-phenylene rings could easily rotate in solution. Due to the structural flexibility, temperature-dependent 1H NMR chemical shifts were observed, which arose from a conformational switch between Hückel (278-H) and Möbius topologies (278-M). Three-level topology changes in 278 were demonstrated by titration experiments with TFA or dichloroacetic acid without changing their oxidation states.335 p-Benzi[28]hexaphyrin adopted either figure-eight Hückel antiaromatic, Möbius-twisted aromatic,or diprotonated planar Hückel antiaromatic states 279. This switching was realized in three- or four-step cycles under thermodynamic and kinetic controls. A,C-di-p-Benzi[24]pentaphyrin 280a,b, N-fused A-p-benzi[24]pentaphyrin 281b, and A,C-di-p-benzi[28]hexaphyrin 282a,b were obtained in a similar acid-catalyzed condensation (Figure 31).336 Among them, 281b showed a temperaturedependent Möbius−Hückel topology switch similarly to 278. On 2609


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[28]hexaphyrin Ni(II) complex 151 and Pt(II) complex 153 also displayed distinct Möbius aromaticity as indicated by the Δδ values of 5.90 ppm for 151 and 6.37 ppm for 153. As suggested by Heilbronner, large annulenes should be more suitable as a platform of Möbius aromatic molecule, because they can mitigate structural constraints associated with a molecular twist. To test this possibility, Pd(II) metalation of decaphyrin 10a was examined with Pd2(dba)3 in the presence of sodium acetate, and Pd(II) complex 283 was obtained in 55% yield (Figure 33).339 This complex possessed the Pd(II) ion bound to

Figure 31. p-Benzipentaphyrins and p-benzihexaphyrins.

the other hand, 280a,b and 282a,b exhibited antiaromatic characters in accordance with their Hückel topology. Protonation of 282a with TFA produced its dication, which exhibited a diatropic ring-current effect and hence a Möbius aromaticity. 4.3. Möbius Aromaticity of Regular Expanded Porphyrins

4.3.1. Metal Complexes. In 2008, Osuka and co-workers discovered the spontaneous formations of Möbius aromatic molecules upon Pd(II) metalation of meso-aryl-substituted expanded porphyrins. Representative examples were [28]hexaphyrin mono-Pd(II) complex 152, [32]heptaphyrin mono-Pd(II) complex 257b, and [36]octaphyrin di-Pd(II) complex 180.208 Figure 32 shows the X-ray structures and

Figure 33. Decaphyrin Pd(II) complexes (Ar = pentafluorophenyl).

the NNNC cavity and was thus elucidated to be a topologically untwisted Hückel aromatic species with a 46π electron circuit. The complex 283 was oxidized with DDQ to provide the corresponding 44π species 284, which was assigned as a Hückel antiaromatic molecule on the basis of its figure-eight structure with Hückel topology and the observed paratropic ring current. [44]Decaphyrin 284 was found to isomerize to the corresponding Möbius aromatic species 285 just on standing in solution. Fortunately, preferential precipitation allowed the isolation of 285 in a pure form. Complex 285 was revealed to be a Möbius aromatic molecule on the basis of its twisted structure and diatropic ring current. Importantly, the largest dihedral angles were only 23−24° in 285, allowing relatively smooth conjugation. This favorable structural feature may come from its pseudotriangular shape, whereby the three 180° contorted dipyrromethene units are located at the corners to accommodate 180° topological inversion. It is interesting to note that 284 and 285 represented the largest Hückel antiaromatic and Möbius aromatic molecules, respectively, at the time of its publication. In the opposite sense, an important question is how small of a ring can realize Möbius aromatic molecules. In this context, the tetrapyrrolic porphyrin skeleton has been demonstrated to be not large enough to implement a Möbius twisted topology within a molecule, because the reported [16]porphyrins and [20]porphyrins were largely distorted but not twisted and thus were nonaromatic or antiaromatic.340−349 N-Fused [22]pentaphyrins 5a,b′ were larger than porphyrins and might be a potential platform to realize a Möbius twisted topology.350 N-Fused [22]pentaphyrins 5a,b′ and N-fused [24]pentaphyrin 286a,b were interconvertible through two-electron redox reactions (Scheme 47). Rh(I) metalation of 286a provided Rh(I) Nfused [24]pentaphyrin 287, which exhibited distinct aromaticity as judged from their 1H NMR chemical shift differences (Δδ =

Figure 32. [28]Hexaphyrin Pd(II) complex, [32]heptaphyrin Pd(II) complex, and [36]octaphyrin Pd(II) complex.

graphical representations of π-electron phases to visualize the singly twisted molecular topologies. Importantly, the 1H NMR spectra of 152, 257b, and 180 exhibited signals due to the outer pyrrolic β-protons in the deshielded region and those due to the inner pyrrolic β-protons in the shielded region. The chemical shift differences (Δδ values) were large: 7.05 ppm for 152, 9.97 ppm for 257b, and 11.12 ppm for 180, indicating the large diatropic ring currents. In addition, these complexes displayed characteristic absorption features such as sharp Soret-like bands and distinct Q-like bands, characteristic of aromatic porphyrinoids. On the basis of these observations, complexes 152, 257b, and 180 were assigned as Möbius aromatic molecules. These Möbius aromatic molecules were quite stable under ambient conditions, and the 1H NMR chemical shifts were almost independent of temperature. It was also shown that these Möbius aromatic molecules exhibited large two-photon absorption crosssection values as compared with nonaromatic and antiaromatic counterparts.337,338 Other group 10 metal complexes such as 2610


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Scheme 47. N-Fused Pentaphyrin Rh(I) Complex

Oxidative demetalation of boron(III) complex 246 followed by the reduction with NaBH4 gave [28]hexaphyrin(2.1.1.0.1.1) 292, which was metalated with Pd(OAc)2 in the presence of t r i e t h y l a m i n e t o a ff o r d M ö b i u s a r o m a t i c [ 2 8 ] hexaphyrin(2.1.1.0.1.1) Pd(II) complex 293 in 36% yield (Scheme 48).353 This complex was a rare example of Möbius aromatic expanded porphyrins possessing the irregular alternate arrangement of pyrroles and methine carbons. Scheme 48. [28]Isohexaphyrin Pd(II) Complex (Ar = Pentafluorophenyl)

8.23 ppm). The structure of 287 was unambiguously determined to be singly twisted, thus allowing its assignment as a Möbius aromatic molecule. Interestingly, upon oxidation with DDQ, 287 underwent a pivot-like Rh(I) walk to give 288 quantitatively, presumably via preformed [22]pentaphyrin mono-Rh(I) complex with the same coordination mode as 287.113 Complex 288 has been assigned as a Hückel aromatic molecule on the basis of the 22π electronic system and pseudo-planar structure. Silicon(IV) ion was found to induce a smoothly twisted Möbius conformation when incorporated into [28]hexaphyrin skeleton. Treatment of [28]hexaphyrin 150a with CH3SiCl3 in the presence of N,N-diisopropylethylamine gave Si(IV) complex 289a and its N-fused product 290a, both of which have distinct Möbius aromatic characters as evinced by their 1H NMR spectra (Δδ = 8.42 ppm for 289a) (Figure 34).351,352 The coordinated Si

A,C-p-Benzi[28]hexaphyrin 282a was metalated with PdCl2 to give N-fused mono-Pd(II) complex 294 (Figure 35).336 In this

Figure 35. A,C-p-Benzihexaphyrin Pd(II) complex and N-confused [28]hexaphyrin Pd(II) complex (Ar = pentafluorophenyl).

complex, the palladium ion was coordinated to the two imine nitrogen atoms, the β-position of the inverted pyrrole, and one of the carbon atoms of the 1,4-phenylene ring. The Pd coordination fixed the twisted structure so that 294 acquired Möbius aromaticity. In a different case, Furuta and co-workers synthesized Pd(II) complex of singly N-confused hexaphyrin 295, which had a Möbius-strip-like twist with the central palladium ion in the NNNN-type coordinated fashion.354 However, this molecule did not exhibit a diatropic ring current (Δδ = 1.78 ppm), probably because of the interrupted annulenic conjugation at the N-confused pyrrole segment. 4.3.2. Fusion Reactions. Möbius aromatic molecules without metal coordination were highly desired to elucidate their intrinsic properties in both the ground state and the excited state without influence of a transition metal ion. In 2009, Osuka and co-workers found the facile and spontaneous formation of benzopyrane-fused [28]hexaphyrin 296 by simple heating of [26]hexaphyrin 6a in acetic acid (Figure 36).355 [28]Hexaphyrin 296 displayed a distinct diamagnetic ring-current effect with Δδ = 6.41 ppm at room temperature, which was largely retained over a wide temperature range from −100 to 100 °C. Compared with the metal complexes, the conjugation was relatively smoothly held with HOMA of 0.73 and with the largest dihedral angle of 36° in the solid state. The singlet (π−π*) excited-state lifetime of 296 was determined to be ∼40 ps by femtosecond transient absorption spectroscopy. This lifetime was shorter than that of parent [28]hexaphyrin 150a (183 ps), probably due to the conformational flexibility. In addition, the NIR fluorescence spectrum was observed at 1058 nm with a vibronic structure.

Figure 34. [28]Hexaphyrin Si(IV) complexes (Ar = pentafluorophenyl).

atom took a typical trigonal bipyramidal coordination to induce a Möbius twist. The Si-CH3 group offered a small influence on the electronic state by causing red-shift in the absorption spectrum due to substantial σ−π interaction. This was also the case for other substituents on the central silicon atom such as vinyl (289b, 290b), phenyl (289c), hydroxy (289d), and hydrogen (289e) groups. Reflecting the electron-donating characters, the absorption spectra were red-shifted in the order of 289d < 289e < 289a < 289b < 289c. Reaction of 150a with HSiCl3 in the presence of N,N-diisopropylethylamine provided other Si(IV) complex 291 along with 289e. Complex 291 was revealed to be a di-Si(IV) complex with two pyrrole groups hydrogenated. It is noteworthy that these are the first examples of Si(IV)incorporated expanded porphyrins. 2611


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As a unique example, treatment of [26]hexaphyrin 6a with triethylamine in the presence of BF3·OEt2 and O2 furnished a diastereomeric mixture of diethylamine-bearing [28]hexaphyrin 303 in 45% yield (Scheme 50).357 The diatropic ring current in Scheme 50. Synthesis of Diethylamine-Fused [28]Hexaphyrin (Ar = Pentafluorophenyl)

Figure 36. Benzopyrane-fused [28]hexaphyrins.

Oxidation of 296 with DDQ was attempted with an expectation to form Möbius antiaromatic species, but instead it gave a 4:1 mixture of Hückel aromatic [26]hexaphyrins 297 and 298. Therefore, this oxidation triggered a spontaneous Möbius-toHückel topology switch. In a different case, heating of 5,20-di(3-thienyl)-substituted hexaphyrin 81b in toluene led to the formation of two compounds 299 and 300 (Scheme 49).356 Hexaphyrin 299

303 was large as judged from the observed large chemical shift difference (Δδ = 12.79 and 12.82 ppm). The strong Möbius aromaticity may arise from the smooth conjugation network of their internally multiply bridged structures. A similar amine incorporation was reported with core-modified hexaphyrins.358 Reduction of 303 by NaBH4 resulted in C−N bond dissociation to give rectangular Hückel antiaromatic [28]hexaphyrin 304 in 25% yield. Very recently, the group of Chandrashekar reported two kinds of core-modified heptaphyrins possessing a planar dithienothiophene segment and thiophene or selenophene as parts of the macrocycles (Figure 37).359,360 Both thiophene-incorporated

Scheme 49. Thiophene-Fused [28]Hexaphyrins (Ar = Pentafluorophenyl)

Figure 37. Core-modified [32]heptaphyrins with a dithienothiophene segment.

(305 and 307) and selenophene-incorporated (306 and 308) heptaphyrins were regarded as a 32π conjugated system. Interestingly, [32]heptaphyrins 307 and 308 were characterized as the first examples of Möbius aromatic core-modified expanded porphyrins on the basis of moderate diatropic ring currents and twisted molecular structures. On the other hand, [32]heptaphyrins 305 and 306 were characterized as Hückel antiaromatic molecules on the basis of their paratopic ring currents. 4.3.3. Temperature and Solvent Control. At room temperature, the 1H NMR spectra of [28]hexaphyrin 150a−e indicated a D2h symmetry assignable to the rectangular shape with a non-negligible diatropic ring current in spite of its [4n]π electronic system. For some time, these spectra had been puzzling, because 150a−e should be strongly Hückel antiaromatic, given the conformationally rigid rectangular shapes. To explain this contradictory situation, the possibility of a dynamic

had a doubly spiro-annulated structure with the two sp3-carbons at the 5,20-positions, and its overall conjugation circuit was interrupted. The other product 300 was revealed to be a [28]hexaphyrin, in which one of the pyrroles was fused with the neighboring thiophene. This product existed as a 10:1 conformational mixture of Möbius aromatic and Hückel antiaromatic species in solution. To circumvent this situation, mono-(3thienyl)-substituted [26]hexaphyrin 301 was synthesized and heated in toluene, to give thienyl-fused [28]hexaphyrin 302 quantitatively. Hexaphyrin 302 was revealed to be a Möbius aromatic molecule with a distinct diatropic ring-current effect (Δδ = 11.4 ppm) in solution. 2612


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Hückel antiaromatic species 7a-H was dominant, while 7a showed Möbius aromatic characters (7a-M) in polar solvents such as THF. In this case, the aromaticity−antiaromaticity switch did not occur with temperature variation. In a similar case, mesotrifluoromethyl-substituted [28]hexaphyrin 31 exhibited a solvent- and temperature-dependent conformational change as probed by variable-temperature 1H NMR spectra in various solvents as well as UV/vis absorption profiles.363 4.3.4. Protonation and Deprotonation. Protonation was shown to be very effective to switch [4n]π expanded porphyrins from antiaromatic Hückel species to Möbius aromatic species. Protonation of [32]heptaphyrins 7a with TFA allowed the formation of several protonated species, one of which was revealed to be a Möbius aromatic monoprotonated [32]heptaphyrin, 309 (Figure 41).365 The 1H NMR spectrum of

equilibrium between Möbius aromatic conformer (150-M) and planar Hückel antiaromatic conformer (150-H) was considered (Figure 38). When the conformational dynamics are faster than

Figure 38. Equilibrium of [28]hexaphyrin. meso-Aryl substituents are omitted and only NH hydrogens are shown.

the 1H NMR time scale, the observed 1H NMR spectrum should record averaged chemical shifts. Actually, the low-temperature 1 H NMR analyses revealed predominant population of 150a-M, which was energetically lower (3.7 kcal/mol) than 150a-H. In addition, several meso-aryl-substituted [28]hexaphyrins 150b−e were examined carefully by using NMR and UV/vis absorption spectroscopic methods.361−363 Finally, the structures of both Hückel topology and Möbius topology were revealed by X-ray diffraction analyses of different crystals obtained from meso-2,6difluorophenyl-substituted [28]hexaphyrin 150c (Figure 39). These crystals were obtained, just depending upon the crystallization solvents.

Figure 41. Protonated [32]heptaphyrin and [36]octaphyrin (Ar = pentafluorophenyl).

309 exhibited a distinct diatropic ring-current effect (Δδ = 8.55 ppm) in CD2Cl2 at 223 K. A similar behavior was observed for [36]octaphyrin 8a.366 The structure of diprotonated [36]octaphyrin 310 was revealed by X-ray diffraction analysis, and the 1 H NMR titration by TFA or MSA supported the appearance of Möbius aromatic species. Protonation behaviors of [28]hexaphyrin 150a were finally elucidated in 2014 (Scheme 51).367 Titration of 150a with TFA Scheme 51. Protonation of [28]Hexaphyrin and Hexaphenyl [28]Hexaphyrin (Ar = Pentafluorophenyl)

Figure 39. X-ray crystal structures of meso-2,6-difluorophenylsubstituted [28]hexaphyrin 150c obtained from different solvent systems.

A solvent-polarity-dependent conformational change between Hückel and Möbius topologies was found for [32]heptaphyrin 7a (Figure 40).364 In nonpolar solvents such as toluene, the

led to the formation of a single Möbius aromatic species as indicated by a sharp and intense Soret-like band and distinct Qlike bands as well as a set of 1H NMR signals showing a diatropic ring current. Finally, the structure was revealed to be a monoprotonated Möbius-twisted form, 311. On the other hand, protonation with MSA, a stronger acid than TFA, effected diprotonation and caused a large structural change to diprotonated triangular [28]hexaphyrin 312, which displayed a

Figure 40. Solvent-polarity-dependent conformational change in [32]heptaphyrin. 2613


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blue-shift of the absorption band and a very broad absorption tail up to 1200 nm as evidence of Hückel antiaromatic character. To increase the contribution of the triangular conformer, 2,3,12,13,22,23-hexaphenyl [28]hexaphyrin 313 was synthesized and shown to form diprotonated triangular antiaromatic [28]hexaphyrin 314 readily upon protonation with MSA. This observation suggested the importance of charge repulsion in conformational preference of protonated expanded porphyrins. In another case, 2,3,17,18-tetraphenyl-[28]hexaphyrin 315, whose structure was identified as a figure-eight conformer in its freebase form, also showed a structural change to Möbius twisted form 316 upon protonation with TFA (Scheme 52).368,369

Scheme 53. [52]-, [50]-, and [48]Dodecaphyrins and the Xray Crystal Structure of 321

Scheme 52. Protonation of Tetraphenyl [28]Hexaphyrin and Its Pd(II) Complexes (Ar = Pentafluorophenyl)

Although one of two isomers of 316 was confirmed by X-ray diffraction analysis, Pd(II) metalation furnished two structural isomers 317 and 318, both of which were identified by X-ray diffraction analysis. Deprotonation was also shown to be an effective means to induce conformational change as well as enhancing aromatic character. Upon treatment with tetrabutylammonium fluoride, [32]heptaphyrin 7a underwent a conformational change to form Möbius aromatic species as indicated by the appearance of a sharp Soret-like band and distinct Q-like bands, as well as enhancement of two-photon absorption cross-section values.370−372 It was conceived that the removal of pyrrolic NH protons caused a disruption of intramolecular hydrogen bonding, hence driving a conformation change from a figure-eight to a twisted extended Möbius conformation. Giant expanded porphyrins tended to take on nonplanar coiled conformations owing to multiple intramolecular hydrogen-bonding interactions. Following this trend, [52]dodecaphyrin 68 exhibited a nonaromatic character due to its coiled conformation (Scheme 53). Oxidation of 68 with DDQ gave [50]dodecaphyrin 319, which was further oxidized with MnO2 to provide [48]dodecaphyrin 320 as a stable compound.373 These dodecaphyrins were fully characterized by 1H NMR, UV/vis, cyclic voltammetry, and X-ray diffraction analaysis. These characterizations concluded that [50]dodecaphyrin 319 was weakly aromatic in spite of its twisted structure and [48]dodecaphyrin 320 is nonaromatic rather than Möbius aromatic. Interestingly, protonation of 319 with MSA furnished tetraprotonated species 321, which structure was shown to be planar. The 1H NMR and UV/vis/NIR absorption spectra of 321 indicated its distinct aromatic character arising from its 50π electron circuit. The extended and planar structure of 321 was revealed by X-ray structural analysis. This molecule

represented the largest Hückel aromatic molecule at the time of its publication. 4.4. Möbius Antiaromaticity

The Möbius aromatic rule proposed by Heilbronner predicted not only [4n]π aromaticity but also [4n+2]π Mö b ius antiaromaticity for annulenes with a singly twisted topology. However, experimental observations of Möbius antiaromaticity were quite difficult, in that two unfavorable factors such as electronic destabilization due to antiaromaticity and structural constraint associated with conformational twist have to be implemented within a single molecule. Indeed, oxidation of [4n]π Möbius aromatic molecules such as 152 and 296 was attempted with a hope to obtain corresponding Möbius antiaromatic species but resulted in formations of nontwisted [4n+2]π Hückel aromatic species with concomitant topological changes. Latos-Grażyński and co-workers obtained insight into the existence of a paratropic ring current in vacataporphyrin Pd(II) complex 324 (Scheme 54).374 Vacataporphyrin is an azadeficient porphyrin (or butadieneporphyrin), acquiring a certain structural flexibility as compared with [18]porphyrin.375,376 Pd(II) complex 323 was at first synthesized by treatment of vacataporphyrin freebase 322 with Pd(PhCN)2Cl2 in the presence of triethylamine. Pd(II) complex 323 exhibited fairly 18π aromatic character. Photoirradiation of 323 followed by 2614


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5. EXPANDED PORPHYRIN ORGANIC RADICALS In recent years, stable organic radicals have attracted broad interest in both basic and applied fields.379,380 In general, organic radicals are highly reactive and therefore short-lived. Steric protection is a well-known method to make organic radicals longlived. Placement of heteroatoms such as nitrogen atom or sulfur atom adjacent to the radical center is often effective to stabilize organic radicals. Another promising strategy is to delocalize the spin density over large π-conjugated systems. In this sense, porphyrins and expanded porphyrins can stabilize radicals owing to their radical-delocalizing abilities. Although a variety of openshell molecules based on porphyrin, phthalocyanine, diazaporphyrin, corrole, and subporphyrin have been recently reported,381−388 here several open-shell species based on expanded porphyrins will be highlighted.

Scheme 54. Vacataporphyrin Pd(II) Complex (Ar = 4Methylphenyl)

protonation with HBF4 gave cationic Pd(II) complex 324, which exhibited a twisted conformation with Pd(II)-η2-alkene coordination. The 1H NMR spectrum of 324 displayed a pronounced paratropic ring current, indicating a contribution of Möbius antiaromaticity of this [4n+2]π electron system. In the course of studies of phosphorus insertion into expanded porphyrins, Osuka and co-workers found that monophosphorus insertion into a hexaphyrin macrocycle afforded [28]hexaphyrin monophosphorus(V) complex 325a,b and successive phosphorus insertion furnished [30]hexaphyrin diphosphorus(V) complex 326a,b (Figure 42).377,378 These complexes took

5.1. meso-Oxygenated Hexaphyrins

In 2008, Osuka and co-workers reported the synthesis of mesofree [26]hexaphyrin 106.164 In this synthesis, an oxygenated product, 107, was obtained as a side product, which turned out to be a stable radical on the basis of the silent 1H NMR spectrum, active ESR spectrum, magnetic susceptibility in the solid state, and broad and low energy NIR absorption band. Demetalation of di-Ni(II) complex 167 with MSA gave meso,meso-dioxygenated hexaphyrin 329 in 24% yield (Scheme 55).217 This compound Scheme 55. Synthesis of meso-Dioxygenated Hexaphyrin (Ar = Pentafluorophenyl)

was characterized as a non-Kekulé singlet biradicaloid molecule, because it exhibited the temperature-dependent ESR intensity and magnetic susceptibility that were analyzed by Bleaney− Bowers equation to give an estimation for the singlet−triplet energy gap of 2.56 kcal/mol.389 The calculated SOMO orbitals and spin-density distribution showed that two unpaired electrons were fully delocalized over each tripyrromethene unit. The singlet biradical character of 329 was estimated to be 62% on the basis of CASSCF(2,2) calculation. Biradicaloid 329 was reduced with hydrazine to give closed-shell diketohexaphyrin 330 in 44% yield, which displayed nonaromatic character due to the disrupted macrocyclic conjugation.

Figure 42. [28]Hexaphyrin and [32]heptaphyrin P(V) complexes.

similar twisted structures in which PO moieties were bound to the NNN cavity or NNC cavity. The 1H NMR spectra of 325a,b exhibited moderate diatropic ring currents due to the presence of 28π Möbius aromaticity, while 326a,b displayed paratropic ring currents as seen in the downfield-shifted signals due to the inner-β protons. These data clearly substantiated 30π Möbius antiaromaticity of 326a,b, as the first structurally characterized Möbius antiaromatic compounds. It was considered that a highly reduced [30]hexaphyrin system was stabilized owing to the presence of two electron-withdrawing phosphamide moieties, which also contributed to structural rigidification of the molecule. Phosphorus insertion to pentafluorophenyl-substituted [32]heptaphyrin 7a led to the formation of doubly twisted monophosphorus(V) complex 328a, which was assigned as a weakly Hückel aromatic molecule. In contrast, a similar phosphorus insertion to 2,6-dichlorophenyl-substituted [32]heptaphyrin 7b furnished Möbius antiaromatic phosphorus(V) complex 327 as a kinetically controlled product. This molecule underwent a thermal rearrangement to a more stable Hückel aromatic complex 328b quantitatively.

5.2. Hexaphyrin Palladium Complexes

Pd(II) complexes of hexaphyrins such as mono-Pd(II) complexes 152 and 162 and di-Pd(II) complex 163 were discussed in section 3. Treatment of [28]hexaphyrin 150a with PdCl2 in CH2Cl2/MeOH afforded a new di-Pd(II) complex 331 in 40% yield along with 152 (Scheme 56).390 The X-ray analysis of 331 revealed its twisted structure in which each Pd(II) ion was coordinated by the three nitrogen atoms and one μ-chloride ligand. The 1H NMR spectrum of 331 was found to be silent, and the ESR spectrum suggested its monoradical nature. The calculated spin density indicated that 331 consisted of two neutral Pd(II) centers and a nonplanar neutral hexaphyrin πradical, which can be regarded as a formally 27π species. Recently, Furuta and co-workers synthesized contracted doubly N-confused dioxo[26]hexaphyrin(1.1.1.1.1.0) 332 from 2615


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was conducted with Cu(OAc)2·H2O to give Zn−Cu−Zn heterometal complex 336 in 72% yield. The structure of 336 was almost the same as that of 335, but the central copper ion was bound to only two pyrroles, clearly indicating the linear bidentate coordination. Interestingly, the ESR spectrum of 336 in toluene at 20 K exhibited a relatively sharp signal with g = 2.0177 and no signals in the half-field region, indicating its organic radical nature. On the basis of these data, complex 336 was assigned to be a Zn(II)−Cu(I)−Zn(II) heterometal complex with oneelectron oxidized [45]decaphyrin ligand.392

Scheme 56. Hexaphyrin and Doubly N-Confused Hexaphyrin Di-Pd(II) Complexes (Ar = Pentafluorophenyl)

5.4. Modified Sapphyrins

Naruta and co-workers reported modified sapphyrin-like macrocycle 337 bearing two exocyclic double bonds at the mesopositions and a 1,10-phenanthroline moiety on its bent structure (Scheme 58).393−395 Interestingly, protonation of 337 resulted in the corresponding N-confused [26]hexaphyrin. Hexaphyrin 332 possessed unsymmetrical NNNO coordinating cavities. Pd(II) metalation of 332 gave di-Pd(II) complex 333 as a stable radical species under ambient conditions.391 The spin-density calculation indicated that the unpaired electron was delocalized over the whole π-framework of the planar hexaphyrin ligand with marginal spin densities on the palladium centers. 333 exhibited a remarkably narrow HOMO−LUMO energy gap of 0.47 eV.

Scheme 58. Protonation Behaviors of Modified Sapphyrin (Ar = 4-Methylphenyl)

5.3. Decaphyrin Zinc−Copper Heterometal Complex

Decaphyrin offered a more extended π-framework favorable for spin delocalization. [44]Decaphyrin 10a was interconvertible with [46]decaphyrin 334 via two-electron redox reactions (Scheme 57). Decaphyrin 334 exhibited a C2-symmetric

the formation of tricationic species 338. Biradicaloid character of 338 was proven by various spectroscopic methods as well as electrochemistry. This acid-triggered conversion was reversible, being applicable to a switching system between closed-shell molecules and biradicaloid species. Similar 1,10-phenanthrolineembedded porphyrin analogues have been reported as fluorescent Mg(II) ion sensors.394

Scheme 57. Decaphyrin Di-Zn(II) Complex and Zn(II)− Cu(I)−Zn(II) Complex (Ar = Pentafluorophenyl)

5.5. Core-Modified Pentaphyrins

Recently, Anand and co-workers achieved the synthesis and characterization of air- and water-stable neutral 25π pentathiophene macrocyclic radical 339 (Scheme 59).396 The structure of Scheme 59. Redox Behaviors of Core-Modified Pentaphyrin

339 was unambiguously revealed to be quite planar. The ESR spectrum of 339 displayed a signal with g = 2.0026, which was characteristic of organic radicals. Moreover, redox reactions of 339 were investigated to lead to the isolation of monoanion 340 and monocation 341, both of which were 1H NMR active and showed strong paratropic and diatropic ring-current effects,

crescent-like conformation similar to decaphyrin 10a. Zn(II) metalation of 334 was performed by refluxing in the presence of Zn(OAc)2 and sodium acetate to give di-Zn(II) complex 335 in 61% yield. Complex 335 displayed a twisted structure with the two Zn(II) ions bound to the NNNN cavity at both sides and the two amino-type pyrroles at the center. Cu(II) metalation of 335 2616


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porphyrins, and (iii) to change overall macrocyclic conformations. Despite these potentials, only a limited number of peripheral modifications were accomplished for expanded porphyrins, mainly due to their intrinsic high reactivities. This situation was in contrast to the rich chemistry of peripheral modifications of porphyrins.400−405 SNAr reaction at the para-position of the meso-pentafluorophenyl substituents was a convenient means to tune the properties of expanded porphyrins. Treatment of [26]hexaphyrin 6a with various nucleophiles such as alcohols, amines, and thiols in the presence of a base furnished 6-fold parasubstitution in good yields (Scheme 61).406−408 This method

respectively. Thus, an amphoteric switch to aromatic and antiaromatic states from a neutral radical was clearly demonstrated. Mixing of solutions of 340 and 341 led to comproportionation reaction to regenerate 339, in line with the high stability of the neutral radical. An analogous molecule, βdecaethylpentathiophene trication, was pioneered by Vogel et al. as early as 1996.397 5.6. Annulated Rosarins

Sessler, Kim, Lee, Fukuzumi, and co-workers reported intriguing redox reactivities of tris(β,β-phenylene)-bridged rosarin 342 upon protonation.398,399 The structure of annulated rosarin 342 was revealed to be planar, in line with its 24π antiaromatic nature in the neutral state. Nonplanar rosarin 345 was also synthesized for comparison. Protonation of 342 by HCl or HBr produced stable triprotonated monoradical dication 343, which was spontaneously, or by an addition of reductant, converted to a triprotonated monocationic [26]rosarin. Such a stepwise nature of the proton-coupled reduction was not seen for nonplanar 345. Furthermore, rosarin 342 was readily triprotonated with trifluoroacetic acid at low temperature to produce ground-state triplet diradical 344 (Scheme 60). The temperature dependence

Scheme 61. Nucleophilic Aromatic Substitution Reactions

Scheme 60. Protonation Behaviors of Annulated Rosarins

allowed the adjustment of solubility of [26]hexaphyrins toward various solvents including water. Using this method, Tomé and co-workers recently developed [28]hexaphyrin derivatives that served as an anion receptor in organic and aqueous media.408 Recently, peripherally double-strapped [26]hexaphyrin 349 and [28]hexaphyrin 350 were obtained by olefin metathesis of 4allyloxyl-2,3,5,6-tetrafluorophenyl-substituted [26]hexaphyrin 348 that was prepared by nucleophilic substitution reaction of A2B4-hexaphyrin 80a (Scheme 62).409 With the aid of these two peripheral straps, these hexaphyrins were forced to adopt fairly planar structures. Owing to this conformational restriction, [28]hexaphyrin 350 displayed distinct Hückel antiaromatic Scheme 62. Synthesis of Peripherally Strapped Hexaphyrins

of the singlet−triplet equilibrium was examined by means of EPR experiments in comparison with other rosarin derivative 346. These results demonstrated that the electronic state of antiaromatic expanded porphyrins can be readily switched by an external chemical stimulus.

6. APPLICATIONS OF EXPANDED PORPHYRINS 6.1. Reactivities

Peripheral substituents of expanded porphyrins played important roles: (i) to increase (or decrease) solubility in organic solvents, (ii) to influence the electronic properties of expanded 2617


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properties. As another means to modify the meso-substituents, iridium-catalyzed direct borylation reaction was applied to 2,6dichlorophenyl-substituted [28]hexaphyrins 351a and A3B3-type [28]hexaphyrin 351b to give [28]hexaphyrin 352a and 352b bearing 2,6-disubstituted 4-pinacolatoborylphenyl groups, respectively (Scheme 63).410 352a was further converted to 2,6dichloro-4-phenyl-substituted [26]hexaphyrin 353 by Suzuki− Miyaura coupling reaction with iodobenzene followed by oxidation.

produced a single regioisomer of Pd(II) complex 357 with clear Möbius aromatic character.412,413 Recently, Meyer and co-workers found intramolecular fusion between the meso-phenyl group and the neighboring pyrazole NH group in the oxidation of Siamese-twin porphyrin 120 with DDQ (Scheme 64). This reaction provided singly fused product Scheme 64. N-Fusion Reaction of Siamese-Twin Porphyrin

Scheme 63. Borylation of [28]Hexaphyrins

358 and doubly fused product 359 in a stepwise manner.173 The doubly folded Siamese-twin porphyrin 359 showed a highly twisted rectangular box-like structure. As an effective means to extend the π-conjugated network of porphyrins, the oxidative fusion reactions with aromatic segments have been successfully employed.414−417 However, such examples were rarely reported for expanded porphyrins, mainly because most of the expanded porphyrins were not tolerated under the harsh reaction conditions required for the oxidative fusion reactions. Di-Au(III) complex of [26]hexaphyrin 360 was designed for this purpose, in that 360 was certainly robust due to the incorporated two Au(III) ions and carried electron-donating and sterically encumbering dimesityloxyanthracene substituents that were suitable for fusion reaction. Oxidation of 360 with DDQ and Sc(OTf)3 in refluxing toluene gave doubly anthracene-fused hexaphyrin 361 in 24% yield (Scheme 65).418 Fused hexaphyrin 361 displayed a coplanar structure and a remarkably red-shifted absorption band at 1467 nm.

As seen in the case of pentaphyrin and heptaphyrin, N-fusion reactions between the ortho-position of the pentafluorophenyl substituent and the outer-pointing pyrrolic NH gave N-fused [28]hexaphyrins. Actually, doubly N-fused hexaphyrins 354a,b and 355 were isolated and confirmed to be anti- and syn-double N-fusion products, respectively (Figure 43).411 Singly N-fused [28]hexaphyrin 356 showed a rather complicated 1H NMR spectral feature, reflecting its conformational dynamics among various Hückel and Möbius forms, but Pd(II) metalation of 356

Scheme 65. Oxidative Fusion Reaction of AnthraceneAppended [26]Hexaphyrin Di-Au(III) Complex

Figure 43. N-Fused [28]hexaphyrins. 2618


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N-Fused [22]pentaphyrin 5a′ had a nonsymmetric structure. Suzuki and Neya isolated compound 362 in 15% yield in bromination of 5a′ with N-bromo succinimide (NBS) in the presence of pyridine (Scheme 66).419 The structure of 362 was

rigid Au(III) complex 158. Under refluxing in bromine in the presence of Fe powder for 3 days followed by oxidation with MnO2, β-octabromohexaphyrin di-Au(III) complex 368 was obtained in 94% yield.210 This perbromide 368 was further transformed to octaphenylated hexaphyrin 370a and octakis(phenylethynyl)-substituted hexaphyrin 370b by cross-coupling reactions. Mono-Pd(II) complex of [26]hexaphyrin 162 was prepared by oxidation of [28]hexaphyrin mono-Pd(II) complex 152 with tris(4-bromophenyl)aminium hexachloroantimonate.214 Complex 162 was halogenated to give monohalogenated [28]hexaphyrin Pd(II) complex 371 and 372 in good yields (Scheme 68).421 This reaction was considered to proceed with

Scheme 66. Bromination of N-Fused [22]Pentaphyrin and Subsequent Substitutions (Ar = Pentafluorophenyl)

Scheme 68. Peripheral Substitutions of Hexaphyrin MonoPd(II) Complexes (Ar = Pentafluorophenyl)

revealed to possess a unique consecutive pentacyclic structure brominated at the inner pyrrolic position. It seemed likely that the pyrrolic β-position next to the fused structure in 5a′ was the most reactive site and was brominated to trigger unprecedented skeletal rearrangements. Compound 362 was highly reactive toward nucleophiles and was transformed into the corresponding substitution products such as aminated (363a−c), hydrogenated (363d), and keto (364) derivatives. Halogenation of 6a with sulfuryl chloride gave trans-vicinaldichlorinated hexaphyrin 365, while the acid-assisted hydrohalogenation of 6a followed by oxidation with DDQ afforded a mixture of halogenated regioisomers (described as 366 and 367, Scheme 67).420 Perbromination was conducted with structurally

regioselective nucleophilic addition to the β-carbon atom adjacent to the Pd(II) ion, followed by N−Pd bond formation with concomitant topology change from a Hückel rectangular form to a Möbius twisted form. This mechanistic consideration was supported by a deuteration experiment. Reduction of 162 with sodium cyanoborodeuteride afforded selectively deuterated [28]hexaphyrin Pd(II) complex 152.214 With the brominated Möbius aromatic molecules 372 in hand, further peripheral fabrications were examined (Scheme 69). By Scheme 69. Peripheral Substitution of [28]Hexaphyrin Mono-Pd(II) Complexes (Ar = Pentafluorophenyl)

Scheme 67. Peripheral Substitutions of [26]Hexaphyrins (Ar = Pentafluorophenyl)

using a specific palladium catalyst, Stille coupling of 372 with ethynyltin reagents afforded various ethynylated hexaphyrins 373b−e in moderate to good yields.421 As an alternative modification, regioselective substitution reaction at the peripheral position of Möbius aromatic [28]hexaphyrin mono-Pd(II) complex 152 was serendipitously discovered.422 In the course of 2619


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the study on enantioselective synthesis of 152 in the presence of chiral BINAP ligands under aerobic conditions, different Pd complex 374 was isolated in 39% yield. The structure of 374 was revealed by X-ray diffraction analysis to possess a Möbius twisted form, in which the second palladium atom was bound at the pyrrolic β-position and the nitrogen atom. The rigorous regioselectivity may be ascribed to the least steric hindrance at this position and to the N−Pd(II) coordination that may serve to direct the following C−Pd bond formation. After a chloride substitution on the palladium(II) atom in 374, Stille coupling reaction was performed to give β-ethynylated products 375a,b in moderate yields. Thus, complementary substitution reactions at the 2- or 3-postions of Möbius aromatic [28]hexaphyrin Pd(II) complex 152 were developed. Mö bius aromatic [28]hexaphyrin phosphonium adducts 376a,b were obtained by addition reactions of triphenylphosphine or tricyclohexylphosphine to [26]hexaphyrin 6a (Figure 44).423 On the other hand, regioselective nucleophilic additions

Scheme 70. 1,3-Dipolar Cycloaddition Reaction of [36]Octaphyrin (Ar = Pentafluorophenyl)

36π conjugated circuit, were changed to sp3 carbons at the pyrrolidine-fused segments, while the overall figure-eight conformation was preserved. 6.2. Expanded Porphyrin Oligomers

Efforts have been devoted to synthesize covalently linked expanded porphyrin oligomers, because such compounds are thought to be promising in applications such as photonic materials, NIR dyes, and energy-transfer pigments. Nevertheless, oligomeric expanded porphyrins have been quite limited mainly due to synthetic difficulty. This is in sharp contrast to the chemistry of porphyrin oligomers that has been extensively studied in relation to artificial photosynthesis, oxygen-reducing catalysts, host−guest chemistry, and others.427−429 Diels−Alder reaction of [26]hexaphyrin 6a with o-xylylene in situ generated from benzosultine 381 followed by oxidation with DDQ afforded naphthalene-fused hexaphyrin 383 in 55% yield (Figure 45).430 The central pyrrolic β-positions at the long side

Figure 44. Phosphine adducts of [28]hexaphyrins (Ar = pentafluorophenyl).

to [26]hexaphyrin palladium(II) complex 162 afforded other phosphonium adducts 377a,b.424 Two possible resonance forms were considered, i.e., a phosphonium ylide form and a phosphorane form. The phosphonium ylide resonance structure allowed them to take a 28π Möbius aromatic electronic conjugation, while the phosphorane structure was unfavorable for the overall macrocyclic conjugation due to its crossconjugated electronic network. Interestingly, the contribution of the phosphorane form was prevailing in 376a,b, while the phosphonium ylide contribution and hence the Möbius aromatic character was more significant in 377a,b. A similar phosphine adduct, 378, was obtained upon addition of triphenylphosphine to gold(III) hexaphyrin 158 in the presence of TFA.425 The phosphine adduct 378 preserved Hückel antiaromatic nature. In the solid state, the phosphorane form was found to be a dominant species. 1,3-Dipolar cycloaddition of [36]octaphyrin 8a with an azomethine ylide in situ generated from N-methylglycine and paraformaldehyde provided mono- and dipyrrolidine-fused adducts 379 and 380, respectively (Scheme 70).426 This cycloaddition reaction proceeded regioselectively and stereoselectively as confirmed by the structures determined by X-ray analysis. The β−β double bonds, which were not involved in the

Figure 45. Cycloadducts of [26]hexaphyrins (Ar = pentafluorophenyl).

were the most reactive toward the diene, but the second addition took place at the β-position of the corner pyrrole. Using bisbenzosultine 382, anthracene-bridged hexaphyrin dimer 384 was synthesized in 20% yield. The structure of 384 was revealed to be a Z-shaped, double-decker structure with an interplanar distance of two hexaphyrins of 6.34 Å. 1,4-Phenylene-bridged hexaphyrin dimer 385 was prepared by the acid-catalyzed condensation reaction using a tetracarbinol of 1,4-phenylene-bridged bisdipyrromethane 108 with tetrapyrrane 44 (Figure 46).431 In the crystal structure of 385, two [26]hexaphyrin segments were connected through a 1,4phenylene bridge at the shorter side, but other structural isomers were observed in solution. Chandrashekar and co-workers synthesized meso−meso-linked oxasmaragdyrin dimers 386a−c by oxidative coupling reaction of the corresponding meso-free oxasmaragdyrin monomer using AgPF6.432 The dihedral angles between two oxasmaragdyrin 2620


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character. The antiaromatic character in the porphyrin− hexaphyrin hybrid was more pronounced in its di-Rh(I) complex 390.434 More recently, porphyrin−hexaphyrin−porphyrin hybrid 3911 was synthesized.435,436 Oxidative coupling of the triad 3911 with DDQ-Sc(OTf)3 afforded a further longer oligomer with n = 2, 3, and 4. Oxidative fusion reactions of Ni(II) porphyrin−[26]hexaphyrin hybrid 392 and Zn(II) porphyrin− [26]hexaphyrin hybrid 393 gave hybrid tapes 394 and 395, respectively, which displayed remarkably red-shifted absorption bands at 1657 and 1912 nm, respectively. BODIPY-appended hexaphyrins with variable numbers of phenylene spacers were prepared as a novel excitation energy donor−acceptor system (Figure 47).437,438 The two BODIPY

Figure 46. Phenylene-bridged [26]hexaphyrin dimer and oxasmaragdyrin dimer.

units were calculated to be 64−71°, suggesting the existence of a considerable degree of π-conjugation. Indeed, dimer 386a−c displayed red-shifted absorption bands as compared with the monomer. Recently, Osuka and co-workers have developed directly linked porphyrin−hexaphyrin dyad 387 by mixed acid-catalyzed condensation reaction followed by oxidation with DDQ (Scheme 71).433 Further oxidative fusion reaction of 387 Scheme 71. Porphyrin−Hexaphyrin Hybrids

Figure 47. BODIPY−hexaphyrin hybrids.

parts were attached at the short side of the rectangular [26]hexaphyrin segment in 396n, whereas the [28]hexaphyrin segment in the hybrid 397n took temperature-dependent conformational dynamics. These hybrids showed efficient singlet excitation energy transfer from the BODIPY segments to the hexaphyrin segments. Excitation energy transfer rates in the hybrids were controlled by the center-to-center distance, which was probed by detailed photophysical studies. 6.3. Chirality

The three-dimensional structure of expanded porphyrins, especially those with figure-eight or Möbius conformers, drove many researchers to consider their chirality, i.e., either P-twist or M-twist forms in analogy to the helical chirality. Vogel and coworkers shed light on such a possibility in their octaphyrin(2.1.0.1.2.1.0.1) 399 in 1999 (Figure 48).439,440 Octaphyrin 399 with a figure-eight structure was successfully separated on a chiral high-performance liquid chromatography (HPLC) column in a preparative scale. The circular dichroism (CD) spectra of the enantiomers of 399 showed mirror-imaged Cotton effects. The tetrahydro derivative 398 can be also separated at 15 °C, but it was racemized even at room temperature, indicating a lower inversion barrier than that of 399 owing to the more flexible structure. Therefore, metal complexation was performed to increase the structural rigidity. Octaphyrin di-Pd(II) complexes 400 and 401 and di-Cu(II) complex 402 were thus prepared and readily separated by HPLC. Among them, the absolute conformation of Pd(II) complex 401 was determined by X-ray diffraction analysis for the first time. Setsune and co-workers investigated the dynamic behaviors of their octaphyrins(1.0.1.0.1.0.1.0) with various peripheral substituents by 1H NMR, UV/vis, and CD spectra and confirmed

furnished triply linked porphyrin−hexaphyrin hybrid tape 388 in 72% yield. While 387 exhibited the absorption spectrum almost as a superposition of those of the porphyrin and hexaphyrin units, hybrid tape 388 displayed considerably redshifted absorption bands due to the effectively extended πframework. Hybrid tape 388 was reduced to the corresponding [28]hexaphyrin congener, which showed a weakly antiaromatic 2621


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The twisted structures of Möbius conformers were also enantiomerically separated first by Herges and co-workers and later by Osuka and co-workers.330,447,448 In the earlier study, the hetero[16]annulene 276 was separated on the chiral HPLC column, and the separated enantiomers showed mirror-imaged CD spectra (Figure 29). Expanded porphyrins 151, 152, and 153, which had robust Möbius-twisted structures due to the coordinated Ni(II), Pd(II), and Pt(II) ions, respectively, were all separated by chiral HPLC (Figure 11). The optical activity was verified by their strong Cotton effects in the CD spectra. In addition, the absolute configuration of 152 was confirmed by Xray diffraction analysis of an enantiomer for the first time. Enantioselective Pd(II) complexation on [26]hexaphyrin 6a was achieved in 23% enantiomeric excess by using BINAP-PdCl2 as a chiral Pd(II) source. [28]Hexaphyrin monophosphorus complex 324a and [30]hexaphyrin diphosphorus complex 325a were also found to be optically separated as a pair of examples for Möbius aromatic and Möbius antiaromatic species (Figure 42).448 As highlighted above, the chiroptical properties of expanded porphyrins have been interesting subjects for both experimental and theoretical studies.449−463 Combined with the detailed computational analysis, further elaboration to unveil the electronic properties of expanded porphyrins may find applications in due course. Very recently, Le Gac and co-workers synthesized hexaphyrin−cyclodextrin hybrid molecules by an acid-catalyzed condensation protocol using a cyclodextrin trialdehyde (Figure 50).464 In the hybrid, the hexaphyrin scaffold was triply capped

Figure 48. Figure-eight octaphyrin(2.1.0.1.2.1.0.1).

the fast chiral interconversion equilibrium at room temperature (Figure 49).441−446 In such a situation, the supramolecular

Figure 50. Hexaphyrin−cyclodextrin hybrids (Ar = pentafluorophenyl). Figure 49. Figure-eight octaphyrins.

by the cyclodextrin base connecting to the meso-substituents at the 5, 15, and 25-positions. Redox interconversion was possible between [26]hexaphyrin hybrid 406 and [28]hexaphyrin hybrid 407, the latter of which exhibited a distinct antiaromatic character due to the forced planar structure. The aromaticity switching behaviors were traced by chiroptical measurements. Conformational analysis revealed discrimination of the two coordination sites of hexaphyrins and fluxional rotational motion.

chirality induction was a useful means for the determination of the absolute configuration. For example, hexadecaethyl-substituted octaphyrin 61a was found to sense a variety of carboxylic acids at sub-mM concentrations at room temperature, showing induced CD spectral change. A different helicate molecule 403 was prepared in which a pair of electronically isolated bisdipyrrin chromophores cross over. Optical resolution of di-Cu(II) complex 404 and Co(II)−Cu(II) heterometal complex 405 was achieved. These complexes showed mirror-imaged CD spectra, which were assigned with the help of theoretical calculations. Furthermore, enantioselective formation of Cu(II) complexes was examined by using CuCl2·2H2O in the presence of (S)-(+)-mandelic acid sodium salt or (R)-(+)-1-(1-phenyl)ethylamine as chiral sources. As a result, 19% enantiomeric excess was observed for Cu(II) metalation for the first time, and the second Cu(II) metalation to give 404 scored 33% enantiomeric excess. Helical molecule 403 was also applied to the recognition of oligonucleotides in water.

6.4. Metal Ion Sensing

In earlier works, expanded porphyrins such as β-alkyl-type [22]pentaphyrin(1.1.1.1.1) and [24]hexaphyrin(1.0.1.0.0.0) were invented as actinide cation complexation agents.465−468 Later, Wong and co-workers reported the synthesis of meso-aryltype [26]hexaphyrin(1.1.1.1.1.0) 408 as a highly sensitive nearinfrared-fluorescent chemodosimeter selective for Hg2+ ions (Figure 51).469−471 The observed high sensitivity (enough at concentrations in the ppb range) was preserved even in the presence of other cations such as environmentally relevant alkali and alkaline earth metal ions. Ag(I) sensing by the parent 2622


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aromaticity, and conformation. Expanded porphyrins can stabilize a radical by virtue of the effective spin-delocalization abilities. Rich metalation chemistry of expanded porphyrins is also notable, allowing for preparation of versatile metal complexes with various electronic states. Some metalation triggered unprecedented irreversible chemical transformations. As often emphasized in the text, expanded porphyrins possess characteristic structural (conformational) and electronic flexibilities. These characteristics lead to their capabilities to realize Möbius aromatic and antiaromatic molecules with distinct diatropic and paratropic ring currents. Impressively, meso-arylsubstituted expanded porphyrins are so smart as to know what to do when they have 4nπ-conjugated electrons, that is, to spontaneously twist a π-conjuagted molecular plane to acquire the stabilization due to Möbius aromaticity. Many examples of Möbius aromatic and antiaromatic molecules reported herein are useful to understand the interplay between molecular twists and aromaticities. [50]Dodecaphyrin(1.1.0.1.1.0.1.1.0.1.1.0) has been shown to become the largest Hückel aromatic molecule in its tetraprotonated state. It is thus an interesting question how large of aromatic systems can be realized. meso-Aryl-substituted expanded porphyrins may be promising candidates to surpass the reported limit. Exploration of triply or more twisted conjugated molecules may be another target of meso-aryl expanded porphyrins. Conjugated macrocycles possessing dual electronic networks and stable polyradicals are also attractive targets. In spite of the extraordinary progress, the chemistry of expanded porphyrins is still in its infancy and more challenges will come. In particular, the chemistry of expanded porphyrins has been rather confined within basic fields. Therefore, it is desirable that expanded porphyrins will find applications in biochemical and materials sciences. For this purpose, more should be studied on water-soluble expanded porphyrins and the aggregations of expanded porphyrins in bulk and film states.

Figure 51. Mercury ion sensors.

[26]hexaphyrin 6a was also reported by Wong’s group.472 Shen, You, Rurack, and co-workers reported the Hg(II) ion sensing ability of their phenanthrene-fused core-modified rubyrin 409 bearing a soft coordination pocket. With the aid of polyurethane hydrogel, the sensing limit of ca. 1 ppm was achieved in aqueous media.473 Ikawa, Furuta, and co-workers synthesized Zn(II) complex of doubly N-confused hexaphyrin 411a as a Zn(II) ion sensor by Zn(II) metalation of doubly N-confused hexaphyrin 410a (Scheme 72).474,475 Water-soluble derivative 410b bearing two Scheme 72. Zinc Ion Sensing by Doubly-N-Confused Hexaphyrin

APPENDIX 1: SUMMARY OF CCDC NUMBERS APPENDIX 2: SYNTHETIC PROCEDURE FOR TRIPYRRANE 5-Pentafluorophenyldipyrromethane (2a)102

To prewarmed water (1478 mL) in a 2 L round-bottomed flask at 40 °C was added 24 mL of 35% aqueous HCl, pentafluorobenzaldehyde (19 mL, 0.15 mol), and pyrrole (31 mL, 0.45 mol). The reaction mixture was vigorously stirred at room temperature (CAUTION: keep the magnetic stirring rate at 1000 rpm; otherwise sticky precipitates make the stirring tip unmovable). After 40 min, the white precipitates were filtered off and washed with water. The precipitates were once dissolved in dichloromethane, dried over anhydrous Na2SO4, and then evaporated to dryness. The residue was passed through a silica gel column (C-300). Recrystallization from dichloromethane/nhexane afforded 2a (33.6 g, 72%) as off-white amorphous solids.

octa-arginine peptide arms was prepared that showed enhanced NIR fluorescence around 1050 nm upon uptake of Zn(II) ion in aqueous solutions. Since the fluorescence of Zn(II) complex 411b was strongly quenched in the presence of Cu2+, 411b was proposed to serve as a promising platform for a switch-off NIR fluorescent sensor for Cu2+ ion. More recently, the same group reported a similar hexaphyrin Zn(II) complex 412 bearing six pyridinium groups. The NIR fluorescence of 412 was quenched by subsequent addition of double-stranded DNA (dsDNA), presumably due to the further formation of a ternary complex.

7. CONCLUSION AND OUTLOOK There is no doubt that sizable progress has been made in the chemistry of expanded porphyrins, particularly since the discovery of the one-pot synthesis of a series of meso-arylsubstituted expanded porphyrins in 2001. Throughout the studies summarized in this Review, it is now apparent that various expanded porphyrins have been explored with regards to ring size, ring connectivity, coordinated metal, peripheral substituent,

1-Pentafluorobenzoyl-5-pentafluorophenyldipyrromethane (S1)105

A solution of EtMgBr (100 mL, 1.6 M in THF) was carefully added via syringe to a stirred solution of 2a (17.2 g, 55 mmol) in THF (100 mL) under Ar. The mixture was stirred at room temperature for 1 h and then cooled to −78 °C. To this solution was added a solution of (pentafluorobenzoyl)pyridyl thioester (16.8 g, 55 mmol) in THF (100 mL). The solution was 2623


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Table 1 compound

number of pyrroles

5a′ 5b′ 6a 7a 8a 9a 10a 16 19 24 30 31 33 34 39f 39l 41 45a 47d 50a 53 55 56a 58a 62b 63a 66 67 68 69 71 74c 74d 75c 76 78 80a 80c 81a 81b (Type-I) 81b (Type-II) 81c 81d 81e 82 83 84a 84d 86d 87a 89a 92 93 94 95 96a 96b 96c 97b 98c 99 100 101

5 5 6 7 8 9 10 8 6 6 5 6 10 12 6 6 6 8 5 5 6 7 4 8 12 16 6 9 12 15 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 14 14 6 6 6 8 6 6 8 8 8 6 8 6 6 6

metal(s)

Pd, Pd Pd, Pd Zn, Zn Cu, Cu Pd, Pd

Rh, Rh Rh, Rh 2624

CCDC number

ref

1476532 255770 212196 604429 1101537 622794 647365 185847 185845 205408, 205409 296241 296243 296244 296245 1103794 1103795 212655 853155 975004 165498 165499 173526 1402375 1402377 135356 159607 258963 258966 658457 658458 258965 931825 906720 931826 906721 987120 607198 606797 606795 606798 606799 1025770 1025771 1025772 270833 270834 622031 1423313 1423312 622032 638473 1405973 1045059 1405976 1405977 1045060 1045061 1045062 899973 899975 988116 988118 988117

113, 203 113 92, 93 95 56, 219 96 97 106 106 107 108 108 108 108 110 110 112 120 128 130 130 133 134 134 135 136 141 141 142 142 141 146 147 146 147 151 152 152 152 152 152 153 153 153 111 111 155 157 157 155 158 159 160 159 159 160 160 160 161 161 162 162 162 DOI: 10.1021/acs.chemrev.6b00371 Chem. Rev. 2017, 117, 2584−2640

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Table 1. continued compound

number of pyrroles

102 105 106 107 109 110 112a 114 117 118 120 124 125a 125b 126 127 128a 129b 130 131 132 133 137 138 140 141 142 143 144 145 146 147 149 150a (Hückel) 150a (Möbius) 150c (Hückel) 150c (Möbius) 151 152 153 155 156 158 160 161 162 163 164 165 166 167 168 169 170 171 172 173 175 177a 178 179 180 181

6 5 6 6 10 10 8 6 6 7 4 8 8 8 6 7 8 8 8 8 8 8 3 4 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 7 8 7 7 7 7 7 8 8 8 8 8

metal(s) Rh, Rh Rh, Rh

Pd, Pd

Cu, Cu Cu, Cu Cu, Cu Cu, Cu Cu, Cu Zn, Zn Cd, Cd Zn, Zn

Ni Pd Pt Hg, Hg Au Au, Au Rh, Rh Rh, Rh Pd Pd, Pd Zn Zn, Zn Cu, Cu Ni, Ni Zn Zn, Zn Cu Cu, Cu Cu, Zn Cu, Cu Cu Co Co Pd, Pd Pd, Pd 2625

CCDC number

ref

988119 864560 693211 693212 280277 280276 280278 958400 958401 958402 1424113 950924 607622 176189 213326 213327 1402378 834694, 840710 800736 800737 931043, 931044 931045 878906 878908 1020552 243024 243025 243026 243027 638475 653746 653747 622033 709963 976943 709964 709965 277404 277402, 767696 652606 653719 275593 275594 720405 720404 790146 928797 716609 716608 716607 764066 643985 716724 648936 727381 727383 727382 731675 603486 716725 1435698 652604 652603

162 163 164 164 165 165 165 166 166 166 173 175 179 177 178 178 134 190, 191 196 196 199 199 201 201 202 205 205 205 205 158 206 206 152 361 367 361 361 207 207, 447 208 206 209 209 212 212 214 215 216 216 216 217 218 219 218 220 220 220 220 222 219 208 208 DOI: 10.1021/acs.chemrev.6b00371 Chem. Rev. 2017, 117, 2584−2640

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Table 1. continued compound 182 183 184 185a 185b 186a 186b 187 188a 188b 191 192 193 194 195 196 197 198 201 202 204 205 206 208 209 211 213 215 217 221 222b 223a 224b 225 227 228 229 230 231 232 233 238c 239 241 244 246 248 250 252 254 255 257b 258b 259a 260 262 264 265 266 267 268 269 271

number of pyrroles

metal(s)

8 8 9 9 9 9 9 9 9 9 6 6 6 6 6 4 4 4 6 6 6 6 6 9 9 9 9 6 6 6 2 4 6 8 8 8 5 5 5 8 4 7 3 7 6 6 8 8 8 8 8 7 7 7 7 7 8 8 6 4a 6 5a 5a

Pd, Pd Ir Ni Ni Ni Ni Ni Ni Ni Zn, Zn Co, Co Rh, Rh Ir Ir Ni Ni, Ni Cu, Cu Au, Cu Au, Rh Au, Rh Au, Ir Pd, Ag Zn Cu Cu, Pd, Pd Ni, Rh B, B B, B B, B B B, B B, B, B P P, P P, P P P P Cu, Cu Cu B B Cu B, B B, B Pd, Pd Ni, Ni Ni, Ni Ni Ni, Ni Pd Pd Pd, Pd Zn, Pd Cu, Cu Ag, Ag Ag, Ag

Au, Ir Au, Ir Au, Ir 2626

CCDC number 716726 1006188 620155 1014020 1014025 1014022 1014026 1014021 1014023 1014027 658459 1054740 1054739 1054741 1054742 1020365 906533 792555 694128 694129 694130 927539 790147 620156 620157 620158 1014028 228378 228380 1020555 795975 795976 795977 747644 747645 747646 827555 827556 827554 234533 234534 604431 643987 763253 765595 765596 194720 194718 194719 836925 836924 652605 807128 807129 807130 264267 264302 264268 257493 257494 927538 927536 927537

ref 219 224 96 225 225 225 225 225 225 225 142 226 226 226 226 227 228 169 232 232 232 233 214 96 96 96 225 256 256 202 261 261 261 262 262 262 263 263 263 267 267 95 218 273 274 274 275 275 275 277 277 208 278 278 278 279 279 279 280 280 233 233 233 DOI: 10.1021/acs.chemrev.6b00371 Chem. Rev. 2017, 117, 2584−2640

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Table 1. continued compound 272 273 274 275 276 277 278 283 284 285 286a 286b 287 288 289a 289c 289d 290a 290b 291 293 295 296 299 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 323 325a 326a 327 328b 329 331 333 334 335 336 337 338 339 340 341 342 349 350

number of pyrroles

metal(s)

6 7 8 0 0 0 4 10 10 10 5 5 5 5 6 6 6 6 6 6 6 6 6 6 6 6 6 3 3 4 4 7 8 6 6 6 6 6 6 6 6 12 12 12 3 6 6 7 7 6 6 6 10 10 10 2 2 0 0 0 6 6 6

Cu, Cu Cu, Cu Cu, Cu

Pd Pd Pd

Rh Rh Si Si Si Si Si Si, Si Pd Pd

Pd Pd

Pd P P, P P P Pd, Pd Pd, Pd Zn, Zn Zn, Cu, Zn

2627

CCDC number 280247 751203 751202 621585 223927 618717 635756 1017503 1017504 1017505 147880 255771 255772 255773 993761 1059063 1059064 993762 1059062 1059065 811685 846699 722625 729195 729196 1038037 1038038 1013536 1013537 1431633 1431632 701508 753343 976944 976945 976947 976949 728273 728272 767702 767703 1048769 1048770 1048771 695082 760748 760749 899879 899878 764067 752335 1050934 1404015 1404016 1404017 758900 758901 1007164 1007166 1007165 889352 1447668 1447669

ref 281 283 283 330 330 330 287 339 339 339 350 113 113 113 351 352 352 351 352 352 353 354 355 356 356 357 357 359 359 360 360 365 366 367 367 367 367 368 368 369 369 373 373 373 374 377 377 378 378 217 390 391 392 392 392 393 393 396 396 396 398 409 409 DOI: 10.1021/acs.chemrev.6b00371 Chem. Rev. 2017, 117, 2584−2640

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Table 1. continued compound 354b 355 357 358 359 360 361 362 363d 364 365 369 370a 370b 371 374 376a 376b 377a 377b 378 379 380 384 385 389 392 395 398 399 400 401 402 404 408 409 410a 411a a

number of pyrroles

metal(s)

6 6 6 4 4 6 6 5 5 5 6 6 6 6 6 6 6 6 6 6 6 8 8 12 12 10 14 14 8 8 8 8 8 8 6 2 6 6

Pd

Au, Au Au, Au

Au, Au Au, Au Au, Au Pd Pd (Pd)

Au, Au

Ni, Rh, Rh Ni, Ni Zn, Zn

Pd, Pd Pd, Pd Cu, Cu Cu, Cu

Zn, Zn

CCDC number 242617 242615 792529 1424114 1424115 884116 884113 970858 970859 970860 289048 645989 645990 645991 903593 870061 277896 816375 816377 816376 1437331 610358 610359 248444 859719 858943 1036493 1036494 120832 120833 121628 120834−120836 120837 698457 295258 659352 204939 759874

ref 410 410 412 173 173 418 418 419 419 419 420 210 210 210 421 422 423 424 424 424 425 426 426 430 431 434 435 435 439 439 439 439 439 443 471 473 282 474

Several pyrrole segments have been rearranged.

maintained at −78 °C for 2 h, and then the cooling bath was removed. After the complete consumption of the starting material, the reaction mixture was quenched by the addition of aqueous NaHCO3. The organic phase was extracted with ethyl acetate, washed with water and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure. The resulting dark brown oil was purified by column chromatography with silica gel (C-300) using dichloromethane/n-hexane (v/v = 1/2) as an eluent, which afforded S1 (22.1 g, 79%) as off-white amorphous solids (Scheme 73).

Scheme 73. Synthesis of Tripyrrane

5,10-Bis(pentafluorophenyl)tripyrrane (3a)

S1 (8.1 g, 16 mmol) was dissolved in THF (50 mL) under nitrogen atmosphere. To the solution was added sodium borohydride (6.1 g, 161 mmol) and then MeOH (5 mL) carefully at 0 °C in the dark. After the complete consumption of the starting material, the reaction mixture was quenched by addition of aqueous NH4Cl solution. The organic phase was extracted with ethyl acetate, washed with water, and dried over anhydrous Na2SO4. The solvent was removed under reduced pressure to afford carbinol S2, which was used in the next step without further purification. To the solution of the crude S2 in

pyrrole (50 mL) was added trifluoroacetic acid (TFA) (1.2 mL, 15.7 mmol), and the resulting mixture was stirred for 1 h under nitrogen atmosphere at room temperature. After the mixture was quenched with triethylamine, the excess pyrrole was removed under reduced pressure. The resulting dark brown oil was purified by the column chromatography with silica gel (C-300) using dichloromethane/n-hexane (v/v = 1/2) as an eluent, which afforded 3a (7.8 g, 88%) as off-white amorphous solids. 2628


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H NMR (CDCl3, 600 MHz, 293 K): δ [ppm] = 8.11 (s, 2H, NH), 8.02-7.98 (s, 1H, NH), 6.72 (m, 2H, pyrrole-H), 6.14 (m, 2H, pyrrole-H), 5.99 (m, 2H, pyrrole-H), 5.92 (d, J = 2.7 Hz, 2H, pyrrole-H), and 5.82 (s, 2H, methine). 19F NMR (CDCl3) δ [ppm] = −141.64 (m, 4F ortho-C6F5), −155.27 (m, 2F paraC6F5), and −160.92 (m, 4F meta-C6F5). 1

(9) Broadhurst, M. J.; Grigg, R.; Johnson, A. W. Sulphur Extrusion Reactions Applied to the Synthesis of Corroles and Related Systems. J. Chem. Soc., Perkin Trans. 1 1972, 1124−1135. (10) Franck, B. Topical Problems in the Biosynthesis of Red Blood Pigment. Angew. Chem., Int. Ed. Engl. 1982, 21, 343−353. (11) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted and Isomeric Porphyrins; Pergamon: New York, 1997; pp 1−520. (12) Sessler, J. L.; Gebauer, A.; Weghorn, S. J. Expanded Porphyrins. In The Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 2, pp 55−124. (13) Day, V. W.; Marks, T. J.; Wachter, W. A. Large Metal IonCentered Template Reactions. A Uranyl Complex of Cyclopentakis(2iminoisoindoline). J. Am. Chem. Soc. 1975, 97, 4519−4527. (14) Berger, R. A.; LeGoff, E. The Synthesis of A 22π-Electron Tetrapyrrolic Macrocycle [1,3,1,3] Platyrin. Tetrahedron Lett. 1978, 19, 4225−4228. (15) Rexhausen, H.; Gossauer, A. The Synthesis of a New 22πElectron Macrocycle: Pentaphyrin. J. Chem. Soc., Chem. Commun. 1983, 275−275. (16) Gossauer, A. Bull. Soc. Chim. Belg. 1983, 92, 793−795. (17) LeGoff, E.; Weaver, O. G. Synthesis of a [1,5,1,5]Platyrin, a 26πElectron Tetrapyrrolic Annulene. J. Org. Chem. 1987, 52, 710−711. (18) König, H.; Eickmeier, C.; Möller, M.; Rodewald, U.; Franck, B. Synthesis of a Bisvinylogous Octaethylporphyrin. Angew. Chem., Int. Ed. Engl. 1990, 29, 1393−1395. (19) Sessler, J. L.; Cyr, M. J.; Lynch, V.; McGhee, E.; Ibers, J. A. Synthetic and Structural Studies of Sapphyrin, a 22-π-Electron Pentapyrrolic “Expanded Porphyrin. J. Am. Chem. Soc. 1990, 112, 2810−2813. (20) Charriere, R.; Jenny, T. A.; Rexhausen, H.; Gossauer, A. The Chemistry of Polyphyrins 2. Syntheses of Hexaphyrins and Their Metal Complexes. Heterocycles 1993, 36, 1561−1575. (21) Sessler, J. L.; Hemmi, G.; Mody, T. D.; Murai, T.; Burrell, A.; Young, S. W. Texaphyrins: Synthesis and Applications. Acc. Chem. Res. 1994, 27, 43−50. (22) Mody, T. D.; Sessler, J. L. Porphyrin- and Expanded PorphyrinBased Diagnostic and Therapeutic Agents. In Supramolecular Materials and Technologies; Wiley: Chichester, U.K., 1999; Vol. 4, pp 245−294. (23) Karlin, K. D.; Mody, T. D.; Fu, L.; Sessler, J. L. Texaphyrins: synthesis and development of a novel class of therapeutic agents. In Progress in Inorganic Chemistry; Wiley: Hoboken, NJ, 2001; Vol. 49, pp 551−598. (24) Mody, T. D.; Sessler, J. L. Texaphyrins: a new approach to drug development. J. Porphyrins Phthalocyanines 2001, 05, 134−142. (25) Magda, D. J.; Wang, Z.; Gerasimchuk, N.; Wei, W.; Anzenbacher, P., Jr.; Sessler, J. L. Synthesis of texaphyrin conjugates. Pure Appl. Chem. 2004, 76, 365−374. (26) Arambula, J. F.; Preihs, C.; Borthwick, D.; Magda, D.; Sessler, J. L. Texaphyrins: tumor localizing redox active expanded porphyrins. AntiCancer Agents Med. Chem. 2011, 11, 222−232. (27) Preihs, C.; Arambula, J. F.; Magda, D.; Jeong, H.; Yoo, D.; Cheon, J.; Siddik, Z. H.; Sessler, J. L. Recent Developments in Texaphyrin Chemistry and Drug Discovery. Inorg. Chem. 2013, 52, 12184−12192. (28) Preihs, C.; Magda, D. J.; Sessler, J. L. Texaphyrins and watersoluble zinc(II) ionophores: development, mechanism of anticancer activity, and synergistic effects. BioInorg. React. Mech. 2013, 9, 3−14. (29) Sessler, J. L.; Murai, T.; Lynch, V.; Cyr, M. An ″Expanded Porphyrin″: The Synthesis and Structure of a New Aromatic Pentadentate Ligand. J. Am. Chem. Soc. 1988, 110, 5586−5588. (30) Sessler, J. L.; Mody, T. D.; Hemmi, G. W.; Lynch, V. Synthesis and Structural Characterization of Lanthanide(III) Texaphyrins. Inorg. Chem. 1993, 32, 3175−3187. (31) Young, S. W.; Qing, F.; Harriman, A.; Sessler, J. L.; Dow, W. C.; Mody, T. D.; Hemmi, G. W.; Hao, Y.; Miller, R. A. Gadolinium(III) texaphyrin: A tumor selective radiation sensitizer that is detectable by MRI. Proc. Natl. Acad. Sci. U. S. A. 1996, 93, 6610−6615. (32) Sessler, J. L.; Tvermoes, N. A.; Guldi, D. M.; Mody, T. D.; Allen, W. E. One-Electron Reduction and Oxidation Studies of the Radiation

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Takayuki Tanaka was born in 1984 in Osaka, Japan. He received his B.Sc. (2007), M.Sc. (2009), and Ph.D. (2012) degrees from Kyoto University. He was selected as a JSPS Research Fellow for Young Scientists in 2009 and a JSPS Postdoctoral Fellow for Research Abroad in 2012. In 2013, he came back to Kyoto University, where he has been working as an Assistant Professor. Atsuhiro Osuka received a Ph.D. from Kyoto University in 1982. In 1979, he started his academic carrier at Ehime University as an Assistant Professor. In 1984, he came back to Kyoto University, where he has been a Professor since 1996. His research interests cover many aspects of synthetic approaches toward novel porphyrin-related compounds with intriguing structures, properties, and functions, which were recognized with the Chemical Society of Japan Award in 2010. Representative molecules explored in his laboratory include artificial photosynthetic reaction center models, meso−meso-linked porphyrin arrays, porphyrin tapes, expanded porphyrins, subporphyrins, and Möbius aromatic and antiaromatic molecules.

ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant no. 25220802. The chemistry of meso-aryl-substituted expanded porphyrins in our group has been developed by our talented students including Dr. J.-Y. Shin, Dr. S. Shimizu, Dr. M. Suzuki, Dr. S. Mori, Dr. Y. Tanaka, Dr. S. Saito, Dr. M. Inoue, Dr. T. Koide, Dr. T. Higashino, Dr. T. Yoneda, Dr. H. Mori, K. Naoda, S. Ishida, and T. Soya. The long collaboration with Prof. D. Kim of Yonsei University was highly appreciated. REFERENCES (1) Dolphin, D. The Porphyrins; Academic Press: New York, 1978− 1979; Vols. 1−8. (2) Smith, K. M. Porphyrins and Metalloporphyrins; Elsevier: Amsterdam, 1975. (3) Sessler, J. L.; Seidel, D. Synthetic Expanded Porphyrin Chemistry. Angew. Chem., Int. Ed. 2003, 42, 5134−5175. (4) Franck, B.; Nonn, A. Novel Porphyrinoids for Chemistry and Medicine by Biomimetic Syntheses. Angew. Chem., Int. Ed. Engl. 1995, 34, 1795−1811. (5) Jasat, A.; Dolphin, D. Expanded Porphyrins and Their Heterologs. Chem. Rev. 1997, 97, 2267−2340. (6) Sessler, J. L. Porphyrin analogues. J. Porphyrins Phthalocyanines 2000, 04, 331−336. (7) First reported by Woodward, R. B. In Aromaticity: An International Symposium, Sheffield, U.K., 1966; Special publication no. 21; The Chemical Society London: London, 1966. (8) Bauer, V. J.; Clive, D. L. J.; Dolphin, D.; Paine, J. B., III; Harris, F. L.; King, M. M.; Loder, J.; Wang, S.-W. C.; Woodward, R. B. Sapphyrins: Novel Aromatic Pentapyrrolic Macrocycles. J. Am. Chem. Soc. 1983, 105, 6429−6436. 2629


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Sensitizer Gadolinium(III) Texaphyrin (PCI-0120) and Other Water Soluble Metallotexaphyrins. J. Phys. Chem. A 1999, 103, 787−794. (33) Sessler, J. L.; Miller, R. A. New Drugs with Diverse Clinical Applications in Radiation and Photodynamic Therapy. Biochem. Pharmacol. 2000, 59, 733−739. (34) Hannah, S.; Lynch, V.; Guldi, D. M.; Gerasimchuk, N.; MacDonald, C. L. B.; Magda, D.; Sessler, J. L. Late First-Row Transition-Metal Complexes of Texaphyrin. J. Am. Chem. Soc. 2002, 124, 8416−8427. (35) Král, V.; Králová, J.; Kaplánek, R.; Bříza, T.; Martásek, P. Quo vadis porphyrin chemistry? Physiol. Res. 2006, 55, S3−S26. (36) Tian, J.; Ding, L.; Xu, H.-J.; Shen, Z.; Ju, H.; Jia, L.; Bao, L.; Yu, J.S. Cell-Specific and pH-Activatable Rubyrin-Loaded Nanoparticles for Highly Selective Near-Infrared Photodynamic Therapy against Cancer. J. Am. Chem. Soc. 2013, 135, 18850−18858. (37) Miller, R. A.; Woodburn, K.; Fan, Q.; Renschler, M. F.; Sessler, J. L.; Koutcher, J. A. In vivo animal studies with gadolinium (III) texaphyrin as a radiation enhancer. Int. J. Radiat. Oncol., Biol., Phys. 1999, 45, 981−989. (38) Magda, D.; Gerasimchuk, N.; Lecane, P.; Miller, R. A.; Biaglow, J. E.; Sessler, J. L. Motexafin gadolinium reacts with ascorbate to produce reactive oxygen species. Chem. Commun. 2002, 2730−2731. (39) Wei, W.-H.; Wang, Z.; Mizuno, T.; Cortez, C.; Fu, L.; Sirisawad, M.; Naumovski, L.; Magda, D.; Sessler, J. L. New polyethyleneglycolfunctionalized texaphyrins: synthesis and in vitro biological studies. Dalton Trans. 2006, 1934−1942. (40) Evans, J. P.; Xu, F.; Sirisawad, M.; Miller, R.; Naumovski, L.; de Montellano, P. R. O. Motexafin Gadolinium-Induced Cell Death Correlates with Heme oxygenase-1 Expression and Inhibition of P450 Reductase-Dependent Activities. Mol. Pharmacol. 2007, 71, 193−200. (41) Patel, H.; Mick, R.; Finlay, J.; Zhu, T. C.; Rickter, E.; Cengel, K. A.; Malkowicz, S. B.; Hahn, S. M.; Busch, T. M. Motexafin LutetiumPhotodynamic Therapy of Prostate Cancer: Short- and Long-Term Effects on Prostate-Specific Antigen. Clin. Cancer Res. 2008, 14, 4869− 4876. (42) Mehta, M. P.; Shapiro, W. R.; Phan, S. C.; Gervais, R.; Carrie, C.; Chabot, P.; Patchell, R. A.; Glantz, M. J.; Recht, L.; Langer, C.; et al. Motexafin gadolinium combined with prompt whole brain radiotherapy prolongs time to neurologic progression in non-small-cell lung cancer patients with brain metastases: results of a phase III trial. Int. J. Radiat. Oncol., Biol., Phys. 2009, 73, 1069−1076. (43) Zahedi Avval, F.; Berndt, C.; Pramanik, A.; Holmgren, A. Mechanism of inhibition of ribonucleotide reductase with motexafin gadolinium (MGd). Biochem. Biophys. Res. Commun. 2009, 379, 775− 779. (44) Arambula, J. F.; Sessler, J. L.; Siddik, Z. H. Overcoming biochemical pharmacologic mechanisms of platinum resistance with a texaphyrin-platinum conjugate. Bioorg. Med. Chem. Lett. 2011, 21, 1701−1705. (45) Arambula, J. F.; Sessler, J. L.; Siddik, Z. H. A texaphyrin-oxaliplatin conjugate that overcomes both pharmacologic and molecular mechanisms of cisplatin resistance in cancer cells. MedChemComm 2012, 3, 1275−1281. (46) Thiabaud, G.; Arambula, J. F.; Siddik, Z. H.; Sessler, J. L. Photoinduced Reduction of PtIV within an Anti-Proliferative PtIV− Texaphyrin Conjugate. Chem. - Eur. J. 2014, 20, 8813. (47) Stępień, M.; Sprutta, N.; Latos-Grażyński, L. Figure Eights, Mö bius Bands, and More: Conformation and Aromaticity of Porphyrinoids. Angew. Chem., Int. Ed. 2011, 50, 4288−4340. (48) Saito, S.; Osuka, A. Expanded Porphyrins: Intriguing Structures, Electronic Properties, and Reactivities. Angew. Chem., Int. Ed. 2011, 50, 4342−4373. (49) Osuka, A.; Saito, S. Expanded porphyrins and aromaticity. Chem. Commun. 2011, 47, 4330−4339. (50) Roznyatovskiy, V. V.; Lee, C.-H.; Sessler, J. L. π-Extended isomeric and expanded porphyrins. Chem. Soc. Rev. 2013, 42, 1921− 1933. (51) Lash, T. D. Metal Complexes of Carbaporphyrinoid Systems. Chem. - Asian J. 2014, 9, 682−705.

(52) Pawlicki, M.; Latos-Grażyński, L. Aromaticity Switching in Porphyrinoids. Chem. - Asian J. 2015, 10, 1438−1451. (53) Gosmann, M.; Franck, B. Synthesis of a Fourfold Enlarged Porphyrin with an Extremely Large, Diamagnetic Ring-Current Effect. Angew. Chem., Int. Ed. Engl. 1986, 25, 1100−1101. (54) Wessel, T.; Franck, B.; Möller, M.; Rodewald, U.; Läge, M. Porphyrins with Aromatic 26π-Electron Systems. Angew. Chem., Int. Ed. Engl. 1993, 32, 1148−1151. (55) Osuka, A.; Noya, G.; Taniguchi, S.; Okada, T.; Nishimura, Y.; Yamazaki, I.; Mataga, N. Energy-Gap Dependence of Photoinduced Charge Separation and Subsequent Charge Recombination in 1,4Phenylene-Bridged Zinc-Free-Base Hybrid Porphyrins. Chem. - Eur. J. 2000, 6, 33−46. (56) Shin, J.-Y.; Furuta, H.; Yoza, K.; Igarashi, S.; Osuka, A. meso-ArylSubstituted Expanded Porphyrins. J. Am. Chem. Soc. 2001, 123, 7190− 7191. (57) Osuka, A. Towards meso−meso-Linked Porphyrin Arrays and meso-Aryl Expanded Porphyrins. Chem. Rec. 2015, 15, 143−159. (58) Shimizu, S.; Osuka, A. meso-aryl expanded porphyrins: synthesis, structures, and coordination chemistry. J. Porphyrins Phthalocyanines 2004, 08, 175−181. (59) Vogel, E. Novel porphyrinoids. Pure Appl. Chem. 1990, 62, 557− 564. (60) Vogel, E. The porphyrins from the ‘annulene chemist’s’ perspective. Pure Appl. Chem. 1993, 65, 143−152. (61) Vogel, E. Porphyrinoid macrocycles: A cornucopia of novel chromophores. Pure Appl. Chem. 1996, 68, 1355−1360. (62) Vogel, E. Novel Porphyrinoid Macrocycles and their Metal Complexes. J. Heterocycl. Chem. 1996, 33, 1461−1487. (63) Sessler, J. L.; Gebauer, A.; Vogel, E. Porphyrin isomers. In The Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 2, pp 1−54. (64) Vogel, E. From Small Carbocyclic Rings to Porphyrins: A Personal Account of 50 Years of Research. Angew. Chem., Int. Ed. 2011, 50, 4278−4287. (65) Callaway, W. B.; Veauthier, J. M.; Sessler, J. L. Schiff-base porphyrin and expanded porphyrin analogs. J. Porphyrins Phthalocyanines 2004, 08, 1−25. (66) Love, J. B. A macrocyclic approach to transition metal and uranyl Pacman complexes. Chem. Commun. 2009, 3154−3165. (67) The first report of superphthalocyanine: Bloor, J. E.; Schlabitz, J.; Walden, C. C.; Demerdache, A. Organic Complexes of Uranium: PART I. The Synthesis and Spectrum of Uranyl Phthalocyanine. Can. J. Chem. 1964, 42, 2201−2208. (68) Recent examples of expanded phthalocyanines: Matsushita, O.; Derkacheva, V. M.; Muranaka, A.; Shimizu, S.; Uchiyama, M.; Luk’yanets, E. A.; Kobayashi, N. Rectangular-Shaped Expanded Phthalocyanines with Two Central Metal Atoms. J. Am. Chem. Soc. 2012, 134, 3411−3418. (69) Furuyama, T.; Ogura, Y.; Yoza, K.; Kobayashi, N. Superazaporphyrins: Meso-Pentaazapentaphyrins and One of Their LowSymmetry Derivatives. Angew. Chem., Int. Ed. 2012, 51, 11110−11114. (70) Furuyama, T.; Sato, T.; Kobayashi, N. A Bottom-up Synthesis of Antiaromatic Expanded Phthalocyanines: Pentabenzotriazasmaragdyrins, i.e. Norcorroles of Superphthalocyanines. J. Am. Chem. Soc. 2015, 137, 13788−13791 and references therein.. (71) Furuta, H.; Maeda, H.; Osuka, A. Confusion, inversion, and creation−a new spring from porphyrin chemistry. Chem. Commun. 2002, 1795−1804. (72) Srinivasan, A.; Furuta, H. Confusion Approach to Porphyrinoid Chemistry. Acc. Chem. Res. 2005, 38, 10−20. (73) Toganoh, M.; Furuta, H. Blooming of confused porphyrinoids− fusion, expansion, contraction, and more confusion. Chem. Commun. 2012, 48, 937−954. (74) D’Souza, F. Expanded Porphyrins: More Confusion All the Time. Angew. Chem., Int. Ed. 2015, 54, 4713−4714. (75) Lash, T. D. Carbaporphyrinoids: Taking the Heterocycle Out of Nature’s [18]Annulene. Synlett 2000, 31, 279−295. 2630


Chemical Reviews

Review

(76) Stępień, M.; Latos-Grażyński, L. Benziporphyrins: Exploring Arene Chemistry in a Macrocyclic Environment. Acc. Chem. Res. 2005, 38, 88−98. (77) Lash, T. D. Recent Advances on the Synthesis and Chemistry of Carbaporphyrins and Related Porphyrinoid Systems. Eur. J. Org. Chem. 2007, 2007, 5461−5481. (78) Stępień, M.; Latos-Grażyński, L. Aromaticity and Tautomerism in Porphyrins and Porphyrinoids. In Topics in Heterocyclic Chemistry: Springer: 2009; Vol. 19, pp 83−153. (79) Lash, T. D. Carbaporphyrins, porphyrin isomers and the legacy of Emanuel Vogel. J. Porphyrins Phthalocyanines 2012, 16, 423−433. (80) Lash, T. D. Benziporphyrins, a unique platform for exploring the aromatic characteristics of porphyrinoid systems. Org. Biomol. Chem. 2015, 13, 7846−7878. (81) Lash, T. D. Out of the Blue! Azuliporphyrins and Related Carbaporphyrinoid Systems. Acc. Chem. Res. 2016, 49, 471−482. (82) Lash, T. D. Synthesis of Novel Porphyrinoid Chromophores. In The Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 2, pp 125−200. (83) Lash, T. D. Carbaporphyrins and Related Systems. Synthesis, Characterization, Reactivity and Insights into Porphyrinoid Aromaticity. In Handbook of Porphyrin Science; World Scientific: Singapore, 2012; Vol. 16, pp 1−330. (84) Pushpan, S. K.; Venkatraman, S.; Anand, V. G.; Sankar, J.; Rath, H.; Chandrashekar, T. K. Inverted porphyrins and expanded porphyrins: An overview. Proc. - Indian Acad. Sci., Chem. Sci. 2002, 114, 311−338. (85) Chandrashekar, T. K.; Venkatraman, S. Core-Modified Expanded Porphyrins: New Generation Organic Materials. Acc. Chem. Res. 2003, 36, 676−691. (86) Misra, R.; Chandrashekar, T. K. Structural Diversity in Expanded Porphyrins. Acc. Chem. Res. 2008, 41, 265−279. (87) Matano, Y.; Imahori, H. Phosphole-Containing Calixpyrroles, Calixphyrins, and Porphyrins: Synthesis and Coordination Chemistry. Acc. Chem. Res. 2009, 42, 1193−1204. (88) Arnold, L.; Müllen, K. Modifying the porphyrin core−a chemist’s jigsaw. J. Porphyrins Phthalocyanines 2011, 15, 757−779. (89) Brückner, C.; Sternberg, E. D.; Boyle, R. W.; Dolphin, D. 5,10Diphenyltripyrrane, a useful building block for the synthesis of mesophenyl substituted expanded macrocycles. Chem. Commun. 1997, 1689−1690. (90) Chmielewski, P. J.; Latos-Grażyński, L.; Rachlewicz, K. 5,10,15,20-Tetraphenylsapphyrin-Identification of a Pentapyrrolic Expanded Porphyrin in the Rothemund Synthesis. Chem. - Eur. J. 1995, 1, 68−73. (91) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M. Rothemund and Adler-Longo Reactions Revisited: Synthesis of Tetraphenylporphyrins under Equilibrium Conditions. J. Org. Chem. 1987, 52, 827−836. (92) Neves, M. G. P. M. S.; Martins, R. M.; Tomé, A. C.; Silvestre, A. J. D.; Silva, A. M. S.; Félix, V.; Drew, M. G. B.; Cavaleiro, J. A. S. mesoSubstituted expanded porphyrins: new and stable hexaphyrins. Chem. Commun. 1999, 385−386. (93) Simkhovich, L.; Goldberg, I.; Gross, Z. Easy Preparation of Cobalt Corrole and Hexaphyrin and Isolation of New Oligopyrroles in the Solvent-Free Condensation of Pyrrole with Pentafluorobenzaldehyde. Org. Lett. 2003, 5, 1241−1244. (94) Shin, J.-Y.; Furuta, H.; Osuka. N-Fused Pentaphyrin. A. Angew. Chem., Int. Ed. 2001, 40, 619−621. (95) Saito, S.; Osuka, A. N-Fusion Reaction Sequence of Heptaphyrin(1.1.1.1.1.1.1): Singly, Doubly, and Quadruply N-Fused Heptaphyrins. Chem. - Eur. J. 2006, 12, 9095−9102. (96) Kamimura, Y.; Shimizu, S.; Osuka, A. [40]Nonaphyrin(1.1.1.1.1.1.1.1.1) and Its Heterometallic Complexes with Palladium− Carbon Bonds. Chem. - Eur. J. 2007, 13, 1620−1628. (97) Tanaka, Y.; Shin, J.-Y.; Osuka, A. Facile Synthesis of Large mesoPentafluorophenyl-Substituted Expanded Porphyrins. Eur. J. Org. Chem. 2008, 2008, 1341−1349.

(98) Krygowski, T. M.; Cyrański, M. Separation of the Energetic and Geometric Contributions to the Aromaticity of π-Electron carbocyclics. Tetrahedron 1996, 52, 1713−1722. (99) Krygowski, T. M.; Cyrański, M. Separation of the Energetic and Geometric Contributions to the Aromaticity. Part IV. A General Model for the π-Electron Systems. Tetrahedron 1996, 52, 10255−10264. (100) Maes, W.; Vanderhaeghen, J.; Dehaen, W. meso-Dichloropyrimidinyl substituted expanded porphyrins. Chem. Commun. 2005, 2612−2614. (101) Kang, S.; Hayashi, H.; Umeyama, T.; Matano, Y.; Tkachenko, N. V.; Lemmetyinen, H.; Imahori, H. meso-3,5-Bis(trifluoromethyl)phenylSubstituted Expanded Porphyrins: Synthesis, Characterization, and Optical, Electrochemical, and Photophysical Properties. Chem. - Asian J. 2008, 3, 2065−2074. (102) Rohand, T.; Dolusic, E.; Ngo, T. H.; Maes, W.; Dehaen, W. Efficient synthesis of aryldipyrromethanes in water and their application in the synthesis of corroles and dipyrromethenes. ARKIVOC 2007, 307−324. (103) Král, V.; Vašek, P.; Dolensky, B. Green Chemistry for Preparation of Oligopyrrole Macrocycles Precursors: Novel Methodology for Dipyrromethanes and Tripyrromethanes Synthesis in Water. Collect. Czech. Chem. Commun. 2004, 69, 1126−1136. (104) Taniguchi, R.; Shimizu, S.; Suzuki, M.; Shin, J.-Y.; Furuta, H.; Osuka, A. Ring size selective synthesis of meso-aryl expanded porphyrins. Tetrahedron Lett. 2003, 44, 2505−2507. (105) Rao, P. D.; Littler, B. J.; Geier, G. R., III; Lindsey, J. S. Efficient Synthesis of Monoacyl Dipyrromethanes and Their Use in the Preparation of Sterically Unhindered trans-Porphyrins. J. S. J. Org. Chem. 2000, 65, 1084−1092. (106) Shimizu, S.; Shin, J.-Y.; Furuta, H.; Ismael, R.; Osuka, A. Perfluorinated meso-Aryl-Substituted Expanded Porphyrins. Angew. Chem., Int. Ed. 2003, 42, 78−82. (107) Krivokapic, A.; Cowley, A. R.; Anderson, H. L. Contracted and Expanded meso-Alkynyl Porphyrinoids: from Triphyrin to Hexaphyrin. J. Org. Chem. 2003, 68, 1089−1096. (108) Shimizu, S.; Aratani, N.; Osuka, A. meso-TrifluoromethylSubstituted Expanded Porphyrins. Chem. - Eur. J. 2006, 12, 4909−4918. (109) Hiroto, S.; Shinokubo, H.; Osuka, A. Porphyrin Synthesis in Water Provides New Expanded Porphyrins with Direct Bipyrrole Linkages: Isolation and Characterization of Two Heptaphyrins. J. Am. Chem. Soc. 2006, 128, 6568−6569. (110) Suzuki, M.; Osuka, A. Improved Synthesis of meso-ArylSubstituted [26]Hexaphyrins. Org. Lett. 2003, 5, 3943−3946. (111) Suzuki, M.; Osuka, A. Reversible caterpillar-motion like isomerization in a N,N′-dimethyl hexaphyrin(1.1.1.1.1.1) induced by two-electron oxidation or reduction. Chem. Commun. 2005, 3685−3687. (112) Krivokapic, A.; Anderson, H. L. Synthesis and crystal structure of a meso-trialkynyl-[28]hexaphyrin. Org. Biomol. Chem. 2003, 1, 3639− 3641. (113) Mori, S.; Shin, J.-Y.; Shimizu, S.; Ishikawa, F.; Furuta, H.; Osuka, A. N-Fused Pentaphyrins and Their Rhodium Complexes: OxidationInduced Rhodium Rearrangement. Chem. - Eur. J. 2005, 11, 2417−2425. (114) Mori, H.; Aratani, N.; Osuka, A. Synthesis of A2B6-Type [36]Octaphyrins: Copper(II)-Metalation-Induced Fragmentation Reactions to Porphyrins and N-Fusion Reactions of meso-(3-Thienyl) Substituents. Chem. - Asian J. 2012, 7, 1340−1346. (115) Sessler, J. L.; Weghorn, S. J.; Hiseada, Y.; Lynch, V. Hexaalkyl Terpyrrole: A New Building Block for the Preparation of Expanded Porphyrins. Chem. - Eur. J. 1995, 1, 56−67. (116) Hannah, S.; Seidel, D.; Sessler, J. L.; Lynch, V. New chemistry of amethyrin. Inorg. Chim. Acta 2001, 317, 211−217. (117) Sessler, J. L.; Weghorn, S. J.; Morishima, T.; Rosingana, M.; Lynch, V.; Lee, V. Rosarin: A New, Easily Prepared Hexapyrrolic Expanded Porphyrin. J. Am. Chem. Soc. 1992, 114, 8306−8307. (118) Sessler, J. L.; Morishima, T.; Lynch, V. Rubyrin: A New Hexapyrrolic Expanded Porphyrin. Angew. Chem., Int. Ed. Engl. 1991, 30, 977−980. 2631


Chemical Reviews

Review

From Porphyrin Isomers to Octapyrrolic “Figure Eight” Macrocycles. Angew. Chem., Int. Ed. Engl. 1995, 34, 2511−2514. (140) Wytko, J. A.; Michels, M.; Zander, L.; Lex, J.; Schmickler, H.; Vogel, E. Competitive Formation of Helical Cycloocta- and Cyclododecapyrroles. J. Org. Chem. 2000, 65, 8709−8714. (141) Shimizu, S.; Taniguchi, R.; Osuka, A. meso-Aryl-Substituted [26]Hexaphyrin(1.1.0.1.1.0) and [38]Nonaphyrin(1.1.0.1.1.0.1.1.0) from Oxidative Coupling of a Tripyrrane. Angew. Chem., Int. Ed. 2005, 44, 2225−2229. (142) Shimizu, S.; Cho, W.-S.; Sessler, J. L.; Shinokubo, H.; Osuka, A. meso-Aryl Substituted Rubyrin and Its Higher Homologues: Structural Characterization and Chemical Properties. Chem. - Eur. J. 2008, 14, 2668−2678. (143) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Sapphyrin Supramolecules through C−H···S and C−H··· Se Hydrogen Bonds. First Structural Characterization of meso-Arylsapphyrins Bearing Heteroatoms. Angew. Chem., Int. Ed. 1998, 37, 3394− 3397. (144) Narayanan, S. J.; Sridevi, B.; Chandrashekar, T. K.; Vij, A.; Roy, R. Novel Core-Modified Expanded Porphyrins with meso-Aryl Substituents: Synthesis, Spectral and Structural Characterization. J. Am. Chem. Soc. 1999, 121, 9053−9068. (145) Xie, Y.-S.; Yamaguchi, K.; Toganoh, M.; Uno, H.; Suzuki, M.; Mori, S.; Saito, S.; Osuka, A.; Furuta, H. Triply N-Confused Hexaphyrins: Near-Infrared Luminescent Dyes with a Triangular Shape. Angew. Chem., Int. Ed. 2009, 48, 5496−5499. (146) Higashino, T.; Osuka, A. 2,3,17,18-Tetrahalohexaphyrins and the First Phlorin-type Hexaphyrins. Chem. - Asian J. 2013, 8, 1994− 2002. (147) Higashino, T.; Osuka, A. 2,3,17,18-Tetraethylsulfanyl [30]hexaphyrin(1.1.1.1.1.1) as the first aromatic isophlorin-type free-base. Chem. Sci. 2013, 4, 1087−1091. (148) Yamada, H.; Okujima, T.; Ono, N. Organic semiconductors based on small molecules with thermally or photochemically removable groups. Chem. Commun. 2008, 2957−2974. (149) Ito, S.; Murashima, T.; Ono, N. A new synthesis of pyrroles fused with polycyclic skeletons. J. Chem. Soc., Perkin Trans. 1 1997, 3161− 3166. (150) Ito, S.; Murashima, T.; Uno, H.; Ono, N. A new synthesis of benzoporphyrins using 4,7-dihydro-4,7-ethano-2H-isoindole as a synthon of isoindole. Chem. Commun. 1998, 1661−1662. (151) Ishida, S.-i.; Kuzuhara, D.; Yamada, H.; Osuka, A. Synthesis of opp-Dibenzohexaphyrins(1.1.1.1.1.1) by Retro-Diels-Alder Reaction. Asian J. Org. Chem. 2014, 3, 716−722. (152) Suzuki, M.; Osuka, A. Conformational Control of [26]Hexaphyrins(1.1.1.1.1.1) by meso-Thienyl Substituents. Chem. - Eur. J. 2007, 13, 196−202. (153) Mori, H.; Suzuki, M.; Kim, W.; Lim, J. M.; Kim, D.; Osuka, A. 5,20-Bis(α-oligothienyl)-substituted [26]hexaphyrins possessing electronic circuits strongly perturbed by meso-oligothienyl substituents. Chem. Sci. 2015, 6, 1696−1700. (154) A similar dynamic behavior was reported ealier as “conveyorbelt-like movement” in octa(p-tolyl)-tetrathia[36]octaphyrin(1.1.1.1.1.1.1.1). Sprutta, N.; Latos-Grażyński, L. Figure-Eight Tetrathiaoctaphyrin and Dihydrotetrathiaoctaphyrin. Chem. - Eur. J. 2001, 7, 5099−5112. (155) Suzuki, M.; Osuka, A. Cross-Bridging Reaction of 5,20Diethynyl Substituted Hexaphyrins to Vinylene-Bridged Hexaphyrins. J. Am. Chem. Soc. 2007, 129, 464−465. (156) Yoon, M.-C.; Cho, S.; Suzuki, M.; Osuka, A.; Kim, D. Aromatic versus Antiaromatic Effect on Photophysical Properties of Conformationally Locked trans-Vinylene-Bridged Hexaphyrins. J. Am. Chem. Soc. 2009, 131, 7360−7367. (157) Honjo, K.; Mori, H.; Osuka, A. An Ethyne-Bridged PorphyrinHexaphyrin-Porphyrin Triad That Undergoes a Thermal Transannular Cyclization. Asian J. Org. Chem. 2016, 5, 196−200. (158) Suzuki, M.; Osuka, A. Intramolecular [3 + 2] Annulation of 5Aryl-20-Ethynyl-Substituted [26]Hexaphyrin(1.1.1.1.1.1) Triggered by

(119) Sessler, J. L.; Weghorn, S. J.; Lynch, V.; Johnson, M. R. Turcasarin, the Largest Expanded Porphyrin to Date. Angew. Chem., Int. Ed. Engl. 1994, 33, 1509−1512. (120) Sessler, J. L.; Lisowski, J.; Boudreaux, K. A.; Lynch, V.; Barry, J.; Kodadek, T. J. Synthesis and Characterization of Diaryl Sapphyrins Prepared under Lindsey-Type Conditions. J. Org. Chem. 1995, 60, 5975−5978. (121) Sessler, J. L.; Davis, J. M. Sapphyrins: Versatile Anion Binding Agents. Acc. Chem. Res. 2001, 34, 989−997. (122) Sessler, J. L.; Camiolo, S.; Gale, P. A. Pyrrolic and polypyrrolic anion binding agents. Coord. Chem. Rev. 2003, 240, 17−55. (123) Geier, G. R., III; Ciringh, Y.; Li, F.; Haynes, D. M.; Lindsey, J. S. A Survey of Acid Catalysts for Use in Two-Step, One-Flask Syntheses of Meso-Substituted Porphyrinic Macrocycles. Org. Lett. 2000, 2, 1745− 1748. (124) Gross, Z.; Galili, N.; Simkhovich, L.; Saltsman, I.; Botoshansky, M.; Bläser, D.; Boese, R.; Goldberg, I. Solvent-Free Condensation of Pyrrole and Pentafluorobenzaldehyde: A Novel Synthetic Pathway to Corrole and Oligopyrromethenes. Org. Lett. 1999, 1, 599−602. (125) Simkhovich, L.; Rosenberg, S.; Gross, Z. Facile synthesis of a novel sapphyrin and its rhodium(I) complex. Tetrahedron Lett. 2001, 42, 4929−4931. (126) Ghosh, A. A Perspective of One-Pot Pyrrole−Aldehyde Condensations as Versatile Self-Assembly Processes. Angew. Chem., Int. Ed. 2004, 43, 1918−1931. (127) Yuan, M.; Ou, Z.; Fang, Y.; Huang, S.; Xue, Z.; Lu, G.; Kadish, K. M. Synthesis, Characterization, Protonation Reactions, and Electrochemistry of Substituted Open-Chain Pentapyrroles and Sapphyrins in Nonaqueous Media. Inorg. Chem. 2013, 52, 6664−6673. (128) Huang, W.; Ou, Z.; Chen, X.; Xue, Z.; Lu, G.; Wang, H.; Xiao, J.; Fang, Y.; Kadish, K. M. A facile synthetic route to meso-tetraaryl substituted N-5 sapphyrins and first single crystal X-ray analysis confirming the pyrrole inverted structure. J. Porphyrins Phthalocyanines 2015, 19, 794−802. (129) Narayanan, S. J.; Sridevi, B.; Srinivasan, A.; Chandrashekar, T.; Roy, R. One Step Synthesis of Sapphyrin and N-Confused Porphyrin using Dipyrromethane. Tetrahedron Lett. 1998, 39, 7389−7392. (130) Sessler, J. L.; Seidel, D.; Bucher, C.; Lynch, V. Novel terpyrrolecontaining aromatic expanded porphyrins. Tetrahedron 2001, 57, 3743− 3752. (131) Sessler, J. L.; Seidel, D.; Bucher, C.; Lynch, V. [26]Hexaphyrin(1.1.1.1.0.0): an all-aza isomer of rubyrin with an inverted pyrrole subunit. Chem. Commun. 2000, 1473−1474. (132) Bucher, C.; Seidel, D.; Lynch, V.; Král, V.; Sessler, J. L. Novel Synthesis of Hybrid Calixphyrin Macrocycles. Org. Lett. 2000, 2, 3103− 3106. (133) Bucher, C.; Seidel, D.; Lynch, V.; Sessler, J. L. [30]Heptaphyrin(1.1.1.1.1.0.0): an aromatic expanded porphyrin with a ‘figure eight’ like structure. Chem. Commun. 2002, 328−329. (134) Anguera, G.; Kauffmann, B.; Borrell, J. I.; Borrós, S.; SánchezGarcía, D. Quaterpyrroles as Building Blocks for the Synthesis of Expanded Porphyrins. Org. Lett. 2015, 17, 2194−2197. (135) Setsune, J.-i.; Katakami, Y.; Iizuna, N. [48]Dodecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0) and [64]Hexadecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0): The Largest Cyclopolypyrroles. J. Am. Chem. Soc. 1999, 121, 8957−8958. (136) Setsune, J.-i.; Maeda, S. Bis(azafulvene) as a Versatile Building Block for Giant Cyclopolypyrroles: X-ray Crystal Structure of [64]Hexadecaphyrin(1.0.1.0.1.0.1.0.1.0.1.0.1.0.1.0). J. Am. Chem. Soc. 2000, 122, 12405−12406. (137) Lash, T. D. Giant Porphyrinoids: From Figure Eights to Nanomolecular Cavities. Angew. Chem., Int. Ed. 2000, 39, 1763−1767. (138) Bröring, M.; Jendrny, J.; Zander, L.; Schmickler, H.; Lex, J.; Wu, Y.-D.; Nendel, M.; Chen, J.; Plattner, D. A.; Houk, K. N.; Vogel, E. Octaphyrin-(1.0.1.0.1.0.1.0). Angew. Chem., Int. Ed. Engl. 1995, 34, 2515−2517. (139) Vogel, E.; Bröring, M.; Fink, J.; Rosen, D.; Schmickler, H.; Lex, J.; Chan, K. W. K.; Wu, Y.-D.; Plattner, D. A.; Nendel, M.; Houk, K. N. 2632


Chemical Reviews

Review

Molecular Compression through a Dynamic Conformational Change. Angew. Chem., Int. Ed. 2007, 46, 5171−5174. (159) Naoda, K.; Mori, H.; Oh, J.; Park, K. H.; Kim, D.; Osuka, A. 5,20Di(pyridin-2-yl)-[28]hexaphyrin(1.1.1.1.1.1): A Stable Hückel Antiaromatic Hexaphyrin Stabilized by Intramolecular Hydrogen Bonding and Protonation-Induced Conformational Twist To Gain Möbius Aromaticity. J. Org. Chem. 2015, 80, 11726−11733. (160) Mori, H.; Osuka, A. Bismetal Complexes of 5,20-Bis(5-formyl-2pyrrolyl)-[26]hexaphyrin(1.1.1.1.1.1) Exhibiting Strong Near-Infrared Region Absorptions. Chem. - Eur. J. 2015, 21, 7007−7011. (161) Mori, H.; Sung, Y. M.; Lee, B. S.; Kim, D.; Osuka, A. Antiaromatic Hexaphyrins and Octaphyrins Stabilized by the HydrogenBonding Interactions of meso-Imidazolyl Groups. Angew. Chem., Int. Ed. 2012, 51, 12459−12463. (162) Naoda, K.; Sung, Y. M.; Lim, J. M.; Kim, D.; Osuka, A. CrossConjugated Hexaphyrins and Their Bis-Rhodium Complexes. Chem. Eur. J. 2014, 20, 7698−7705. (163) Yoneda, T.; Mori, H.; Sun Lee, B.; Yoon, M.-C.; Kim, D.; Osuka, A. A non-fused mono-meso-free pentaphyrin and its rhodium(I) complex. Chem. Commun. 2012, 48, 6785−6787. (164) Koide, T.; Kashiwazaki, G.; Suzuki, M.; Furukawa, K.; Yoon, M.C.; Cho, S.; Kim, D.; Osuka, A. A Stable Radical Species from Facile Oxygenation of meso-Free 5,10,20,25-Tetrakis(pentafluorophenyl)Substituted [26]Hexaphyrin(1.1.1.1.1.1). Angew. Chem., Int. Ed. 2008, 47, 9661−9665. (165) Anand, V. G.; Saito, S.; Shimizu, S.; Osuka, A. Internally 1,4Phenylene-Bridged meso Aryl-Substituted Expanded Porphyrins: The Decaphyrin and Octaphyrin Cases. Angew. Chem., Int. Ed. 2005, 44, 7244−7248. (166) Mori, H.; Lim, J. M.; Kim, D.; Osuka, A. Modulation of Dual Electronic Circuits of [26]Hexaphyrins Using Internal Aromatic Straps. Angew. Chem., Int. Ed. 2013, 52, 12997−13001. (167) Karthik, G.; Sneha, M.; Raja, V. P.; Lim, J. M.; Kim, D.; Srinivasan, A.; Chandrashekar, T. K. Core-Modified meso-Aryl Hexaphyrins with an Internal Thiophene Bridge: Structure, Aromaticity, and Photodynamics. Chem. - Eur. J. 2013, 19, 1886−1890. (168) The structure of Siamese-twin porphyrin was first conceived by Lind in 1987, but the synthesis was unsuccessful. Lind, E. J. Ph.D. Thesis, Michigan State University, East Lansing, MI, U.S.A., 1987. (169) Frensch, L. K.; Pröpper, K.; John, M.; Demeshko, S.; Brückner, C.; Meyer, F. Siamese-Twin Porphyrin: A Pyrazole-Based Expanded Porphyrin Providing a Bimetallic Cavity. Angew. Chem., Int. Ed. 2011, 50, 1420−1424. (170) Katsiaouni, S.; Dechert, S.; Brückner, C.; Meyer, F. A versatile building block for pyrazole−pyrrole hybrid macrocycles. Chem. Commun. 2007, 951−953. (171) Katsiaouni, S.; Dechert, S.; Briñas, R. P.; Brückner, C.; Meyer, F. Schiff Base Macrocycles Containing Pyrroles and Pyrazoles. Chem. - Eur. J. 2008, 14, 4823−4835. (172) Sachse, A.; Penkova, L.; Noël, G.; Dechert, S.; Varzatskii, O. A.; Fritsky, I. O.; Meyer, F. Efficient Synthesis of Some Versatile 3,5Bifunctional Pyrazole Building Blocks. Synthesis 2008, 2008, 800−806. (173) Vogel, A.; Dechert, S.; John, M.; Brückner, C.; Meyer, F. Siamese-Twin Porphyrin Origami: Oxidative Fusing and Folding. Chem. - Eur. J. 2016, 22, 2307−2316. (174) Ito, T.; Hayashi, Y.; Shimizu, S.; Shin, J.-Y.; Kobayashi, N.; Shinokubo, H. Gram-Scale Synthesis of Nickel(II) Norcorrole: The Smallest Antiaromatic Porphyrinoid. Angew. Chem., Int. Ed. 2012, 51, 8542−8545. (175) Kido, H.; Shin, J.-Y.; Shinokubo, H. Selective Synthesis of a [32]Octaphyrin(1.0.1.0.1.0.1.0) Bis(palladium) Complex by a MetalTemplated Strategy. Angew. Chem., Int. Ed. 2013, 52, 13727−13730. (176) Murakami, K.; Yamamoto, Y.; Yorimitsu, H.; Osuka, A. Demetalation of Metal Porphyrins via Magnesium Porphyrins by Reaction with Grignard Reagents. Chem. - Eur. J. 2013, 19, 9123−9126. (177) Seidel, D.; Lynch, V.; Sessler, J. L. Cyclo[8]pyrrole: A Simple-toMake Expanded Porphyrin with No Meso Bridges. Angew. Chem., Int. Ed. 2002, 41, 1422−1425.

(178) Köhler, T.; Seidel, D.; Lynch, V.; Arp, F. O.; Ou, Z.; Kadish, K. M.; Sessler, J. L. Formation and Properties of Cyclo[6]pyrrole and Cyclo[7]pyrrole. J. Am. Chem. Soc. 2003, 125, 6872−6873. (179) Bucher, C.; Devillers, C. H.; Moutet, J.-C.; Pécaut, J.; Sessler, J. L. Electrochemical synthesis of cyclo[8]pyrrole. Chem. Commun. 2006, 3891−3893. (180) Buda, M.; Iordache, A.; Bucher, C.; Moutet, J.-C.; Royal, G.; Saint-Aman, E.; Sessler, J. L. Electrochemical Syntheses of Cyclo[n]pyrrole. Chem. - Eur. J. 2010, 16, 6810−6819. (181) Eller, L. R.; Stępień, M.; Fowler, C. J.; Lee, J. T.; Sessler, J. L.; Moyer, B. A. Octamethyl-octaundecylcyclo[8]pyrrole: A Promising Sulfate Anion Extractant. J. Am. Chem. Soc. 2007, 129, 11020−11021. (182) Karnas, E.; Kim, S. K.; Johnson, K. A.; Sessler, J. L.; Ohkubo, K.; Fukuzumi, S. Stopped-Flow Kinetic Analysis of the Interaction of Cyclo[8]pyrrole with Anions. J. Am. Chem. Soc. 2010, 132, 16617− 16622. (183) Lee, C.-H.; Miyaji, H.; Yoon, D.-W.; Sessler, J. L. Strapped and other topographically nonplanar calixpyrrole analogues. Improved anion receptors. Chem. Commun. 2008, 24−34. (184) Köhler, T.; Ou, Z.; Lee, J. T.; Seidel, D.; Lynch, V.; Kadish, K. M.; Sessler, J. L. Reductive N Alkylation of Cyclo[8]pyrroles. Angew. Chem., Int. Ed. 2005, 44, 83−87. (185) Gorski, A.; Köhler, T.; Seidel, D.; Lee, J. T.; Orzanowska, G.; Sessler, J. L.; Waluk, J. Electronic Structure, Spectra, and Magnetic Circular Dichroism of Cyclohexa-, Cyclohepta-, and Cyclooctapyrrole. Chem. - Eur. J. 2005, 11, 4179−4184. (186) Melfi, P. J.; Kim, S. K.; Lee, J. T.; Bolze, F.; Seidel, D.; Lynch, V. M.; Veauthier, J. M.; Gaunt, A. J.; Neu, M. P.; Ou, Z.; Kadish, K. M.; Fukuzumi, S.; Ohkubo, K.; Sessler, J. L. Redox Behavior of Cyclo[6]pyrrole in the Formation of a Uranyl Complex. Inorg. Chem. 2007, 46, 5143−5145. (187) Stępień, M.; Donnio, B.; Sessler, J. L. Supramolecular Liquid Crystals Based on Cyclo[8]pyrrole. Angew. Chem., Int. Ed. 2007, 46, 1431−1435. (188) Sessler, J. L.; Karnas, E.; Kim, S. K.; Ou, Z.; Zhang, M.; Kadish, K. M.; Ohkubo, K.; Fukuzumi, S. Umpolung” Photoinduced Charge Separation in an Anion-bound Supramolecular Complex. J. Am. Chem. Soc. 2008, 130, 15256−15257. (189) Ohkubo, K.; Mase, K.; Karnas, E.; Sessler, J. L.; Fukuzumi, S. Cyclo[8]pyrrole: An Androgynous Expanded Porphyrin That Acts as Both an Electron Donor and Acceptor in Anion-Bound Supramolecular Electron Donor−Acceptor Complexes. J. Phys. Chem. C 2014, 118, 18436−18444. (190) Sarma, T.; Panda, P. K. Cyclo[4]naphthobipyrroles: Naphthobipyrrole-Derived Cyclo[8]pyrroles with Strong Near-Infrared Absorptions. Chem. - Eur. J. 2011, 17, 13987−13991. (191) Roznyatovskiy, V. V.; Lim, J. M.; Lynch, V. M.; Lee, B. S.; Kim, D.; Sessler, J. L. π-Extension in Expanded Porphyrins: Cyclo[4]naphthobipyrrole. Org. Lett. 2011, 13, 5620−5623. (192) Kee, S.-Y.; Lim, J. M.; Kim, S.-J.; Yoo, J.; Park, J.-S.; Sarma, T.; Lynch, V. M.; Panda, P. K.; Sessler, J. L.; Kim, D.; Lee, C.-H. Conformational and spectroscopic properties of π-extended, bipyrrolefused rubyrin and sapphyrin derivativesw. Chem. Commun. 2011, 47, 6813−6815. (193) Roznyatovskiy, V.; Lynch, V.; Sessler, J. L. Dinaphthoporphycenes. Org. Lett. 2010, 12, 4424−4427. (194) Roznyatovskiy, V. V.; Roznyatovskaya, N. V.; Weyrauch, H.; Pinkwart, K.; Tübke, J.; Sessler, J. L. Naphthobipyrrole: Versatile Synthesis and Electropolymerization. J. Org. Chem. 2010, 75, 8355− 8362. (195) Sarma, T.; Panda, P. K.; Anusha, P. T.; Rao, S. V. Dinaphthoporphycenes: Synthesis and Nonlinear Optical Studies. Org. Lett. 2011, 13, 188−191. (196) Okujima, T.; Jin, G.; Matsumoto, N.; Mack, J.; Mori, S.; Ohara, K.; Kuzuhara, D.; Ando, C.; Ono, N.; Yamada, H.; Uno, H.; Kobayashi, N. Cyclo[8]isoindoles: Ring-Expanded and Annelated Porphyrinoids. Angew. Chem., Int. Ed. 2011, 50, 5699−5703. 2633


Chemical Reviews

Review

fluorophenyl)-Substituted [26]Hexaphyrin(1.1.1.1.1.1). Inorg. Chem. 2009, 48, 4595−4597. (217) Koide, T.; Furukawa, K.; Shinokubo, H.; Shin, J.-Y.; Kim, K. S.; Kim, D.; Osuka, A. A Stable Non-Kekulé Singlet Biradicaloid from mesoFree 5,10,20,25-Tetrakis(Pentafluorophenyl)-Substituted [26]Hexaphyrin(1.1.1.1.1.1). J. Am. Chem. Soc. 2010, 132, 7246−7247. (218) Saito, S.; Kim, K. S.; Yoon, Z. S.; Kim, D.; Osuka, A. Extrusion of Boron(III) Subporphyrin from meso-Heptakis(pentafluorophenyl)[32]heptaphyrin upon Cooperative CuII and BIII Metalation. Angew. Chem., Int. Ed. 2007, 46, 5591−5593. (219) Tanaka, Y.; Shinokubo, H.; Yoshimura, Y.; Osuka, A. Metathesislike Splitting Reactions of Metallated [36]Octaphyrins(1.1.1.1.1.1.1.1): Experimental and Computational Investigations. Chem. - Eur. J. 2009, 15, 5674−5685. (220) Saito, S.; Furukawa, K.; Osuka, A. T-Shaped Three-Coordinate Copper(II) Heptaphyrin Complexes. Angew. Chem., Int. Ed. 2009, 48, 8086−8089. (221) Chmielewski, P. J. Lucky Seven: Characterization of Stable TShaped Copper(II) Complexes of [32]Heptaphyrins. Angew. Chem., Int. Ed. 2010, 49, 1359−1361. (222) Setsune, J.-i.; Mori, M.; Okawa, T.; Maeda, S.; Lintuluoto, J. M. Synthesis and structures of cobalt(II) complexes of mesotetraphenyloctaphyrin(1.0.1.0.1.0.1.0)s. J. Organomet. Chem. 2007, 692, 166−174. (223) Mori, M.; Setsune, J.-i. Regioselective Metallation of Octaphyrin(1.0.1.0.1.0.1.0) with Mixed Bipyrrole Units by Using CuII or CuI. Chem. Lett. 2007, 36, 244−245. (224) Naoda, K.; Osuka, A. Iridium Complexes of [26]Hexaphyrin(1.1.1.1.1.1) and [36]Octaphyrin(1.1.1.1.1.1.1.1). J. Porphyrins Phthalocyanines 2014, 18, 652−658. (225) Soya, T.; Naoda, K.; Osuka, A. NiII Metalations of [40]- and [42]Nonaphyrins(1.1.1.1.1.1.1.1.1): The Largest Doubly Twisted Hückel Antiaromatic Molecule. Chem. - Asian J. 2015, 10, 231−238. (226) Soya, T.; Osuka, A. Group 9 Metal Complexes of meso-ArylSubstituted Rubyrin. Chem. - Eur. J. 2015, 21, 10639−10644. (227) Blusch, L. K.; Mitevski, O.; Martin-Diaconescu, V.; Pröpper, K.; DeBeer, S.; Dechert, S.; Meyer, F. Selective Synthesis and Redox Sequence of a Heterobimetallic Nickel/Copper Complex of the Noninnocent Siamese-Twin Porphyrin. Inorg. Chem. 2014, 53, 7876− 7885. (228) Blusch, L. K.; Hemberger, Y.; Pröpper, K.; Dittrich, B.; Witterauf, F.; John, M.; Bringmann, G.; Brückner, C.; Meyer, F. Siamese-Twin Porphyrin: A Pyrazole-Based Expanded Porphyrin of Persistent Helical Conformation. Chem. - Eur. J. 2013, 19, 5868−5880. (229) Blusch, L. K.; Craigo, K. E.; Martin-Diaconescu, V.; McQuarters, A. B.; Bill, E.; Dechert, S.; DeBeer, S.; Lehnert, N.; Meyer, F. Hidden Non-Innocence in an Expanded Porphyrin: Electronic Structure of the Siamese-Twin Porphyrin’s Dicopper Complex in Different Oxidation States. J. Am. Chem. Soc. 2013, 135, 13892−13899. (230) Bley-Escrich, J.; Gisselbrecht, J.-P.; Vogel, E.; Gross, M. Electrochemical and Spectroelectrochemical Investigations of “FigureEight” Octapyrrolic Macrocycles, Free Bases, and Their Mononuclear, Homo- and Heterodinuclear Complexes with Divalent Transition Metals. Eur. J. Inorg. Chem. 2002, 2002, 2829−2837. (231) Bley-Escrich, J.; Gisselbrecht, J.-P.; Michels, M.; Zander, L.; Vogel, E.; Gross, M. Electrochemical and Spectroelectrochemical Investigations of Mono- and Binuclear Cobalt(II) Complexes of “Figure-Eight” Octapyrrolic Macrocycles. Eur. J. Inorg. Chem. 2004, 2004, 492−499. (232) Mori, S.; Osuka, A. Heterobismetal Complexes of [26]Hexaphyrin(1.1.1.1.1.1). Inorg. Chem. 2008, 47, 3937−3939. (233) Naoda, K.; Mori, H.; Osuka, A. GoldIII−IridiumIII Hybrid Complexes of Hexaphyrin(1.1.1.1.1.1). Chem. - Asian J. 2013, 8, 1395− 1398. (234) Yamamoto, Y.; Akiba, K.-y. The chemistry of Group 15 element porphyrins bearing element−carbon bonds: synthesis and properties. J. Organomet. Chem. 2000, 611, 200−209. (235) Brothers, P. J. Organoelement Chemistry of Main-Group Porphyrin Complexes. Adv. Organomet. Chem. 2001, 48, 289−342.

(197) Swain, D.; Anusha, P. T.; Sarma, T.; Panda, P. K.; Rao, S. V. Dispersion studies of optical nonlinearity and excited state dynamics in cyclo[4]naphthobipyrroles. Chem. Phys. Lett. 2013, 580, 73−77. (198) Kowalska, P.; Gawinkowski, S.; Sarma, T.; Panda, P. K.; Waluk, J. Structure, Electronic States, and Anion-Binding Properties of Cyclo[4]naphthobipyrroles. J. Phys. Chem. A 2014, 118, 1038−1046. (199) Okujima, T.; Ando, C.; Mack, J.; Mori, S.; Hisaki, I.; Nakae, T.; Yamada, H.; Ohara, K.; Kobayashi, N.; Uno, H. Acenaphthylene-Fused Cyclo[8]pyrroles with Intense Near-IR-Region Absorption Bands. Chem. - Eur. J. 2013, 19, 13970−13978. (200) Okujima, T.; Ando, C.; Mori, S.; Nakae, T.; Yamada, H.; Uno, H. Synthesis and Molecular Structure of Cyclo[8](9,10-dihydro-9,10anthraceno)pyrrole. Heterocycles 2014, 88, 417−424. (201) Zhang, Z.; Lim, J. M.; Ishida, M.; Roznyatovskiy, V. V.; Lynch, V. M.; Gong, H.-Y.; Yang, X.; Kim, D.; Sessler, J. L. Cyclo[m]pyridine[n]pyrroles: Hybrid Macrocycles That Display Expanded π-Conjugation upon Protonation. J. Am. Chem. Soc. 2012, 134, 4076−4079. (202) Zhang, Z.; Cha, W.-Y.; Williams, N. J.; Rush, E. L.; Ishida, M.; Lynch, V. M.; Kim, D.; Sessler, J. L. Cyclo[6]pyridine[6]pyrrole: A Dynamic, Twisted Macrocycle with No Meso Bridges. J. Am. Chem. Soc. 2014, 136, 7591−7594. (203) Shimizu, S.; Osuka, A. Metalation Chemistry of meso-ArylSubstituted Expanded Porphyrins. Eur. J. Inorg. Chem. 2006, 2006, 1319−1335. (204) Sessler, J. L.; Tomat, E. Transition-Metal Complexes of Expanded Porphyrins. Acc. Chem. Res. 2007, 40, 371−379. (205) Shimizu, S.; Anand, V. G.; Taniguchi, R.; Furukawa, K.; Kato, T.; Yokoyama, T.; Osuka, A. Biscopper Complexes of meso-ArylSubstituted Hexaphyrin: Gable Structures and Varying Antiferromagnetic Coupling. J. Am. Chem. Soc. 2004, 126, 12280−12281. (206) Mori, S.; Shimizu, S.; Shin, J.- Y.; Osuka, A. Group 12 Metal Complexes of [26]Hexaphyrin(1.1.1.1.1.1). Inorg. Chem. 2007, 46, 4374−4376. (207) Mori, S.; Shimizu, S.; Taniguchi, R.; Osuka, A. Group 10 Metal Complexes of meso-Aryl-Substituted [26]Hexaphyrins with a MetalCarbon Bond. Inorg. Chem. 2005, 44, 4127−4129. (208) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata, N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; et al. Metalation of Expanded Porphyrins: A Chemical Trigger Used To Produce Molecular Twisting and Möbius Aromaticity. Angew. Chem., Int. Ed. 2008, 47, 681−684. (209) Mori, S.; Osuka, A. Aromatic and Antiaromatic Gold(III) Hexaphyrins with Multiple Gold-Carbon Bonds. J. Am. Chem. Soc. 2005, 127, 8030−8031. (210) Mori, S.; Kim, K. S.; Yoon, Z. S.; Noh, S. B.; Kim, D.; Osuka, A. Peripheral Fabrications of a Bis-Gold(III) Complex of [26]Hexaphyrin(1.1.1.1.1.1) and Aromatic versus Antiaromatic Effect on Two-Photon Absorption Cross Section. J. Am. Chem. Soc. 2007, 129, 11344−11345. (211) Naoda, K.; Mori, H.; Osuka, A. Selective Mono- and Bis-Au(III) Metalations of 5,10,15,20,25,30-Hexaaryl-[26]hexaphyrins(1.1.1.1.1.1). Chem. Lett. 2013, 42, 22−24. (212) Rath, H.; Aratani, N.; Lim, J. M.; Lee, J. S.; Kim, D.; Shinokubo, H.; Osuka, A. Bis-rhodium hexaphyrins: metalation of [28]hexaphyrin and a smooth Hückel aromatic−antiaromatic interconversionw. Chem. Commun. 2009, 3762−3764. (213) Sung, Y. M.; Yoon, M.-C.; Lim, J. M.; Rath, H.; Naoda, K.; Osuka, A.; Kim, D. Reversal of Hückel (anti)aromaticity in the lowest triplet states of hexaphyrins and spectroscopic evidence for Baird’s rule. Nat. Chem. 2015, 7, 418−422. (214) Inoue, M.; Osuka, A. Redox-Induced Palladium Migrations that Allow Reversible Topological Changes between Palladium(II) Complexes of Möbius Aromatic [28]Hexaphyrin and Hückel Aromatic [26]Hexaphyrin. Angew. Chem., Int. Ed. 2010, 49, 9488−9491. (215) Yoneda, T.; Osuka, A. Synthesis of a [26]Hexaphyrin Bis-PdII Complex with a Characteristic Aromatic Circuit. Chem. - Eur. J. 2013, 19, 7314−7318. (216) Koide, T.; Kashiwazaki, G.; Furukawa, K.; Osuka, A. Homo- and Heterobismetal Complexes of 5-Hydroxy-10,15,25,30-tetrakis-(penta2634


Chemical Reviews

Review

(236) Brothers, P. J. Recent developments in the coordination chemistry of porphyrin complexes containing non-metallic and semimetallic elements. J. Porphyrins Phthalocyanines 2002, 06, 259−267. (237) Brothers, P. J. Boron complexes of porphyrins and related polypyrrole ligands: unexpected chemistry for both boron and the porphyrin. Chem. Commun. 2008, 2090−2102. (238) Brothers, P. J. Boron Complexes of Pyrrolyl Ligands. Inorg. Chem. 2011, 50, 12374−12386. (239) Pareek, Y.; Ravikanth, M.; Chandrashekar, T. K. Smaragdyrins: Emeralds of Expanded Porphyrin Family. Acc. Chem. Res. 2012, 45, 1801−1816. (240) Higashino, T.; Osuka, A. Boron and Phosphorus Complexes of meso-Aryl Expanded Porphyrins. Heterocycles 2013, 87, 31−54. (241) Geyer, M.; Plenzig, F.; Rauschnabel, J.; Hanack, M.; del Rey, B.; Sastre, A.; Torres, T. Subphthalocyanines: Preparation, Reactivity and Physical Properties. Synthesis 1996, 1996, 1139. (242) Claessens, C. G.; Blau, W. J.; Cook, M.; Hanack, M.; Nolte, R. J. M.; Torres, T.; Wöhrle, D. Phthalocyanines and Phthalocyanine Analogues: The Quest for Applicable Optical Properties. Monatsh. Chem. 2001, 132, 3−11. (243) Claessens, C. G.; González-Rodríguez, D.; Torres, T. Subphthalocyanines: Singular Nonplanar Aromatic CompoundsSynthesis, Reactivity, and Physical Properties. Chem. Rev. 2002, 102, 835− 854. (244) Torres, T. From Subphthalocyanines to Subporphyrins. Angew. Chem., Int. Ed. 2006, 45, 2834−2837. (245) Medina, A.; Claessens, C. G. Subphthalocyanines: synthesis, selforganization, properties and applications. J. Porphyrins Phthalocyanines 2009, 13, 446−454. (246) Osuka, A.; Tsurumaki, E.; Tanaka, T. Subporphyrins: A Legitimate Ring-Contracted Porphyrin with Versatile Electronic and Optical Properties. Bull. Chem. Soc. Jpn. 2011, 84, 679−697. (247) Claessens, C. G.; González-Rodríguez, D.; Rodríguez-Morgade, M. S.; Medina, A.; Torres, T. Subphthalocyanines, Subporphyrazines, and Subporphyrins: Singular Nonplanar Aromatic Systems. Chem. Rev. 2014, 114, 2192−2277. (248) Ziessel, R.; Ulrich, G.; Harriman, A. The chemistry of Bodipy: A new El Dorado for fluorescence tools. New J. Chem. 2007, 31, 496−501. (249) Loudet, A.; Burgess, K. BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties. Chem. Rev. 2007, 107, 4891− 4932. (250) Ulrich, G.; Ziessel, R.; Harriman, A. The Chemistry of Fluorescent Bodipy Dyes: Versatility Unsurpassed. Angew. Chem., Int. Ed. 2008, 47, 1184−1201. (251) Benniston, A. C.; Copley, G. Lighting the way ahead with boron dipyrromethene (Bodipy) dyes. Phys. Chem. Chem. Phys. 2009, 11, 4124−4132. (252) Boens, N.; Leen, V.; Dehaen, W. Fluorescent indicators based on BODIPY. Chem. Soc. Rev. 2012, 41, 1130−1172. (253) Ni, Y.; Wu, J. Far-red and near infrared BODIPY dyes: synthesis and applications for fluorescent pH probes and bio-imaging. Org. Biomol. Chem. 2014, 12, 3774−3791. (254) Lu, H.; Mack, J.; Yang, Y.; Shen, Z. Structural modification strategies for the rational design of red/NIR region BODIPYs. Chem. Soc. Rev. 2014, 43, 4778−4823. (255) Loudet, A.; Burgess, K. BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties. In Handbook of Porphyrin Science; World Scientific: Singapore, 2010; Vol. 8, pp 1−164. (256) Köhler, T.; Hodgson, M. C.; Seidel, D.; Veauthier, J. M.; Meyer, S.; Lynch, V.; Boyd, P. D. W.; Brothers, P. J.; Sessler, J. L. Octaethylporphyrin and expanded porphyrin complexes containing coordinated BF2 groups. Chem. Commun. 2004, 1060−1061. (257) Rao, M. R.; Ravikanth, M. Boron Complexes of Oxasmaragdyrin, a Core-Modified Expanded Porphyrin. J. Org. Chem. 2011, 76, 3582− 3587. (258) Pareek, Y.; Ravikanth, M. Synthesis of Mono-Functionalized Core-Modified Expanded Porphyrin Building Blocks and Covalently Linked Expanded Porphyrin Dyads. Eur. J. Org. Chem. 2011, 2011, 5390−5399.

(259) Mane, S. B.; Hu, J.-Y.; Chang, Y.-C.; Luo, L.; Diau, E. W.-G.; Hung, C.-H. Novel expanded porphyrin sensitized solar cells using boryl oxasmaragdyrin as the sensitizer. Chem. Commun. 2013, 49, 6882−6884. (260) Mane, S. B.; Hung, C.-H. Molecular Engineering of Boryl Oxasmaragdyrins through Peripheral Modification: Structure-Efficiency Relationship. Chem. - Eur. J. 2015, 21, 4825−4841. (261) Sakida, T.; Yamaguchi, S.; Shinokubo, H. Metal-Mediated Synthesis of Antiaromatic Porphyrinoids from a BODIPY Precursor. Angew. Chem., Int. Ed. 2011, 50, 2280−2283. (262) Miura, T.; Higashino, T.; Saito, S.; Osuka, A. Phosphorus Complexes of the First Expanded Isophlorins. Chem. - Eur. J. 2010, 16, 55−59. (263) Higashino, T.; Osuka, A. Phosphorus complexes of a triply-fused [24]pentaphyrin. Chem. Sci. 2012, 3, 103−107. (264) Latos-Grażyński, L. Bimetallic Figure-Eight Octaphyrins Split into Four-Pyrrolic Macrocycles. Angew. Chem., Int. Ed. 2004, 43, 5124− 5128. (265) Senge, M. O.; Sergeeva, N. N. Metamorphosis of Tetrapyrrole Macrocycles. Angew. Chem., Int. Ed. 2006, 45, 7492−7495. (266) Inokuma, Y.; Osuka, A. Subporphyrins: emerging contracted porphyrins with aromatic 14π-electronic systems and bowl-shaped structures: rational and unexpected synthetic routes. Dalton Trans. 2008, 2517−2526. (267) Tanaka, Y.; Hoshino, W.; Shimizu, S.; Youfu, K.; Aratani, N.; Maruyama, N.; Fujita, S.; Osuka, A. Thermal Splitting of Bis-Cu(II) Octaphyrin(1.1.1.1.1.1.1.1) into Two Cu(II) Porphyrins. J. Am. Chem. Soc. 2004, 126, 3046−3047. (268) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoo, M.-C.; Kim, D.; Osuka, A. Tribenzosubporphines: Synthesis and Characterization. Angew. Chem., Int. Ed. 2006, 45, 961−964. (269) Kobayashi, N.; Takeuchi, Y.; Matsuda, A. meso-Aryl Subporphyrins. Angew. Chem., Int. Ed. 2007, 46, 758−760. (270) Inokuma, Y.; Yoon, Z. S.; Kim, D.; Osuka, A. meso-ArylSubstituted Subporphyrins: Synthesis, Structures, and Large Substituent Effects on Their Electronic Properties. J. Am. Chem. Soc. 2007, 129, 4747−4761. (271) Kitano, M.; Hayashi, S.; Tanaka, T.; Yorimitsu, H.; Aratani, N.; Osuka, A. Effective meso Fabrications of Subporphyrins. Angew. Chem., Int. Ed. 2012, 51, 5593−5597. (272) Tanaka, T.; Kitano, M.; Hayashi, S.; Aratani, N.; Osuka, A. Rational Synthesis of A2B-type meso-Triarylsubporphyrins. Org. Lett. 2012, 14, 2694−2697. (273) Sakamoto, R.; Saito, S.; Shimizu, S.; Inokuma, Y.; Aratani, N.; Osuka, A. meso-Trifluoromethyl-substituted Subporphyrin from Ringsplitting Reaction of meso-Trifluoromethyl-substituted [32]Heptaphyrin(1.1.1.1.1.1.1). Chem. Lett. 2010, 39, 439−441. (274) Moriya, K.; Saito, S.; Osuka, A. Boron(III) Induced Skeletal Rearrangement of Hexaphyrin(1.1.1.1.1.1) to Hexaphyrin(2.1.1.0.1.1). Angew. Chem., Int. Ed. 2010, 49, 4297−4300. (275) Vogel, E.; Michels, M.; Zander, L.; Lex, J.; Tuzun, N. S.; Houk, K. N. Spirodiporphyrins−As Binuclear Metal Complexes. Angew. Chem., Int. Ed. 2003, 42, 2857−2862. (276) Hohlneicher, G.; Bremm, D.; Wytko, J.; Bley-Escrich, J.; Gisselbrecht, J.-P.; Gross, M.; Michels, M.; Lex, J.; Vogel, E. Spiroconjugation in Spirodicorrolato-Dinickel(II). Chem. - Eur. J. 2003, 9, 5636−5642. (277) Tanaka, Y.; Mori, H.; Koide, T.; Yorimitsu, H.; Aratani, N.; Osuka, A. Rearrangements of a [36]Octaphyrin Triggered by Nickel(II) Metalation: Metamorphosis to a Directly meso-β-Linked Diporphyrin. Angew. Chem., Int. Ed. 2011, 50, 11460−11464. (278) Yoneda, T.; Saito, S.; Yorimitsu, H.; Osuka, A. Palladium(II)Triggered Rearrangement of Heptaphyrins to N-Confused Porphyrins. Angew. Chem., Int. Ed. 2011, 50, 3475−3478. (279) Shimizu, S.; Tanaka, Y.; Youfu, K.; Osuka, A. Dicopper and Disilver Complexes of Octaphyrin(1.1.1.1.1.1.1.1): Reversible Hydrolytic Cleavage of the Pyrrolic Ring to a Keto−Imine. Angew. Chem., Int. Ed. 2005, 44, 3726−3729. (280) Inokuma, Y.; Matsunari, T.; Ono, N.; Uno, H.; Osuka, A. A Doubly N-Fused Benzohexaphyrin and Its Rearrangement to a 2635


Chemical Reviews

Review

Fluorescent Macrocycle upon DDQ Oxidation. Angew. Chem., Int. Ed. 2005, 44, 1856−1860. (281) Suzuki, M.; Yoon, M.-C.; Kim, D. Y.; Kwon, J. H.; Furuta, H.; Kim, D.; Osuka, A. A New Entry to Doubly N-Confused[26]Hexaphyrins(1.1.1.1.1.1) from Normal [26]Hexaphyrins(1.1.1.1.1.1) through an Unprecedented Double Pyrrolic Rearrangement. Chem. Eur. J. 2006, 12, 1754−1759. (282) Srinivasan, A.; Ishizuka, T.; Osuka, A.; Furuta, H. Doubly NConfused Hexaphyrin: A Novel Aromatic Expanded Porphyrin that Complexes Bis-metals in the Core. J. Am. Chem. Soc. 2003, 125, 878− 879. (283) Saito, S.; Furukawa, K.; Osuka, A. Fully π-Conjugated Helices from Oxidative Cleavage of meso-Aryl-Substituted Expanded Porphyrins. J. Am. Chem. Soc. 2010, 132, 2128−2129. (284) Heilbronner, E. Hückel molecular orbitals of Möbius-type conformations of annulenes. Tetrahedron Lett. 1964, 5, 1923−1928. (285) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Synthesis of a Möbius aromatic hydrocarbon. Nature 2003, 426, 819−821. (286) Herges, R. Topology in Chemistry: Designing Mö bius Molecules. Chem. Rev. 2006, 106, 4820−4842. (287) Stępień, M.; Latos-Grażyński, L.; Sprutta, N.; Chwalisz, P.; Szterenberg, L. Expanded Porphyrin with a Split Personality: A Hückel− Möbius Aromaticity Switch. Angew. Chem., Int. Ed. 2007, 46, 7869− 7873. (288) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; van Eikema Hommes, N. J. R. Nucleus-independent chemical shifts: a simple and efficient aromaticity probe. J. Am. Chem. Soc. 1996, 118, 6317−6318. (289) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Nucleus-independent chemical shifts (NICS) as an aromaticity criterion. Chem. Rev. 2005, 105, 3842−3888. (290) Kawase, T.; Oda, M. Mö bius Aromatic Hydrocarbons: Challenges for Theory and Synthesis. Angew. Chem., Int. Ed. 2004, 43, 4396. (291) Rzepa, H. S. Möbius Aromaticity and Delocalization. Chem. Rev. 2005, 105, 3697−3715. (292) Spitler, E. L.; Johnson, C. A., II; Haley, M. M. Renaissance of Annulene Chemistry. Chem. Rev. 2006, 106, 5344−5386. (293) Jux, N. The Porphyrin Twist: Hückel and Möbius Aromaticity. Angew. Chem., Int. Ed. 2008, 47, 2543−2546. (294) Yoon, Z. S.; Osuka, A.; Kim, D. Möbius aromaticity and antiaromaticity in expanded porphyrins. Nat. Chem. 2009, 1, 113−122. (295) Castro, C.; Karney, W. L. Mechanisms and Möbius strips: understanding dynamic processes in annulenes. J. Phys. Org. Chem. 2012, 25, 612−619. (296) Schaller, G. R.; Herges, R. Möbius molecules with twists and writhes. Chem. Commun. 2013, 49, 1254−1260. (297) Tanaka, T.; Osuka, A. Möbius aromatic and antiaromatic expanded porphyrins. In Chemical Science of π-Electron Systems; Springer: Tokyo, 2015; pp 257−272. (298) Zimmerman, H. E. On Molecular Orbital Correlation Diagrams, the Occurrence of Möbius Systems in Cyclization Reactions, and Factors Controlling Ground- and Excited-State Reactions. I. J. Am. Chem. Soc. 1966, 88, 1564−1565. (299) Zimmerman, H. E. Möbius-Hückel Concept in Organic Chemistry. Application of Organic Molecules and Reactions. Acc. Chem. Res. 1971, 4, 272−280. (300) Martín-Santamaría, S.; Lavan, B.; Rzepa, H. S. Hückel and Möbius aromaticity and trimerous transition state behaviour in the pericyclic reactions of [10], [14], [16] and [18]annulenes. J. Chem. Soc., Perkin Trans. 2 2000, 1415−1417. (301) Martín-Santamaría, S.; Rzepa, H. S. Möbius and Hückel molecular orbitals arising from CCC components in annulene rings. J. Chem. Soc., Perkin Trans. 2 2000, 2372−2377. (302) Martín-Santamaría, S.; Lavan, B.; Rzepa, H. S. Möbius aromatics arising from a CCC ring component. Chem. Commun. 2000, 1089−1090.

(303) Karney, W. L.; Kastrup, C. J.; Oldfield, S. P.; Rzepa, H. S. Möbius aromatic forms of 8-π electron heteropines. J. Chem. Soc., Perkin Trans. 2 2002, 388−392. (304) Kastrup, C. J.; Oldfield, S. P.; Rzepa, H. S. The aromaticity and Möbius characteristics of carbeno[8]heteroannulenes and triplet state annulenes. Chem. Commun. 2002, 642−643. (305) Kastrup, C. J.; Oldfield, S. V.; Rzepa, H. S. An ab initio computational study of monodentate palladium ligand complexes with Möbius-aromatic chiral character. Dalton Trans. 2002, 2421−2422. (306) Rzepa, H. S.; Taylor, K. R. Möbius and Hückel spiroaromatic systems. J. Chem. Soc., Perkin Trans. 2 2002, 1499−1501. (307) Hall, D.; Rzepa, H. S. Möbius bis and tris-spiroaromatic systems. Org. Biomol. Chem. 2003, 1, 182−185. (308) Rzepa, H. S. A Double-Twist Möbius-Aromatic Conformation of [14]Annulene. Org. Lett. 2005, 7, 4637−4639. (309) Rzepa, H. S. Double-twist Möbius aromaticity in a 4n+2 electron electrocyclic reaction. Chem. Commun. 2005, 5220−5222. (310) Fowler, P. W.; Rzepa, H. S. Phys. Chem. Chem. Phys. 2006, 8, 1775−1777. (311) Rzepa, H. S. Lemniscular Hexaphyrins as Examples of Aromatic and Antiaromatic Double-Twist Möbius Molecules. Org. Lett. 2008, 10, 949−952. (312) Allan, C. S. M.; Rzepa, H. S. Chiral Aromaticities. AIM and ELF Critical Point and NICS Magnetic Analyses of Mö bius-Type Aromaticity and Homoaromaticity in Lemniscular Annulenes and Hexaphyrins. J. Org. Chem. 2008, 73, 6615−6622. (313) Rappaport, S. M.; Rzepa, H. S. Intrinsically Chiral Aromaticity. Rules Incorporating Linking Number, Twist, and Writhe for HigherTwist Möbius Annulenes. J. Am. Chem. Soc. 2008, 130, 7613−7619. (314) Mauksch, M.; Gogonea, V.; Jiao, H.; Schleyer, P. v. R. Monocyclic (CH)9+− A Heilbronner Mö bius Aromatic System Revealed. Angew. Chem., Int. Ed. 1998, 37, 2395−2397. (315) Fokin, A. A.; Jiao, H.; Schleyer, P. v. R. From Dodecahedrapentaene to the “[n]Trannulenes”. A New In-Plane Aromatic Family. J. Am. Chem. Soc. 1998, 120, 9364−9365. (316) Gogonea, V.; Schleyer, P. v. R.; Schreiner, P. R. Angew. Chem., Int. Ed. 1998, 37, 1945−1948. (317) Castro, C.; Isborn, C. M.; Karney, W. L.; Mauksch, M.; Schleyer, P. v. R. Aromaticity with a Twist: Möbius [4n]Annulenes. Org. Lett. 2002, 4, 3431−3434. (318) Castro, C.; Chen, Z.; Wannere, C. S.; Jiao, H.; Karney, W. L.; Mauksch, M.; Puchta, R.; Hommes, N. J. R. v. E.; Schleyer, P. v. R. Investigation of a Putative Möbius Aromatic Hydrocarbon. The Effect of Benzannelation on Möbius [4n]Annulene Aromaticity. J. Am. Chem. Soc. 2005, 127, 2425−2432. (319) Wannere, C. S.; Rzepa, H. S.; Rinderspacher, B. C.; Paul, A.; Allan, C. S. M.; Schaefer, H. F., III; Schleyer, P. v. R. The Geometry and Electronic Topology of Higher-Order Charged Möbius Annulenes. J. Phys. Chem. A 2009, 113, 11619−11629. (320) McKee, W. C.; Wu, J. I.; Rzepa, H. S.; Schleyer, P. v. R. A Hückel Theory Perspective on Möbius Aromaticity. Org. Lett. 2013, 15, 3432− 3435. (321) Bucher, G.; Grimme, S.; Huenerbein, R.; Auer, A. A.; Mucke, E.; Köhler, F.; Siegwarth, J.; Herges, R. Is the [9]Annulene Cation a Möbius Annulene? Angew. Chem., Int. Ed. 2009, 48, 9971−9974. (322) Mucke, E.-K.; Köhler, F.; Herges, R. The [13]Annulene Cation Is a Stable Möbius Annulene Cation. Org. Lett. 2010, 12, 1708−1711. (323) Mucke, E.-K.; Schönborn, B.; Köhler, F.; Herges, R. Stability and Aromaticity of Charged Möbius[4n]Annulenes. J. Org. Chem. 2011, 76, 35−41. (324) Castro, C.; Karney, W. L.; Valencia, M. A.; Vu, C. M. H.; Pemberton, R. P. Möbius Aromaticity in [12]Annulene: Cis-Trans Tsomerization via Twist-Coupled Bond Shifting. J. Am. Chem. Soc. 2005, 127, 9704−9705. (325) Pemberton, R. P.; McShane, C. M.; Castro, C.; Karney, W. L. Dynamic Processes in [16]Annulene: Möbius Bond-Shifting Routes to Configuration Change. J. Am. Chem. Soc. 2006, 128, 16692−16700. 2636


Chemical Reviews

Review

of 16π-Electron Octaisobutyltetraphenylporphyrin. J. Am. Chem. Soc. 2010, 132, 12627−12638. (346) Sugawara, S.; Kodama, M.; Hirata, Y.; Kojima, S.; Yamamoto, Y. Synthesis and characterization of the most distorted 16π porphyrin: 16π Octaisopropyltetraphenylporphyrin (OiPTPP). J. Porphyrins Phthalocyanines 2011, 15, 1326−1334. (347) Kakui, T.; Sugawara, S.; Hirata, Y.; Kojima, S.; Yamamoto, Y. Anti-Aromatic 16π Porphyrin-Metal Complexes with meso-Alkyl Substituents. Chem. - Eur. J. 2011, 17, 7768−7771. (348) Sugawara, S.; Hirata, Y.; Kojima, S.; Yamamoto, Y.; Miyazaki, E.; Takimiya, K.; Matsukawa, S.; Hashizume, D.; Mack, J.; Kobayashi, N.; et al. Synthesis, Characterization, and Spectroscopic Analysis of Antiaromatic Benzofused Metalloporphyrins. Chem. - Eur. J. 2012, 18, 3566−3581. (349) Hiramatsu, S.; Sugawara, S.; Kojima, S.; Yamamoto, Y. Synthesis of the most anti-aromatic 16π porphyrin: an octaethylporphyrin zinc(II) complex with no meso-substituents. J. Porphyrins Phthalocyanines 2013, 17, 1183−1187. (350) Park, J. K.; Yoon, Z. S.; Yoon, M.-C.; Kim, K. S.; Mori, S.; Shin, J.Y.; Osuka, A.; Kim, D. Möbius Aromaticity in N-Fused [24]Pentaphyrin upon Rh(I) Metalation. J. Am. Chem. Soc. 2008, 130, 1824−1825. (351) Ishida, S.-i.; Tanaka, T.; Lim, J. M.; Kim, D.; Osuka, A. SiIV Incorporation into a [28]Hexaphyrin That Triggered Formation of Möbius Aromatic Molecules. Chem. - Eur. J. 2014, 20, 8274−8278. (352) Ishida, S.-i.; Osuka, A. meso-Aryl [28]Hexaphyrin Silicon Complexes Bearing Various Si-Substituents and 1,16-Dihydrohexaphyrin bis-Chlorosilicon Complex. Chem. - Asian J. 2015, 10, 2200−2206. (353) Moriya, K.; Yoneda, T.; Saito, S.; Osuka, A. A Möbius Aromatic Pd(II) Complex of [28]Hexaphyrin(2.1.1.0.1.1). Chem. Lett. 2011, 40, 455−457. (354) Gokulnath, S.; Toganoh, M.; Yamaguchi, K.; Mori, S.; Uno, H.; Furuta, H. Confusion of Möbius aromaticity: disruption of annulenic pathway in singly N-confused [28]hexaphyrin and its mono-Pd(II) complex. Dalton Trans. 2012, 41, 6283−6290. (355) Tokuji, S.; Shin, J.-Y.; Kim, K. S.; Lim, J. M.; Youfu, K.; Saito, S.; Kim, D.; Osuka, A. Facile Formation of a Benzopyrane-Fused [28]Hexaphyrin That Exhibits Distinct Möbius Aromaticity. J. Am. Chem. Soc. 2009, 131, 7240−7241. (356) Inoue, M.; Kim, K. S.; Suzuki, M.; Lim, J. M.; Shin, J.-Y.; Kim, D.; Osuka, A. Thermal Fusion Reactions of meso-(3-Thienyl) Groups in [26]Hexaphyrins to Produce Möbius Aromatic Molecules. Angew. Chem., Int. Ed. 2009, 48, 6687−6690. (357) Higashino, T.; Soya, T.; Kim, W.; Kim, D.; Osuka, A. A Möbius Aromatic [28]Hexaphyrin Bearing a Diethylamine Group: A Rigid but Smooth Conjugation Circuit. Angew. Chem., Int. Ed. 2015, 54, 5456− 5459. (358) Hung, C.-H.; Jong, J.-P.; Ho, M.-Y.; Lee, G.-H.; Peng, S.-M. CuMediated Syntheses of N-Fused and Ring-Modified Trithiahexaphyrins. Chem. - Eur. J. 2002, 8, 4542−4548. (359) Karthik, G.; Srinivasan, A.; Suresh, C. H.; Chandrashekar, T. K. Fused core-modified planar antiaromatic 32π heptaphyrins: unusual synthesis and structural diversity. Chem. Commun. 2014, 50, 12127− 12130. (360) Ghosh, A.; Chaudhary, A.; Srinivasan, A.; Suresh, C. H.; Chandrashekar, T. K. [32]π Fused Core-Modified Heptaphyrin with Möbius Aromaticity. Chem. - Eur. J. 2016, 22, 3942−3946. (361) Sankar, J.; Mori, S.; Saito, S.; Rath, H.; Suzuki, M.; Inokuma, Y.; Shinokubo, H.; Kim, K. S.; Yoon, Z. S.; Shin, J.-Y.; et al. Unambiguous Identification of Möbius Aromaticity for meso-Aryl-Substituted [28]Hexaphyrins(1.1.1.1.1.1). J. Am. Chem. Soc. 2008, 130, 13568−13579. (362) Kim, K. S.; Yoon, Z. S.; Butler Ricks, A.; Shin, J.-Y.; Mori, S.; Sankar, J.; Saito, S.; Jung, Y. M.; Wasielewski, M. R.; Osuka, A.; Kim, D. Temperature-Dependent Conformational Change of meso-Hexakis(pentafluorophenyl) [28]Hexaphyrins(1.1.1.1.1.1) into Möbius Structures. J. Phys. Chem. A 2009, 113, 4498−4506. (363) Yoon, M.-C.; Kim, P.; Yoo, H.; Shimizu, S.; Koide, T.; Tokuji, S.; Saito, S.; Osuka, A.; Kim, D. Solvent- and Temperature-Dependent Conformational Changes between Hückel Antiaromatic and Möbius

(326) Moll, J. F.; Pemberton, R. P.; Gutierrez, M. G.; Castro, C.; Karney, W. L. Configuration Change in [14]Annulene Requires Möbius Antiaromatic Bond Shifting. J. Am. Chem. Soc. 2007, 129, 274−275. (327) Braten, M. N.; Castro, C.; Herges, R.; Köhler, F.; Karney, W. L. The [12]Annulene Global Minimum. J. Org. Chem. 2008, 73, 1532− 1535. (328) Okoronkwo, T.; Nguyen, P. T.; Castro, C.; Karney, W. L. [14]Annulene: Cis/Trans Isomerization via Two-Twist and Nondegenerate Planar Bond Shifting and Möbius Conformational Minima. Org. Lett. 2010, 12, 972−975. (329) Santander, M. V.; Pastor, M. B.; Nelson, J. N.; Castro, C.; Karney, W. L. Hü c kel and Mö b ius Bond-Shifting Routes to Configuration Change in Dehydro[4n+2]annulenes. J. Org. Chem. 2013, 78, 2033−2039. (330) Ajami, D.; Hess, K.; Köhler, F.; Näther, C.; Oeckler, O.; Simon, A.; Yamamoto, C.; Okamoto, Y.; Herges, R. Synthesis and Properties of the First Möbius Annulenes. Chem. - Eur. J. 2006, 12, 5434−5445. (331) Werner, C.; Hopf, H.; Grunenberg, J.; Ernst, L.; Jones, P. G.; Köhler, F.; Herges, R. Transferring Sondheimer’s Annulene Chemistry into Three-Dimensional Space. Eur. J. Org. Chem. 2009, 2009, 2621− 2626. (332) Mohebbi, A. R.; Mucke, E.-K.; Schaller, G. R.; Köhler, F.; Sönnichsen, F. D.; Ernst, L.; Näther, C.; Herges, R. Singly and Doubly Twisted [36]Annulenes: Synthesis and Calculations. Chem. - Eur. J. 2010, 16, 7767−7772. (333) Schaller, G. R.; Topić, F.; Rissanen, K.; Okamoto, Y.; Shen, J.; Herges, R. Design and synthesis of the first triply twisted Möbius annulene. Nat. Chem. 2014, 6, 608−613. (334) Stępień, M.; Szyszko, B.; Latos-Grażyński, L. Steric Control in the Synthesis of p-Benziporphyrins. Formation of a Doubly N-Confused Benzihexaphyrin Macrocycle. Org. Lett. 2009, 11, 3930−3933. (335) Stępień, M.; Szyszko, B.; Latos-Grażyński, L. Three-Level Topology Switching in a Molecular Möbius Band. J. Am. Chem. Soc. 2010, 132, 3140−3152. (336) Szyszko, B.; Sprutta, N.; Chwalisz, P.; Stępień, M.; LatosGrażyński, L. Hückel and Möbius Expanded para-Benziporphyrins: Synthesis and Aromaticity Switching. Chem. - Eur. J. 2014, 20, 1985− 1997. (337) Shin, J.-Y.; Kim, K. S.; Yoon, M.-C.; Lim, J. M.; Yoon, Z. S.; Osuka, A.; Kim, D. Aromaticity and photophysical properties of various topology-controlled expanded porphyrins. Chem. Soc. Rev. 2010, 39, 2751−2767. (338) Pawlicki, M.; Collins, H. A.; Denning, R. G.; Anderson, H. L. Two-Photon Absorption and the Design of Two-Photon Dyes. Angew. Chem., Int. Ed. 2009, 48, 3244−3266. (339) Yoneda, T.; Sung, Y. M.; Lim, J. M.; Kim, D.; Osuka, A. PdII Complexes of [44]- and [46]Decaphyrins: The Largest Hückel Aromatic and Antiaromatic, and Möbius Aromatic Macrocycles. Angew. Chem., Int. Ed. 2014, 53, 13169−13173. (340) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. An Antiaromatic Porphyrin Complex: Tetraphenylporphyrinato(silicon)(L)2 (L = THF or Pyridine). J. Am. Chem. Soc. 2005, 127, 12212−12213. (341) Yamamoto, Y.; Yamamoto, A.; Furuta, S.-y.; Horie, M.; Kodama, M.; Sato, W.; Akiba, K.-y.; Tsuzuki, S.; Uchimaru, T.; Hashizume, D.; Iwasaki, F. Synthesis and Structure of 16π Octaalkyltetraphenylporphyrins. J. Am. Chem. Soc. 2005, 127, 14540−14541. (342) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. The Doubly Oxidized, Antiaromatic Tetraphenylporphyrin Complex [Li(TPP)][BF4]. Org. Lett. 2006, 8, 2401−2404. (343) Liu, C.; Shen, D.-M.; Chen, Q.-Y. Synthesis and Reactions of 20π-Electron β-Tetrakis(trifluoromethyl)-meso-tetraphenylporphyrins. J. Am. Chem. Soc. 2007, 129, 5814−5815. (344) Cissell, J. A.; Vaid, T. P.; Yap, G. P. A. Reversible Oxidation State Change in Germanium(tetraphenylporphyrin) Induced by a Dative Ligand: Aromatic GeII(TPP) and Antiaromatic GeIV(TPP) (pyridine)2. J. Am. Chem. Soc. 2007, 129, 7841−7847. (345) Yamamoto, Y.; Hirata, Y.; Kodama, M.; Yamaguchi, T.; Matsukawa, S.; Akiba, K.-y.; Hashizume, D.; Iwasaki, F.; Muranaka, A.; Uchiyama, M.; et al. Synthesis, Reactions, and Electronic Properties 2637


Chemical Reviews

Review

Aromatic Species in meso-Trifluoromethyl Substituted [28]Hexaphyrins. J. Phys. Chem. B 2011, 115, 14928−14937. (364) Yoon, M.-C.; Shin, J.-Y.; Lim, J. M.; Saito, S.; Yoneda, T.; Osuka, A.; Kim, D. Solvent-Dependent Aromatic versus Antiaromatic Conformational Switching in meso-(Heptakis)pentafluorophenyl [32]Heptaphyrin. Chem. - Eur. J. 2011, 17, 6707−6715. (365) Saito, S.; Shin, J.-Y.; Lim, J. M.; Kim, K. S.; Kim, D.; Osuka, A. Protonation-Triggered Conformational Changes to Möbius Aromatic [32]Heptaphyrins(1.1.1.1.1.1.1). Angew. Chem., Int. Ed. 2008, 47, 9657−9660. (366) Lim, J. M.; Shin, J.-Y.; Tanaka, Y.; Saito, S.; Osuka, A.; Kim, D. Protonated [4n]π and [4n+2]π Octaphyrins Choose Their Möbius/ Hückel Aromatic Topology. J. Am. Chem. Soc. 2010, 132, 3105−3114. (367) Ishida, S.-i.; Higashino, T.; Mori, S.; Mori, H.; Aratani, N.; Tanaka, T.; Lim, J. M.; Kim, D.; Osuka, A. Diprotonated [28]Hexaphyrins(1.1.1.1.1.1): Triangular Antiaromatic Macrocycles. Angew. Chem., Int. Ed. 2014, 53, 3427−3431. (368) Koide, T.; Youfu, K.; Saito, S.; Osuka, A. Multiple conformational changes of β-tetraphenyl meso-hexakis(pentafluorophenyl) substituted [26] and [28]hexaphyrins(1.1.1.1.1.1). Chem. Commun. 2009, 6047−6049. (369) Koide, T.; Osuka, A. Möbius Aromatic Palladium(II) Complexes of a β-Tetraphenyl meso-Hexakis(pentafluorophenyl) Substituted Hexaphyrin(1.1.1.1.1.1). Bull. Chem. Soc. Jpn. 2010, 83, 877−879. (370) Cha, W.-Y.; Yoneda, T.; Lee, S.; Lim, J. M.; Osuka, A.; Kim, D. Deprotonation induced formation of Mö b ius aromatic [32]heptaphyrins. Chem. Commun. 2014, 50, 548−550. (371) Cha, W.-Y.; Lim, J. M.; Yoon, M.-C.; Sung, Y. M.; Lee, B. S.; Katsumata, S.; Suzuki, M.; Mori, H.; Ikawa, Y.; Furuta, H.; et al. Deprotonation-Induced Aromaticity Enhancement and New Conjugated Networks in meso-Hexakis(pentafluorophenyl)[26]hexaphyrin. Chem. - Eur. J. 2012, 18, 15838−15844. (372) Cha, W.-Y.; Soya, T.; Tanaka, T.; Mori, H.; Hong, Y.; Lee, S.; Pa r k, K . H.; Osu ka , A.; K im, D. M ul tifa ce te d [36] octaphyrin(1.1.1.1.1.1.1.1): deprotonation-induced switching among nonaromatic, Möbius aromatic, and Hückel antiaromatic species. Chem. Commun. 2016, 52, 6076−6078. (373) Soya, T.; Kim, W.; Kim, D.; Osuka, A. Stable [48]-, [50]-, and [52]Dodecaphyrins(1.1.0.1.1.0.1.1.0.1.1.0): The Largest Hückel Aromatic Molecules. Chem. - Eur. J. 2015, 21, 8341−8346. (374) Pacholska-Dudziak, E.; Skonieczny, J.; Pawlicki, M.; Szterenberg, L.; Ciunik, Z.; Latos-Grażyński, L. Palladium Vacataporphyrin Reveals Conformational Rearrangements Involving Hückel and Möbius Macrocyclic Topologies. J. Am. Chem. Soc. 2008, 130, 6182− 6195. (375) Pacholska-Dudziak, E.; Latos-Grażyński, L. Aza-deficient porphyrin as a ligand. Coord. Chem. Rev. 2009, 253, 2036−2048. (376) Pacholska, E.; Latos-Grażyński, L.; Ciunik, Z. A Direct Link between Annulene and Porphyrin Chemistry−21-Vacataporphyrin. Chem. - Eur. J. 2002, 8, 5403−5406. (377) Higashino, T.; Lim, J. M.; Miura, T.; Saito, S.; Shin, J.-Y.; Kim, D.; Osuka, A. Möbius Antiaromatic Bisphosphorus Complexes of [30]Hexaphyrins. Angew. Chem., Int. Ed. 2010, 49, 4950−4954. (378) Higashino, T.; Lee, B. S.; Lim, J. M.; Kim, D.; Osuka, A. A Möbius Antiaromatic Complex as a Kinetically Controlled Product in Phosphorus Insertion to a [32]Heptaphyrin. Angew. Chem., Int. Ed. 2012, 51, 13105−13108. (379) Abe, M. Diradicals. Chem. Rev. 2013, 113, 7011−7088. (380) Zeng, Z.; Shi, X.; Chi, C.; López Navarrete, J. T.; Casado, J.; Wu, J. Pro-aromatic and anti-aromatic π-conjugated molecules: an irresistible wish to be diradicals. Chem. Soc. Rev. 2015, 44, 6578−6596. (381) Hiroto, S.; Shinokubo, H.; Furukawa, K.; Osuka, A. Synthesis and Biradicaloid Character of Doubly Linked Corrole Dimers. J. Am. Chem. Soc. 2006, 128, 12380−12381. (382) Schweyen, P.; Brandhorst, K.; Wicht, R.; Wolfram, B.; Bröring, M. The Corrole Radical. Angew. Chem., Int. Ed. 2015, 54, 8213−8216. (383) Shimizu, D.; Oh, J.; Furukawa, K.; Kim, D.; Osuka, A. mesoHydroxysubporphyrins: A Cyclic Trimeric Assembly and a Stable mesoOxy Radical. Angew. Chem., Int. Ed. 2015, 54, 6613−6617.

(384) Shimizu, D.; Oh, J.; Furukawa, K.; Kim, D.; Osuka, A. Triarylporphyrin meso-Oxy Radicals: Remarkable Chemical Stabilities and Oxidation to Oxophlorin π-Cations. J. Am. Chem. Soc. 2015, 137, 15584−15594. (385) Esdaile, L. J.; Rintoul, L.; Goh, M. S.; Merahi, K.; Parizel, N.; Wellard, R. M.; Choua, S.; Arnold, D. P. Nickel(II) mesoHydroxyporphyrin Complexes Revisited: Palladium-Catalysed Synthesis, Electronic Structures of Derived Oxy Radicals, and Oxidative Coupling to a Dioxoporphodimethene Dyad. Chem. - Eur. J. 2016, 22, 3430−3446. (386) Zeng, W.; Lee, S.; Son, M.; Ishida, M.; Furukawa, K.; Hu, P.; Sun, Z.; Kim, D.; Wu, J. Phenalenyl-fused porphyrins with different ground states. Chem. Sci. 2015, 6, 2427−2433. (387) Yoshida, T.; Zhou, W.; Furuyama, T.; Leznoff, D. B.; Kobayashi, N. An Extremely Air-Stable 19π Porphyrinoid. J. Am. Chem. Soc. 2015, 137, 9258−9261. (388) Satoh, T.; Minoura, M.; Nakano, H.; Furukawa, K.; Matano, Y. Redox-Switchable 20π-, 19π-, and 18π-Electron 5,10,15,20-Tetraaryl5,15-diazaporphyrinoid Nickel(II) Complexes. Angew. Chem., Int. Ed. 2016, 55, 2235−2238. (389) Ishida, M.; Shin, J.-Y.; Lim, J. M.; Lee, B. S.; Yoon, M.-C.; Koide, T.; Sessler, J. L.; Osuka, A.; Kim, D. Neutral Radical and Singlet Biradical Forms of Meso-Free, -Keto, and -Diketo Hexaphyrins(1.1.1.1.1.1): Effects on Aromaticity and Photophysical Properties. J. Am. Chem. Soc. 2011, 133, 15533−15544. (390) Rath, H.; Tokuji, S.; Aratani, N.; Furukawa, K.; Lim, J. M.; Kim, D.; Shinokubo, H.; Osuka, A. A Stable Organic Radical Delocalized on a Highly Twisted π System Formed Upon Palladium Metalation of a Möbius Aromatic Hexaphyrin. Angew. Chem., Int. Ed. 2010, 49, 1489− 1491. (391) Hisamune, Y.; Nishimura, K.; Isakari, K.; Ishida, M.; Mori, S.; Karasawa, S.; Kato, T.; Lee, S.; Kim, D.; Furuta, H. Stable π Radical from a Contracted Doubly N-Confused Hexaphyrin by Double Palladium Metalation. Angew. Chem., Int. Ed. 2015, 54, 7323−7327. (392) Tanaka, Y.; Yoneda, T.; Furukawa, K.; Koide, T.; Mori, H.; Tanaka, T.; Shinokubo, H.; Osuka, A. A Stable Organic π-Radical of a Zinc(II)−Copper(I)−Zinc(II) Complex of Decaphyrin. Angew. Chem., Int. Ed. 2015, 54, 10908−10911. (393) Ishida, M.; Karasawa, S.; Uno, H.; Tani, F.; Naruta, Y. Protonation-Induced Formation of a Stable Singlet Biradicaloid Derived from a Modified Sapphyrin Analogue. Angew. Chem., Int. Ed. 2010, 49, 5906−5909. (394) Ishida, M.; Naruta, Y.; Tani, F. A Porphyrin-Related Macrocycle with an Embedded 1,10-Phenanthroline Moiety: Fluorescent Magnesium(II) Ion Sensor. Angew. Chem., Int. Ed. 2010, 49, 91−94. (395) Ishida, M.; Lim, J. M.; Lee, B. S.; Tani, F.; Sessler, J. L.; Kim, D.; Naruta, Y. Photophysical Analysis of 1,10-Phenanthroline-Embedded Porphyrin Analogues and Their Magnesium(II) Complexes. Chem. Eur. J. 2012, 18, 14329−14341. (396) Gopalakrishna, T. Y.; Reddy, J. S.; Anand, V. G. An Amphoteric Switch to Aromatic and Antiaromatic States of a Neutral Air-Stable 25π Radical. Angew. Chem., Int. Ed. 2014, 53, 10984−10987. (397) Vogel, E.; Pohl, M.; Herrmann, A.; Wiss, T.; Konig, C.; Lex, J.; Gross, M.; Gisselbrecht, J. P. Porphyrinoid Macrocycles Based on Thiophene−The Octaethyltetrathiaporphyrin Dication. Angew. Chem., Int. Ed. Engl. 1996, 35, 1520−1524. (398) Ishida, M.; Kim, S.-J.; Preihs, C.; Ohkubo, K.; Lim, J. M.; Lee, B. S.; Park, J. S.; Lynch, V. M.; Roznyatovskiy, V. V.; Sarma, T.; Panda, P. K.; Lee, C.-H.; Fukuzumi, S.; Kim, D.; Sessler, J. L. Protonation-coupled redox reactions in planar antiaromatic meso-pentafluorophenyl-substituted o-phenylene-bridged annulated rosarins. Nat. Chem. 2013, 5, 15−20. (399) Fukuzumi, S.; Ohkubo, K.; Ishida, M.; Preihs, C.; Chen, B.; Borden, W. T.; Kim, D.; Sessler, J. L. Formation of Ground State Triplet Diradicals from Annulated Rosarin Derivatives by Triprotonation. J. Am. Chem. Soc. 2015, 137, 9780−9783. (400) Senge, M. O. Nucleophilic Substitution as a Tool for the Synthesis of Unsymmetrical Porphyrins. Acc. Chem. Res. 2005, 38, 733− 743. 2638


Chemical Reviews

Review

(423) Youfu, K.; Osuka, A. Phosphonium Ylides from Nucleophilic Addition of Triphenylphosphine to [26]Hexaphyrin(1.1.1.1.1.1). Org. Lett. 2005, 7, 4381−4384. (424) Inoue, M.; Yoneda, T.; Youfu, K.; Aratani, N.; Osuka, A. Möbius Aromatic [28]Hexaphyrin Phosphonium Adducts. Chem. - Eur. J. 2011, 17, 9028−9031. (425) Naoda, K.; Osuka, A. A Bis-Au(III) [28]hexaphyrin triphenylphosphine adduct. J. Porphyrins Phthalocyanines 2016, 1. (426) Hata, H.; Kamimura, Y.; Shinokubo, H.; Osuka, A. Synthesis of Pyrrolidine-Fused [34]- and [36]Octaphyrins via 1,3-Dipolar Cycloaddition. Org. Lett. 2006, 8, 1169−1172. (427) Sanders, J. K. M. Coordination Chemistry of Oligoporphyrins. In Porphyrin Handbook; Academic Press: San Diego, 2000; Vol. 3, pp 347−368. (428) Aratani, N.; Osuka, A. Synthetic Strategies Toward Multiporphyrinic Architectures. In Handbook of Porphyrin Science; World Scientific: Singapore, 2010; Vol. 1, pp 1−132. (429) Tanaka, T.; Osuka, A. Conjugated porphyrin arrays: synthesis, properties and applications for functional materials. Chem. Soc. Rev. 2015, 44, 943−969. (430) Hata, H.; Shinokubo, H.; Osuka, A. Anthracene-Bridged ZShaped [26]Hexaphyrin(1.1.1.1.1.1.1) Dimer from the Regioselective Diels−Alder Reaction of a Hexaphyrin with Bis-o-xylylene Equivalents. Angew. Chem., Int. Ed. 2005, 44, 932−935. (431) Mori, H.; Aratani, N.; Osuka, A. 1,4-Phenylene-Bridged Hexaphyrin Dimers. Eur. J. Org. Chem. 2012, 2012, 1913−1919. (432) Misra, R.; Kumar, R.; Chandrashekar, T. K.; Suresh, C. H. Mesomeso linked core modified 22π smaragdyrins with unusual absorption properties. Chem. Commun. 2006, 4584−4586. (433) Tanaka, T.; Aratani, N.; Lim, J. M.; Kim, K. S.; Kim, D.; Osuka, A. Porphyrin-hexaphyrin hybrid tapes. Chem. Sci. 2011, 2, 1414−1418. (434) Tanaka, T.; Aratani, N.; Osuka, A. Aromatic-to-Antiaromatic Switching in Triply Linked Porphyrin Bis(rhodium(I)) Hexaphyrin Hybrids. Chem. - Asian J. 2012, 7, 889−893. (435) Mori, H.; Tanaka, T.; Lee, S.; Lim, J. M.; Kim, D.; Osuka, A. meso-meso Linked Porphyrin-[26]Hexaphyrin-Porphyrin Hybrid Arrays and Their Triply Linked Tapes Exhibiting Strong Absorption Bands in the NIR Region. J. Am. Chem. Soc. 2015, 137, 2097−2106. (436) Mori, H.; Osuka, A. meso-meso Directly-linked trimeric and pentameric electron-deficient porphyrin-hexaphyrin hybrid arrays. J. Porphyrins Phthalocyanines 2016, 20, 245−253. (437) Shin, J.-Y.; Tanaka, T.; Osuka, A.; Miao, Q.; Dolphin, D. BODIPY−Hexaphyrin Hybrids. Chem. - Eur. J. 2009, 15, 12955−12959. (438) Shin, J.-Y.; Kim, K.; Lim, J. M.; Tanaka, T.; Kim, D.; Kim, K.; Shinokubo, H.; Osuka, A. Photodynamics of [26]- and [28]HexaphyrinBodipy Hybrids. Chem. - Eur. J. 2014, 20, 4574−4582. (439) Werner, A.; Michels, M.; Zander, L.; Lex, J.; Vogel, E. Figure Eight” Cyclooctapyrroles: Enantiomeric Separation and Determination of the Absolute Configuration of a Binuclear Metal Complex. Angew. Chem., Int. Ed. 1999, 38, 3650−3653. (440) Haridas, V.; Singh, H.; Sharma, Y. K.; Lal, K. Engineering macrocyclic figure−eight motif. Proc. - Indian Acad. Sci., Chem. Sci. 2007, 119, 219−230. (441) Lintuluoto, J. M.; Nakayama, K.; Setsune, J.-i. Direct determination of absolute configuration of carboxylic acids by cyclooctapyrrole. Chem. Commun. 2006, 3492−3494. (442) Mori, M.; Okawa, T.; Iizuna, N.; Nakayama, K.; Lintuluoto, J. M.; Setsune, J.-i. Dynamic Figure Eight Loop Structure of mesoTetraaryl[32]octaphyrins(1.0.1.0.1.0.1.0). J. Org. Chem. 2009, 74, 3579−3582. (443) Setsune, J.-i.; Tsukajima, A.; Okazaki, N.; Lintuluoto, J. M.; Lintuluoto, M. Enantioselective Induction of Helical Chirality in Cyclooctapyrroles by Metal-Complex Formation. Angew. Chem., Int. Ed. 2009, 48, 771−775. (444) Setsune, J.-i.; Tsukajima, A.; Watanabe, J. Synthesis and chiroptical property of C2-symmetric cyclohexapyrrole. Tetrahedron Lett. 2007, 48, 1531−1535. (445) Setsune, J.-i. Chiroptical sensing of oligonucleotides with a cyclic octapyrrole. J. Chem. Sci. 2012, 124, 1151−1163.

(401) Suijkerbuijk, B. M. J. M.; Klein Gebbink, R. J. M. Merging Porphyrins with Organometallics: Synthesis and Applications. Angew. Chem., Int. Ed. 2008, 47, 7396−7421. (402) Ren, T. Peripheral Covalent Modification of Inorganic and Organometallic Compounds through C−C Bond Formation Reactions. Chem. Rev. 2008, 108, 4185−4207. (403) Shinokubo, H.; Osuka, A. Marriage of porphyrin chemistry with metal-catalysed reactions. Chem. Commun. 2009, 1011−1021. (404) Senge, M. O. Stirring the porphyrin alphabet soup functionalization reactions for porphyrins. Chem. Commun. 2011, 47, 1943−1960. (405) Yorimitsu, H.; Osuka, A. Organometallic Approaches for Direct Modification of Peripheral C−H Bonds in Porphyrin Cores. Asian J. Org. Chem. 2013, 2, 356−373. (406) Suzuki, M.; Shimizu, S.; Shin, J.-Y.; Osuka, A. Regioselective nucleophilic substitution reaction of meso-hexakis(pentafluorophenyl) substituted [26]hexaphyrin. Tetrahedron Lett. 2003, 44, 4597−4601. (407) Figueira, F.; Farinha, A. S. F.; Tomé, A. C.; Cavaleiro, J. A. S.; Almeida Paz, F. A.; Tomé, J. P. C. Synthesis of hexaphyrins and N-fused pentaphyrins bearing pyridin-4-ylsulfanyl groups. J. Porphyrins Phthalocyanines 2014, 18, 824−831. (408) Figueira, F.; Farinha, A. S. F.; Muteto, P. V.; Polêto, M. D.; Verli, H.; Gomes, M. T. S. R.; Tomé, A. C.; Cavaleiro, J. A. S.; Tomé, J. P. C. [28]Hexaphyrin derivatives for anion recognition in organic and aqueous media. Chem. Commun. 2016, 52, 2181−2184. (409) Yoneda, T.; Kim, T.; Soya, T.; Neya, S.; Oh, J.; Kim, D.; Osuka, A. Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps. Chem. - Eur. J. 2016, 22, 4413−4417. (410) Mori, G.; Shinokubo, H.; Osuka, A. Highly selective Ir-catalyzed direct sixfold borylation of peripheral aromatic substituents on hexakisaryl-substituted [28]hexaphyrin(1.1.1.1.1.1). Tetrahedron Lett. 2008, 49, 2170−2172. (411) Suzuki, M.; Taniguchi, R.; Osuka, A. Doubly N-fused meso-aryl substituted hexaphyrins(1.1.1.1.1.1). Chem. Commun. 2004, 2682− 2683. (412) Higashino, T.; Inoue, M.; Osuka, A. Singly N-Fused Möbius Aromatic [28]Hexaphyrins(1.1.1.1.1.1). J. Org. Chem. 2010, 75, 7958− 7961. (413) Lim, J. M.; Inoue, M.; Sung, Y. M.; Suzuki, M.; Higashino, T.; Osuka, A.; Kim, D. Conformation dynamics of non-, singly- and doublyN-fused [28]hexaphyrins revealed by photophysical studies. Chem. Commun. 2011, 47, 3960−3962. (414) Davis, N. K. S.; Pawlicki, M.; Anderson, H. L. Expanding the Porphyrin π-System by Fusion with Anthracene. Org. Lett. 2008, 10, 3945−3947. (415) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. BisAnthracene Fused Porphyrins: Synthesis, Crystal Structure, and NearIR Absorption. Org. Lett. 2010, 12, 2124−2127. (416) Davis, N. K. S.; Thompson, A. L.; Anderson, H. L. A Porphyrin Fused to Four Anthracenes. J. Am. Chem. Soc. 2011, 133, 30−31. (417) Mori, H.; Tanaka, T.; Osuka, A. Fused porphyrinoids as promising near-infrared absorbing dyes. J. Mater. Chem. C 2013, 1, 2500−2519. (418) Naoda, K.; Mori, H.; Aratani, N.; Lee, B. S.; Kim, D.; Osuka, A. Hexaphyrin Fused to Two Anthracenes. Angew. Chem., Int. Ed. 2012, 51, 9856−9859. (419) Suzuki, M.; Hoshino, T.; Neya, S. Skeletal Recombination Reaction of N-Fused Pentaphyrin(1.1.1.1.1) via Bromination. Org. Lett. 2014, 16, 327−329. (420) Youfu, K.; Osuka, A. Regioselective vicinal-dichlorination of meso-aryl [26]hexaphyrin(1.1.1.1.1.1). Tetrahedron Lett. 2006, 47, 1381−1384. (421) Yoneda, T.; Aratani, N.; Osuka, A. Regioselective fabrications of a Möbius aromatic [28]hexaphyrin palladium(II) complex. J. Porphyrins Phthalocyanines 2013, 17, 665−672. (422) Tanaka, T.; Osuka, A. Regioselective Palladation of a Möbius Aromatic [28]Hexaphyrin(1.1.1.1.1.1) PdII Complex. Chem. - Eur. J. 2012, 18, 7036−7040. 2639


Chemical Reviews

Review

and 1,2-Diamino-4,5-dimethoxybenzene. Inorg. Chem. 1992, 31, 529− 531. (467) Sessler, J. L.; Mody, T. D.; Dulay, M. T.; Espinoza, R.; Lynch, V. The template synthesis and X-ray characterization of pyrrole-derived hexadentate uranyl(V1) Schiff-base macrocyclic complexes. Inorg. Chim. Acta 1996, 246, 23−30. (468) Sessler, J. L.; Seidel, D.; Vivian, A. E.; Lynch, V.; Scott, B. L.; Keogh, D. W. Hexaphyrin(1.0.1.0.0.0): An Expanded Porphyrin Ligand for the Actinide Cations Uranyl (UO22+) and Neptunyl (NpO2+). Angew. Chem., Int. Ed. 2001, 40, 591−594. (469) Ding, Y.; Tang, Y.; Zhu, W.; Xie, Y. Fluorescent and colorimetric ion probes based on conjugated oligopyrroles. Chem. Soc. Rev. 2015, 44, 1101−1112. (470) Ding, Y.; Zhu, W.-H.; Xie, Y. Development of Ion Chemosensors Based on Porphyrin Analogues. Chem. Rev. 2016. (471) Zhu, X.-J.; Fu, S.-T.; Wong, W.-K.; Guo, J.-P.; Wong, W.-Y. A Near-Infrared-Fluorescent Chemodosimeter for Mercuric Ion Based on an Expanded Porphyrin. Angew. Chem., Int. Ed. 2006, 45, 3150−3154. (472) Zhu, X.; Fu, S.; Wong, W.-K.; Wong, W.-Y. A near-infrared fluorescent chemodosimeter for silver(I) ion based on an expanded porphyrin. Tetrahedron Lett. 2008, 49, 1843−1846. (473) Wu, D.; Descalzo, A. B.; Weik, F.; Emmerling, F.; Shen, Z.; You, X.-Z.; Rurack, K. A Core-Modified Rubyrin with meso-Aryl Substituents and Phenanthrene-Fused Pyrrole Rings: A Highly Conjugated NearInfrared Dye and Hg2+ Probe. Angew. Chem., Int. Ed. 2008, 47, 193−197. (474) Ikawa, Y.; Takeda, M.; Suzuki, M.; Osuka, A.; Furuta, H. Watersoluble doubly N-confused hexaphyrin: a near-IR fluorescent Zn(II) ion sensor in water. Chem. Commun. 2010, 46, 5689−5691. (475) Ikawa, Y.; Katsumata, S.; Sakashita, R.; Furuta, H. Spectrometric Detection of DNA by the Bis-Zn(II) Complex of a Water-soluble Doubly N-Confused Hexaphyrin. Spectrometric Detection of DNA by the Bis-Zn(II) Complex of a Water-soluble Doubly N-Confused Hexaphyrin. Chem. Lett. 2014, 43, 1929−1931.

(446) Ie, M.; Setsune, J.-i.; Eda, K.; Tsuda, A. Chiroptical sensing of oligonucleotides with a cyclic octapyrrole. Org. Chem. Front. 2015, 2, 29−33. (447) Tanaka, T.; Sugita, T.; Tokuji, S.; Saito, S.; Osuka, A. Metal Complexes of Chiral Möbius Aromatic [28]Hexaphyrin(1.1.1.1.1.1): Enantiomeric Separation, Absolute Stereochemistry, and Asymmetric Synthesis. Angew. Chem., Int. Ed. 2010, 49, 6619−6621. (448) Mori, T.; Tanaka, T.; Higashino, T.; Yoshida, K.; Osuka, A. Combined Experimental and Theoretical Investigations on Optical Activities of Möbius Aromatic and Möbius Antiaromatic Hexaphyrin Phosphorus Complexes. J. Phys. Chem. A 2016, 120, 4241−4248. (449) Rzepa, H. S. The Chiro-optical Properties of a Lemniscular Octaphyrin. Org. Lett. 2009, 11, 3088−3091. (450) Liu, Z.; Tian, Z.; Li, W.; Meng, S.; Wang, L.; Ma, J. Chiral Interconversions of Pd and/or Au Bis-Metalated [32]Octaphyrins(1,0,1,0,1,0,1,0) Involving Hückel and Möbius Macrocyclic Topologies: A Theoretical Prediction. J. Org. Chem. 2012, 77, 8124−8130. (451) Alonso, M.; Geerlings, P.; de Proft, F. Viability of Möbius Topologies in [26]- and [28]Hexaphyrins. Chem. - Eur. J. 2012, 18, 10916−10928. (452) Alonso, M.; Geerlings, P.; de Proft, F. Topology Switching in [32]Heptaphyrins Controlled by Solvent, Protonation, and meso Substituents. Chem. - Eur. J. 2013, 19, 1617−1628. (453) Alonso, M.; Geerlings, P.; de Proft, F. Conformational Control in [22]- and [24]Pentaphyrins(1.1.1.1.1) by Meso Substituents and their N-Fusion Reaction. J. Org. Chem. 2013, 78, 4419−4431. (454) Alonso, M.; Geerlings, P.; De Proft, F. Exploring the structure− aromaticity relationship in Hückel and Möbius N-fused pentaphyrins using DFT. Phys. Chem. Chem. Phys. 2014, 16, 14396−14407. (455) Alonso, M.; Pinter, B.; Geerlings, P.; de Proft, F. Metalated Hexaphyrins: From Understanding to Rational Design. Chem. - Eur. J. 2015, 21, 17631−17638. (456) Torrent-Sucarrat, M.; Anglada, J. M.; Luis, J. M. Evaluation of the Nonlinear Optical Properties for Annulenes with Hückel and Möbius Topologies. J. Chem. Theory Comput. 2011, 7, 3935−3943. (457) Marcos, E.; Anglada, J. M.; Torrent-Sucarrat, M. Theoretical Study of the Switching between Hückel and Möbius Topologies for Expanded Porphyrins. J. Phys. Chem. C 2012, 116, 24358−24366. (458) Torrent-Sucarrat, M.; Anglada, J. M.; Luis, J. M. Evaluation of the nonlinear optical properties for an expanded porphyrin Hückel-Möbius aromaticity switch. J. Chem. Phys. 2012, 137, 184306. (459) Marcos, E.; Anglada, J. M.; Torrent-Sucarrat, M. Effect of the Meso-Substituent in the Hückel-to-Möbius Topological Switches. J. Org. Chem. 2014, 79, 5036−5046. (460) Toganoh, M.; Furuta, H. Theoretical Study on Conformation and Electronic State of Hückel-Aromatic Multiply N-Confused [26]Hexaphyrins. J. Org. Chem. 2010, 75, 8213−8223. (461) Toganoh, M.; Furuta, H. Theoretical Study on the Conformation and Aromaticity of Regular and Singly N-Confused [28]Hexaphyrins. J. Org. Chem. 2013, 78, 9317−9327. (462) Sun, G.; Li, H.; Jiang, S.; Wang, X.; Liu, C.; Fu, Q. Theoretical study on electronic spectrum properties of group 12 bis-metal(II) complexes of [26]hexaphyrin(1.1.1.1.1.1). Comput. Theor. Chem. 2011, 976, 68−75. (463) Hršak, D.; Pertejo, M. A.; Lamsabhi, A. M.; Muranaka, A.; Ceulemans, A. Explanation of the sign inversion in the magnetic circular dichroism of Möbius [28] hexaphyrin. Chem. Phys. Lett. 2013, 586, 148− 152. (464) Ménand, M.; Sollogoub, M.; Boitrel, B.; Le Gac, S. HexaphyrinCyclodextrin Hybrids: A Nest for Switchable Aromaticity, Asymmetric Confinement, and Isomorphic Fluxionality. Angew. Chem., Int. Ed. 2016, 55, 297−301. (465) Burrell, A. K.; Hemmi, G.; Lynch, V.; Sessler, J. L. Uranylpentaphyrin: An Actinide Complex of an Expanded Porphyrin. J. Am. Chem. Soc. 1991, 113, 4690−4692. (466) Sessler, J. L.; Mody, T. D.; Lynch, V. Synthesis and X-ray Characterization of a Uranyl(VI) Schiff Base Complex Derived from a 2:2 Condensation Product of 3,4-Diethylpyrrole-2,5-dicarbaldehyde 2640


Chemistry of meso-Aryl-Substituted Expanded Porphyrins: Aromaticity

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