Synthesis of Aza-, Oxa-, and Thiaporphyrins and Related Compounds

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Synthesis of Aza‑, Oxa‑, and Thiaporphyrins and Related Compounds Yoshihiro Matano* Department of Chemistry, Faculty of Science, Niigata University, Nishi-ku, Niigata 950-2181, Japan ABSTRACT: Chemical modification at the periphery with nitrogen or chalcogens is a highly promising strategy to diversify the optical, electrochemical, magnetic, and coordination properties of the porphyrin family. Indeed, various kinds of phthalocyanines and related benzo-annelated azaporphyrinoids have been synthesized, and their fundamental properties have been extensively investigated. However, the synthesis of heteroatom-containing porphyrins in which the peripheral methine groups are partially replaced with nitrogen or chalcogens remains a considerable challenge. In this review, we will focus mainly on recent advances in the synthesis of aza-, oxa-, and thiaorphyrins and related compounds, including historically important examples. 3.4. Heterocorroles Containing One meso-Chalcogen Atom 4. Conclusions Author Information Corresponding Author Notes Biography Acknowledgments Dedication Abbreviations References

CONTENTS 1. Introduction 2. Azaporphyrins and Related Compounds 2.1. Azaporphyrins Containing One meso-Nitrogen Atom 2.1.1. Synthesis 2.1.2. Structures and Properties 2.2. Partially Reduced Azaporphyrins Containing One meso-Nitrogen Atom 2.3. Azaporphyrins and Azachlorins Containing One β-Nitrogen Atom 2.4. Azaporphyrins Containing Two meso-Nitrogen Atoms 2.4.1. Synthesis 2.4.2. Structures and Properties 2.5. Partially Reduced Diazaporphyrins 2.5.1. Synthesis 2.5.2. Structures and Properties 2.6. Azaporphyrins Containing Two β-Nitrogen Atoms 2.7. Azaporphyrins Containing Three meso-Nitrogen Atoms 2.8. Azaporphycenes 2.9. Azahomoporphyrins and Azahemiporphycenes 2.10. Azacorroles 2.11. Subtriazaporphyrins 2.12. Expanded Azaporphyrins 3. Oxa- and Thiaporphyrins and Related Compounds 3.1. Oxaporphyrins Containing One meso- or βOxygen Atom 3.1.1. Synthesis 3.1.2. Structures and Properties 3.2. Thiaporphyrins Containing One meso-Sulfur Atom 3.3. Thiaporphyrins Containing Two meso-Sulfur Atoms

© XXXX American Chemical Society

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1. INTRODUCTION Porphyrins and phthalocyanines are two of the most widely studied groups of tetrapyrrole macrocycles because of their ability to bind metals to form interesting complexes such as heme, chlorophyll, and phthalocyanine blue.1 In light of their fascinating colors and various functions, these two groups of macrocyclic compounds can be found in a wide range of photofunctional and redox-active materials in both naturally and artificially produced products.2−4 Azaporphyrins are related macrocycles, where the meso-carbons (i.e., the methine carbon atoms connecting the pyrrole rings) or the β-carbons (the βpyrrolic carbon atoms) of the parent porphyrin structure are completely or partially replaced by nitrogens, maintaining the π-electron conjugation. According to this classification, phthalocyanines are regarded as tetrabenzo-annelated 5,10,15,20-tetraazaporphyrins.5−8 Porphyrins and azaporphyrins are both aromatic compounds, with 18π electrons distributed over their macrocyclic structures. However, despite this similarity, these two groups of compounds show completely different optical and electrochemical properties depending on the number and position of nitrogen atoms at the periphery. These differences can not only be attributed to the

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins

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Received: July 15, 2016

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DOI: 10.1021/acs.chemrev.6b00460 Chem. Rev. XXXX, XXX, XXX−XXX

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development of artificial azaporphyrin-based materials for use in optoelectronic devices such as bulk-heterojunction and dyesensitized solar cells, semiconductors, and nonlinear optics, as well as biomedical agents for imaging diagnosis and photodynamic therapy (PDT). As was comprehensively summarized by Kobayashi and Mack in their review, which was published in 2011,9 low symmetry phthalocyanines and their partially azasubstituted analogues have been extensively investigated for over 80 years. Recent advances in azaporphyrin chemistry have also highlighted the aza analogues of expanded and contracted porphyrin derivatives. Considerable research efforts have been devoted to the development of efficient methods for the chemical modification of the meso and β-carbon atoms of porphyrins and related macrocycles with nitrogen atoms. The resulting azaporphyrin derivatives have exhibited a wide range of optical, electrochemical, and coordination properties that reflect the electronic and steric effects of the nitrogens positioned on the periphery of these macrocyclic structures.9,11 It is noteworthy, however, that there are still many difficulties associated with the organic synthesis of partially aza-substituted porphyrins, namely MAP, DAP, cis-DAP, and TrAP, as well as several other related compounds. One of the main reasons for this synthetic problem is the limited number of methodologies available for the selective insertion of nitrogen atoms around the periphery of these structures. Furthermore, well-established protocols for the synthesis of regular D4h-symmetrical TAPs are not always applicable to the synthesis of lower symmetry azaporphyrins. Some of the synthetic methods typically used to prepare symmetrical TAPs and phthalocyanines are shown in Scheme 1.23−30 For example, the metal complexes of radially symmetrical phthalocyanines and TAPs are generally obtained by the cyclotetramerization reactions of the corresponding phthalonitriles and 2,3-disubstituted maleonitriles, respectively. For the synthesis of phthalocyanines, it is also possible to use phthalic acid anhydrides, phthalimides, phthalamides, and 1,3diiminoisoindolines as substrates instead of phthalonitriles. Several examples of peripheral substituents of the radially symmetrical TAPs that have recently been synthesized are encircled with a broken-line frame in Scheme 1.31−41 The research groups of Helberger and Linstead independently reported their pioneering studies on the synthesis of TBMAP and TBTrAP in the late 1930s.42−46 Since then, several methodologies have been developed for the preparation of low symmetry phthalocyanine derivatives. The most straightforward and convenient method for the synthesis of tetrabenzoazaporphyrins involves the mixed condensation reaction of dialkylated phthalonitriles with differing amounts of Grignard reagents or metallic lithium, which produces a mixture of TBMAP, cis-TBDAP, TBDAP, and TBTrAP.21,22,47−51 Several recent examples of this strategy, including those reported by the research groups of Cammidge, Cook, Kobayashi, and Swarts, are summarized in Scheme 2.22,50 The distribution and the relative yields of the cyclized products prepared in this way are strongly dependent on the ratio of the phthalonitrile to methylmagnesium bromide, as represented by the data that are listed in a table (Scheme 2). The metal-templated mixed condensation reactions of isoindoline-1,3-diimine with phenylacetic acid or aliphatic acids have also been applied to the one-pot synthesis of partially aza-substituted tetrabenzoporphyrins, as exemplified by the work of Shaposhnikov and co-workers, some of which are shown in Scheme 3.52−60 For example, zinc(II) complexes

electronic effects of the nitrogen atoms, but can also be caused by the effect that the introduction of peripheral nitrogen atoms has on lowering of the molecular symmetry of the πsystems.9−15 For example, when we consider the planar metal(II) complexes of regular porphyrin and five different meso-azaporphyrins, we would expect the π-systems of these complexes to exist in the following space groups: D 4h (porphyrin), C2v (5-monoazaporphyrin, MAP), C2v (5,10diazaporphyrin, cis-DAP), D2h (5,15-diazaporphyrin, DAP), C2v (5,10,15-triazaporphyrin, TrAP), and D4h (5,10,15,20tetraazaporphyrin or porphyrazine, TAP). Consequently, Gouterman’s four orbitals (i.e., HOMO−1, HOMO, LUMO, and LUMO+1) of the porphyrin would be partially or completely nondegenerated in the azaporphyrins.16−18 It is well-known that meso-aza substitution lowers the energy levels of both the HOMO/HOMO−1 and the LUMO/LUMO+1 because of the large electronegativity of nitrogen (3.04, Pauling units) compared with carbon (2.55). This difference therefore implies that azaporphyrin π-systems would be harder to oxidize and easier to reduce than porphyrin π-systems. Furthermore, the relieved degeneracies of the HOMO/HOMO−1 and LUMO/LUMO+1 makes the HOMO-to-LUMO transition being allowed, leading to a red shift (bathochromic shift) in the Q bands corresponding to the HOMO-to-LUMO transitions of the azaporphyrins, as well as an increase in their intensity, compared with porphyrin.19,20 In contrast, the Soret bands of azaporphyrins are much broader and weaker than those of the corresponding porphyrins, because of the mixing of several different kinds of π−π* excitation. Figure 1 summarizes the

Figure 1. UV−vis absorption spectra of copper(II) complexes of TBP, TBMAP, cis-TBDAP, and TBTrAP in THF. Reproduced from ref 21. Copyright 2015 American Chemical Society.

UV−vis absorption spectra of the copper(II) complexes of tetrabenzoporphyrin (TBP) and partially aza-substituted tetrabenzoporphyrins (TBMAP, cis-TBDAP, and TBTrAP) which were recently reported by Swarts and co-workers.21 These spectra clearly highlight remarkable electronic effects of the aza substitution on the electronic transition energies of the TBP π-system. The optical properties of a series of tetrabenzoannulated derivatives (TBP, TBMAP, TBDAP, cis-TBDAP, TBTrAP, and Pc) substituted with eight alkyl groups were also studied in detail.22 A clear understanding of the optical and electrochemical properties of azaporphyrins is of utmost importance, with research to date leading to significant advances in the B


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of TBMAP, cis-TBDAP, TBDAP, and TBTrAP were isolated in 7, 7, 12, and 4% yields, respectively (R = OC16H33).57 The cross-condensation reactions of different kinds of phthalonitriles or their equivalents, as well as the ring expansion reactions of subphthalocyanines, are also effective for the synthesis of specific low symmetry phthalocyanine derivatives. For example, Luk’yanets et al. obtained a mixture of unsymmetrically substituted tetrabenzoazaporphyrins by reacting a phthalimide derivative with phthalonitrile or 3iminophthalimide in the presence of zinc acetate (Scheme 4).61 However, the synthesis of partially aza-substituted tetrabenzoporphyrins is often complicated by poor regioselectivity and difficulties associated with the isolation of the products. The development of efficient methods for the selective synthesis of tetrabenzoazaporphyrins has recently been explored by several research groups (vide infra). From a fundamental perspective, peripheral modification, namely, replacing one or more of the meso and/or β-pyrrolic carbons, with heteroatoms other than nitrogen, also represents a promising approach for creating novel heteroatom-containing π-conjugated macrocycles. These peripherally modified heteroporphyrins and related compounds are particularly attractive because it would be possible to tune their structural, charge, aromaticity, excitation energy, and redox potential characteristics by varying the heteroatoms on their periphery. Indeed, meso-oxaporphyrins, meso-thiaporphyrins, meso-dithiaporphyrins, and their ring-contracted analogues continue to attract considerable research interest. Furthermore, meso-oxaporphyrins have received significant attention from the field of bioinorganic chemistry in terms of their application as models of heme oxygenases.

Scheme 1. General Methods for the Synthesis of Phthalocyanine (Pc) and Radially Symmetrical TAPs

Scheme 2. Synthesis of TBMAPs, cis-TBDAPs, TBDAPs, and TBTrAPs from 3,6-Dialkylphthalonitriles

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Scheme 3. Synthesis of TBMAPs, cis-TBDAPs, TBDAPs, and TBTrAPs from Isoindoline-1,3-diimine

Scheme 4. Synthesis of TBMAP, cis-TBDAP, TBDAP, and TBTrAP from a Phthalimide Derivative

This review mainly focuses on recent advances in the synthesis of aza-, oxa-, and thiaporphyrins, as well as several related compounds, with a particular emphasis on historically important examples from the literature (Chart 1). With regard to azaporphyrins, we have included MAP, DAP, TrAP, and some peripherally aza-substituted tetra-/tripyrrolic macrocycles whereas tetraazaporphyrins (porphyrazines), phthalocyanines, and tetrabenzo-annelated derivatives (TBMAP, TBDAP, cisTBDAP, and TBTrAP) have been deliberately excluded, unless otherwise noted, because their syntheses and properties have been summarized elsewhere.9−11 The two main sections of this review have been divided according to the heteroatoms incorporated at the periphery, namely azaporphyrins (section 2) and chalcogen-containing porphyrins (section 3). Each of these sections has been further divided into subsections according to the number of heteroatoms and/or the structure of the π-frameworks. Representative reactions have been provided in all of these subsections, together with valuable insights for designing new heteroporphyrins. Because of page limitations, we have only included the structural, spectroscopic, optical, and electrochemical properties of selected compounds. Furthermore, we have only provided a brief discussion pertaining to the structure−property relationships and

applications of these materials as optoelectronic and biomedical agents for the same reason. We would therefore recommend that readers refer to the original studies if they would like to consider the reactions described in this review in greater detail. It should be noted that we have not cited all the references concerning the chemistry of aza, oxa-, and thiaporphyrins in this review. Several leading reviews and books covering the general properties of these compounds have been cited in the appropriate sections, together with several reports covering special topics in this area. The nomenclature and numbering systems for the porphyrins and precursory oligopyrrole compounds described in this review have been applied in accordance with the IUPAC recommendations,62 although some common names have been used in some cases. The nomenclature and numbering systems used in this review may therefore be different from those used in the original papers.

2. AZAPORPHYRINS AND RELATED COMPOUNDS 2.1. Azaporphyrins Containing One meso-Nitrogen Atom

2.1.1. Synthesis. The first examples of 5-monoazaporphyrin (MAP) compounds containing one meso-nitrogen atom were reported in the late 1930s. Fischer and co-workers found that D


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Chart 1. Macrocyclic Skeletons Described in This Reviewa

a

M = metal or 2H. R = alkyl, aryl, or H. The section numbers are indicated in parentheses.

condensation of two different dipyrrins in the presence of stannic chloride. When 1 was heated in DMF in the absence of NaN3, a reductive C−C coupling reaction occurred between C1 and C19 to yield the corresponding corrole instead of MAP. The source of the meso-nitrogen atom of 2 was therefore confirmed to be the azide ion. Building on this work, the cyclization of 1,19-dibromo-l,19dideoxybiladiene-ac dihydrobromides with NaN3 has become one of the commonly used synthetic routes for the preparation of MAPs.67−71 It is noteworthy that Dolphin et al. achieved dramatic improvements in the yields of MAPs by adding

several kinds of octaalkylated MAPs were produced in low yield by heating a pyridine solution of 1,9-dibromodipyrromethene (dipyrrin) hydrobromide in the presence of sodium hydroxide.63−65 This finding was the starting point of azaporphyrin chemistry. In 1966, Harris and co-workers developed a more robust method for the synthesis of MAP, involving the cyclization of 1,19-dibromo-1,19-dideoxybiladiene-ac dihydrobromide 1 with sodium azide (NaN3) in boiling methanol (Scheme 5).66 According to this method, MAP 2 was isolated as a free base in 42% yield after the esterification of its carboxy group. Biladiene-ac 1 was prepared in 72% yield by the E


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prepare. The use of 1,19-dideoxybiladiene-ac as a precursor for the preparation of MAP is therefore much more desirable than the use of 1,19-dihalobiladiene. With this in mind, Neya et al. developed an efficient one-pot method for the synthesis of MAP from 1,19-dideoxybiladiene-ac (Scheme 8).74 According

Scheme 5. Synthesis of MAP 2

Scheme 8. Synthesis of MAPs from 1,19-Dideoxybiladiene-ac

to their optimized reaction conditions, a mixture of iodine and potassium iodide was used to functionalize the 1 and 19 positions of biladiene-ac, and the resulting 1,19-diiodobiladiene-ac was subjected to an in situ intramolecular N-promoted ring-closing reaction to give the desired MAPs. Ammonium hydroxide (NH4OH) and NaN3 both performed well as nitrogen sources in this reaction, and MAPs were isolated in 19−33% yields. The photoirradiation of a MeOH solution containing 1,19dideoxybiladiene-ac dihydrobromide and ammonium hydroxide with a medium-pressure mercury lamp for 10 min produced MAP as a minor product (Scheme 9), in addition to corrole.75

dibenzo-18-crown-6 as a phase transfer agent, as exemplified by the reactions shown in Scheme 6.72 Given that 1,9Scheme 6. Synthesis of MAPs from 1,19-Dibromobiladienesac

Scheme 9. Synthesis of MAP from 1,19-Dideoxybiladiene-ac

dibromobiladienes-ac can be prepared from two differently substituted dipyrrins, this cyclization strategy can be applied to the synthesis of both symmetrically and unsymmetrically substituted MAPs. Notably, 1,19-diiodobiladienes-ac can be used instead of 1,19-dibromobiladienes-ac as a starting material in the cyclization reaction with NaN3 (Scheme 7).73 However, the synthesis of the 1,19-dihalobiladiene precursors for this reaction via the coupling of dipyrromethane and 2-bromo-5-formylpyrrole requires several steps from commercially available substrates, whereas 1,19-dideoxybiladiene-ac is much easier to

Most of the known MAPs have alkyl groups on their βpyrrolic carbons, and there have been very few reports pertaining to the synthesis of MAPs bearing any other kinds of peripheral substituents. The treatment of 1-bromobenzopyrromethene hydrobromide with an excess of NaN3 in DMF at 140 °C gave dibenzo-fused MAP 3 as a single regioisomer in 84% yield (Scheme 10).76 The same condensation reaction occurred with quinolone, but the yield of 3 was much lower (35%). In this report, Bonnett proposed a reaction mechanism

Scheme 7. Synthesis of MAPs from 1,19-Diiodobiladienes-ac

Scheme 10. Synthesis of Dibenzo-MAP 3

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for the head-to-head cyclization, including a base-assisted demethylation step. Considerable interest has been directed toward azaporphyrins in terms of their role as potential photosensitizers in PDT. In an attempt to resolve issues surrounding the selective uptake (cancerous vs healthy tissue) and water solubility of porphyrin systems, Croisy et al. prepared glycoconjugated MAPs 6, bearing two glucose or maltose moieties via a phenylene spacer, from 7,13-diarylbiladiene-ac 4, which was itself prepared by the acid-promoted condensation of 2,8-diaryldipyrromethane dicarboxylic acid with 2-formylpyrrole (Scheme 11).77 The

Scheme 12. Synthesis of MAPs 9 and 10

Scheme 11. Synthesis of Glycoconjugated MAPs 6

Montforts et al. reported a similar method starting from bilindione 11, which was initially treated with Meerwein reagent in the presence of Hünig’s base to give 1,19diethoxybiline 12. The subsequent treatment of a zinc complex of crude 12 with gaseous ammonia, followed by heating at 180 °C in 1,3-dimethyl-2-imidazolidinone (DMI) afforded ZnMAP 13 in 33% yield (Scheme 13).80 In this case, the zinc ion that acted as a template for the cyclization was removed by TFA, yielding the free base 14 in 78% yield. Scheme 13. Synthesis of MAPs 13 and 14

cyclization of 4 with NH4OH in the presence of K3Fe(CN)6 afforded 12,18-diaryl-MAP 5 in 9% yield. The subsequent glycosylation of this material with α-1-bromo-per-acetyl-sugars in a mixture of acetonitrile and triethanolamine, followed by the deprotection of the hydroxy groups, gave 6. Similarly, glycoconjugated MAP without an arene spacer was prepared from the corresponding dipyrromethane dicarboxylic acid. It is noteworthy that these compounds were the first reported examples of sugar-appended, hydrophilic MAPs. Fuhrhop et al. synthesized zinc(II) complex of MAP (ZnMAP) 9 by the metal-templated cyclization of the zinc complex of 1-amino-substituted biliverdin dimethyl ester 8 (Scheme 12).78,79 The precursory biliverdin derivative 8 was prepared from zinc(II) complex of 5-oxoniaporphyrin 7 via a C−O bond cleavage reaction (vide infra). Accordingly, this method is formally regarded as a two-step meso-aza-substitution of a meso-oxoniaporphyrin. The dehydration reaction from 1aminobiliverdin 8 to MAP 9 was also promoted by dicyclohexylcarbodiimide (DCC), but the yield was much lower than that of the corresponding thermal dehydration procedure (i.e., reflux in DMF). ZnMAP 9 was easily converted to the free base 10 by treatment with trifluoroacetic acid (TFA).

Saito and co-workers reported the development of a new method for the direct conversion of iron(II) complex of 2,3,7,8,12,13,17,18-octaethyl-5-oxoniaporphyrin (FeIIOEOP) to FeIIMAP. The treatment of FeIIOEOP(py)2 15 with concentrated ammonia, followed by p-toluenesulfonylmethyl isocyanide (TsCH2NC) resulted in a stepwise O-to-N exchange reaction to afford octahedral FeIIL2MAP (L = CNCH2Ts) 16 in 36% yield (Scheme 14).81 This product was subsequently demetalated with a concentrated solution of HCl, saturated with FeSO4 in acetic acid, yielding the free base 17 in 63% G


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Scheme 14. Synthesis of MAPs 16, 17, and 19

Scheme 16. Synthesis of MAPs 23

concluded that the ease with which 22 underwent N-insertion reactions stemed from its low redox potential. The proposed reaction mechanism for this transformation involved the nucleophilic attack of ammonia to an oxidatively generated metal/π-radical cation yielding a ring-opened, brominated biladiene intermediate. The metal complexes of porphyrins are generally prepared by the insertion of a metal into the core of the free base. The metal complexes of MAPs can also be prepared in a similar manner, although there have been very few reports describing the synthesis of these complexes. Berezin et al. provided an excellent summary of the kinetics associated with the coordination reactions of various M(OAc)2 with the free bases of several different azaporphyrins.88−91 Zvezdina et al. reported the metal exchange reaction of cadmium complex of MAP with zinc(II) and copper(II) chlorides in DMSO.92 Grigg et al. reported that the reaction of N-methyl-MAP 24 with [RhCl(CO)2]2 allowed for the insertion of rhodium into the macrocyclic structure to give RhIII(Me)MAP 25 (Scheme 17).93,94 This reaction most likely occurred by the conversion

yield. Similar procedures have also been reported for the preparation of the iron complexes of 5-azaprotoporphyrin IX dimethyl ester,82 5-azamesoporphyrin IX dimethyl ester,83 and 5-aza-2,3,7,8,12,13,17,18-octaethylporphyrin84 and zinc(II) complex of 5-azaprotoporphyrin IX dimethyl ester85 from the corresponding metal complexes of 5-oxoniaporphyrins. The conversion from FeII complex 18 to FeIIIMAP(Cl) 19 is also shown in Scheme 14.83 The synthesis of the precursory mesooxaporphyrins is described in section 3.1. The replacement of the meso sulfur atom of 5-thiaporphyrin with a nitrogen atom was also reported by Harris. The treatment of 5-thiaporphyrin hydrobromide 20 with gaseous ammonia in MeOH resulted in a nucleophilic C−S bondcleavage reaction, generating a ring-opened intermediate, 1aminobiline. This intermediate subsequently recyclized via the elimination of hydrogen sulfide to give MAP 21 as a minor product (Scheme 15).86

Scheme 17. Synthesis of RhIIIMAPs 25 from N-Alkyl-MAPs 24

Scheme 15. Synthesis of MAP 21

of 24 to the corresponding rhodium(I) complex, followed by the migration of the methyl group from nitrogen to rhodium to give 25. Overall, this reaction therefore represents the oxidative addition of an Me−N bond to a rhodium(I) center, resulting in the formation of a square pyramid rhodium(III) complex 25 bearing an axial methyl group. Stuzhin et al. reported a coordination reaction of MAP with metallic iron. When a solution of the free base of octaalkylMAP 26 was heated at reflux in glacial acetic acid in the presence of a 10-fold excess of iron powder for 2−3 h, binuclear μ-oxo-bridged iron(III) complex 27 was obtained in 65−75% yield (Scheme 18).95 IR analysis of 27 revealed an asymmetrical stretching vibration at 880 cm−1, which was attributed to the μoxo bridge (Fe−O−Fe). Grigg et al. reported that the N-methylation of MAPs generally led to the formation of isomeric mixture of mono-, di-

Palmer et al. reported a unique ring expansion reaction from corrole to MAP.87 During their attempts to chemically oxidize the iridium(III) bis(ammonia) complex of 5,10,15-tris(pentafluorophenyl)corrole 22 with NBS in the presence of NH4OH, they observed the unexpected formation of a mixture of iridium(III) bis(ammonia) complexes of MAP 23 (Scheme 16). Interestingly, the iridium(III) bis(trimethylamine) and bis(pyridine) complexes of the same corrole failed to undergo the N-insertion reaction. Based on these findings, it was H


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Scheme 18. Synthesis of μ-Oxo-Bridged FeIIIMAP 27 from MAP 26

and tri-N-alkylated MAPs.96 For example, the mono- and tri-Nmethyl-MAPs, 28 and 29, were obtained in 85 and 45% yields, respectively, when the MAPs were heated under reflux conditions in a mixture of CHCl3 and iodomethane containing K 2 CO 3 (Scheme 19). These N-alkylated MAPs were subsequently used to study coordination reactions with rhodium, as described above. Scheme 19. Synthesis of N-Alkyl-MAPs 28 and 29 Figure 2. Crystal structures of 30 and 31. Hydrogen atoms in side view are omitted for clarity.

than the meso-C−C bond lengths (1.391 Å for 30 and 1.375 Å for 31). Grigg et al. also provided an in-depth discussion of the underlying reasons for the considerable differences in the predicted stabilities of the core NH and meso-NH tautomers of the free bases (Figure 3).68 They ultimately concluded that the

Figure 3. Two tautomeric structures of MAP.

2.1.2. Structures and Properties. Very few crystal structures of MAPs have been reported in the literature. Grigg et al. reported the first successful analysis of the cobalt(II) complex of 13,17-diethyl-2,3,7,8,12,18-hexamethylMAP 30 (Figure 2),68 whose bond lengths and bond angles were consistent with those of the cobalt(II) complexes of porphyrins97 and phthalocyanines.98 The cobalt center adopted a square planar geometry with the highly planar MAP ring. In 1993, Balch et al. reported their results for the X-ray structural analysis of the chloroiron(III) complex of octaethyl-MAP 31 (Figure 2).84 It is noteworthy, however, that Marsh et al. later concluded that the interpretation of these results with regard to the inclusion of dinitrogen in the crystal structure was erroneous.99 In the crystalline state of 31, the iron center was five-coordinate, and its structural parameters were consistent with those of the high-spin, five-coordinate iron(III) porphyrins.100 The average Fe−N bond length (2.044 Å) was slightly shorter than the range (2.06−2.09 Å) typically observed for porphyrin analogues. In complex 31, the MAP π-system was slightly domed with a distance of 0.54 Å from the iron center to the mean MAP plane. In these MAPs, the average meso-N−C bond lengths (1.321 Å for 30 and 1.361 Å for 31) were shorter

electronic, geometric, and steric characteristics of these πsystems greatly favored the protonation of the core nitrogen atoms over the meso-nitrogen atoms. In acidic proton-donor media, the meso-nitrogen atom of the core NH tautomer could be protonated.101 Most of the MAPs reported to date have alkyl substituents on their β carbons. Dolphin et al. reported the 1H NMR spectra of several 2,3,7,8,12,13,17,18-octaalkyl-MAPs recorded in CDCl3, in which the meso-CH peaks were observed in the ranges δ 9.84−9.87 ppm (C10 and C20 positions) and 10.01− 10.03 ppm (C15 position), with one inner NH peak being detected in the range δ −2.72 to −3.28 ppm.72 The 1H NMR spectrum of octamethyl-MAP recorded in a mixture of CDCl3/ TFA exhibited the corresponding protons at δ 10.21, 10.48, and −2.03 ppm, respectively. The methyl protons on the β carbons also appeared to be relatively downfield (δ 3.38−3.51 ppm). These spectral features were attributed to the diatropic ring currents from the macrocycle, namely the aromaticity of the 18π MAP ring. Saito et al. conducted a detailed study of the 1H NMR spectra of the free bases, zinc(II) complexes, and iron(II) complexes of 5-azaprotoporphyrin IX dimethyl ester and some I


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of its derivatives.82 Based on a comparison of the downfield/ upfield shifts of the β-pyrrolic and NH protons of MAP and porphyrin, it was concluded that the ring-current effects of the 18π-electron system in MAP were small compared with those observed for the 18π-electron system of porphyrin. In the 1980s, Vysotskii and co-workers used the π-electron SCF-MO LCAO method to study a scale of aromaticity for azaporphyrins as well as porphyrins on the basis of the calculated π-electron contribution to the diamagnetic susceptibility and the ring currents.102−104 The effects of various additives and solvents on the 1H NMR spectra of FeIIIMAPs have been investigated in detail to identify the oxidation and spin states of the metal center.84 It is noteworthy that the addition of HCl to FeIIIMAP(Cl) led to a peripherally N5-protonated, six-coordinate iron(III) complex with two chlorine ligands at the axial positions. This process also led to the broadening of the NH peak, the assignment of which was verified by deuterium labeling. Interestingly, the ability of the meso-nitrogen atom in MAP to accept a proton converted the N4 core from a dianionic to a monoanionic ligand. The 1H NMR spectrum of iridium(III) bis(ammine) complex 23 (X1 = X2 = H) in DMSO-d6 displayed a singlet resonance far upfield (δ −3.66 ppm) and doublet signals far downfield (δ 8.9−9.2 ppm) assigned to the axial ammine ligands and the β protons, respectively, both of which indicated the aromatic character of its MAP ring.87 More than half a century ago, Gouterman predicted the electronic effects of meso-aza substitution on the absorption spectra of porphyrin during the course of his theoretical studies on the excited states of porphyrin.16 The replacement of a mesomethine group with a nitrogen atom lowers the symmetry of the porphyrin macrocycle and breaks the degeneracy of its frontier orbitals, leading to changes in the excitation energies and relative intensities of the Soret and Q bands, as mentioned in the Introduction. Kobayashi and co-workers compared the optical and redox properties of a series of MAPs, DAPs, and TAPs on the basis of UV−vis absorption and magnetic circular dichroism (MCD) spectra, cyclic voltammetry, and density functional theory (DFT) calculations.105 It was found that the introduction of nitrogen atoms at the meso positions of porphyrins caused the blue shift of the B band and the red shift of the Q band, with the concomitant decrease and increase of the absorption coefficients, respectively. Some data for the optical properties of selected MAPs bearing β-alkyl substituents are summarized in Table 1. Solov’ev et al. experimentally investigated the fluorescence and phosphorescence of MAPs, DAPs, and TAPs.106,107 Rochester et al. studied the protonation behavior of monoaza-etioporphyrin I (5-aza-2,7,12,17-tetraethyl-3,8,13,18-tetramethylporphyrin) in solutions by UV−vis absorption spectroscopy.108,109 With increasing the number of meso-nitrogen atomsin the order OEP, MAP, DAP, and TAPthe first oxidation and reduction potentials became more positive because of the electron-withdrawing effects of the meso-nitrogen atoms.105 Neya and co-workers used the iron complex of α-azamesoporphyrin XIII combined with apomyoglobin to reveal the influence of the meso-nitrogen atom on the ligand-binding properties in the reconstituted protein.110 The electrochemical behavior of monoazahemin-reconstituted myoglobin was compared with that of native myoglobin.111 Fuhrhop et al. reported the EPR study of dimeric forms of copper(II) complex of octaethyl-MAP.112

Table 1. UV−Vis Absorption Data for Selected MAPs and Related Compoundsa

a λ max (log ε), absorption maxima (logarithms of extinction coefficients). For details, see refs 72, 74, 78, 80, 81, and 84.

2.2. Partially Reduced Azaporphyrins Containing One meso-Nitrogen Atom

Some of the reactions of MAPs bearing vinyl or exomethylene functions on their peripheral β carbons were used to synthesize azachlorins and azabacteriochlorins. As part of their research toward the preparation of azaporphyrin-based potential sensitizers for PDT,113 Montforts et al. reported the preparation of azachlorins via 5-azaprotoporphyrin IX dimethyl ester 33, which was itself prepared in several steps from the naturally occurring bile pigment bilirubin 32.85 The key step in this transformation involved the Diels−Alder reaction of a peripheral diene moiety with the singlet oxygen, which was generated by photosensitization of 33. This reaction led to the formation of the corresponding endoperoxides, which underwent a rearrangement reaction to give a mixture of 5azachlorins 34 and 35 in an overall yield of 90% (Scheme 20). It is noteworthy that this photosensitized cycloaddition reaction proceeded 5 times faster than that of the corresponding 5-carbon analogue, protoporphyrin IX dimethyl J


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tetracyanoethylene and dimethyl acetylene dicarboxylate to give azachlorins 39 and 40, respectively (Scheme 22).116

Scheme 20. Synthesis of 5-Azachlorins 34 and 35 from MAP 33

Scheme 22. Synthesis of 5-Azachlorins 39 and 40 from MAP 38

ester. This result therefore implied that the energy transfer from the excited azachlorin dye 33 to ground-state oxygen occurred much more efficiently than that of the excited chlorin dye. Furthermore, 34 was converted to the more stable geminally dialkylated azachlorin 37 via 2-formyl-MAP 36 (Scheme 21).114,115 The first step of this transformation involved the

The treatment of MAP 41 with osmium tetroxide (OsO4), followed by H2S gas, led to the formation of azachlorin diol 42 as the main product, together with a small amount of azabacteriochlorin-tetraol 43 (Scheme 23).116 Notably, however, this reaction afforded 43 as the sole product when it was conducted in the presence of a large excess of OsO4. The acid treatment of azachlorin 42 resulted in a pinacol−pinacolone rearrangement process to give the ethyl-migrated azaketochlorin 44 in 40% yield, which reacted with OsO4 to give azabacteriochlorin-diol 45 in 60% yield.116 The UV−vis absorption spectra of these pyrrole-ringmodified MAPs displayed bathochromically shifted and intense Q-like bands due to lowering of the symmetry. For example, the lowest-energy absorption bands of 34, 35, 36, and 37 in CHCl3 appeared at λmax (log ε) 674 (4.82), 665 (4.63), 627 (4.00), and 669 nm (4.76), respectively.115 The cyclohexeneannelated monoazachlorins 39 and 40 in CH2Cl2 also showed the Q-like bands at λmax 666 and 686 nm, respectively.116 In the absorption spectra of azabacteriochlorin-tetraol 43, azaketochlorin 44, and azabacteriochlorin-diol 45 in CH2Cl2, the intense bands appeared at λmax 724, 640, and 698 nm, respectively. The strong absorption at long wavelengths indicated that these partially ring-reduced derivatives would be potential candidates as photosensitizers, like regular chlorins and bacteriochlorins, for PDT.

Scheme 21. Synthesis of 5-Azachlorin 37 from 5-Azachlorin 34

2.3. Azaporphyrins and Azachlorins Containing One β-Nitrogen Atom

oxidative C−C bond cleavage of a glycol intermediate, while the second step involved the reduction of the aldehyde function in 36 with NaBH4, followed by the amide acetal Claisen rearrangement of the resulting allylic alcohol intermediate. The yields for these steps were 11 and 17%, respectively. Smith et al. conducted an independent study of azaporphyrin-based sensitizers for PDT, which resulted in the development of several other methods for the preparation of partially reduced MAPs, azachlorins, and azabacteriochlorins. For example, β-vinyl-MAP 38, which was prepared from a 1,19dibromobiladine-ac, underwent Diels−Alder reactions with

In this section, we have described the synthesis of βmonoazaporphyrins (β-MAPs) and azachlorins containing one nitrogen atom at their β-pyrrolic position. These macrocyclic compounds may be regarded as core-modified porphyrins, where one of the pyrrole rings has been replaced with an imidazole ring. Structurally related N-confused porphyrins in which one or more pyrrole rings are inverted have been excluded from this review, because they are regarded as pure porphyrin isomers rather than azaporphyrins.117−119 K


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peripherally N-methylated β-MAP iminium salt 48 in 73% yield (Scheme 25).121 The UV−vis absorption spectra of these two

Scheme 23. Synthesis of 5-Azachlorin-diol 42, Azabacteriochlorin-tetraol 43, Azaketochlorin 44, and Azabacteriochlorin-diol 45 from MAP 41

Scheme 25. Synthesis of β-N-Methyl-β-MAP 48 from β-MAP 47

compounds in CHCl3 revealed that the Soret and lowestenergy Q bands of 48 (λmax 420 and 633 nm) had bathochromically shifted compared with those of 47 (λmax 407 and 582 nm). The cationic β-MAP 48 showed a remarkable increase in its photodynamic activity for the degradation of a 2′,3′-O-isopropylidene guanosine substrate compared with the standard photosensitizer hematoporphyrin and neutral β-MAP 47. Brückner and co-workers have developed an extensive series of methods for the modification of pyrrole rings of 5,10,15,20tetraphenylporphyrin (TPP) and established new pathways to formally replace one of the β-carbons of TPP with a nitrogen atom.122 Their first reported example of this work is shown in Scheme 26. The reaction of meso-tetraphenylsecochlorin bisaldehyde 49 with hydroxylamine in refluxing pyridine produced meso-tetraphenylimidazoloporphyrin (β-MAP) 50 Masaki et al. prepared β-MAPs as their free bases 46 via the acid-promoted [3 + 1] condensation of tripyrrane α,α′dicarboxylic acids with 2,5-bis[(trimethylammonio)methyl]-4methylimidazole triflate in a mixture of DMF and THF (20/1 v/v) in the presence of Zn(OAc)2, followed by acidolysis with TFA (Scheme 24).120 The UV−vis absorption spectrum of 46

Scheme 26. Synthesis of β-MAPs 50 and 52

Scheme 24. Synthesis of β-MAP 46

(R = Me) in CHCl3 displayed rhodo-type spectral features, with the Soret and lowest-energy Q bands occurring at λmax 400 and 620 nm, respectively. The addition of 1 equiv of TFA led to the bathochromic shift of the latter of these two bands, although the same phenomenon was not observed for OEP under these conditions. The results therefore indicated that the protonation was most likely occurring on the outer β-nitrogen atom. The reaction of the zinc(II) complex of β-MAP 47 with iodomethane in acetone under reflux conditions gave the L


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any distortion (Figure 4). In CH2Cl2, the β-MAP free base 50 displayed a typical etio-type UV−vis absorption spectrum,

and 2-nitro-TPP in 10−15% and 60−75% yields, respectively.123 These unexpected products were formed by two diverging pathways of the presumed monooxime monoaldehyde intermediate shown in Scheme 26. Brückner et al. also reported a mechanistic interpretation for the formation of the nitroporphyrin. Alternatively, the nickel complex of β-MAP 52 was prepared by the treatment of the nickel(II) complex of meso-tetraphenyl-1-formylchlorophin 51 with hydroxylamine in pyridine (Scheme 26), although Brückner et al. stated that they experienced some difficulty preparing larger quantities of 51. The 1H NMR spectrum of β-MAP 52 in CDCl3 was consistent with a porphyrin system without axial symmetry and contained one unusual singlet at δ 10.2 ppm for the imidazole-ring proton. The same group established another route for the synthesis of β-MAP from porpholactones (Scheme 27), which were Scheme 27. Synthesis of β-MAPs 50 and 56 from Porpholactone 53

Figure 4. Crystal structure of 50. Hydrogen atoms except for NH are omitted for clarity.

which was bathochromically shifted by 3−5 nm compared with that of TPP. In a similar manner to β-MAPs 46, the peripheral nitrogen atom in 50 showed sufficient basicity in solution. The treatment of 50 with 2% TFA resulted in a dramatic bathochromic shift, as well as the intensification of the Q band (from λmax 654 nm, log ε = 3.95, to λmax 717 nm, log ε = 4.39). In contrast, only minor shifts were observed for TPP under similar conditions. These phenomena were attributed to the protonation of the outer nitrogen, as well as the protonation of the inner nitrogen(s), resulting in the distortion of the β-MAP ring. The protonated and neutral forms of βMAP free base 50 were found to be fluorescent in solution. Montforts and co-workers reported the metal-templated synthesis of β-azachlorins 59 and β-azahexadehydrocorrin 60 (Scheme 28).126 Alkaline hydrolysis of the ester group of the nickel(II) complex 57, followed by the decarboxylation, acidpromoted condensation with an imidazole carbaldehyde, and recomplexation with zinc(II) or nickel(II) acetate gave the corresponding tetracyclic metal complexes 58Zn and 58Ni in 66 and 67% yields, respectively. Heating a 1,2,4-trichlorobenzene solution of Zn-azabilin 58Zn at 220 °C for 30 min yielded β-azachlorin 59Zn as the major product together with a trace amount of the CN-substituted derivative 59Zn-CN. In contrast, the thermal reaction of Ni-azabilin 58Ni under similar conditions afforded mainly the nickel(II) complex of βazahexadehydrocorrin 60Ni, together with small amounts of β-azachlorin 59Ni and the CN-substituted β-azahexadehydrocorrin 60Ni-CN. The different reaction modes of 58Zn and 58Ni were interpreted by DFT calculations on their cyclization processes. The UV−vis absorption spectra of 59Zn and 59Ni in CHCl3 exhibited their Soret bands at λmax 404 and 410 nm and the lowest-energy Q bands at λmax 620 and 656 nm, respectively.126 The Q band of 59Zn was hypsochromically shifted on protonation of the peripheral nitrogen atom.

available from the corresponding TPPs.124,125 The reaction of porpholactone 53 with hydrazine hydrate in refluxing THF over several days resulted in the formation of N-aminoporpholactams 54 as the major products. The hydrazine derivatives 54 were found to be susceptible to the reductive cleavage of their N−N bonds using samarium(II) iodide, resulting in the formation of the parent porpholactams 55, although this reduction only proceeded at elevated temperatures (in refluxing o-dichlorobenzene, bp 180.5 °C). The treatment of 55 (Ar = Ph) with POCl3 under reflux conditions in toluene proceeded cleanly to give chloro-β-MAP 56, which was subsequently reduced with Zn dust in H2SO4 to give βMAP 50 in 10% yield, together with the major hydrolysis product porpholactam 55.125 The structures of 50 and 56 were determined by X-ray crystallography.122,125 The results revealed that the chlorinesubstituted imidazole ring in 56 was slightly tilted out of the mean β-MAP π-plane, whereas the structure of 50 did not show

2.4. Azaporphyrins Containing Two meso-Nitrogen Atoms

2.4.1. Synthesis. 5,15-Diazaporphyrins (DAPs) contain two meso-nitrogen atoms that act as linkers between two dipyrromethene (dipyrrin) units. DAPs are therefore structural hybrids of regular porphyrins and TAPs. Ever since Fischer and co-workers reported the first example of DAP over 80 years M


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Scheme 28. Synthesis of β-Azachlorins 59

Scheme 29. Synthesis of DAP 61 from a 1,9Diaminodipyrrin

Scheme 30. Synthesis of DAP 62 from a 1,9Dibromodipyrrin

reaction was conducted in the absence of Pb(acac)2. These results suggested that the weak Pb−N bonds of the initial product, PbDAP, were hydrolyzed under the reaction conditions, leading to the exclusive formation of the free base 62. It is therefore likely that the lead(II) ion worked as a metal template. In 1966, Johnson et al. reported a highly efficient method for the synthesis of the copper(II) complex of DAP (CuDAP) 63, where the copper(II)−bis(1,9-dibromo-3,7-diethyl-2,8-dimethyldipyrrin) complex was converted to 63 in 75% yield when it was treated with NaN3 in boiling DMF for 15 min (Scheme 31).66

ago, DAPs have continued to attract much higher levels of interest than any of the other partially aza-substituted porphyrins. In this section, we have provided a summary of the methods currently available for the synthesis of DAPs bearing alkyl or aryl substituents at their periphery. Except for one example, the synthesis of TBDAP has been excluded from this section. To the best of our knowledge, there have been no reports pertaining to the synthesis of 5,10-diazaporphyrin (cisDAP) without annelated benzene rings. In 1936, Fischer et al. reported the first example of a DAP bearing alkyl substituents at all of its β-pyrrolic positions, which was produced in very low yield ( 400 nm) under ambient conditions in air resulted in the rapid and quantitative conversion of the green-brown colored 139Mn to the bright-green colored 143.235 The 1H NMR spectrum of isolated 143 showed several peaks in the diamagnetic region (δ 0−10 ppm), which strongly supported the low-spin state of the d2 manganese(V) center in 143. The triazacorrole ring of 143 was chemically oxidized using Magic blue to give a π-radical cation.233 Goldberg et al. studied the reactivities of the high-valent metal complexes of triazacorroles extensively, especially those of manganese, iron, and cobalt.214 Several metal complexes of triazacorroles have been structurally characterized by X-ray diffraction analysis. The results revealed that the triazacorrole rings in most of these complexes exhibited high levels of planarity. For example, the crystal structure of the cobalt(III)−phosphine complex 139Co (L = PPh3) showed that the mean displacement of the 23 atoms of the triazacorrole core from the π-plane was 0.10 Å, despite steric crowding from the eight peripheral aryl substituents (Figure 23).229 The core sizes of the triazacorrole ligands were intrinsically small compared with those of the corresponding regular corrole ligands. Indeed, the fivecoordinate cobalt(III) ion in the phosphine complex was displaced out of the plane of the core (0.44 Å) to a much greater extent than the cobalt(III) ion in the analogous corrole complexes.240 In contrast, the six-coordinate cobalt(III) ion in the bis(pyridine) complex 139Co (L = pyridine) was

137 with Na/NH3 in THF under low temperature conditions afforded the free base of triazacorrole 138, which underwent complexation reactions with various transition metals, including vanadium,224 chromium,225 manganese,225,226 rhenium,227 iron,228 cobalt,229,230 and copper213,224 to give the corresponding metal complexes of triazacorrole 139 (Scheme 67). Kobayashi et al. reported the synthesis of triazacorroles bearing tert-butyl groups on their β-pyrrolic carbons. The treatment of tert-butylated H2TAP 140 (a mixture of regioisomers) with HSiCl3 in the presence of tributylamine in refluxing benzene afforded the silicon(IV) complex of triazacorrole 141 and its μ-oxo-bridged dimer 142 as the major products (Scheme 68).231 The yield of monomer 141 (X = OSiMe3) increased up to 40% when the reaction time was shortened, whereas the yield of dimer 142, which was produced by the dehydration of 141 (X = OH), increased up to 85% when the reaction time was lengthened. As mentioned above, triazacorroles typically behave as trianionic N4 ligands, stabilizing the trivalent, tetravalent, and pentavalent oxidation states of the central metal ions.232 The metal complexes of triazacorroles and their regular corrole counterparts have consequently been used to study the catalytic cycles of oxo-transfer reactions using various high-valent metal−oxo complexes. The research groups of Goldberg and Ghosh independently studied the synthesis and oxo-transfer reactions of the manganese−oxo complexes of triazacorroles.232−239 One of the results reported by Goldberg’s group AE


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18π-electron ring systems. The Soret band of the μ-oxo-bridged dimer 142 appeared at a shorter wavelength (ca. 360−410 nm) than that of the monomer 141 because of the cofacial arrangement of the two triazacorrole chromophores in 142. The UV−vis absorption spectra of the green-brown MnIII complex 139Mn and the bright-green MnV(O) complex 143 showed Q bands at λmax 687 and 639 nm, respectively (Figure 24).235 UV−vis absorption spectroscopy has been widely used

Figure 23. Crystal structures of 139Co. tert-Butyl groups and hydrogen atoms in side view are omitted for clarity. Figure 24. UV−vis absorption spectra of 139Mn and 143 in cyclohexane. Reproduced from ref 235. Copyright 2012 American Chemical Society.

positioned in the plane of the triazacorrole ring, where it accommodated two axial pyridine ligands (Figure 23).229 In the field of bioinorganic chemistry, it is of critical importance to understand the intrinsic factors controlling the valence tautomerization processes that occur between the metal and ligand in metal−oxo porphyrinoids. In this regard, triazacorrole and corrole rings represent promising model systems. The 1H NMR spectra of metal complexes of triazacorroles clearly reflect the spin states of the metal centers. Representative data for the cobalt and manganese complexes of these ligands are described below. The 1H NMR spectrum of the square planar cobalt(III) complex 139Co displayed sharp peaks in the range δ +27 to −23 ppm.229,230 The large range for the chemical shifts of these protons was consistent with the cobalt(III) ion in 139Co being paramagnetic, namely in the high spin configuration. In contrast, the 1H NMR spectrum of the square pyramidal phosphine complex 139Co (L = PPh3) revealed that its cobalt(III) ion was diamagnetic, namely in the low spin configuration.229 Similarly, the proton signals of the square pyramidal, five-coordinate manganese(V)−oxo complexes 143 were all found in the diamagnetic region because of the low spin state of the manganese(V) ion in 143 (vide supra).226 Interestingly, the addition of a Lewis acid such as a zinc(II) ion and B(C6F5)3 or proton to a solution of 143 resulted in the formation of its valence tautomer (i.e., the manganese(IV)−oxo complex of the triazacorrole radical cation), which was confirmed spectroscopically.232 The UV−vis absorption spectrum of tetrakis(tert-butyl)triazacorrole 141 in CHCl3 showed Soret and Q bands at λmax 407 and 591 nm, respectively,231 whereas that of the free base of octaaryltriazacorrole 138 in pyridine showed Soret and Q bands at λmax 467 and 668 nm, respectively.223 These spectral data indicated that the triazacorrole rings were fully aromatic

to monitor the oxidation and protonation processes of 139M in solution. The redox properties of 137 were studied by cyclic voltammetry in CH2Cl2 with 0.1 M TBAPF6, and the results showed that there were two reversible reduction processes centered at E1/2 = −0.80 and −1.33 V, as well as one irreversible oxidation process at Epa = +1.16 V (vs Ag/AgCl).223 Notably, the triazacorrole ring exhibited good electronaccepting properties because of the involvement of the three meso-nitrogen atoms. 2.11. Subtriazaporphyrins

The chemistries of subphthalocyanines and subtriazaporphyrins (subporphyrazines) have been reviewed in detail by research groups of Torres241 and Kobayashi.242 In this section, we have focused exclusively on the synthesis and selected properties of several subporphyrazines. In 2005, Torres and co-workers reported the development of a boron-templated cyclization method for the synthesis of subporphyrazines.243 When a xylene solution of disubstituted maleonitriles was heated at 140 °C in the presence of BCl3, the cyclotrimerization of the maleonitriles occurred to give the corresponding hexapropyl- or hexaalkylthio-substituted subporphyrazines 144 as boron complexes bearing an axial chlorine ligand in 8−22% yields (Scheme 70).243 Two other groups had previously reported the synthesis of subporphyrazines using similar procedures,244,245 although their spectral data did not match with the data reported for the subporphyrazines that were unambiguously characterized by Torres’s group.241 The isolation of subporphyrazines 144 was complicated by their high solubility in most solvents and poor stability over silica gel, both of which AF


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Scheme 70. Synthesis of Subporphyrazines 144

Scheme 72. Synthesis of Hexaarylsubporphyrazines 149 from 148

resulted in the relatively low yields observed for the cyclotrimerization reactions. Kobayashi et al. also synthesized the hexaethyl-substituted derivative 144 (R = Et) from cis-3,4dicyano-3-hexene in 20% yield (Scheme 70) and characterized the structure of its B−OH derivative by X-ray crystallography.246 Subporphyrazine 144 (R = Pr) underwent an axial substitution reaction on silica gel involving the replacement of its chlorine atom with a hydroxy group to give the B−OH derivative 145.243 The metathesis reactions of 144 with phenol in boiling toluene or a large excess of BF3·OEt2 afforded the phenoxy- and fluorine-substituted derivatives, 146 and 147, respectively (Scheme 71).247 The B−F subporphyrazine 147 Scheme 71. Transformation of B−Cl Subporphyrazine 144 to 145, 146, and 147

Figure 25. Crystal structure of 149 (R = OMe). Axial aryl group in top view and hydrogen atoms in side view are omitted for clarity.

aromatic protons of the axial tert-butylphenoxy ligand (ortho to the oxygen) were shielded (δ 5.3−5.4 ppm) by the diatropic ring-current effects of the 14π-electron subporphyrazine macrocycle. Torres et al. also studied the intrinsic effects of the axial and peripheral substituents on the reactivity, optical and electrochemical properties, and crystal packing characteristics of the subporphyrazine π-systems.249,250 Notably, a reduction in the number of π electrons from 18 (porphyrazines) to 14 (subporphyrazines) led to large hypsochromic shifts in the Soret and Q bands. Alkylthio-substituted derivatives 148 exhibited two broad bands in the visible region; the first of these bands (λmax 444 nm) was assigned to n−π* transitions from the lone pair of electrons on the peripheral sulfur to the subporphyrazine π* orbital, whereas the second band (λmax 559 nm) was assigned to the Q-band of subporphyrazine. Arylsubstituted derivatives 149 also displayed two characteristic bands in the visible region. The first band (λmax 395 nm for R = Ph) was assigned to charge-transfer transitions between the

was also prepared by treatment of the B−OPh derivative 146 with BF3·OEt2 in toluene. In contrast to the labile B−Cl derivative 144, the B−F derivative 147 was quite robust toward hydrolysis. Torres et al. also prepared a series of 2,3,7,8,12,13hexaarylsubporphyrazines 149 in 20−43% yields based on a convergent strategy involving the palladium-catalyzed, copper(I) thiophene-2-carboxylate (CuTC) mediated coupling of 2,3,7,8,12,13-hexa(ethylthio)subporphyrazine 148 with the corresponding arylboronic acids (Scheme 72).248 The axial tert-butylphenoxy group was introduced to enhance the solubility of the intermediates and products. The crystal structure of 149 (R = OMe) revealed the conical shape of its core, which had a bowl depth of 1.742 Å (Figure 25).248 The 1H NMR spectra of 148 and 149 showed that the AG


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peripheral aryl substituents and subporphyrazine π-systems. The second band (λmax 539 nm for R = Ph) was assigned to the Q-band of subporphyrazine. Hexaarylated derivatives 149 bearing various electron-donating para-substituents (e.g., NMe2 and OMe groups) also exerted a significant impact on the electron density of the subporphyrazine π-system. For example, these π-systems showed large bathochromic shifts in their Q bands, as well as the large cathodic shifts in their redox potentials.

Scheme 74. Synthesis of Diarylporphocyanines 153 and Unsymmetrical Porphocyanine 154

2.12. Expanded Azaporphyrins

It is well-known that expanded porphyrins exhibit long wavelength absorptions and that these π-systems could therefore offer distinct advantages over porphyrins as potential photosensitizers for PDT and any other optical devices.251 Dolphin and co-workers reported the first examples of expanded tetrapyrrolic macrocycles containing two additional nitrogen atoms at the periphery, which they named porphocyanines.252 The large ring sizes of porphocyanines make them suitable candidates for the insertion of large metal ions such as gadolinium and technetium with the resulting metal complexes representing promising radiopharmaceuticals and paramagnetic contrast agents. The reduction of 1,9dicyano-2,3,7,8-tetraethyldipyrromethane with LiAlH4 in THF produced the unstable 1,9-bis(aminomethyl)-2,3,7,8-tetraethyldipyrromethane 150, which was used directly in a subsequent self-condensation reaction, where it lost ammonia to give, after an oxidation step, octaethylporphocyanine 151 in 24% yield (Scheme 73).252 The metalation of 151 with zinc(II) chloride

condensation of two different dicyanodipyrromethanes, followed by chromatographic separation (Scheme 74).253 The condensation reaction of 1,9-diformyldipyrromethane with ammonia in EtOH was also used for the synthesis of porphocyanine 155 to avoid the undesired reduction of the ester groups with LiAlH4 (Scheme 75).254

Scheme 73. Synthesis of Octaethylporphocyanines 151 and 152

Scheme 75. Synthesis of Porphocyanine 155

The crystal structure of the zinc(II) complex 152 was evaluated by X-ray diffraction analysis, showing the high planarity of the porphocyanine framework, which gave a mean deviation from the π-plane of 0.0095 Å (Figure 26).252 The distance between the two peripheral imine-type nitrogen atoms was 5.70 Å, suggesting that a large metal ion could be readily inserted into the core of this π-system. The 1H NMR spectrum of 151 in CDC13 with 1% TFA showed two different kinds of peripheral methine protons at δ 11.95 (2H) and 13.75 (4H) ppm, as well as the inner pyrrolicNH protons at δ −5.75 ppm. The large downfield and upfield shifts of these proton signals indicated the aromaticity of the expanded porphyrin-like [22] annulene structure in 151. The UV−vis absorption spectrum of the free base 151 in CH2Cl2 showed an intense Soret-like band at λmax 457 nm, together with some less intense bands in the range 590−800 nm.252,254,255 The lowest-energy Q-like band was observed in the NIR region (λmax 797 nm). The pKa values of the monoand diprotonated forms of 151 were estimated to be 6.0 and

proceeded smoothly in refluxing CH3OH/CH2Cl2 to give the corresponding zinc(II) complex 152. The zinc ion in 152 was tetrahedrally coordinated to two pyrrolic nitrogen atoms and two chlorine atoms, leaving the other two pyrrolic nitrogen atoms to be protonated, forming hydrogen-bonding interactions to the neighboring chlorine atoms. Three different 12,24-diarylporphocyanines 153 were prepared in a similar manner from the corresponding 1,9-dicyano5-aryldipyrromethanes in 30−40% yields (Scheme 74).253 The unsymmetrically substituted 2,3,21,22-tetraethyl-12-phenylporphocyanine 154 was also synthesized in 21% yield by the mixed AH


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was used as a substrate instead of 156, the analogous 30π macrocycles 160 and 161 were prepared, albeit in much lower yields (Scheme 76).257 Scheme 76. Synthesis of Expanded Porphyrins 157 and 160 and Azaporphyrins 158 and 161

Figure 26. Crystal structure of 152. Hydrogen atoms in side view are omitted for clarity.

4.4, respectively, based on spectrophotometric titration measurements, which were conducted in a biphasic system (Figure 27).254 For the titration measurements, the fluorescence spectra of the different protonated forms of 151 were also recorded (λem 715, 764, and 802 nm).255

The expanded aza-porphyrinoids 158 and 161, as well as the corresponding expanded porphyrins 157 and 160, follow Hückel’s (4n + 2) rule when they are held in their planar conformation. The 1H NMR spectrum of 158 in TFA-d showed significant diatropic ring-current effects on the outer and inner methine protons, which were observed at δ 11.26− 12.53 and −4.47 ppm, respectively.256,257 The corresponding protons of 157 in DMSO-d6/TFA-d were observed at δ 12.26− 14.35 and −9.79 ppm, respectively. Franck et al. compared the diatropicity (i.e., the magnetic criterion of aromaticity) characteristics of the expanded azaporphyrins and isoelectronic reference porphyrins using the maximum difference (Δδ) between the δ values of their inner and outer methine protons as a suitable index. The Δδ value observed for 158 (17.0 ppm) was smaller than that observed for 157 (24.1 ppm), suggesting that the diatropicity of the aza[26]porphyrin π-system would be smaller than that of the [26]porphyrin π-system. The UV−vis absorption spectrum of 158 in CH2Cl2 showed a long-wavelength band (λmax 697 nm), which was bathochromically shifted compared with the corresponding band of 157 (λmax 663 nm).256 This shift was caused by the substitution of a methine group on the porphyrinoid π-system by a nitrogen atom. The absorption maxima of the bisprotonated form of 158 were bathochromically shifted by 13−38 nm. The syntheses of several other expanded azaporphyrins such as texaphyrins, accordion porphyrins, and hexapyrrolic nitrogen-bridged macrocycles were summarized by Sessler and co-workers,258,259 and have therefore been excluded from this review.

Figure 27. UV−vis absorption spectra of 151 and its protonated forms in MeOH. Reproduced from ref 254. Copyright 1996 American Chemical Society.

Franck et al. reported the synthesis of two different kinds of expanded azaporphyrins containing one meso-nitrogen atom. The acid-catalyzed condensation of the open-chain tetravinylogous biladiene 156 with formaldehyde in the presence of HBr, followed by the oxidation of the resulting intermediate with DDQ, gave [26]porphyrin 157 in 43% yield. In contrast, the condensation of 156 with ammonia, followed by the oxidation of the resulting intermediate, gave 14-aza[26]porphyrin 158 in 18% yield (Scheme 76).256 When hexavinylogous biladiene 159 AI


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3. OXA- AND THIAPORPHYRINS AND RELATED COMPOUNDS

Scheme 78. Synthesis of Oxoniaporphyrins 163 and 164

3.1. Oxaporphyrins Containing One meso- or β-Oxygen Atom

Heme enzymes such as cytochrome P-450 catalyze a wide range of in vivo oxidations, and considerable efforts have been devoted to the elucidation of the mechanisms of their oxidation processes.260−263 It is widely accepted that heme oxygenases utilize molecular oxygen as an oxidant to induce the regiospecific ring-opening of tetrapyrrolic macrocycles at one of their meso carbons to produce biliverdin, together with the elimination of carbon monoxide and iron (Scheme 77).264,265 Scheme 77. Plausible Mechanism for the Formation of Biliverdin from Heme

chloride/pyridine and with zinc(II) acetate in acetic anhydride, respectively, followed by an anion exchange reaction with sodium tetrafluoroborate. During their studies on the chemistry of verdohemes, Balch and co-workers developed a bioinspired synthesis to prepare a series of metal complexes of 5-oxaporphyrins.270 In particular, they optimized their reaction conditions for the direct oxidation of the metal complexes of OEP (MOEP) to the corresponding metal complexes of OEOP (MOEOP).271−274 For example, the treatment of FeIIOEP(py)2 (py = pyridine) with molecular oxygen in the presence of ascorbic acid as the reducing agent and pyridine afforded [FeIIOEOP(py)2]+ 165 in 50% yield,271 whereas the application of similar conditions to CoIIOEP gave CoIIIOEOP(Cl)2 166 in 64% yield274 (Scheme 79). In the latter reaction, the axial chlorine ligands of the product were supplied by the dichloromethane solvent. The optimization of the reaction conditions revealed that the cooling of the reaction solution in a liquid nitrogen bath was crucial to the success of the transformation. The cooling of the solution in this way presumably led to an increase in the concentration of dioxygen to a level that facilitated the direct oxygenation. The cobalt(III) complex 166 was reduced by aqueous sodium dithionite, resulting in the formation of [CoIIOEOP][PF6] 167Co in 48% yield.274 Balch et al. also reported a convenient method for the synthesis of [CuIIOEOP]+ starting from MgOEP.275 The irradiation of a CH2Cl2 solution of MgOEP in air using a 750 W projector lamp, followed by the transmetalation of the irradiated mixture with copper acetate, resulted in the formation of the copper(II) complex of octaethylformylbiliverdin 168 in 22% yield. The subsequent treatment of 168 with 30% hydrogen peroxide in CH2Cl2, followed by the addition of ammonium hexafluorophosphate, gave [CuIIOEOP][PF6] 167Cu in 84% yield (Scheme 80).275 Carbon monoxide and carbon dioxide were detected during this copper-templated cyclization reaction, which were probably released from the biliverdin.275 The oxidation of CuIIOEP with 30% hydrogen peroxide in CH2Cl2 provided direct access to complex 167Cu, albeit in a lower yield (2%). The agitation of a chloroform solution of the iron(II) 2,3,7,8,12,13,17,18-octaethyloxophlorin radical 169 bearing two axial ligands (xylyl or tert-butyl isocyanide) in air for 1 h, followed by the addition of ammonium hexafluorophosphate,

This transformation is believed to consist of several sequential reactions proceeding via verdoheme (an iron oxoniaporphyrin) as a key intermediate. The in vitro oxidation of iron βoctaalkylated porphyrins with molecular oxygen and a reductant has generally been examined as a model system for this enzymatic oxidation reaction. In this section, we have described the regioselective oxidation of metalloporphyrins to the corresponding metal complexes of oxaporphyrins as a method for the synthesis of meso-oxaporphyrins. In this paper, the metal complex of 5-oxoniaporphyrin has been used as a cationic complex of 5-oxaporphyrin with an anionic counterion. The synthesis of β-oxachlorins is also included in this section. 3.1.1. Synthesis. In the 1960s and 1970s, several groups reported observing the conversion of porphyrins to oxaporphyrins during the course of their mechanistic studies of heme oxygenase.266−269 In the 1980s, Saito et al. reported the synthesis of several metal complexes of OEOP, 5-oxamesoporphyrin IX dimethyl ester, and 5-oxaprotoporphyrin IXa dimethyl ester, which were prepared by the metal-templated cyclization of the corresponding biliverdin derivatives.81−83 Two representative examples of the cationic iron(II) and zinc(II) complexes of 5-oxamesoporphyrin IX dimethyl ester, 163 and 164, are shown in Scheme 78. It is noteworthy that these complexes were prepared in 48−69% yields by the reaction of the common biliverdin precursor 162 with iron(III) AJ


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Scheme 81. Synthesis of FeIIOEOPs 165 and [FeIIIOEOP]2O 170

Scheme 79. Synthesis of MOEOPs 165, 166, and 167Co

Scheme 82. Synthesis of Zinc Complexes of Oxaporphyrins 172

Scheme 80. Synthesis of CuIIOEOP 167Cu

resulted in an oxidative decarbonylation reaction at the meso carbon to give the corresponding [FeIIOEOP(L)2][PF6] 165 (L = 2,6-xylylNC or t-butylNC) in 68−70% yields (Scheme 81).276 In these complexes, the axial isocyanide ligands stabilized the low spin state of the iron(II) ion, which was located at the core of the planar structure. The treatment of a CH2Cl2 solution of [FeIIOEOP][PF6] bearing no axial ligands 167Fe with molecular oxygen gave the μ-oxo-bridged dimer of FeIIIOEOP 170. Mizutani and co-workers reported the metal-templated synthesis of meso-aryl-substituted oxaporphyrins. The treatment of the 10,15,20-triarylbilindiones 171, which were prepared from the corresponding iron(II) complexes of the 5,10,15,20tetraarylporphyrins,277,278 with zinc acetate and acetic anhydride in refluxing chloroform produced the zinc(II) complexes of the 10,15,20-triaryl-5-oxaporphyrins 172, which were isolated as trifluoroacetate salts after purification by column chromatography over silica gel eluting with CH2Cl2/acetone/ TFA (Scheme 82).279 5,10,15,20-Tetraarylporpholactones are easily available from the corresponding 5,10,15,20-tetraarylporphyrins using several different methods such as the stepwise oxidation reactions with

OsO4/pyridine and MnO4−,280−282 the sequential nitration− reduction−oxidation reactions,283 and the direct oxidation with high-valent metal oxidants,284,285 although we have not described the synthesis of porpholactones in this review. Brückner et al. have developed several efficient methods for the synthesis and chemical functionalization of porpholactones.286,287 For example, they reported the synthesis of βoxachlorins by the stepwise reduction of zinc complex of 5,10,15,20-tetraphenylporpholactone 173 (Scheme 83).288 The first step was the reduction of 173 with DIBAL-H, which resulted in the generation of zinc complex of 3-hydroxy2-oxachlorin 174 in near quantitative yield (isolated yield was ca. 80%). The demetalation of the zinc complex 174 with HCl, followed by the acid-catalyzed silane-induced deoxygenation of the lactol hydroxy group of the resulting free base 175, gave the free base of β-oxachlorin 176 in quantitative yield. The AK


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reagent, gave the free base of 3,3-dialkyl-2-oxachlorins 178 in 45−74% yields after demetalation using a 6 M aqueous HCl solution. When the hydrolysis was conducted after the first alkylation of 173, 3-alkyl-3-hydroxy-2-oxachlorins 179 were isolated in 65−82% yields. The hemiketals 179 underwent dehydroxylation reactions by treatment with triethylsilane in the presence of boron trifluoride to give 3-alkyl-2-oxachlorins 180 in 76−88% yields. These mono- and dialkylated 2oxachlorins were found to be stable with respect to oxidation. 3.1.2. Structures and Properties. The structural and spectral features of some of the metal complexes of 5oxaporphyrins are described in this section. The six-coordinate iron complexes [FeIIOEOP(py)2][PF6] 165 (Figure 28),

Scheme 83. Synthesis of 2-Oxachlorins 176 and 177

treatment of 176 with zinc acetate in degassed warm DMF led cleanly to the formation of the zinc(II) complex of β-oxachlorin 177 in 90% yield. Attempts to direct reduction of 173 to 176 using LiAlH4 or DIBAL-H under more forcing conditions resulted in the destruction of the porphyrinic macrocycle without the formation of any major product. The free base 177 was also prepared from the porpholactone 53 via a thiolactone, although the overall yield was low (ca. 10%).289 The 1H NMR spectra of 176 and 177 in CDCl3 displayed the methylene protons at δ 6.54 ppm. β-Oxachlorin 176 was sensitive toward oxidation and reoxidized to the porpholactol 175. With this information in hand, Brückner et al. developed practical methods for the synthesis of 3-alkylated 2-oxachlorins from the zinc complex 173 (Scheme 84).290 The treatment of 173 with an alkyl Grignard reagent, followed by the sequential reactions with trimethylsilyl triflate and the same Grignard Scheme 84. Synthesis of 3-Alkylated 2-Oxachlorins 178, 179, and 180

Figure 28. Crystal structures of 165 and 170. Hydrogen atoms and counteranions are omitted for clarity.

FeIIIOEOP(CN)2, and FeIIIOEOP(Br)2 were found to have an essentially planar oxaporphyrin skeleton, although the structures of the latter two compounds suffered from disorder in terms of the location of the oxygen atom and one of the methine groups.271,291,292 In 165, the in-plane FeII−N bond lengths (1.966−1.977 Å) were shorter than the axial FeII−N bond length (2.029 Å). In FeIIIOEOP(X)2, the FeIII−N bond lengths (1.955−1.976 Å for CN; 1.964 Å for X = Br) were appreciably shorter than those of the corresponding sixcoordinate iron(III) porphyrin complexes. The μ-oxo-bridged dimers of FeIIIOEOP 170 were also characterized by X-ray crystallography. In 170, the FeIII−O bond lengths and Fe−O− Fe bond angle were 1.75−1.78 Å and 180°, respectively (Figure 28).293,294 The crystal structure of the square planar, four-coordinate [CoIIOEOP][PF6] 167Co was characterized without disorder (Figure 29), whereas that of the octahedral, six-coordinate CoIIIOEOP(Cl)2 was characterized with partial disorder.274 Both of these cobalt complexes had an almost planar OEOP AL


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of FeIIIOEOP(CN)2 was suggested by the results of variable temperature 1H NMR measurements and further confirmed by magnetic susceptibility and EPR analyses.292 Several cobalt complexes have also been identified based on their 1H NMR spectra. For example, CoIIIOEOP(Cl)2 166 exhibited two peaks for the meso-methine protons at δ 9.71 and 9.70 ppm in CDCl3, whereas [CoIIOEOP][PF6] 167Co showed meso-methine protons with marked hyperfine chemical shifts at δ 17.1 and 23.7 ppm.274 The UV−vis absorption spectra of oxaporphyrins typically show split Soret bands and intensified Q bands similar to those observed for MAPs. These features have been attributed to the fact that the replacement of the meso methine group with an oxygen atom breaks the degeneracy of the HOMO/LUMO of porphyrin, allowing for the Q-band transition. Furthermore, this replacement intensifies the transition electric dipole moment along the O5−C15 axis of the oxaporphyrin ring. For example, the intensities of the Q bands of 172 in CHCl3 (λmax 641−644 nm) were comparable to or even larger than those of the Soret band (Figure 30).279 The zinc(II) complexes 172 were fluorescent (λem 657 nm), with fluorescence quantum yields of 0.050−0.071 in CHCl3. Figure 29. Crystal structures of 167Co and 172. Hydrogen atoms, counteranions (for 167Co), and para-substituents (for 172) are omitted for clarity.

ring, with Co−N bond lengths of 1.940−1.949 and 1.956− 1.964 Å for the cobalt(II) and cobalt(III) complexes, respectively. The X-ray crystal structures of the square pyramidal chloro−zinc(II) complex of oxa-mesoporphyrin dimethyl ester and the trifluoroacetoxy−zinc(II) complex of 10,15,20-triaryl-5-oxaporphyrin 172 (Ar = p-MeO2CC6H4) revealed that their oxaporphyrin rings were almost planar (Figure 29).78,279 The C−O bond lengths of these 5oxaporphyrin complexes were in the range 1.33−1.37 Å, making them longer than a typical carbonyl CO bond (1.23 Å) and closer to that of a furan C−O bond (1.36 Å). This result therefore suggested that there were weak resonance interactions between the meso oxygen and the adjacent carbon atoms. The 1H NMR spectra of the zinc(II) complex of the oxoniaporphyrin 164 showed that the methine protons at the meso positions appeared in the range δ 8.96−9.25 ppm in CDCl3.83 The spectral features of the 5-oxaporphyrin complexes, however, varied considerably depending on the oxidation/spin states and coordination number of the metal centers, the axial ligands, and the charges on the macrocyclic ligands. The 1H NMR spectra of 172 in CDCl3 contained four different kinds of β-pyrrolic protons in the range δ 7.76−8.22 ppm, whereas those of the bilindiones 171 appeared in the range δ 6.26−6.94 ppm.279 These data indicated that the peripheral protons of 164 and 172 imparted some diatropic ring-current effects from the 18π electrons of the aromatic 5oxaporphyrin ring. However, the magnetic criterion for the aromaticity of 172 was lower than that of ZnTPP (δ 8.8 ppm for the β-pyrrolic protons). Based on the results of several COSY experiments and ab initio calculations involving the NMR chemical shifts, the two deshielded peaks (δ 8.09 and 8.22 ppm) were assigned to the protons bound to the C3 and C2 carbons of the meso-oxygen-bridged pyrrole rings. 1H NMR spectroscopy has been frequently used to characterize the iron complexes of 5-oxaporphyrins. For example, the low-spin state

Figure 30. UV−vis absorption spectra of 172 in CHCl3. Reproduced from ref 279. Copyright 2012 American Chemical Society.

The UV−vis absorption spectra of [FeIIOEOP(py)2]Cl 165, FeIIIOEOP(Cl)2, and FeIIOEOP(Cl) in CH2Cl2 showed relatively intense bands with λmax values of 384/526/651 nm (with 1% pyridine), 372/627/712 nm, and 394/662 nm, respectively, indicating that the electronic transitions of the 5oxaporphyrin π-systems were being strongly influenced by the oxidation state and axial ligands of the iron center (Figure 31).271 The same was true for the cobalt complexes, CoIIIOEOP(Cl)2 166 and CoIIOEOP(PF6) 167Co, which showed different absorption spectra.274 The strong absorption bands observed in the range 600−700 nm were typical of oxoniaporphyrin complexes. The electrochemical reduction of [ZnOEOP]+ 164 occurred irreversibly at +0.17 V (vs SCE) in CH2Cl2 with TBAP.267 However, in the presence of a base, electrochemical reduction of 164 occurred at −0.29 V, indicating the extremely high electron affinity of the oxoniaporphyrin π-system.267 Indeed, these electron-deficient porphyrins reacted with a variety of different nucleophiles, including alcohols, amides, hydroxide, AM


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hydrobromic acid. The resulting solution was buffered by the addition of anhydrous sodium acetate and then aerated to afford monothiaporphyrin (MTP) 181 as its hydrobromide salt in 25−30% yield (Scheme 85).86 The subsequent treatment of Scheme 85. Synthesis of MTP 181

Figure 31. UV−vis absorption spectra of (a) 165, (b) FeIIIOEOP(Cl)2, and (c) FeIIOEOP(Cl) in CH2Cl2. Reproduced from ref 271. Copyright 1993 American Chemical Society.

cyanide, and even DMF under mild conditions to yield the corresponding ring-opened, α-functionalized helical tetrapyrrole derivatives.270 The research groups of Balch and Mizutani independently studied the mechanism responsible for the ringopening reactions of these meso-oxaporphyrin systems and concluded that these reactions involved nucleophile attack at the α carbon adjacent to the meso oxygen atom, followed by the cleavage of the C−O bond. Furthermore, the results of a series of kinetic measurements revealed that 172 was rapidly solvated in primary and secondary alcohols, whereas a tertiary alcohol and water were much slower by a factor of 102−103.279 In the crystal structure of the 3,3-bis(isopropyl)-2-oxachlorin 178 (R = iPr), the β-oxaporphyrin ring was primarily coplanar with the mean deviation of 0.11 Å (Figure 32).290 The Cα−O bond length (1.359 Å) in the oxazole ring was appreciably shorter than the Cβ−O bond length (1.448 Å), indicative of a significant degree of the double bond character for the Cα−O bond. The UV−vis absorption spectra of the free base of oxachlorin 176, 3-alkyl-2-oxachlorins 180, and 3,3-dialkyl-2oxachlorins 178 exhibited the lowest-energy Q-like bands at almost the same wavelengths (λmax 668, 664−667, and 667− 668 nm, respectively).290 However, the extinction coefficients of 178 were distinctly lower than those of 180. The alkylated 2oxachlorins were moderately fluorescent (Φf = 0.19−0.30).

181 with ammonia resulted in the formation of the corresponding aminobilatriene, which recyclized with the concomitant elimination of H2S to give MAP bearing the same β-substituents (vide supra). In a similar manner, Grigg et al. condensed a dipyrrolyl sulfide dialdehyde with the same dipyrromethane 1,9-diacid in chloroform at 0 °C, using dry hydrogen chloride as a catalyst, to obtain MTP 182 as a blue, air-stable solid (Scheme 86).295 The alkylation of 182 with methyl iodide gave the N-methyl derivative 183 in 75% yield, whereas the oxidation of 182 with DDQ proceeded instantaneously to give the free base of MTP 184, which was isolated as the more stable cationic zinc(II) complex 185 in 75% yield after a metal insertion reaction with zinc acetate. The acid-catalyzed condensation reactions of diformyl difuryl sulfide with dipyrromethane 1,9-diacids resulted in the formation of core-modified corroles 187, which were most likely formed by the sulfur extrusion reaction of the coremodified meso-thiaporphyrin intermediates 186 (Scheme 87).296 Indeed, MTP 184 underwent a sulfur extrusion reaction to give the corresponding corrole product when it was heated in 1,2-dichlorobenzene for 2 h.295 Fuhrhop et al. synthesized the zinc complexes of 5thioniaporphyrins (ZnMTPs) 189 by the metal-templated cyclization of the zinc complexes of the corresponding 1mercaptobiliverdin dimethyl esters 188 (Scheme 88).78

3.2. Thiaporphyrins Containing One meso-Sulfur Atom

The first examples of meso-thiaporphyrins were independently reported by the research groups of Harris and Grigg at almost the same time. Harris condensed a dipyrrolyl sulfide dialdehyde with a dipyrromethane 1,9-diacid in glacial acetic acid− AN


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The 1H NMR spectra of ZnMTPs 189 in CDCl3 revealed that the meso-methine protons appeared at δ 9.63−9.99 ppm, which was consistent with the strong diatropic ring-current effect from the 18π electrons of the aromatic thioniaporphyrin ring.78,295 In sharp contrast, evidence for the nonaromatic mesothiaphlorin structure was provided by its NMR spectrum, where the observed chemical shifts for the meso and imino protons were consistent with the absence of a diamagnetic ring current. The meso-methine protons of the zinc complex of the aromatic ZnMTP 185 appeared at δ 10.47 and 9.64 ppm, whereas the corresponding protons of the nonaromatic mesothiaphlorin free base 182 appeared at δ 7.57 and 6.3 ppm, with the NH protons appearing at δ 4.56 ppm. The UV−vis absorption spectra of these two kinds of macrocycle also exhibited differences in the aromaticity of their π-systems. For example, Soret- and Q-like bands of 185 (λmax 424 and 703 nm in EtOH) were bathochromically shifted compared with the related intense absorption bands of 182 (λmax 396 and 639 nm in CHCl3).295 Grigg et al. investigated the metal insertion reactions of 5thiaphlorins with rhodium salts and discovered a potential redox system between rhodium(I)−thiaphlorin and rhodium(III)−thiaporphyrin (Scheme 89).297,298 The reaction of the

Scheme 86. Synthesis of 5-Thiaporphyrin Derivatives 182, 183, 184, and 185

Scheme 89. Synthesis of Rhodium Complexes of Thiaporphyrin Derivatives 190 and 191

Scheme 87. Synthesis of Dioxacorrole 187 via Core-Modified MTP 186

Scheme 88. Synthesis of ZnMTP 189

free base of meso-thiaphlorin 182 with [RhCl(CO)2]2 gave the dicarbonylrhodium(I) complex 190, in which the rhodium atom was bridged with the nitrogen atoms at the 22 and 23 positions of the thiaphlorin ring. In contrast to the rhodium(I) complexes of porphyrins and azaporphyrins, 190 reacted with acetic and propionic acid anhydrides to give the neutral diacyloxy-rhodium(III) complexes of MTP 191. The change in the oxidation state of the MTP ring during the acyloxylations was evidenced by the aromatic ring-current effects in the 1H NMR spectra. Thus, 191 (R = OCOMe) showed the meso and acetyl protons at δ 11.32/10.06 and −0.78 ppm, respectively. 3.3. Thiaporphyrins Containing Two meso-Sulfur Atoms

Compounds 189 were reconverted to the biliverdin compounds when they were treated with strong acids or bases, and the stability sequence of the meso-heteroatom-substituted porphyrins was found to be 5-aza ≫ 5-thionia > 5-oxonia.

Grigg et al. synthesized 192 as the first example of a 5,15dithiaporphyrin (DTP) by the acid-mediated condensation of a dipyrrolyl sulfide diacid with a dipyrrolyl sulfide dialdehyde AO


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(Scheme 90).299 The treatment of the free base of DTP 192 with zinc(II) and palladium(II) acetates yielded the corresponding metal complexes of DTP 193. Scheme 90. Synthesis of DTPs 192 and 193

Shinokubo et al. reported the synthesis of β-unsubstituted derivatives of DTP.300,301 The treatment of the zinc(II) complex of bis(1,9-dichloro-5-mesityldipyrrin) with sodium sulfide hydrate in DMF at 65 °C gave the free base of 10,20dimesityl-DTP 194 in 53% yield (Scheme 91), together with a

Figure 33. Crystal structures of 195Ni and 195Zn. Hydrogen atoms in side view are omitted for clarity.

195Ni, 195Pt, and 195Zn, respectively. These structural features could be related to the different reactivity of 195 during the sulfur extrusion to give the thiacorroles (vide infra). In the crystalline state, 195Cu showed a similar bent structure with a folding angle of 135.7°, and existed as a dimeric form, in which the meso-sulfur atom served as the axial ligand to each pentacoordinated copper(II) center.301 The temperature dependent magnetic susceptibility of 195Cu was measured, and the results revealed the presence of a weak antiferromagnetic interaction between the two copper(II) centers in the solid state. Given that sulfur has two lone pairs of electrons, there has been considerable interest in determining the intrinsic role of the bridging, meso-sulfur atoms in DTP in terms of their global π-conjugation over the tetrapyrrole macrocycle. The 1H NMR spectrum of the β-octaalkyl DTP 192 showed that the meso-C− H protons appeared at δ 7.77 ppm, whereas the β-pyrrolic protons of the meso-diaryl DTP 194 appeared at δ 6.25−6.31 ppm in CDCl3.299 The NH protons of 194 were observed at δ 13.00 ppm, most likely because of the intramolecular NH−N hydrogen-bonding interactions. Taken together, these spectral data indicated that the DTPs would exhibit nonaromatic characteristics, irrespective of their peripheral substituents, and these compounds were subsequently regarded as sulfur-bridged bis(dipyrrin)s. On the basis of both experimental and theoretical results, Shinokubo et al. concluded that the substantially bent structure and the long C−S bonds in DTPs would prevent effective π-conjugation between the sulfur lone pairs and the dipyrrin π orbitals, lacking in effective global πconjugation over the macrocycle. The UV−vis absorption spectra of the β-alkylated DTP free base 192, zinc complex 193Zn, and palladium complex 193Pd showed the lowest-energy bands at λmax 593, 551, and 581 nm, respectively,299 whereas those of the 10,20-dimesityl-DTP free base 194, zinc complex 195Zn, and nickel complex 195Ni exhibited the corresponding bands at λmax 483, 529, and 556 nm, respectively.300 Furthermore, CV measurements revealed that ZnDTP 195Zn showed reversible redox processes, with its first oxidation potential occurring at −0.08 V (vs Fc/Fc+; in CH2Cl2 with TBAPF6).

Scheme 91. Synthesis of DTPs 194 and 195

small amount of its zinc(II) complex, whereas a similar reaction involving the nickel(II) complex of bis(1,9-dichlorodipyrrin) afforded the nickel(II) complex of 5,15-diaryl-10-thiacorrole as the sole product (vide infra). The same DTP 194 was directly prepared in 64% yield by the reaction of 1,9-dibromo-5mesityldipyrrin 68 with sodium sulfide. The free base 194 underwent coordination reactions with copper(II), zinc(II), nickel(II), palladium(II), and platinum(II) salts to give the corresponding metal complexes (MDTPs) 195 (Scheme 91). Furthermore, MDTPs 195 were efficiently converted to the corresponding metal complexes of 10-thiacorrole when they were heated in refluxing toluene (vide infra). These metal complexes were subjected to X-ray diffraction analysis, which revealed that the DTP rings showed considerable folding at the two sulfur atoms, with folding (dihedral) angles of 117.6, 134.3, and 143.8° for 195Ni, 195Pt, and 195Zn, respectively (Figure 33).300 The folding angles were reflected in the average distances between the two αpyrrolic carbon atoms, which were 2.64, 2.71, and 2.79 Å for AP


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3.4. Heterocorroles Containing One meso-Chalcogen Atom

Scheme 94. Synthesis of Copper Complexes of Corrole 202 and 10-Oxacorrole 203

As described above, Johnson et al. established the metaltemplated cyclization reaction of 1,1′-bis(9-bromodipyrrin)s for the synthesis of 10-azacorroles. The same strategy was also applied to the synthesis of 10-chalcogenacorroles.218 In refluxing EtOH, the palladium complex of 1,1′-bis(9bromodipyrrin) 196 reacted with hydrochloric acid and sodium sulfide to give 10-oxacorrole 197 and 10-thiacorrole 198, respectively, in 45−58% yields (Scheme 92). Scheme 92. Synthesis of 10-Oxacorrole 197 and 10Thiacorrole 198

Scheme 95. Synthesis of 10-Chalcogenacorroles 207−212 In contrast, the metal-templated cyclization reaction of the ring-opened tetrapyrrolic chloro-amide 199 with copper(II) and nickel(II) salts gave the corresponding metal complexes of 10-oxacorrole 197 in 80−84% yields (Scheme 93).218 The free base 200 was obtained in 40% yield by the acidolysis of the copper(II) complex 197Cu with concentrated sulfuric acid. Scheme 93. Synthesis of 10-Oxacorroles 197 and 200

The oxidation state of the metal centers in corrole complexes has long been discussed in relation to the innocent/noninnocent character of the N4-corrole ligand. To provide a suitable reference system for evaluating this issue, Bröring and co-workers focused their attention on the use of a fully substituted oxacorrole. Notably, Bröring’s group observed that, upon treatment with excess anhydrous copper(II) sulfate in DMF, the fully substituted 1,1′-bis(dipyrrin) 201 was selectively transformed into the fully substituted corrole 202 or 10-oxacorrole 203, depending on the reaction conditions. For example, 202 was obtained in 65% yield under dry oxygenfree conditions, whereas 203 was formed in 6% yield in the presence of oxygen (Scheme 94).302 Following on from this work, Bröring et al. reported the development of a modified method for the synthesis of 203 and its heavier congeners in much better yields (Scheme 95).303 The treatment of the fully substituted 1,9-dibromodipyrrin 204 with Cu(OAc)2 in DMF at 110 °C for 15 min afforded the copper(II) complex of oxacorrole 203 in 57% yield. In contrast,

the sequential treatment of 204 with (i) Cu(OAc)2 (room temperature, 5 min) and Na2S (110 °C, 15 min) in DMF or (ii) Cu(OAc)2 (room temperature, 15 min) and KSeCN (65 °C, 12−16 h) in THF gave the copper(II) complexes of 10thiacorrole 205 and 10-selenacorrole 206 in 34 and 50% yields, respectively. Furthermore, the free bases of the 10-heterocorroles 207, 208, and 209 were synthesized by the reductive demetalation of the corresponding copper(II) complexes 203, 205, and 206 with SnCl2/HCl304 in a mixture of CH2Cl2 and AQ


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acetonitrile. Similar procedures were also applied for the synthesis of the 2,3,7,8,12,13,17,18-octaethyl-10-heterocorroles 210, 211, and 212, which are shown in Scheme 95. Bröring et al. also reported another approach for the synthesis of the 10-thiacorrole 211, which proceeded by the MacDonald-type [2 + 2] condensation of tetraethyl-2,2′bipyrrole and bis(3,4-diethyl-5-formylpyrrole) sulfide to give the free base in 29% yield (Scheme 96).305 Furthermore, the treatment of 211 with ferrous chloride and sodium acetate in a mixture of MeOH and CH2Cl2 under air gave the chloridoiron(III) complex 212-Cl in 74% yield.305

Scheme 97. Synthesis of 10-Thiacorroles 214a,b

Scheme 96. Synthesis of 10-Thiacorroles 211 and 213-Cl

Scheme 98. Synthesis of Iron Complexes of 10-Thiacorroles 213-X

Heating a 1,2,4-trichlorobenzene solution of the free base of DTP 192 or ZnDTP 193Zn in the presence of triphenylphosphine under reflux conditions led to the extrusion of a sulfur atom from the macrocycle, affording 50:50 mixtures of the two regioisomers of 10-thiacorroles 214a and 214b in 42 and 66% yields, respectively (Scheme 97).299 The rate of this sulfur extrusion process was much slower than that of the corresponding meso-thiaphlorin case. The axial ligand exchange reactions of the iron(III) complex 213-Cl with fluoride, bromide, and iodide ions were achieved by metathesis reactions with the appropriate anion sources in one or two steps, affording the homologous halogenido complexes 213-X (X = F, Br, I) in 90−99% yields.305 Similarly, azido, triiodo, and the cationic five- or six-coordinate iron(III) complexes were directly prepared from 213. Selected examples are shown in Scheme 98.305 Bröring et al. characterized a series of 10-heterocorroles in considerable detail. The results for the X-ray diffraction analysis of 207, 208, and 209 revealed planar macrocycles with small saddle-shaped distortions, which were accompanied by the binding of both of the inner hydrogen atoms to the bipyrrolic N-donor atoms (Figure 34).303 The chalcogen atom at the 10position was bound almost symmetrically with average C−X bond lengths of 1.36 Å (X = O), 1.73 Å (X = S), and 1.88 Å (X = Se), together with average C−X−C bond angles of 125.0° (X = O), 112.8° (X = S), and 110.3° (X = Se). These changes in the bond parameters resulted in a continuous increase in the size of the N4 cavity from 207 to 208 to 209. The EPR spectra of copper(II) complexes 203, 205, and 206 revealed that the development of the ACu hyperfine coupling constant of the z

component of the g tensor was of the order 242 G (X = O) > 228 G (X = S) > 221 G (X = Se), which followed the order of their cavity sizes. This result therefore implied that the steric properties of the cavities of the 10-heterocorroles governed their hyperfine interactions. The 1H NMR spectra of 210, 211, and 212 in CDCl3 revealed that the meso C−H and N−H protons appeared at δ 8.28 and 7.07 ppm for 210, δ 8.91 and 3.60 ppm for 211, and δ 8.55 and 5.00 ppm for 212.303 It is therefore likely that the aromatic character, as well as the π-conjugation of the heterocorrole ring, increased in the order O < Se < S. This was also supported by the UV−vis absorption spectra, where the heavier chalcogen analogues 208 and 209 showed intense Soret bands and several weaker Q bands around 400 and 580 nm, respectively.303 In contrast, 207 showed blue-shifted and AR


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When mCPBA was used as an oxidant, 216 was produced in 51% yield at −30 °C in 3 h. The treatment of the nickel(II) complex of dichlorodipyrrin 65-Cl with sodium sulfide in DMF at 100 °C gave the nickel(II) complex of 10-thiacorrole 217 in 41% yield (Scheme 100).300 This mode of cyclization differed considerably from Scheme 100. Synthesis of 10-Thiacorrole 217

that observed for the zinc(II)−dipyrrin complex, which was converted to the zinc(II) complex of DTP (vide supra). Nickel complex 217 was also synthesized by the sulfur-extrusion reaction of a series of DTP derivatives. The treatment of the free base of DTP 194 with Ni(acac)2 or the treatment of NiDTP 195Ni with triphenylphosphine in boiling toluene afforded 217 in 43 and 75% yields, respectively. It is noteworthy that the central metals in these MDTPs played a crucial role in their reactivities, as exemplified by 195M (M = Zn, Pd, Pt), which did not undergo similar sulfur extrusion reactions. Shinokubo et al. studied the reaction mechanism of the sulfur extrusion reaction from DTP to 10-thiacorrole using computational methods and rationalized the different reactivities observed for 195M.300 The results for the X-ray diffraction analysis of 10-oxacorrole 216 (Figure 35) and 10-thiacorrole 217 revealed that the

Figure 34. Crystal structures of 207, 208, and 209. Hydrogen atoms in side view are omitted for clarity.

relatively intense Q bands, which were consistent with the diminished aromaticity of its oxacorrole ring. Shinokubo et al. reported a practical method for the synthesis of the nickel(II) complex of 5,15-dimesityl-10-oxacorrole 216 via nickel(II) 5,15-dimesitylnorcorrole 215 (Scheme 99).306 Scheme 99. Synthesis of 10-Oxacorrole 216 via Norcorrole 215


The nickel(0)-mediated intramolecular reductive homocoupling of the nickel(II) complex of 1,9-dibromodipyrrin 65-Br in DMF gave 215 in 90% yield. Although norcorrole 215 was found to be stable both in solution and in the solid state under ambient conditions, it was slowly oxidized under air at 100 °C in DMF to afford 10-oxacorrole 216 in 47% yield after 5 days.

heterocorrole π planes of these compounds were highly planar, with C−O and C−S bond lengths of 1.34−1.35 and 1.69 Å, respectively.300,306 The average C−S bond length of 217 was much shorter than that of NiDTP 195Ni (1.74 Å). This result therefore indicated that 10-thiacorrole 217 contained an 18πAS


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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI (15H00931, 15K13762).

electron system, including the lone pair on the meso-sulfur atom. The 1H NMR spectra of 10-oxacorrole 216 and 10thiacorrole 217 in CDCl3 exhibited doublet peaks in the aromatic region (δ 7.5−7.9 ppm for 216 and δ 7.9−8.4 ppm for 217) corresponding to the β-pyrrolic protons.300,306 Notably, the aromatic character of the 18π-electron system of thiacorrole was much more pronounced than that of oxacorrole, which was consistent with the results observed for the corresponding octaethyl derivatives. The UV−vis absorption spectrum of 216 in CH2Cl2 exhibited its Soret and lowest-energy Q bands at λmax 389 and 680 nm, respectively, whereas that of 217 displayed split Soret bands (λmax 390 and 406 nm) and a slightly blue-shifted Q-band (λmax 674 nm).300,306 These optical data provided further evidence in support of the aromatic character of these 10-heterocorroles.

DEDICATION This work is dedicated to the memory of Prof. Kazuhiro Maruyama. ABBREVIATIONS BOC tert-butoxycarbonyl COD cycloocta-1,4-diene COZY correlation spectroscopy CV cyclic voltammetry DABCO 1,4-diazabicyclo[2.2.2]octane DAP 5,15-diazaporphyrin cis-DAP 5,10-diazaporphyrin DCC dicyclohexylcarbodiimide DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DFT density functional theory DIBAL-H diisobutylaluminum hydride DMSO dimethyl sulfoxide dppf 1,1′-bis(diphenylphosphino)ferrocene DPV differential pulse voltammetry DTP 5,15-dithiaporphyrin EPR electron paramagnetic resonance Fc/Fc+ ferrocene/ferrocenium HOMO highest occupied molecular orbital LCAO linear combination of atomic orbitals LUMO lowest unoccupied molecular orbital Magic blue tris(4-bromophenyl)aminium hexachloridoantimonate MAP 5-monoazaporphyrin (meso-monoazaporphyrin) β-MAP β-monoazaporphyrin MCD magnetic circular dichroism mCPBA m-chloroperbenzoic acid mesityl 2,4,6-trimethylphenyl MO molecular orbital MTP 5-monothiaporphyrin NBS N-bromosuccinimide NIR near-infrared NMR nuclear magnetic resonance OEP 2,3,7,8,12,13,17,18-octaethylporphyrin OEOP 2,3,7,8,12,13,17,18-octaethyl-5-oxaporphyrin PDT photodynamic therapy SCE saturated calomel reference electrode SCF self-consistent field TAP 5,10,15,20-tetraazaporphyrin TBAP tetrabutylammonium perchlorate TBAPF6 tetrabutylammonium hexafluorophosphate TBMAP tetrabenzo-5-monoazaporphyrin TBDAP tetrabenzo-5,15-diazaporphyrin cis-TBDAP tetrabenzo-5,10-diazaporphyrin TBTrAP tetrabenzo-5,10,15-triazaporphyrin TD-DFT time-dependent density functional theory TFA trifluoroacetic acid TPP 5,10,15,20-tetraphenylporphyrin TrAP 5,10,15-triazaporphyrin

4. CONCLUSIONS Recent extensive investigations concerning azaporphyrins and their chalcogen analogues have demonstrated that the incorporation of nitrogen or chalcogen atoms into the macrocyclic framework dramatically alters the structural, optical, electronic, magnetic, and coordinating properties of these porphyrin compounds. In addition, recent progress toward the synthesis of these π-systems has highlighted potential applications for the peripherally heteroatom-modified porphyrins in the fields of biomimetic chemistry, materials science, and medical research. However, the development of new methods for the selective synthesis of specific low symmetry azaporphyrins is still required to further develop these areas of research with particular emphasis on more elaborated methodologies for replacing the peripheral carbons with heteroatoms. Furthermore, the existing methods developed by the pioneers in these fields need to be revisited in terms of their practical applications. Comprehensive research programs toward the optoelectrochemical applications of azaporphyrin- and heteroporphyrin-based π systems have just started, and we hope these will mark the beginning of a long and rich period of research in the fields of porphyrin and materials chemistry. AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. Biography Yoshihiro Matano was born in Tokyo, Japan. He obtained his bachelor’s degree in 1987 and master’s degree in 1989 from Kyoto University under the supervision of Prof. Kazuhiro Maruyama. He became an assistant professor at the Faculty of Science, Kyoto University, in 1990 and obtained his Ph.D. from Kyoto University in 1994 under the supervision of Prof. Hitomi Suzuki. From 1996 to 1997, he joined Prof. James Mayer’s group (University of Washington) for one year on a Monbu-sho research scholarship. He moved to the Graduate School of Engineering, Kyoto University, as an associate professor in 2002. He was appointed as a full professor at the Faculty of Science, Niigata University, in 2013. His research interests include organoelement chemistry, azaporphyrin chemistry, and redox-active ligands.

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