Recent Advances in Subporphyrins and Triphyrin Analogues

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Review pubs.acs.org/CR

Recent Advances in Subporphyrins and Triphyrin Analogues: Contracted Porphyrins Comprising Three Pyrrole Rings Soji Shimizu* Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-0395, Japan Center for Molecular Systems (CMS), Kyushu University, Fukuoka 819-0395, Japan ABSTRACT: Subporphyrinato boron (subporphyrin) was elusive until the syntheses of tribenzosubporphine in 2006 and meso-aryl-substituted subporphyrin in 2007. These novel contracted analogues possess a 14π-electron conjugated system embedded in a bowl-shaped structure. They exhibit absorption and fluorescence in the UV/vis region and nonlinear optical properties due to their octupolar structures. The unique coordination geometry around the central boron atom in the structure of subporphyrin enabled investigation of rare boron species, such as borenium cations, boron hydrides, and boron peroxides. Along with the burgeoning development of the chemistry of subporphyrins, analogous triphyrin systems have also emerged. Their rich coordination chemistry as a result of their free-base structures, which are different from the boroncoordinating structure of subporphyrins, has been intensively investigated. On the basis of the unique structures and reactivities of subporphyrins and their related triphyrin analogues, supramolecular architectures and covalently linked multicomponent systems have also been actively pursued. This Review provides an overview of the development of subporphyrin and triphyrin chemistry in the past decade and future prospects in this field, which may inspire molecular design toward applications based on their unique properties.

CONTENTS 1. Introduction 2. Subporphyrins 2.1. Synthesis of Subporphyrins 2.1.1. Synthesis of Tribenzosubporphines 2.1.2. Synthesis of A3-Type meso-Aryl-Substituted Subporphyrins 2.1.3. Synthesis of A2B-Type meso-Aryl-Substituted Subporphyrins 2.1.4. Synthesis of ABC-Type meso-Aryl-Substituted Subporphyrins 2.1.5. Synthesis of meso-Alkyl-Substituted Subporphyrins 2.1.6. Synthesis of meso-Aryl-Substituted Subporphyrins from Heptaphyrins 2.1.7. Synthesis of Subpyriporphyrin 2.1.8. Synthesis of Subporphyrin−Subphthalocyanine Hybrid 2.2. Characterization of Subporphyrins 2.2.1. X-ray Crystal Structures 2.2.2. Aromaticity of Subporphyrins and NMR Spectroscopy 2.3. Optical and Electrochemical Properties of Subporphyrins 2.3.1. UV/Vis Absorption Spectra and Basic Theoretical Description 2.3.2. MCD Spectroscopy 2.3.3. Fluorescence Spectra and Excited States 2.3.4. Redox Properties

© XXXX American Chemical Society

2.4. Reactivities of Subporphyrins and Properties of Functionalized Subporphyrins 2.4.1. Subchlorin and Subbacteriochlorin 2.4.2. Subporphyrin Borenium Cations 2.4.3. Functionalization of the Axial Position of the Central Boron 2.4.4. Functionalization of the para-Positions of meso-Aryl-Substituted Subporphyrins 2.4.5. Functionalization of the meso-Positions of Subporphyrins 2.4.6. Peripheral Functionalization of mesoAryl-Substituted Subporphyrins 2.5. Subporphyrin Dimers and Multicomponent Systems of Subporphyrins 2.5.1. Subporphyrin Dimers Linked at the Axial Position of the Central Boron 2.5.2. Subporphyrin Dimers Linked at the meso-Position 2.5.3. Subporphyrin Dimers Linked at the Peripheral β-Position 2.5.4. Multicomponent Systems of Subporphyrins 2.5.5. Supramolecular Systems of Subporphyrins 2.6. Further Topics of Subporphyrins

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: June 24, 2016

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Chemical Reviews 2.6.1. Single Molecular Electrical Properties of Subporphyrin and its Borenium Cation 2.6.2. Photophysical Properties of Subporphyrins on Anionic Clay Surface 3. Triphyrins 3.1. Synthesis and Coordination Chemistry of [14]Triphyrins(2.1.1) 3.1.1. Synthesis of [14]Triphyrins(2.1.1) 3.1.2. X-ray Crystal Structures of [14]Triphyrins(2.1.1) 3.1.3. Aromaticity of [14]Triphyrins(2.1.1) and NMR Spectroscopy 3.1.4. Photophysical Properties of [14]Triphyrins(2.1.1) 3.1.5. Redox Properties of [14]Triphyrins(2.1.1) 3.1.6. Coordination Chemistry of [14]Triphyrins(2.1.1) 3.1.7. Further Topics of [14]Triphyrin(2.1.1) 3.2. Other Recent Progress in Triphyrin Systems 4. Conclusions and Future Prospects Author Information Corresponding Author Notes Biography Acknowledgments References

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other heteroaromatic rings has been intensively investigated owing to possible tuning of the unique optical and electrochemical properties of porphyrin toward a wide range of applications in photoenergy conversion and molecular electronics.12 In general, these modified porphyrin analogues can be categorized into expanded, contracted, and isomeric porphyrins (Chart 2). This terminology was introduced in Expanded,

AO AP AQ AQ AQ

Chart 2. Representative Examples of Expanded, Contracted, and Isomeric Porphyrins and the Relationship between Porphyrin and Porphyrin Analogues

AR AR AR AT AT AV AW AX AY AY AY AY AY AZ

1. INTRODUCTION Fundamental research of porphyrins, which are naturally occurring red or green dyes, was initiated because of the interest in their crucial roles in vital activities, photosynthesis of plants and respiration of animals. It is known that the tetrapyrrolic macrocyclic structure of porphyrin with an 18π-electron conjugated system, in which pyrrole rings are connected by methine carbon atoms, namely, at meso positions, gives rise to visible light absorption of chlorophyll for light-energy harvesting and energy transfer in photosynthesis of plants1−4 and dioxygen binding of heme for oxygen transport in blood (Chart 1).5−9

Contracted, and Isomeric Porphyrins by Sessler and Weghorn in 1997.13 Among the porphyrin analogues, considerable work has been devoted to expanded and isomeric porphyrins.12−16 As an example of expanded porphyrins, sapphyrin17 is shown in Chart 2. Expanded porphyrins consist of more than four pyrrole rings and possess larger macrocyclic conjugated systems than the 18πelectron conjugated system of porphyrin. As examples of isomeric porphyrins, N-confused porphyrin18,19 and porphycene20 are also shown in Chart 2. They are constitutional isomers that share the C20H14N4 core component with porphyrin. In contrast to the variety of expanded and isomeric porphyrins known to date, contracted porphyrin analogues have rather been limited. Among the contracted porphyrins, corrole, which was synthesized by Johnson and Kay in 1964 (Chart 2),21 has been well investigated. The structure of corrole has one less mesocarbon atom than porphyrin, but the 18π-electron conjugated system is retained. Owing to the unique trivalent tetradentate coordination geometry, which can stabilize high-valent transition metal ions, metallocorroles have been intensively studied.22 Removing one pyrrole ring from the structure of corrole leads to a subporphyrin structure with more contracted 14π-electron conjugation than 18π-electron conjugation of porphyrin and corrole. Subporphyrin is named after subphthalocyanine, a contracted analogue of phthalocyanine. Subphthalocyanine consists of three isoindole rings and three meso-nitrogen atoms rather than pyrrole rings and meso-carbon atoms (Chart 3). Since its serendipitous discovery by Meller and Ossko in 1972,23 subphthalocyanine has been widely investigated in a variety of fields, such as organic photovoltaics, light-emitting diodes, fieldeffect transistors, and nonlinear optics, owing to its unique bowlshaped structure, 14π-electron aromatic conjugation, and intense Q-band absorption in the visible region.24−26 Subphthalocyanine

Chart 1. Porphyrin (Middle), Chlorophyll a (Right), and Heme B (Left)

Along with synthetic investigations, theoretical research has also been performed to give insight into the electrochemical and photophysical properties of porphyrin and its analogues. One of the theoretical achievements in this field is the so-called fourorbital theory developed by Gouterman, which clearly explains the origin of characteristic Soret and Q-band absorption and the difference in the relative intensities of these bands.10,11 From the early stage of the porphyrin chemistry, structural modification of porphyrin by changing the number of pyrrole rings and/or meso-carbon atoms or replacing pyrrole rings with B

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the initial studies on triphyrins is the synthesis of vacataporphyrin ([18]triphyrin(6.1.1)) by Latos-Grażyński and co-workers31 and the unique switching behavior of its palladium complex between Hückel and Möbius topologies.32 Other types of triphyrins, such as [18]triphyrin(4.1.1)33,34 and [15]triphyrin(3.1.1),35 have also been reported (Chart 4). These early contributions in the chemistry of triphyrin with conjugated systems larger than 14πelectron conjugation were comprehensively overviewed in other review articles.13,36,37 In 2006, Osuka and co-workers successfully synthesized tribenzosubporphine under the rather classical synthetic conditions of tetrabenzoporphine developed by Gouterman and co-workers38 using boric acid instead of zinc acetate as a templating reagent (Chart 5).28 In the same year, Latos-

Chart 3. Subphthalocyanine and Phthalocyanine, which are meso-Nitrogen Counterparts of Subporphyrin and Porphyrin, Respectively

is a boron complex that is conventionally synthesized in moderate yields from phthalonitrile or its derivatives using a boron trihalide (BCl3 or BBr3) as a template. The facile synthesis of subphthalocyanine presumably encouraged a number of porphyrin chemists to attempt the synthesis of subporphyrin. Although subporphyrin was elusive for more than 30 years until its seminal synthesis of tribenzosubporphine by Osuka and co-workers in 2006,27,28 an important contribution has been devoted to pioneer the synthesis of tripyrrolic macrocyclic systems (referred to as triphyrin) with larger conjugated systems than 14π-electron conjugation. The first triphyrins can be traced back to 1964, when Badger et al. reported so-called heteroatom-bridged [18]annulenes, some of which retained a porphyrin-like 18πelectron conjugated system (Chart 4).29 From the initial work by Badger et al., other groups developed syntheses of this kind of macrocycle. The groups of Vogel and Cava established McMurry-type coupling reactions of 2,5-diformyl-substituted heteropentacycle precursors.30 Non-negligible contribution in

Chart 5. Representative Examples of Subporphyrin and Triphyrin Analogues with Contracted 14π-Electron Conjugation

Chart 4. Representative Examples of Triphyrin with Larger Conjugated Systems than 14π-Electron Conjugation Grażyński and co-workers reported the first free-base form of a subporphyrin species, subpyriporphyrin, which was synthesized in a stepwise manner using a tripyrrolylmethane-like [2 + 1]-type precursor with a pyridine unit at the center.39 In porphyrin chemistry, meso-aryl-substituted porphyrins have been investigated as mainstream compounds owing to their facile preparation from pyrrole and arylaldehyde under Rothemund− Lindsey reaction conditions40 and sufficient solubility to common organic solvents. Analogous to the chemistry of porphyrin, meso-aryl-substituted subporphyrin became the next target to increase the availability of subporphyrin species. Unlike subphthalocyanine synthesis, template synthesis is rather unfavorable because the macrocyclization reaction of pyrrole and arylaldehyde is generally carried out under acidic conditions, which prevents coordination of pyrrole nitrogen atoms to boron templates. Tri-N-pyrrolylborane, a boron complex with three pyrrole ligands, was found to be a reasonable precursor for the synthesis of meso-aryl-substituted subporphyrin. Although this compound was reported in 196841 and pioneering research established its synthesis under milder conditions,42−44 the importance of these molecules as a synthetic precursor was ignored until the first synthesis of a meso-aryl-substituted subporphyrin by Kobayashi et al.45,46 Subsequently, using a pyridine adduct of tri-N-pyrrolylborane (pyridine-tri-N-pyrrolylborane) that is more moisture- and oxygen-stable than tri-Npyrrolylborane, Osuka and co-workers optimized the reaction C

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conditions to establish a more reliable synthetic route to obtain meso-aryl-substituted subporphyrins.47 In addition, they also developed two other approaches: a reaction using a boron complex of tripyrromethane as a precursor,48 which enables preparation of meso-free subporphyrin and A2B- and ABC-type meso-aryl-substituted subporphyrins, and an extrusion reaction of subporphyrins with electron-deficient pentafluorophenyl or trifluoromethyl substituents from the corresponding [32]heptaphyrins upon cooperative coordination of copper(II) and boron(III).49,50 With these synthetic achievements, unique reactivities at the axial, peripheral, and meso-positions and nonlinear optical properties have been revealed in the past decade.51,52 During the rapid growth of subporphyrin chemistry, triphyrins with 14π-electron conjugated system have also emerged.36,37 In 2008, Shen, Yamada, and co-workers made a seminal report on the synthesis of [14]triphyrin(2.1.1).53 One of the strikingly different properties of [14]triphyrin(2.1.1) from those of subporphyrins is its rich coordination chemistry owing to the nonboron-containing free-base structure. In this Review, research of subporphyrin and its related analogues with 14π-electron contracted conjugation is reviewed, including their synthesis, functionalization, physical properties, and theoretical calculations. Subphthalocyanine and subporphyrazine, which are meso-aza counterparts of subporphyrin, are out of the scope of this Review because Torres and co-workers have already published two comprehensive review articles of subphthalocyanine and related analogues.24,25 Although subporphyrin and triphyrins were also described in one of their review articles,25 it is necessary to review this fast-growing field again because of the significant progress in the past decade. Throughout this Review, the systematic nomenclature proposed by Gosmann and Franck,54 which is used for expanded porphyrins, is applied to triphyrin systems, except for tribenzosubporphine, subporphyrin, and subpyriporphyrin, because these names are accepted as their common names. For example, according to this nomenclature, subporphyrin is called [14]triphyrin(1.1.1), in which the numbers in the square brackets and parentheses (i.e., 14 and 1.1.1, respectively) indicate the number of π-electrons in the conjugation circuit and the number of bridging carbon atoms between the pyrrole rings, respectively. Although subporphyrin is more appropriately referred to as “a boron complex of subporphyrin” or “subporphyrinato boron”, considering the fact that subporphyrins hitherto reported contain a boron atom at the center, “subporphyrin” is used for convenience in this Review.

Scheme 1. Synthesis of Tribenzosubporphine 1

(B-hydroxy form). Because the axial ligand is susceptible for substitution, the hydroxy group is easily replaced by a methoxy group on dissolving 1 in methanol. The reactivity of axial ligand exchange is detailed in the following section. Tribenzosubporphine derivatives with aryl substituents at the meso-positions can be synthesized in a similar manner. Kobayashi, Luk’yanets, and co-workers reported the synthesis of meso-phenyl-substituted tribenzosubporphyrin 2 in 7.8% yield from 3-benzalphthalimidine using boric acid as a template (Scheme 2).55 The same compound was also prepared from Scheme 2. Synthesis of meso-Phenyl-Substituted Tribenzosubporphyrin 2

phthalimide and phenylacetic acid in a similar yield. In addition to the phenyl substituent, substituted aryl groups, such as 2,6dichlorophenyl groups, can be introduced using aryl acetic acids with the corresponding substituents. Similar to 1, a facile axial ligand exchange from the B-hydroxy form to the B-ethoxy form was observed for 2 when ethanol was used during the purification procedure. 2.1.2. Synthesis of A3-Type meso-Aryl-Substituted Subporphyrins. meso-Aryl-substituted subporphyrins with the same aryl substituents at three meso-positions, which can be referred to as A3-type compounds, were synthesized for the first time by Kobayashi et al.,45,46 using tri-N-pyrrolylborane41−44 as a key precursor under Adler reaction conditions (Scheme 3).56 Tri-N-pyrrolylborane dispersed in propionic acid was added dropwise to a solution of arylaldehyde in propionic acid at 0.1 mol/L under reflux conditions. After tedious purification by silica gel, alumina, and preparative silica gel thin-layer chromatography, meso-aryl-substituted subporphyrins 3−8 were obtained in 4−8% yields. Reactions of arylaldehydes with electrondonating substituents provided higher yields of subporphyrins than those with electron-deficient substituents. Despite the successful synthesis of meso-aryl-substituted subporphyrins, the first synthetic method was found to be rather problematic because of the following reasons. First, air- and moisture-sensitive tri-N-pyrrolylborane should be handled under inert conditions. Second, the pure crystalline solids of tri-Npyrrolylborane prepared by sublimation are poorly soluble in dichloromethane and chloroform, which are common solvents for syntheses of porphyrin analogues. Finally, the reaction

2. SUBPORPHYRINS 2.1. Synthesis of Subporphyrins

2.1.1. Synthesis of Tribenzosubporphines. In 2006, Osuka and co-workers reported the first synthesis of subporphyrin, tribenzosubporphine 1,28 by modifying reaction conditions that were developed for the synthesis of tetrabenzoporphine by Gouterman and co-workers.38 (3-Oxo-2,3-dihydro1H-isoindol-1-yl)acetic acid and boric acid were ground into a fine powder and subsequently heated up to 350 °C for 3.5 h under nitrogen atmosphere to provide 1 in 1.4% yield after repeated chromatographic purification (Scheme 1). As a template, phenylboronic acid or 4-methoxyphenylboronic acid can also be utilized in the place of boric acid, and 1 was obtained in comparable yields. The synthesis of 1 was also carried out under microwave irradiation (500 W) for 30 min to provide 1 in 1.2% yield. 1 was obtained as an axially hydroxy-substituted form D

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Scheme 3. Synthesis of A3-Type meso-Aryl-Substituted Subporphyrins 3−8

Scheme 4. Improved Synthetic Methods of A3-Type mesoAryl-Substituted Subporphyrins 9−15

mixture obtained in this method contained a certain amount of meso-aryl-substituted porphyrins and boron dipyrromethene derivatives because of dissociation of pyrrole from tri-Npyrrolylborane under harsh acidic conditions in the Adler-type reaction (Figure 1).

Figure 1. Absorption spectrum of the reaction solution at the end of the condensation reaction of 3, which was diluted with chloroform. (1)−(3) in the figure denote peak positions associated with 3, tetraphenylsubstituted porphyrin (TPP), and the boron dipyrromethene derivative, respectively. Reprinted with permission from ref 46. Copyright 2007 American Chemical Society.

presence of trifluoroacetic acid (TFA) for 1 h. After being neutralized with pyridine, the reaction mixture was refluxed in odichlorobenzene under air for 1 h. In both syntheses, a variety of A3-type meso-aryl-substituted porphyrins 9−15 were isolated in 1.1−5.6% yields. Protocol B provided better yields of subporphyrins with less sterically hindered aryl substituents, while the isolated yields in protocol B significantly dropped in the case of ortho-substituted arylaldehydes. As pointed out by Kobayashi et al. in their first report on the synthesis of meso-aryl-substituted subporphyrins, it was tedious to purify meso-aryl-substituted porphyrins by silica gel chromatography because of the highly polar nature of the Bhydroxy forms.45,46 Osuka and co-workers also successfully established the purification procedures by isolating subporphyrins as B-methoxy forms, which can be easily obtained by refluxing the B-hydroxy forms in methanol.47 As detailed in the following section, during the synthesis of meso-aryl-substituted subporphyrins, Osuka and co-workers noticed that a certain amount of subchlorin, which is a peripherally reduced form of subporphyrin, was obtained as a byproduct.58 Because this compound was eluted closely with a fraction containing subporphyrin, contamination of subchlorin

To attain reproducibility of the synthesis of meso-arylsubstituted subporphyrins, Osuka and co-workers examined other tri-N-pyrrolylborane derivatives, and they found that pyridine−tri-N-pyrrolylborane is a suitable precursor owing to its sufficient air and moisture stabilities and better solubility in common organic solvents relative to tri-N-pyrrolylborane.47 Pyridine−tri-N-pyrrolylborane can be easily prepared on a 50− 60 g scale from a reaction of freshly distilled pyrrole and borane triethylamine complex (BH3·NEt3)41 and a subsequent reaction with pyridine.57 They further optimized the reaction conditions and developed two synthetic methods (Scheme 4). In protocol A, meso-aryl-substituted subporphyrins were synthesized from pyridine−tri-N-pyrrolylborane and arylaldehydes under open air refluxing conditions in o-dichlorobenzene in the presence of chloroacetic acid. They used 9 equiv of arylaldehydes to suppress formation of porphyrin and polypyrrolic byproducts caused by dissociation of pyrroles from the precursor. As another method to prevent scrambling of pyridine−tri-N-pyrrolylborane, lowtemperature reaction conditions were also examined. In synthetic protocol B, pyridine−tri-N-pyrrolylborane was reacted with 3 equiv of arylaldehydes at 0 °C in o-dichlorobenzene in the E

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Scheme 5. Synthesis of β-Alkyl-Substituted Subporphyrins 16 and 17

also made isolation of subporphyrin difficult. Osuka and coworkers then introduced an extra oxidation procedure using MnO2 before isolation of subporphyrins by silica gel column chromatography. This further improved the yields of meso-arylsubstituted subporphyrins, for instance, 9 from 3.8% to 6.3%.58 Completely substituted subporphyrins including the βpositions were similarly synthesized by applying the A3-type synthetic procedure to 3,4-diethylpyrrole. Pyridine−tri-N-(3,4diethylpyrrolyl)borane, which was synthesized from 3,4diethylpyrrole according to the synthesis developed by Osuka and co-workers,58 was reacted with benzaldehyde or 1,3,5trioxane to provide β-ethyl-substituted subporphyrins 16 and 17 in 8% and 5% yields, respectively (Scheme 5).59 In the case of the synthesis of 17 using 1,3,5-trioxane, Panda and co-workers found that methanesulfonic acid (MSA) can facilitate the formation of 17 among various acids tested. 2.1.3. Synthesis of A2B-Type meso-Aryl-Substituted Subporphyrins. The initial attempt to obtain meso-arylsubstituted subporphyrins with two different types of aryl substituents was accomplished by Osuka and co-workers, using a 1:2:1 mixture of pyridine−N-tripyrrolylborane and two different arylaldehydes under similar reaction conditions to the synthesis of the A3-type compounds. However, this reaction provided an inseparable mixture of the target A2B-type compound and intractable byproducts. The formation of A2B-type compound was confirmed in the case of the reaction of benzaldehyde and 4bromobenzaldehyde by dimerizing or introducing an alkynyl group via transition metal-catalyzed coupling reactions.60,61 The rational synthesis of A2B-type meso-aryl-substituted subporphyrins was achieved by Osuka and co-workers during the investigation of the synthesis of meso-free subporphyrins.48 Instead of pyridine−tri-N-pyrrolylborane, a triethylamine tripyrromethene borane precursor, which was synthesized in situ from tripyrrane and BH3·NEt3 at 100 °C for 1 h, was reacted with trimethyl orthoformate in o-dichlorobenzene at 100 °C to provide 5,10-diphenyl-substituted subporphyrin 18 in 9.7% yield (Scheme 6). Osuka and co-workers further optimized the reaction conditions toward the synthesis of A2B-type meso-arylsubstituted subporphyrins using the triethylamine tripyrromethene borane precursor. After testing several acid catalysts, they finally found that use of aroyl chloride in the place of arylaldehyde worked sufficiently because HCl would be generated during the reaction. Under these reaction conditions,

Scheme 6. Synthesis of meso-Free Subporphyrin 18

a variety of A2B-type compounds (19−26) were synthesized in moderate yields (2.0−11%, Scheme 7).62 2.1.4. Synthesis of ABC-Type meso-Aryl-Substituted Subporphyrins. Following the successful syntheses of A3-type and A2B-type meso-aryl-substituted subporphyrins, ABC-type compounds with three different aryl substituents at the mesopositions were the next synthetic targets. Osuka and co-workers developed the synthetic route toward ABC-type compounds (27−34) based on the synthesis of A2B-type compounds using AB-type tripyrranes as key precursors in place of A2-type mesodiphenyl-substituted tripyrrane (Scheme 8).63 2.1.5. Synthesis of meso-Alkyl-Substituted Subporphyrins. The synthetic methods of meso-aryl-substituted subporphyrins cannot simply be applied to the synthesis of meso-alkyl-substituted subporphyrins. Osuka and co-workers investigated an indirect synthesis via desulfurization of meso-(2thienyl)-substituted subporphyrins 35−37 with Raney nickel.64 Because this reaction provided several over-reduced products, oxidation of the reaction mixture with o-chloranil was performed to facilitate isolation of meso-alkyl-substituted subporphyrins 38−40 in ca. 50% yields (Scheme 9). 2.1.6. Synthesis of meso-Aryl-Substituted Subporphyrins from Heptaphyrins. The above-mentioned synthetic methods have not been applicable to the synthesis of A3-type subporphyrins with electron-deficient pentafluorophenyl or trifluoromethyl substituents. Osuka and co-workers found that these particular subporphyrins can be synthesized by skeletal rearrangement of [32]heptaphyrin(1.1.1.1.1.1.1)65,66 during metalation of copper(II) and boron(III).49,50 A similar thermal-splitting reactivity was observed for copper complexes of [36]octaphyrins(1.1.1.1.1.1.1.1), which split into two copper porphyrins.67,68 F

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Scheme 7. Synthesis of A2B-Type meso-Aryl-Substituted Subporphyrins 19−26

Scheme 8. Synthesis of ABC-Type meso-Aryl-Substituted Subporphyrins 27−34

Scheme 9. Synthesis of meso-Alkyl-Substituted Subporphyrins 38−40 from meso-(2-Thienyl)-Substituted Subporphyrins 35−37

meso-trifluoromethyl-substituted subporphyrin 42 was obtained in 12% yield from a copper complex of [32]heptaphyrin(1.1.1.1.1.1.1) with meso-trifluoromethyl substituents. 2.1.7. Synthesis of Subpyriporphyrin. Probably owing to the small biting angle between two Cα−Cmeso bonds of a pyrrole ring (Cmeso and Cα denote meso-carbon atoms and α-pyrrolic carbon atoms, respectively), it would be impossible to synthesize subporphyrins without boron templates. In addition, removing the central boron from subporphyrins has not yet been successful. Therefore, a free-base form of subporphyrin has still remained elusive except for a core-modified subporphyrin reported by Latos-Grażyński and co-workers. Before the first synthesis of meso-aryl-substituted subporphyrins by Kobayashi et al.,45,46 they found that core modification with a pyridine ring, which possesses a larger biting angle than that of a pyrrole ring, enabled the synthesis of a free-base form.39 Subpyriporphyrin 43 was synthesized in a stepwise manner from 2,6-bis[hydroxy(mesityl)methyl]pyridine (Scheme 11). The sterically bulky mesityl substituents are necessary for preventing acidolysis. Subpyriporphyrin was then obtained in 6% yield after chromatographic purification. In contrast to subporphyrins, subpyriporphyrin is nonaromatic because the local aromaticity of the pyridine ring is predominant. 2.1.8. Synthesis of Subporphyrin−Subphthalocyanine Hybrid. Substitution of meso-carbon atoms with nitrogen atoms in the structure of porphyrin leads to hybrid structures between porphyrin and phthalocyanine. It is well-known that these so-

Copper metalation of meso-pentafluorophenyl-substituted [32]heptaphyrin(1.1.1.1.1.1.1) provided a twisted figure-eight form of a monometalated complex, in which a subporphyrin-like coordination pocket was formed. meso-Pentafluorophenylsubstituted subporphyrin 41 was subsequently obtained in 36% yield from a boron coordination reaction of this copper complex in CH2Cl2 at room temperature using BBr3 (100 equiv) and EtN(iPr)2 (150 equiv) (Scheme 10).49 In contrast, zinc metalation of [32]heptaphyrin(1.1.1.1.1.1.1) gave a rather complicated mixture containing ΔLUMO for the other [14]triphyrins(2.1.1), which is well-reproduced by the DFT calculations.133 The initial studies on fluorescence of [14]triphyrins(2.1.1) revealed that they are virtually less fluorescent compared to subporphyrins (Figure 59). The fluorescence quantum yields of benzo- and naphtho-fused [14]triphyrins(2.1.1) 250−252 and 256 are ca. 0.01 and 0.04, respectively, whereas those of the others are