Strategies for Corrole Functionalization - ACS Publications - American

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Strategies for Corrole Functionalization Joana F. B. Barata,†,‡ M. Graça P. M. S. Neves,† M. Amparo F. Faustino,† Augusto C. Tomé,† and José A. S. Cavaleiro*,† †

Department of Chemistry and QOPNA, and ‡Department of Chemistry and CICECO, University of Aveiro, 3810-193 Aveiro, Portugal ABSTRACT: This review covers the functionalization reactions of meso-arylcorroles, both at the inner core, as well as the peripheral positions of the macrocycle. Experimental details for the synthesis of all known metallocorrole types and for the N-alkylation reactions are presented. Key peripheral functionalization reactions such as halogenation, formylation, carboxylation, nitration, sulfonation, and others are discussed in detail, particularly the nucleophilic aromatic substitution and the participation of corroles in cycloaddition reactions as 2π or 4π components (covering Diels−Alder and 1,3-dipolar cycloadditions). Other functionalizations of corroles include a large diversity of reactions, namely Wittig reactions, reactions with methylene active compounds, formation of amines, amides, and imines, and metal catalyzed reactions. At the final section, the reactions involving oxidation and ring expansion of the corrole macrocycle are described comprehensively.

CONTENTS 1. Introduction 2. Functionalization at the Inner Core Positions 2.1. Metalation 2.1.1. Group 1 Metallocorroles 2.1.2. Group 4 Metallocorroles 2.1.3. Group 5 Metallocorroles 2.1.4. Group 6 Metallocorroles 2.1.5. Group 7 Metallocorroles 2.1.6. Group 8 Metallocorroles 2.1.7. Group 9 Metallocorroles 2.1.8. Group 10 Metallocorroles 2.1.9. Group 11 Metallocorroles 2.1.10. Group 12 Metallocorroles 2.1.11. Group 13 Metallocorroles 2.1.12. Group 14 Metallocorroles 2.1.13. Group 15 Metallocorroles 2.1.14. Lanthanide Metallocorroles 2.1.15. Actinide Metallocorroles 2.2. Demetalation 2.3. N-Alkylation 3. Functionalization at the Peripheral Positions 3.1. Halogenation 3.1.1. β-Fluorinated Corroles 3.1.2. Chlorination 3.1.3. Bromination 3.1.4. Iodination 3.2. Formylation 3.3. Carboxylation 3.4. Nitration 3.5. Sulfonation/Chlorosulfonation 3.6. Formation of Isocorroles 3.7. Oligomerization 3.8. Other Functionalizations at the Peripheral Positions © XXXX American Chemical Society

3.8.1. 3.8.2. 3.8.3. 3.8.4. 3.8.5.

Borylation Fluoroalkylation Aminomethylation Hydrogenation Corroles as 2π Components in Cycloaddition Reactions 4. Post-Functionalization of Corroles 4.1. Nucleophilic Aromatic Substitutions 4.2. Amines, Amides, and Imines 4.3. Wittig Reaction 4.4. Reactions with Methylene Active Compounds and with Pyrroles 4.5. Corroles as 4π Components in Cycloaddition Reactions 4.6. Metal Catalyzed Reactions 4.6.1. Suzuki−Miyaura Cross-Coupling Reaction 4.6.2. Sonogashira Cross-Coupling Reaction 4.6.3. Heck Cross-Coupling Reaction 4.6.4. Buchwald−Hartwig Cross-Coupling Reaction 4.6.5. Other Cross-Coupling Reactions 5. Other Reactions 5.1. Oxidation 5.2. Ring Expansion 6. Final Remarks Author Information Corresponding Author ORCID Notes

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Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: July 21, 2016

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DOI: 10.1021/acs.chemrev.6b00476 Chem. Rev. XXXX, XXX, XXX−XXX

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refinined after that.9,10 For almost 15 years certain research groups have been looking at corrole potential applications, sometimes in parallel studies with porphyrins. A very good review on such corrole applications was put forward in 2007 by Aviv and Gross.11 Since then, particularly in recent years, several research groups have considered corrole derivatives as their research targets. As a result, it became known that such compounds can play a role in oxidation or group transfer catalysis, in dye-sensitized solar cells, also as corrole based sensors, and in medicinal applications (photodynamic therapy of cancer, inactivation of bacteria, viruses and fungi, biomedical imaging, MRI contrast, etc.).12−26 The potential applications already demonstrated are very significant and point out to new developments and further studies in all those areas. To reach such a goal, researchers must have access to corrole derivatives containing appropriate substituents mainly at the meso or β positions. Corrole derivatives can then be obtained not only by functionalization of easily available corrole macrocycles but also by further postfunctionalization into the target products, by following well established synthetic organic chemistry methods.27,28 This review considers established methods which can be used in the synthesis of a wide range of corrole macrocycles appropriately substituted to be used in planned studies. This includes the functionalization of corrole inner core positions and peripheral positions (halogenation, formylation, carboxylation, nitration/amination, sulfonation/chlorosulfonation, etc.). Other results to be reported are mainly concerned with postfunctionalization procedures involving nucleophilic aromatic substitutions, reactions with methylene active compounds, Wittig reaction, formation of imines, amides, and sulfonamides, and also the use of corrole derivatives as 4π components in cycloaddition reactions. Metal catalyzed reactions discovered a few decades ago brought a great impact in organic synthesis and, definitely, in corrole chemistry. The Suzuki−Miyaura, Sonogashira, Heck, and Buchwald−Hartwig cross-coupling reactions are discussed. Other postfunctionalization procedures involving trifluoromethylation, Migita−Kosugi−Stille transformation, oxidation, and ring expansion are also described.

AX AY AY AY

1. INTRODUCTION When mentioning tetrapyrrolic macrocycles usually the idea of porphyrin derivatives is highlighted in the mind. Presumably that comes from the fact that some of such derivatives play in Nature vital functions and so are of great biological significance. Corrole macrocycles are also tetrapyrrolic derivatives but are non-natural compounds; however, the corrole skeleton is found in cobalamins. Porphyrin and corrole macrocycles have the structural features and IUPAC numbering shown in Figure 1,

Figure 1. Porphyrin and corrole macrocycles and their IUPAC numbering. Identification of the pyrrole rings in corroles using the A− D system.

with corroles having one methine bridge less (CH−20) in their structures in relation with those of porphyrins. However, both groups of compounds are aromatic and as a result they have similar chemical and photophysical properties, although with peculiar differences. Particularly the corroles, with three NH in the inner core, stabilize high-valent transition metal ions; the Ga(III) complex is considered to be the typical metallocorrole, akin to Zn(II) porphyrins.1,2 The porphyrin macrocycle had its structure definitively established by Fischer in 1929 with the synthesis of protoporphyrin IX and also of hemin.3 However, before that several other groups had already tried to understand the action and structure of the natural porphyrin pigments. As a few striking examples, in 1864 Hoppe-Seyler named hemoglobin, in 1871 he was able to isolate porphyrins from blood and in 1879 demonstrated the related structural feature between chlorophyll and heme; in 1918 Milroy put forward a general porphyrin synthesis.3 After the Fischer’s landmark in 1929, several groups all over the world have been improving synthetic methodologies, searching biosynthetic pathways and looking for the mode of action of a great variety of natural and synthetic porphyrin derivatives. Also, the search for potential applications, sometimes as mimics of natural processes, has been an important target over the years. The overall situation with corroles is not yet as developed as porphyrins, but a significant amount of reports have appeared mainly after 1999 due to the introduction of facile one-pot synthetic methods. Although the corrole structure had been established by X-ray studies in 1971,4 it was only after 1999 that efficient synthetic methodologies for corroles appeared.5 These were established mainly by the groups of Gross,6 Paolesse,7 and soon afterward by Gryko,8 although these synthetic approaches have been

2. FUNCTIONALIZATION AT THE INNER CORE POSITIONS Corroles, like other pyrrolic macrocycles, give different types of reactions at the inner core positions, namely protonation, deprotonation, coordination with metals, and N-alkylation and N-acylation reactions. When compared with other tetrapyrrolic macrocycles, namely porphyrins, corroles are more acidic and that is related with the steric relief obtained by the loss of one inner core proton.29 In metalation processes, corroles typically act as trianionic ligands while porphyrins act as dianionic ligands. This substantial difference allows corroles to coordinate high-valent metal ions. However, recent studies have demonstrated noninnocence of the corrole ligand, leading to the reformulation of formally high-valent complexes. In this regard, the corrole ligand is more prone to noninnocent behavior than porphyrin. This subject is discussed briefly in the cases of iron and copper corroles. 2.1. Metalation

Although metalation of corroles is usually not considered functionalization of the ligand but of the metal ions, this aspect has been included in this review for one main reason: most of B


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Figure 2. Periodic table of corroles.

the reactions described in the other sections have actually been performed on metallocorroles and not free-base corroles. This emphasizes the need for outlining the synthetic chemistry required for obtaining the metal complexes, as well as the demetalation procedures. Every year new elements are coordinated to corrole ligand for the first time. Figure 2 shows an updated version of the “periodic table of corroles”30,31 where all elements that have been coordinated to corroles are highlighted. In this section, it is given broad experimental details for the preparation of all types of metallocorroles known. However, the discussion of the structure and properties of the resulting metallocorroles is out of the scope of this review. Several reviews have been published in recent years outlining the synthesis, structural features and applications of metallocorroles.31−35 2.1.1. Group 1 Metallocorroles. 2.1.1.1. Lithium. The only alkali metal corrole complex known was reported by Arnold and co-workers.36 The lithium corrole 2 was prepared in 74% yield by treating the free-base corrole 1 with LiN(SiMe3)2 (3 equiv) in THF at −40 °C (Scheme 1). This new complex was used as a metathesis reagent in the synthesis of group 4 (titanium, zirconium, and hafnium, see Scheme 2), lanthanide (gadolinium, see Scheme 11), and actinide (thorium and uranium, see Figure 16) corrole complexes. 2.1.2. Group 4 Metallocorroles. 2.1.2.1. Titanium, Zirconium, and Hafnium. Oxotitanium β-alkylcorrolates are synthesized by treating free-base corroles with titanyl acetylacetonate [TiO(acac)2] in phenol, in an open tube at ca. 180 °C.37 Using TiCl3 or titanocene in the metalation reaction also leads to the formation of oxotitanium corrolates. In these complexes, the corrole acts as tridentate and dianionic ligand, with one NH not participating in metal-binding.37 Ti(IV), Zr(IV), and Hf(IV) meso-triarylcorrole complexes were prepared via salt metathesis with the lithium corrole 2. Addition of CpZrCl3 to 2 in THF at −40 °C gives the cyclopentadienyl zirconium(IV) corrole complex 3 (Figure 3). The pentamethylcyclopentadienyl titanium(IV) corrole complex 4 was prepared in a similar manner.36 Treatment of 2 with titanium, zirconium, and hafnium tetrachlorides afforded, respectively, the metal(IV) chloride complexes 5−7 (Scheme 2).38 Dimeric

Scheme 1. Synthesis of a Lithium Corrole Complex

complexes 5 and 6 have metal centers joined via bis(μ-chloride) linkages. 2.1.3. Group 5 Metallocorroles. 2.1.3.1. Vanadium. Freebase octaethylcorrole (H3OEC) reacts with vanadyl acetylacetonate [VO(acac)2] in phenol, in an open tube at ca. 180 °C, to give the oxovanadium(IV) corrole complex in high yield.37 In this complex, the corrole acts as tridentate and dianionic ligand, with one NH not participating in metal-binding. The oxovanadium corrolate is formed even when V(acac)3 or VCl3 are used in the metalation reaction.37 2.1.4. Group 6 Metallocorroles. 2.1.4.1. Chromium. The oxochromium(V) complex of 7,8,12,13-tetraethyl-2,3,17,18tetramethylcorrole, prepared by treating the free-base corrole with anhydrous CrCl2 in dry DMF at reflux, was the first wellcharacterized chromium corrole to be reported.39,40 The synthesis of the first (meso-triarylcorrolato)chromium(V) complex was only reported 20 years later. The oxo[trisC


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Scheme 2. Synthesis of Group 4 Corrole Complexes

(ca. 40%).49 Tipically, Mo complexes of meso-triarylcorroles are prepared by treating the free-bases with Mo(CO)6 in decalin at 170−180 °C50,51 or with MoCl4(THF)2, in the presence of ethyldiisopropylamine, in CH2Cl2 at 40 °C.52,53 The strength of the metal−oxo bond in high-valent molybdenum(V)−oxo complexes is strongly solvent-dependent.52,54 Gross and co-workers55 reported the formation of low-valent molybdenum complexes by addition of reducing agents to Mo(V)(TPFC)(O). The addition of either NaBH4, CoCp2, or CoCp*2 to a N2-purged THF solution of Mo(V)(TPFC)(O) leads to the formation of [Mo(IV)(TPFC)(O)]−. The addition of trimesitylvanadium(III) (or bromo(mesityl)magnesium bromide) to an anaerobic solution of Mo(V)(TPFC)(O) in THF induced the immediate formation of the binuclear complex 8 (Figure 4). In this complex, each one of the two metal ions is chelated by a trianionic corrolate and one axial oxygen atom, and these subunits are bridged by a Mg(THF)4 moiety.55

Figure 3. Zirconium(IV) and titanium(IV) corrole complexes.

(pentafluorophenyl)corrolato]chromium(V) was obtained in 76% yield from the aerobic reaction of 5,10,15-tris(pentafluorophenyl)corrole (H3TPFC) with Cr(CO)6 in toluene at reflux.41 The same complex was obtained in 54− 59% yield by treating the free-base corrole with CrCl2 in pyridine at reflux under argon and purification by column chromatography on silica gel.42 Electrochemical and chemical oxidation or reduction of that oxo complex allowed the isolation and characterization of chromium complexes in four oxidation states: [Cr(TPFC•)(O)][SbCl6], Cr(TPFC)(O), [Cr(TPFC)(O)][Cp2Co], and Cr(TPFC)(py)2.42,43 Cr(V) and Cr(VI) nitrido complexes of H3TPFC were prepared by a route that involved the transfer of a nitrogen atom from a stable (nitrido)manganese(V) complex of salophene to Cr(III)(TPFC).44 The first examples of mononuclear imido complexes of Cr(V) were prepared via the reaction of Cr(TPFC)(py)2 with aryl nitrenes, generated via thermal or photochemical decomposition of the corresponding organic azides.45 Oxo and imido Cr(V) corrole complexes show extremely fast electron transfer reactions, in sharp contrast with slow electron transfer reactions of other high-valent metal−oxo and−imido complexes.46 Chromium(III) and (oxo)chromium(V) complexes are appealing catalysts for aerobic oxidations.47 In this context, the synthesis of chiral meso-ABC-corrolatochromium(V) complexes48 is a step forward for the asymmetric oxidation of organic substrates by molecular oxygen. 2.1.4.2. Molybdenum. β-Octaalkylcorroles react with MoCl5 or Mo(CO)6 at 170−180 °C in decalin to give monomeric oxo(corrolato)molybdenum(V) complexes in moderate yields

Figure 4. Binuclear molybdenum corrole complex.

2.1.4.3. Tungsten. A tungsten corrole was prepared by Gross and co-workers56 by heating a solution of H3TPFC and WCl6 in decalin at 170−180 °C, under nitrogen. The structural analysis of the resulting air-stable, EPR-silent, and NMR-active complex revealed a novel trioxo-bridged binuclear tungsten(VI) corrole (9, Figure 5). The tungsten(V) corrole 10 was prepared in 70% yield through a metathesis reaction of the lithium corrole 2 with WCl6 in toluene at −40 °C.57 Surprisingly, the reaction of 5,10,15-trimesitylcorrole with WCl6 and W(CO)6 (2:1) in benzonitrile at 200 °C for 8 h resulted in the formation D


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Figure 5. Tungsten(V) (10) and tungsten(VI) (9 and 11) corrole complexes.

of the air-stable 3,17-dichloro-5,10,15-trimesitylcorrole radical.58 Recently, a series of three tungsten(VI) biscorroles of the general formula W[corrole]2 (11) were prepared by treating the free-base meso-tris(p-X-phenyl)corrole derivatives (X = H, Me,CF3) with W(CO)6 in refluxing decalin.59 2.1.5. Group 7 Metallocorroles. 2.1.5.1. Manganese. Manganese complexes of octaalkylcorroles can be obtained by direct metalation of the free-bases with Mn2(CO)10 in toluene at reflux or with Mn(OAc)3 in refluxing DMF.60,61mesoTriarylcorroles are typically metalated with Mn(OAc)2 in DMF,62,63 in methanol64 or in pyridine.65 High-valent manganese corroles are conveniently prepared by oxidation of Mn(III) corroles: treatment with tris(4-bromophenyl)aminium hexachloroantimonate affords chloromanganese(IV) corroles,66 treatment with molecular bromine or iodine affords, respectively, bromo-66 or iodomanganese(IV) corroles,67 and treatment with PhIO gives oxomanganese(V) corroles.64 Irradiation of a mixture of Mn(III) corroles and NaN3 in CH3CN results in conversion into (nitrido)manganese(V) complexes.66 Methods that work for conversion of other manganese(III) corroles to the corresponding nitridomanganese(V) cannot be used for the synthesis of (nitrido)manganese(V) complexes of 5,10,15-tris(4-nitrophenyl)corrole since nitrido-manganese(V) azahemiporphycenes are formed via ring expansion of the corrole macrocycle (see Scheme 45).63 A series of bismanganese cofacial corrole−porphyrin dyads were synthesized and their electrochemical and spectroelectrochemial properties were investigated. The two chromophore units were linked by 9,9-dimethylxanthene, anthracene, dibenzofuran, or diphenylether units.68 A recent application of manganese corroles involves their use as bifunctional catalysts for the electrocatalytic generation of dioxygen as well as for the reduction of dioxygen in aqueous media.69 Recent developments in the field of manganese corroles have been reviewed.70,71 2.1.5.2. Rhenium. The first rhenium corrole complex was obtained accidentally in the course of the metalation of mesotetrakis(trifluoromethyl)porphyrin with [Re2(CO)10].72 Surprisingly, when the porphyrin was refluxed with [Re2(CO)10] in benzonitrile, a detrifluoromethylation and ring contraction occurred, and the oxorhenium(V) corrolate 12 was formed in ca. 9% yield (Figure 6, see also Scheme 37). A general and facile synthesis of oxorhenium(V) mesotriarylcorroles was, however, recently reported by Ghosh and co-workers.73 They were able to synthesize complexes 13 from the corresponding free-base corroles and [Re2(CO)10] in refluxing decalin in 62−84% yield. 2.1.6. Group 8 Metallocorroles. 2.1.6.1. Iron. Iron(III) octamethylcorrole has been prepared by metalation of

Figure 6. Oxorhenium(V) corrole complexes.

octamethylcorrole (H3OMC) with FeCl3 in DMF at reflux or with Fe(CO)5 in toluene at reflux.60 Metalation of H3OEC with Fe2(CO)9 in toluene at reflux, followed by workup with admission of air, affords the stable oxygen-bridged binuclear iron(IV) complex 14 that presumably arises from the spontaneous air oxidation of Fe(III)(OEC) (Scheme 3).74 This type of diiron(IV) μ-oxo biscorrole complex is also formed with meso-triarylcorroles.75,76 The Fe(OEC)Cl complex (15) is formed by treatment of 14 with 1 M HCl at room temperature while addition of aqueous NaOH to solutions of the chloro complex 15 leads to the rapid reformation of 14. The chloro complex 15 reacts with phenylmagnesium bromide providing access to the stable organometallic σ-phenyl Fe(OEC) complex 16. meso-Triarylcorroles are also efficiently metalated with Fe2(CO)9 in refluxing toluene,75 or with FeCl2 in dry DMF at reflux.77 Metalation of meso-triarylcorroles with FeCl2·4H2O in a refluxing mixture of pyridine/methanol, followed by the addition of an aqueous solution of NaNO2 affords the corresponding Fe(III)(corrole)(NO) complexes.78 In 1994, Vogel and co-workers, assigned an Fe(IV) electronic configuration in complexes 14−16, and at the same time alerted that “in the case of complexes containing ligands having extended π systems (so-called noninnocent ligands) the metal cannot be assigned a definite oxidation state−the reason for this is the mixing of metal and ligand orbitals.”74 Since then, the oxidation state of the metal center in high-valent metal corrolates, namely the formally tetravalent iron corrolates, has been object of an extensive debate. Walker and co-workers concluded that chloroiron corrolates have a Fe(III) center coupled to a corrolate π radical and should be represented as [Fe(III)(corrole•2−)]Cl−.79−81 Ghosh and co-workers also proposed corrole π-cation radicals for Cl(triarylcorrolate)Fe complexes but did not find evidence of a radical character in Fe(IV) corrole μ-oxo dimers complexes.75 The same authors concluded that β-octafluorocorroles act as noninnocent ligands, exhibiting corrole π-cation radical character, in the chloroiron complexes.82 Gross and coworkers found no indications for corrole radicals in chloroironE


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Scheme 3. Synthesis of Iron Corroles

monomeric species was evidenced by mass spectrometry, indicating the presence of strongly bound dimers.96,97 Mononuclear ruthenium corroles were finally reported in 2003.98 Nitrosyl complexes 18 and the binuclear complex 19 were achieved by metalation of the corresponding free-bases with excess [{Ru(cod)Cl2}x] followed by a stream of dry NO bubbled through the hot solution. When H3TPFC is used, the reaction affords two products that can be separated by column chromatography on silica gel: the red-brown 19 (39% yield) and the wine-red 18a (30% yield). In contrast, the reaction with meso-tris(2,6-dichlorophenyl)corrole affords only the mononuclear complex 18b (35−40% yield). When the same procedure is applied to H3TPFC, but without the addition of NO, the binuclear complex 19 is the only isolated product (50% yield). 2.1.6.3. Osmium. The synthesis of osmium corroles was achieved for the first time in 2014. Ghosh and co-workers99 synthesized the nitridoosmium(VI) corroles 20 in ca. 43−60% yield by oxidative metalation of the corresponding free-base meso-triarylcorroles with [Os3(CO)12]/NaN3, under inert atmosphere, in refluxing 1:2 diethylene glycol monomethyl ether/glycol (Figure 8).

(IV) corroles and, accordingly, these complexes should be represented as [Fe(IV)(corrole3−)]Cl−.83,84 However, recent theoretical and spectroscopic characterizations seem to favor the existence of [Fe(III)(corrole•2−)] species.85−87 Using an array of experimental and theoretical methods, the Ghosh group found that the iron centers of diiron μ-oxo biscorrole complexes of meso-tris(4-X-phenyl)corroles (X = OMe, Me, H, CF3) are not Fe(IV) but intermediate-spin Fe(III) coupled to a corrole•2−. DFT calculations indicated that such complexes are best described as assemblages of four open-shell fragments: corrole•2−(↓)−Fe(III)(↑↑↑)−Fe(III)(↓ ↓↓)−corrole•2−(↑), with antiferromagnetic coupling between any two adjacent fragments.88 Iron complexes of β-nitro-substituted corroles,89,90 bis-picket fence nitrosyliron corrolates,91 σ-phenyl iron−meso-triarylcorrole complexes,92 and structurally interesting xanthenemodified and Hangman iron corrolates93,94 have been reported in recent years. A dissolved amphiphilic iron(III) corrole was used as a sensitizer to determine photometrically small amounts of nitric oxide in the subparts-per-million range.95 2.1.6.2. Ruthenium. Initial attempts to metalate H3OMC with RuCl3 in DMF or with [Ru3(CO)12] in toluene lead to the formation of octamethylporphyrin and the corresponding porphyrinatoruthenium(II).60 However, in 2000 it was found that β-octaalkylcorroles react with [(cod)RuCl2]2 (cod = cycloocta-1,5-diene) in refluxing 2-methoxyethanol, in the presence of triethylamine, to afford face-to-face diruthenium(III,III) corrole dimers (17, Figure 7). No trace of a

Figure 8. Nitridoosmium(VI) corroles.

2.1.7. Group 9 Metallocorroles. 2.1.7.1. Cobalt. Freebase octaalkylcorroles and triarylcorroles react with cobalt(II) acetate in pyridine at ca. 100 °C or in boiling methanol in the presence of PPh3 to give, respectively, pyridine- or PPh3coordinated Co(III) complexes.100,101 However, a simple modification of the experimental conditions may lead to

Figure 7. Mononuclear and dinuclear ruthenium corroles. F


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“face-to-face” porphyrin−corrole dyads,114−122 and biscorroles.116,117,120,123,124 The metallic or heterobimetallic complexes of these types of compounds have been extensively studied as catalysts in a range of chemical transformations. Cobalt corroles,125−129 and in particular the Hangman corroles,130−132 show enhanced activity for oxygen reduction and water oxidation. Porphyrins and corroles show very different affinities to divalent metal ions: there is a large affinity for porphyrins but it is virtually zero for corroles. These different chelating properties allow the selective metalation of the porphyrin or corrole units in porphyrin−corrole conjugates.133 The metalation of the penta-chromophore 25, which consists of a central porphyrin decorated with four corrole units, is an interesting example of this subject (Scheme 5).134 Heating a DMF solution of 25 with zinc(II) acetate at 90−100 °C provides complex 26 (84% yield), in which zinc(II) is only chelated by the central porphyrin core. Conversely, stirring a solution of compound 25, Co(OAc)2·4H2O, and PPh3 in pyridine for 1 h at room temperature allows the insertion of a Co−PPh3 moiety into the four corrole units while the central porphyrin remains as a freebase (27). 2.1.7.2. Rhodium. Metalation of 8,12-diethy-2,3,7,17,18hexamethylcorrole with [Rh(CO)2Cl]2 in CHCl3 at room temperature yields the unsymmetrical dicarbonylrhodium(I) complex 28 (Figure 10).135,136 Out-of-plane dicarbonylrhodium(I) complexes are also obtained by metalation of N(21)- and N(22)-methyl corroles.137 Single crystal Xray structures of the N-methyl complexes have shown that the methyl and Rh(CO)2 moieties are trans.138 Conversely, in rhodium(III) corrolates bearing one or two axial ligands the metal ion is lying in the macrocycle plane.60,139,140 The electrochemistry of Rh(III)(OMC)(PPh3) has shown that the oxidations occur at the corrole π-ring system while the reductions occur at the rhodium metal center.141 The first rhodium(III) complex of a meso-triarylcorrole, Rh(TPFC)(PPh3), was obtained by Gross and co-workers by heating H3TPFC with an excess of [Rh(CO)2Cl]2 in benzene and in the presence of PPh3 and K2CO3.101 This complex is an excellent catalyst for the cyclopropanation of olefins by carbenoids.77,142,143 The same research group reported the Xray structures of N(21)- and N(22)-substituted benzyl and 2picolyl derivatives of H3TPFC and the corresponding rhodium(I) and zinc(II) complexes.144 Other rhodium(I) and rhodium(III) corrole complexes, such as 29 and 30, were fully characterized by NMR and X-ray crystallography.145,146 Collman and co-workers have shown that the metalation of meso-triarylcorroles 31 with [Rh(cod)2Cl]2 in dichloromethane, benzene, or methanol, at room temperature and in the presence of a sterically unhindered amine, such as pyridine or diethylamine, affords the expected octahedral Rh(III) corroles 32 with the base coordinated at axial positions (Scheme 6). However, when sterically hindered bases such as triethylamine or 2,6-dimethylpyridine are used, the products have been identified as Rh(III)(corrole)(NHEt2)2 (33), and a mixture of three Rh(III)(corrole)L2 complexes (34−36) in which 3- and/ or 4-methylpyridine are bound at axial positions, respectively. The unusual results show that bulky bases are both dealkylated, and rearrangement is observed in the case of 2,6-dimethylpyridine.147 2.1.7.3. Iridium. Metalation of H3TPFC with [Ir(cod)Cl]2 and K2CO3 in refluxing THF, under Ar, gives Ir(I)(TPFC)(cod), which is converted to an axially trimethylamine-ligated

different products. Metalation of H3TPFC with Co(OAc)2 and PPh3 (initially present) affords the PPh3-coordinated cobalt(III) complex 21 in 90% yield (Scheme 4, path a).102 However, Scheme 4. Metallation of TPFC with Co(OAc)2 under Different Experimental Conditions

addition of the PPh3 after the complete metalation process results in the formation of an additional product, the 3,3′corrole dimer 22 (Scheme 4, path b). The relative yields of 21 and 22 depend mainly on how late the PPh3 is added. Co(III) complexes of the structurally similar 3,3′-corrole trimer (23) and tetramer (24) (Figure 9) were prepared by metalation of

Figure 9. Co(III) complexes of a corrole trimer and a tetramer.

the corresponding free-base oligomers with Co(OAc)2·4H2O in pyridine at 100 °C.103 Cobalt complexes of a 5,5′-linked corrole dimer and a 10,10′-linked corrole dimer were synthesized by metalation with Co(OAc)2·4H2O in 80% and 87% yield, respectively.104 The Co complex of a meso-triaryltetrabenzocorrole was synthesized by reaction of the free-base corrole with Co(OAc)2 in refluxing pyridine.105 The discovery, in the 1980s, of the “face-to-face” (Pacmantype) bisporphyrins,106−113 in which the two porphyrin rings are anchored to a single rigid spacer, enabled the emergence of G


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Scheme 5. Selective Metallation of a Porphyrin−Corrole Conjugate

Figure 10. Rhodium corrole complexes.

The metalation of the “face-to-face” biscorrole 39, rigidly linked by an anthracenyl spacer, with Ni(OAc)2 in pyridine leads to the formation of a 1:1 mixture of the isomeric Ni(II) bisoxocorroles 40 and 41 (Scheme 7).157 Conversely, under the same metalation conditions, no nickel bisoxocorrole is obtained when the biphenylene biscorrole 42 is used. In this case the bisnickel bisradical species 43 is the major product of the reaction; the hydrooxobiscorrole 44 is produced as a minor compound. 2.1.8.2. Platinum. meso-Triarylcorroles 45 react with the tetranuclear platinum acetate complex [Pt(OAc)2]4·2AcOH in benzonitrile, under microwave irradiation at 140−150 °C, to give low but reproducible yields (∼6%) of diamagnetic Pt(IV) corroles 46a−d (Scheme 8).158 Aerobic conditions are critical to the success of the reaction. The axial cyanophenyl ligand may be bound through the o-, m-, or p-carbon, relative to the CN group. Treatment of the Pt(IV) complexes 46 with an aryl

Ir(III) complex 37 upon addition of trimethylamine N-oxide and exposure to air (Figure 11).148 Bromination of 37 with Br2 affords the octabromocorrole complex 38. When the metalation is carried out in the presence of pyridine,149 substituted pyridines,150 triphenylphosphine,149 4,4′-bipyridine,151 or ammonia152,153 iridium(III) complexes with these axial ligands are obtained. The Ir(III) corroles phosphorescence in the nearinfrared region depends strongly on the nature of the axial ligand.154 2.1.8. Group 10 Metallocorroles. 2.1.8.1. Nickel. Nickel complexes of β-octaalkylcorroles are prepared by treating freebase corroles with NiCl2 in CH3Cl/MeOH155 or Ni(OAc)2 in DMF.156 ESR and 1H NMR data indicated that these complexes are paramagnetic and should be formulated as π radical cations of nickel(II) corroles and not as formal nickel(III) complexes.156 H


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Scheme 6. Metallation of meso-Triarylcorroles with [Rh(cod)2Cl]2 in the Presence of Sterically Unhindered or Hindered Amines

ation of the corresponding free-bases with Cu(OAc)2 in pyridine at room temperature.159 The metalation of the “faceto-face” biscorroles 39 and 42, and analogous porphyrin− corrole dyads, with CuCl2 in pyridine at 40 °C gives the corresponding dicopper derivatives.160 Copper complexes of a 5,5′-linked corrole dimer and a 10,10′-linked corrole dimer were synthesized by metalation with Cu(OAc)2·H2O in 75% and 81% yield, respectively.104 The oxidation state of the copper center in Cu corroles has been a topic of debate. Copper octaalkylcorroles were described as Cu(II) complexes of corrole dianions until 1997.155 In that year it was shown that these complexes are formed by a fully deprotonated corrole ligand and a central metal in the formal oxidation state +3.156 Copper meso-triarylcorroles were also assigned as Cu(III) complexes.159,161 The noninnocent character of the corrole ligands in copper corroles was first demonstrated by Bröring and co-workers in 2007,162 suggesting that these complexes are better described as Cu(II) corrole

Figure 11. Iridium(III) corrole complexes.

Grignard reagent affords the air-stable oxidized Pt(IV) (corrole•2−)Ar1Ar2 complexes 47a−d. 2.1.9. Group 11 Metallocorroles. 2.1.9.1. Copper. Octaalkylcorroles are easily metalated with Cu(OAc)2 in pyridine at room temperature,155 or in hot DMF.156 Copper complexes of meso-triarylcorroles are also prepared by metal-

Scheme 7. Metallation of “Face-to-Face” Biscorroles with Ni(OAc)2

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Scheme 8. Synthesis of Pt(IV) Corrole Complexes

Scheme 9. Synthesis of Gold(I) and Gold(III) Corrole Complexes

radical cations. Subsequently, Ghosh163−165 and Pierloot166 provided structural and computational evidence to support this assignment. In a recent communication,167 it was established that the ground state of Cu corroles is best described as an antiferromagnetically coupled Cu(II) corrole radical cation. It was also demonstrated that the Cu(II) oxidation state is maintained upon oxidation and reduction of the complex (i.e., both oxidation and reduction occur on the corrole ring). 2.1.9.2. Silver. meso-Triarylcorroles react with an excess of silver(I) acetate in pyridine at ∼80 °C to give the corresponding Ag(III) corroles in good yields (60−80%).168 The products are air-, light-, and water-stable and dissolve in a wide variety of solvents. The formation of the Ag(III) complexes involves a disproportionation reaction: 3Ag(I) → Ag(III) + 2Ag(0); a fine precipitate of Ag(0) and/or the formation of a silver mirror on the wall of the reaction flask is observed.168 This (or similar) procedure has been used to prepare Ag(III) complexes of A3-corroles,169,170opp-A2B171 corroles, and “face-to-face” biscorroles.172 The nitration of meso-triarylcorroles with AgNO2 leads to the nitro-substituted Ag(III) corrole complexes (see section 3.4).173 Free-base β-octabromo-meso-triarylcorroles react with silver(I) acetate at room temperature in pyridine to afford the corresponding (formally) Ag(III) corroles.174 While silver complexes of “simple” meso-triarylcorroles may be rationalized in terms of an innocent Ag(III)(corrole3−) description, silver β-

octabromo-meso-triarylcorroles are better described as a noninnocent Ag(II)(corrole•2−).174 Ghosh and co-workers recently published a XANES (X-ray absorption near-edge spectroscopy) study that provided direct experimental support for ligand noninnocence in silver octabrominated corroles.175 Silver complexes of a 5,5′-linked corrole dimer and a 10,10′linked corrole dimer were synthesized by metalation with AgOAc in 83% and 67% yield, respectively.104 2.1.9.3. Gold. Free-base β-octabromo-meso-triarylcorrole 48 (H3Br8TPFC) reacts with the gold(I) auration agents [MeAu(PPh3)] or [ClAu(PPh3)] in very mild conditions (toluene, room temperature, 20 min) to give a mixture of gold(I) complexes 49/50 (in a 1:4 ratio) in almost quantitative yield (Scheme 9).176 There is no reaction when THF or chloroform are used as solvents. The nonbrominated corrole H3(TPFC) could not be aurated by any of the applied reaction conditions.176 The oxidation of the chiral gold(I) corroles 49/50 with N-iodosuccinimide (NIS) gives the Au(III) complex 51 in an almost quantitative yield. Alternatively, 51 can be obtained directly from 48 via treatment with [ClAu(tht)] (tht = tetrahydrothiophene).176 Free-base βoctabromo-meso-triarylcorroles also give Au(III) corrolates upon treatment with chloroauric acid and trimethylamine in CH2Cl2.177 A simple and general method for the synthesis of gold(III) complexes of β-unsubstituted meso-triarylcorroles J


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yield, respectively (Scheme 10).189 These are rare examples of nonaromatic zinc(II) corrole complexes. 2.1.11. Group 13 Metallocorroles. 2.1.11.1. Boron. Freebase meso-triarylcorroles react with BF3·OEt2 in the presence of ethyldiisopropylamine to yield boron corrole complexes of the type [HNEtiPr2]+ [B2OF2(corrole)]−.190 These complexes comprise a corrole trianion coordinated to a FBOBF moiety and a ethyldiisopropylammonium countercation. The NMR and X-ray crystal structures show that each boron atom in the FBOBF unit is coordinated to two adjacent nitrogen atoms in the dipyrromethene sites (55, Figure 13). In contrast, the

involves the reaction of the free-base corroles with gold(III) acetate in pyridine at room temperature.178,179 The water-soluble Au(III) complex of 2,17-bis-sulfonato5,10,15-tris(pentafluorophenyl)corrole revealed high toxicity to cisplatin-resistant cancer cells180 and gold(III) complexes of 5,10,15-tris(3-carboxyphenyl)corrole and 5,10,15-tris(4carboxyphenyl)corrole exhibited significant phototoxicity against AY27 rat bladder cancer cells in photodynamic therapy experiments while the free-base corroles proved to be inactive.181 Gold(III) complexes of meso-triarylcorroles have been used in solar cells.181,182 Self-assembled and micelle encapsulated conjugates of Au(III) meso-triarylcorroles and CdSe/ZnS quantum dots have been used as optical oxygen sensors.183 2.1.10. Group 12 Metallocorroles. 2.1.10.1. Zinc. Zinc complexes of N-unsubstituted corroles are generally unstable. However, the Zn(II) complex of octamethylcorrole has been prepared by metalation of H3OMC with Zn(OAc)2 in pyridine. The resulting compound was formulated as {[Zn(OMC)]− [pyH]+}.184 A similar procedure was used to prepare the Zn(II) complex of meso-triphenylcorrole.185 The complexation of H3TPFC with several divalent transition-metal ions, including Zn2+, was studied by liquid secondary ion and electrospray mass spectrometry.186,187 N-Alkyl corroles are easily metalated with zinc(II) acetate in pyridine to yield zinc(II) corrole complexes such as the Zn(II)(N(21)-benzyl-TPFC) (py) (52) and Zn(II)(N(21)picolyl-TPFC) (53) (Figure 12).144,188

Figure 13. Different types of boron corrole complexes.

sterically crowded β-octabromo-meso-tris(4-fluorophenyl)corrole (H3(Br8T(4-FC6H4)corrole), when treated with BF3· OEt2 and NEtiPr2 (in a 1:12:20 ratio) under the same experimental conditions, leads to the formation of the diboryl complex [HNEtiPr 2 ] + [(BF 2 ) 2 (Br 8 T(4-FC 6 H 4 )corrole)] − (56).191 Reducing the BF3·OEt2 and NEtiPr2 to a 1:2:4 ratio allows the isolation of the corresponding monoboryl corrole 57. Free-base meso-triarylcorroles also react with PhBCl2, in the presence of NEtiPr2, to afford complexes of the type Ph2B2H(corrole) (58) with a bridging B−H−B unit.192 The regio- and stereochemical preferences for bonding of one or two boron atoms to the corrole macrocycle have been

Figure 12. Zinc(II) complexes of N-alkyl corroles.

Metalation of the triply linked 2H-corrole dimers (with two inner NH groups in each corrole unit) 54a and 54b with Zn(OAc)2·2H2O in CHCl3 at reflux for 20 h gave the bis(zinc(II)) complexes Zn54a and Zn54b in 76% and 79% Scheme 10. Metallation of 2H-Corrole Dimers with Zn(II)

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the μ-oxo dimer 60 and the methoxy derivative 59d. The chloro complex 59c is obtained by treatment of 59d or 60 with HCl (4 M).215 The Ge(IV)(OH) complex of meso-tri(4pyridyl)corrole has been prepared by treating the free-base with GeCl4 in dry DMF at reflux.216 Methylation of this complex with methyl iodide afforded the corresponding ticationic derivative. Its interaction with single- and double-stranded nucleic acid homopolymers and calf thymus DNA was studied.216 The germanium(IV) complex of a meso-triaryltetrabenzocorrole was also prepared by metalation of the freebase with GeCl4 in DMF.197 The chloro and hydroxo axial ligands in (TPFC)Ge−X (X = Cl, OH) (59a and 59b) are labile and are rapidly substituted by −OMe or −OEt at room temperature when these complexes are dissolved in methanol or ethanol, respectively.217 The axial OH group can also be substituted by phenol groups, such as meso-(4-hydroxyphenyl)porphyrins in toluene at reflux.218 Reduction of (TPFC)Ge−OMe with NaBH4 in methanol/ benzene solution yields the germanium hydride complex (TPFC)Ge−H. This hydride reacts with aldehydes, alkenes and alkyl halides to afford a range of complexes of the types (TPFC)Ge−CH(OH)R or (TPFC)Ge−R (R = alkyl) in 50− 95% yield. As an example, the reaction with paraformaldehyde at 70 °C gives the hydroxymethyl complex (TPFC)Ge− CH2OH in 91% yield.217 The hydride complex (TPFC)Ge−H reacts with two equivalents of TEMPO to afford complex [(TPFC)Ge(TEMPO)] (61) (TEMPO = (2,2,6,6-tetramethylpiperidin-1yl)oxyl)) (Figure 15).219 Under visible-light irradiation, the

established using DFT methods.193 The chemistry of the complexes of boron with ligands containing pyrrolyl motifs has been comprehensively reviewed.194 2.1.11.2. Aluminum. H3TPFC reacts with AlMe3 in a toluene/pyridine mixture at room temperature yielding the Al(III)(TPFC)(py)2 complex in 81% isolated yield.195 A corrole bearing two β-chlorosulfonyl groups was also metalated by this method.196 The aluminum(III) complex of a mesotriaryltetrabenzocorrole has been prepared by treating the freebase corrole with AlEt3 in toluene at room temperature.197 Studies concerning the photophysical properties of Al(III) corrole complexes have been reported.198,199 2.1.11.3. Gallium. H3TPFC reacts with flame-dried GaCl3 in pyridine at reflux under argon to give Ga(TPFC)py in 78% yield.2 This gallium-insertion reaction was applied to other A3triarylcorroles196 and opp-A2B corroles.200 The steady-state absorption and emission spectra and the temporal fluorescence decay profiles of the Ga(TPFC)py complex was reported.198 The gallium(III) complex of a meso-triaryltetrabenzocorrole has been prepared by treating the free-base corrole with Ga(acac)3 in pyridine at reflux.197 The photophysical, electrochemical, and spectroelectrochemical characterization of the complex was reported.197 Gallium complexes of a 5,5′-linked corrole dimer and a 10,10′-linked corrole dimer were synthesized by metalation with GaCl3 in 75% and 73% yield, respectively.104 The gallium(III) complex of 5,10,15-tris(pentafluorophenyl)corrole-2,17-bis-sulfonato binds rapidly and noncovalently to human serum albumin forming extremely stable complexes.201 This property enables the formation of fluorescent bioconjugate assemblies that are highly effective for killing breast cancer cells in vitro202−204 as well as for tumor detection and imaging.205−208 The nonconjugated sulfonated gallium complex possesses cytoprotective and cytotoxic properties209 and carboxylated gallium corroles are promising anticancer agents that can also function as fluorescence imaging agents.21 The water-soluble cationic 5,10,15-tris(N-methyl-4-pyridyl)corrolatogallium(III) exhibited high photocytotoxicity toward Hep G2 cancer cells (IC50 = 60 nM).210 Ga corroles have also been used as colorimetric probes for anions211,212 and as emissive probes.213,214 2.1.12. Group 14 Metallocorroles. 2.1.12.1. Germanium. H3TPFC reacts with excess GeCl4 in refluxing dry DMF to give a mixture of chloro- and hydroxo-coordinated germanium(IV) complexes [Ge(TPFC)Cl] (59a) and [Ge(TPFC)OH] (59b), respectively (Figure 14). Ge(TPFC)Cl is obtained in 90% yield by treating the reaction mixture with HCl (0.1 M) while pure Ge(TPFC)(OH) can be prepared just by passing the chloro complex through a column of silica with CH2Cl2 as eluent.77 Under identical conditions, H3TPC affords, after chromatographic separation of the crude reaction mixture on silica gel,

Figure 15. Germanium corrole complexes with axial TEMPO or amino ligands.

weak Ge−O bond in 61 is cleaved and the tetra-coordinated Ge(III) corrole complex [(TPFC)Ge(III)]• is generated. The Ge(III) radical reacts rapidly with ammonia, primary/secondary aliphatic amines and aniline to produce (TPFC)Ge−NR1R2 complexes (62) (R1R2 = HPr, HiPr, HtBu, HPh, Et2, iPr2) in high yields (65−95%).219,220 In a recent paper, Brothers and Fu reported the generation and reactivity of the highly reactive tetra-coordinate square planar germanium(IV) cation [(TPFC)Ge]+.221 This cation was synthesized quantitatively by the reaction of (TPFC)Ge− H with [Ph3C]+[B(C6F5)4]−. [(TPFC)Ge]+ reacted with benzene at room temperature to form quantitatively the complex (TPFC)Ge−Ph in a few days. In the presence of Na2CO3, [(TPFC)Ge]+ reacted with ethylene in benzene form (TPFC)Ge−CH2CH2Ph quantitatively at room temperature in 1 h. Under similar reaction conditions, the reaction with cyclopropane in benzene afforded (TPFC)Ge− CH2CH2CH2Ph in 58% yield. 2.1.12.2. Tin. The tin(IV) complex Sn(OEC)Cl, was synthesized by heating at reflux a solution of H3OEC and

Figure 14. Germanium corrole complexes. L


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Scheme 11. Synthesis of Lanthanum, Terbium, and Gadolinium Corrole Complexes

(C6H5)2, P(OEC)(O), and [P(OEC)(CH3)]+ ClO4−, were reported by Kadish et al.230 Treatment of the hexacoordinate P(TPC) (OH)2 complex with trifluoroacetic acid (TFA) in CH2Cl2 for 30 min at room temperature gives the pentacoordinate oxophosphorus(V) complex P(TPC)(O) in quantitative yield.232 P(corrole) (OH)2 complexes react with phenols in refluxing toluene to give P(corrole)(OAryl)2 complexes. This method was used to prepare the “vertically aligned” hetero trimeric array P(TPC)(O-porphyrin)2 by treating P(TPC)(OH)2 with a mono-(4hydroxyphenyl)porphyrin.233 The two axial hydroxides in P(corrole)(OH)2 complexes are exchanged by fluorides by treatment with aqueous 40% HF leading to bis-fluoridecoordinated complexes P(corrole)F2 in quantitative yields.234 The phosphorus(V) complex of a meso-triaryltetrabenzocorrole has been prepared by treating the free-base corrole with POCl3 in pyridine at reflux.197 The photophysical, electrochemical, and spectroelectrochemical characterization of the complex was reported.197 Experiments with HeLa cells demonstrated that the fluorescent P(O) 5,10,15-tris(4-methoxycarbonylphenyl)corrole complex is biocompatible and cell-permeable, being potentially useful for bioimaging applications.231 Anionic and cationic P(corrole)(OH)2 complexes have been used for the photodynamic inactivation of mold fungi spores,20 and the inhibition of green algae growth.17 The synthesis of the P[10(4-hydroxyphenyl)-5,15-bis(pentafluorophenyl)corrole](OH)2 complex and its photonuclease activity was recently reported.235 This compound displayed low dark toxicity and high photocytotoxic activity against H460 and A549 tumor cell lines. The ability of P(corrole)(OH)2, P(β-octachlorocorrole) (OH)2 and Ga(corrole)(py) complexes to act as photosensitizers in DSSCs was evaluated. The most efficient one (with 1.9% solar conversion efficiency) was a P(corrole)(OH)2 bearing a 5-(4-carboxyphenyl) group.236 2.1.13.2. Arsenic. H3OEC reacts with AsCl3 in pyridine at room temperature to afford the As(III)(OEC) complex in 94%

excess SnCl2 in DMF.222 This procedure was also used to prepare Sn(TPFC)Cl77 and other Sn(meso-triarylcorrole)Cl complexes.223,224 H3OEC reacts with excess diphenyltin oxide in dry DMF at 140 °C, under an argon atmosphere, to yield the phenyl σ-bonded Sn(IV) complex Sn(OEC)(C6H5).222 Tin(IV) corrolate complexes with alkyl or aryl axial ligands can be prepared by addition of an excess of alkyl- or arylmagnesium bromide or aryllithium to a solution of Sn(TPFC)Cl in dry THF, at room temperatue, under an inert atmosphere.225 Addition of an excess of NaBH4 to Sn(TPFC)Cl under a N2 atmosphere at room temperature leads to the immediate formation of the Sn(II) anion [Sn(TPFC)]−. This anion reacts efficiently with alkenes and alkyl halides to give Sn(IV) complexes of the type (TPFC)Sn−R (R = alkyl).223 The Sn complex of a meso-triaryltetrabenzocorrole was synthesized by reaction of the free-base corrole with SnCl2 in refluxing DMF.105 A heterobinuclear tin corrole−rhodium porphyrin complex, where the two metals are bound via a single metal−metal bond, was prepared by the nucleophilic attack of a rhodium(I) porphyrin on a chlorotin(IV) corrole.226 2.1.12.3. Lead. The only example known of lead corroles were synthesized by Knör and co-workers.227 They converted H3TPC into [Pb(II)(TPC)]− by reaction with Pb(NO3)2, in the presence of tetrabutylammonium tetrafluoroborate, in absolute ethanol at 45 °C. Photoinduced oxidation of the Pb(II) complex (irradiation with 313 nm light) lead to the corresponding Pb(IV)(TPC)(O) derivative. 2.1.13. Group 15 Metallocorroles. 2.1.13.1. Phosphorus. Octaalkylcorroles react with POCl3 in pyridine to give pentavalent P(V) complexes of type [P(corrole)(OH)]+ Cl−,228 while meso-triarylcorroles give hexacoordinate P(V) complexes of type P(corrole)(OH)2.77,229 Both β-octaalkylcorroles230 and meso-triarylcorroles231 react with PCl3 in pyridine to give P(corrole)(O) complexes. The synthesis and electrochemical studies of a series of phosphorus complexes of H3OEC, namely P(OEC)(H)2, P(OEC)(CH3)2, P(OEC)M


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yield.237 Oxidative methylation of As(OEC) with methyl iodide gives the σ-bonded methyl complex [As(V)(OEC)(CH3)]+. The perchlorate salt was obtained by treatment of the above reaction product with perchloric acid. 2.1.13.3. Antimony. H3OEC reacts with SbCl3 in pyridine at 100 °C to afford the Sb(III)(OEC) complex in 90% yield.237 The treatment of H3TPFC with SbCl3 affords the pyridinecoordinated antimony(III) complex Sb(TPFC)(py).50 The (oxo)antimony(V) complex Sb(TPFC)(O) is obtained in quantitative yields by either slow aerobic or fast chemical oxidation (with PhIO) of the corresponding antimony(III) corrole. Treatment of either Sb(TPFC)(py) or Sb(TPFC)(O) with aqueous HF leads to the trans-difluoroantimony(V) complex Sb(TPFC)F2 in 97% yield.50 Antimony corroles display high activity and selectivity as catalysts for the photoinduced oxidation of thioanisole by molecular oxygen, and for the oxidation of allylic and tertiary benzylic CH bonds to the corresponding hydroperoxides.50 2.1.13.4. Bismuth. The Bi(III)(OEC) complex was obtained in 82% yield from the reaction of H3OEC with BiCl3 in DMF at reflux.237 Bismuth meso-triarylcorroles were first reported by Schoefberger, Knör and co-workers in 2011.238 They were able to synthesize the Bi(III)(TPC) complex from the metalation reaction of H3TPC with Bi(OAc)3 in DMF or BiCl3 in pyridine. Due to its low stability, Bi(III)(TPC) was not isolated but its formation was confirmed by NMR and mass spectrometry. Bi(III)(TPC) undergoes photochemical oxidation to highvalent bismuth(V)-oxo species. Soon after, the same group reported the synthesis, isolation, and structural characterization (including by single-crystal X-ray diffraction) of the stable Bi(III)(TPFC) complex.239 The new compound was prepared by metalation of H3TPFC using Bi{N(SiMe3)2}3 as the metalation agent. The reaction was carried out in THF at room temperature and the Bi(III)(TPFC) complex was isolated in 75% yield after column chromatography. The Bi(III) complex of 10-(4-bromophenyl)-5,15-bis(pentafluorophenyl)corrole was used as a platform to a range of meso-functionalized Bi−corroles through palladium cross-coupling reactions (see Figures 73 and 80).240 The Bi(III) complexes are easily demetalated by treatment with HCl in THF at room temperature.240 2.1.14. Lanthanide Metallocorroles. 2.1.14.1. Lanthanum, Europium, Gadolinium, and Terbium. Arnold and coworkers241 described the preparation of the first examples of lanthanide corroles. Lanthanum and terbium complexes 63 and 64 were prepared from the free-base corrole 1 and Ln((NSiMe3)2)3 (Ln = La or Tb) (Scheme 11). Gadolinium complex 65 was prepared by metathesis of the Li corrole 2 and GdCl3 in the presence of 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3TACN). Kadish and co-workers reported the synthesis of the first europium triple-decker compounds which contain one corrole and two phthalocyanine macrocycles. The new compounds 68a,b were prepared from the reaction of the free-base corrole 66 with the “half-sandwich” phthalocyanine complexes 67a,b in octanol containing DBU (Scheme 12).242,243 This method was extended to the synthesis of other europium triple-decker compounds with nitrophenylcorroles244 and to other rare earth corrole−phthalocyanine heteroleptic triple-decker complexes where the metal is Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), or Tb(III).245 2.1.15. Actinide Metallocorroles. 2.1.15.1. Thorium and Uranium. Arnold and co-workers,246 following the salt

Scheme 12. Synthesis of Europium Corrole−Phthalocyanine Heteroleptic Triple-Decker Complexes

metathesis reaction procedure developed for the synthesis of Zr and Hf (Scheme 2) and Gd (Scheme 11) corroles, have prepared, for the first time, actinide corrole complexes. The thorium(IV) and uranium(IV) complexes 69 and 70 were obtained in 93% and 83% yield, respectively, by treating the lithium complex 2 with ThCl4(DME)2 or UCl4 in DME (1,2dimethoxyethane) at room temperature for 24 h (Figure 16).

Figure 16. Thorium(IV) and uranium(IV) corrole complexes.

2.2. Demetalation

The functionalization of the periferic positions of the corrole skeleton is often performed on the generally more stable metallocorroles rather than their free-base counterparts. That is required to protect the trianionic corrole cavity from (de)protonation or to avoid extensive (oxidative) fragmentaN


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Paolesse and co-workers170 reported the demetalation of Ag(III) complexes of meso-triarylcorroles with excess NaBH4 in a CH2Cl2/MeOH solution at room temperature. This method is not effective for demetallating silver 3-nitrocorroles since the peripheral nitro group is reduced by borohydride giving the corresponding free-base 3-aminocorrole. The authors explain that the demetalation/reduction of the silver nitrocorrole to the aminocorrole probably involves the initial reduction of Ag(III) to Ag(I) by NaBH4, Ag(I) is then dissociated from the macrocycle and reduced to Ag(0). This process leads to the formation of Ag(0) nanoparticles that constitute the catalytic system responsible for reduction of the nitro group to an amino group.170 Silver 3-nitrocorroles are, however, successfully demetalated, without reduction of the nitro group, using DBU/THF solutions.170 The DBU/THF method was also used for the demetalation of Ag(III) complexes of 3,17-dibromoand 3,17-diphenylethynylcorrole derivatives.253 An efficient, mild, and one-step method for the conversion of tin(IV)-corroles to the corresponding free-base corroles was developed by Sinha and Kar.224 The reported strategy for the demetalation reaction was based on the use of a Grignard reagent, namely methylmagnesium chloride. This particular protocol has also been proven to be versatile on a wide variety of meso-triarylcorrolato-Sn(IV)-chloride substrates, and the free-base corroles were obtained in 50−90% yield. Other Grignard reagents, such as methyl/phenylmagnesium bromides, however failed to perform the demetalation reaction and rather resulted in the usual σ-methyl/phenyl complexes in 80−87% yield.224

tion of the rather fragile corrole framework. Moreover, the introduction of metal ions that favor specific reactions is also very important. Porphyrin chemistry has well established procedures for reversible coordination of metal ions. However, the removal of the central metal ion in corroles is often problematic, leading to decomposition of the macrocycle and poor free-base yields. The first example of a corrole demetalation was reported in 2001 by Bröring and co-worker.247 They found that treatment of Mn(III)(OEC) with HBr in AcOH affords the corresponding free-base in a quantitative yield. Brückner and co-workers found that Ag(III) complexes of meso-triarylcorroles slowly demetallate in CHCl3 containing acid impurities. Thus, treatment of a CHCl3 or CH2Cl2 solution of Ag(III) complexes with concentrated aqueous HCl in a biphasic system leads within minutes to complete demetalation of the silver complexes and precipitation of silver(I) chloride.168 Paolesse and co-workers248 reported that the use of the acidic mixture CHCl3/H2SO4 allows the almost complete removal of copper ions from meso-triaryl-, β-octaalkyl-, and fully substituted corroles. Both the decomposition of the starting complex and the formation of isocorrole species (typically produced by oxidation of the corrole ring under acidic conditions) are minimized in this procedure. This procedure is also efficient for the demetalation of copper mesotriaryltetrabenzocorroles105 but was found to be less effective for the demetalation of other metal complexes of corrole, such as cobalt, manganese, iron, and germanium derivatives.248 Schoefberger and co-workers240 reported a simple procedure for the acid-catalyzed demetalation of meso-substituted bismuth corroles. It involved the dropwise addition of HCl to a solution of the Bi(corrole) complex in THF at room temperature. The best yields (92%) of the corresponding free-bases were obtained using a concentration of 15 mmol L−1 HCl in the reaction mixture. Dehaen and co-workers249 found, in a serendipitous way, a new procedure for the demetalation of copper mesotriarylcorroles. While attempting to reduce a copper 10-(4nitrophenyl)corrole to the aminophenyl derivative with SnCl2· 2H2O and concentrated aqueous HCl, they obtained the freebase (4-aminophenyl)corrole derivative in 90% yield. This reductive demetalation procedure was then successfully used for the demetalation of a range of Cu-meso-triarylcorroles,200,249 and corrole−porphyrin conjugates with variable metal centers.133 The reductive demetalation of Mn(III) mesotriarylcorroles with SnCl2 as the reductive reagent was also reported:250 while free-bases of electron-rich corroles were obtained in moderate isolated yields (47% yield from Mn(III)TPC), the method was revealed to be ineffective for electron-deficient corroles (11% yield from Mn(III)TPFC). Free-bases of electron-deficient corroles were obtained in higher yields using the acid-induced demetalation in HOAc− H2SO4 (67% yield from Mn(III)TPFC). Ghosh and co-workers reported a variation for the reductive demetalation of Cu-meso-triarylcorroles.251 It involves the treatment of the metallocorrole with concentrated H2SO4 and a large excess of FeCl2 or SnCl2. The authors claim that the yields of the free-bases obtained by this procedure are generally better than those with CHCl3/H2SO4, CH2Cl2/H2SO4, or H2SO4 alone. This procedure is also effective for the demetalation of Cu251,252 and Mn252 β-octabromo-mesotriarylcorroles.

2.3. N-Alkylation

The N-alkylation of β-alkyl corroles was reported in 1965 by Johnson and Kay.155 These authors observed that the addition of iodomethane to the free-base corrole in refluxing acetone, in the presence of anhydrous potassium carbonate, gives a mixture of two isomeric N-methyl corroles: the N(21)-Me (71, 38% yield) and the N(22)-Me (72, 16% yield) corroles (Figure 17).155 Later, it was found that both N(21)-Me and N(22)-Me

Figure 17. N(21)- and N(22)-monomethylcorroles and a N(21),N(22)-dimethylcorrole.

corrole isomers react further with iodomethane (sealed tube, 100 °C, 15 h) to give the same (and only) N,N′dimethylcorrole, the N(21),N(22)-dimethyl isomer (in 81% and 75% yield, respectively).254,255 The same dimethylcorrole was obtained in 71% yield, together with starting material (14%), from the parent β-octaalkylcorrole under identical conditions. Since no mono-N-alkyl corroles were isolated, the rate of introduction of the second methyl group was greater than that of the first. The N-methylation approach was extended to other alkylating agents, allowing the preparation of mixtures of N(21)- and N(22)-alkyl corroles bearing ethyl, allyl, and 3,3-dimethylallyl groups at the inner nitrogens.254 The reaction of meso-aryl corroles with various alkylating agents, O


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such as benzyl bromide,188 2-(chloromethyl)pyridine,188 or ethyl bromoacetate,256 also gave mixtures of N(21)- and N(22)-alkyl corroles. Both N(21)- and N(22)-substituted corroles are chiral and can be separated into their enantiomers by HPLC on a chiral column.188 The preferential formation of the N(21)-substituted isomer results in a less crowded environment and consequently to a smaller deformation of the corrole ring planarity.256 The first example of a doubly inner N-methylation of a meso-triarylcorrole was reported recently by Maeda and co-workers.257 They observed that the reaction of H3TPFC with an excess of iodomethane in the presence of K2CO3 in refluxing acetone gave the N(21),N(22)dimethylcorrole 73 in 71% yield, without the formation of other regioisomers. Unusual N-substituted corroles have been isolated as side reaction products of the Vilsmeier−Haack formylation of freebase corroles258 or during the attempted carboxylation of H3TPFC with phosgene.259 These reactions are discussed in sections 3.2 and 3.3. The use of metalated corroles prevented the occurrence of these reactions in their inner core. Metalation of N-alkyl corroles with zinc(II),144,188 rhodium(I),137,138,144 copper,254 and palladium254,260 afforded the corresponding complexes.

Figure 18. Perfluorinated corrole H3F8TPFC.

3. FUNCTIONALIZATION AT THE PERIPHERAL POSITIONS The reactions described below are related with the development of protocols to introduce key primary groups directly at the corrole peripheral positions. In most cases, these reactions are highly regioselective and, depending on the reaction type, the first substitution occurs tipically at positions 2 or 3. The large difference in the reactivity of the various carbon atoms on the macrocycle is due to electronic rather than steric effects.261 The functionalization of corroles at the peripheral positions has been reviewed.262

Figure 19. Structures of β-chlorinated and meso-chlorinated corroles.

β-octachlorocorrole 75 was prepared in 46% yield by chlorination of the copper complex with N-chlorosuccinimide (NCS) in o-dichlorobenzene at high temperature. The corresponding free-base β-octachlorocorrole was obtained by reductive demetalation of 75 (see section 2.2).249 Gross and co-workers,263 reported the chlorination of Co(III)(TPFC) with Cl2. The fully β-chlorinated corrole complex was isolated in 90% yield after the addition of pyridine and NaBH4 to the reaction mixture. Brö ring and co-workers58 have shown that 5,10,15trimesitylcorrole, when treated with WCl6 and W(CO)6 (2:1) in benzonitrile at 200 °C, originates the air stable 3,17-dichloro radical species 76 in 4−7% yield. The observed regioselectivity presumably is due to nucleophilic attack of chloride ions to an oxidized corrole macrocycle with subsequent abstraction of hydrogen atoms. Recently, Osuka and co-workers264 reported the chlorination of two isomeric corroles: the 5,10- and the 5,15-bis(pentafluorophenyl)corrole. In both cases the reactions were carried out with Palau’chlor (2-chloro-1,3-bis(methoxycarbonyl)guanidine) in CHCl3, in the presence of 1% pyridine, and at room temperature. While the 10-chloro5,15-bis(pentafluorophenyl)corrole 77 was obtained in 88% yield, the 5-chloro isomer was formed in a trace amount only. The low yield of the 5-chloro isomer may be ascribed to the intrinsic high reactivity of 5,10-bis(pentafluorophenyl)corrole toward its oligomerization upon oxidizing conditions.264 Under similar conditions, the chlorination of the Co and Ga complexes of 5,15-bis(pentafluorophenyl)corrole resulted in formation of complex mixtures consisting of regioisomers and multichlorinated products from which it was possible to separate only the Co complex 78 in 21% yield.265

3.1. Halogenation

The synthetic strategies giving access to corroles bearing halogen atoms at β-pyrrolic positions, namely chloro, bromo and iodo atoms, are typically based on the use of conventional halogenating reagents: Cl2, Br2, I2 or the corresponding Nhalosuccinimides. There are no known procedures for the fluorination of corroles. 3.1.1. β-Fluorinated Corroles. The access to β-fluorocorroles requires the use of fluorinated pyrroles in the synthesis of the macrocycle. The 3,4-difluoropyrrole has been selected by several groups as the key synthon to prepare fully β-fluorinated corroles. The first example of this type of corroles was reported by Chang and co-workers in 2003.64 They found that the condensation of 3,4-difluoropyrrole with pentafluorobenzaldehyde, under the solvent-free protocol developed by Gross,6 gives access to the free-base perfluorinated corrole H3F8TPFC (74, Figure 18) in an acceptable yield of 5%. Ghosh and coworkers82 used a similar protocol to synthesize a series of βoctafluoro-meso-tris(para-X-phenyl)corroles (where X = H, Me, OMe, and CF3) that were then used to prepare the corresponding copper and chloroiron complexes. Other metal complexes of β-octafluorocorroles have been prepared and used as catalysts.125,126,131 3.1.2. Chlorination. The fully β-chlorinated copper complex 75 (Figure 19) was synthesized by Maes and coworkers200,249 during their studies concerning the functionalization of corroles bearing meso-pyrimidinyl substituents. The P


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Scheme 13. Bromination of H3TPC and 5,10,15-Tris(4-nitrophenyl)corrole with NBS and Metallation of the Resulting βOctabromoisocorroles

afforded a mixture of 2-bromo- and 3-bromo-5,10,15tritolylcorrole in 14% and 18% yield, respectively.266 The same group reported the bromination of 5,10,15-(4-tbutylphenyl)corrole and its silver complex with NBS in CHCl3.253 The reaction of free-base with NBS (2.1 equiv) for 15 min at 30 °C gave a mixture of 2-bromo- and 2,3dibromocorrole in a 27% overall yield. Conversely, the reaction of the silver corrole with NBS (2.1 equiv) in refluxing CHCl3 for 1 h gave the silver complex of the 3,17-dibromocorrole in 86% yield. The corresponding free-base was obtained by demetalation of the silver complex under basic conditions (DBU). The reaction of the silver complex of 3-nitro-5,10,15tritolylcorrole with NBS gave selectively the 3-Br-17-NO2corrole derivative in 75% yield.271 A series of metal complexes of β-octabromo-5,10,15tris(pentafluorophenyl)corrole (88, Figure 21) were reported

The reaction of 5,10,15-tritolylcorrole (H3TTC) with conc. HCl followed by addition of 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) afforded the corresponding 3-Cl- and the 3,17-dichlorocorrole derivatives in 43% and 9% yield, respectively).266 3.1.3. Bromination. Paolesse and co-workers reported the bromination of H3TPC and 5,10,15-tris(4-nitrophenyl)corrole with NBS (Scheme 13). The products of these reactions were the corresponding β-octabromoisocorroles 79 and 80. However, the treatment of these compounds with Co(OAc)2 and PPh3 yielded the corresponding Co(III) complexes 81 and 82.267 The authors justified the initial formation of isocorroles as a way to reduce the sterical strain induced by the three NH protons, via the displacement of one inner proton to the periphery of the corrole. Chen and co-workers268 obtained the partially β-brominated corroles 83−87 (Figure 20) from the reaction of H3TPFC and

Figure 21. Metal complexes of a β-octabrominated corrole.

by Gross and co-workers in several publications.47,154,272−275 The β-octabromo-5,10,15-tris(pentafluorophenyl)corrolatooxochromium(V) was obtained in 96% yield by reacting Cr(O)TPFC with NBS followed by oxidation with mchloroperbenzoic acid.47 The Ga(Br8TPFC)py272 and Al(Br8TPFC)py2273 complexes were obtained in 90% and 78% yield, respectively, by adding bromine to benzene solutions of Ga(TPFC)py or Al(TPFC)py2 at room temperature. Paolesse and co-workers studied the reaction of a germanium complex of 5,10,15-triphenylcorrole with bromine.215 The authors remarked that by controlling the amount of Br2 it was possible to obtain preferentially the 2,3,17-tribromocorrole or the pentabromo- and hexabromocorrole derivatives, each as a single isomer. The same group also studied the bromination of the phosphorus(V) corrole P(TTC)(OMe)2 with Br2.276 Using two protocols, differing mainly on the ratio corrole/Br2, they obtained, in one case, a mixture of P(2,3,8,12,17,18Br6TTC)(OMe)2 and P(2,3,7,8,12,17,18-Br7TTC)(OMe)2, and in the other one, a mixture of P(3-Br-TTC)(OMe)2 and

Figure 20. Partially β-brominated corroles.

NBS in CH2Cl2/MeCN at room temperature. A careful control of the number of equiv of NBS (between 1.1 and 4.4) allowed obtaining the monobrominated corroles 83 and 84 (as a nonseparable mixture), the di-, tri-, or tetrabrominated derivatives 85, 86, and 87 in moderate to good yields (40− 88%). This method was also applied to the synthesis of partially β-brominated A2B corroles.269 Paolesse and co-workers270 reported the formation of two mono-β-brominated corroles during their studies concerning the synthesis of isocorroles via oxidation of H3TTC with DDQ followed by addition of EtMgBr. From the reaction mixture, together with the expected 5-ethyl- and 10-ethyl-5,10,15tritolylisocorroles, were also isolated the 2-bromo- and 3bromo-5,10,15-tritolylcorrole derivatives. The reaction of H3TTC with HBr in AcOH followed by addition of DDQ Q


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reaction on free-base or metallocorroles. The formylation was the first example of peripheral functionalization of the corrole ring and was reported in 1997 by Paolesse, Smith, and coworkers.279 They found that H3OMC reacts with the Vilsmeier reagent (POCl3/DMF), affording the 10-dimethylaminomethene derivative 91, and not the expected 5-formyl or 10-formyl derivatives (Scheme 15). Similarly, the 10-phenylcorrole 93 affords compound 94. Heating solutions of compounds 91 or 94 with Co(OAc)2 and PPh3 in methanol at reflux causes both metalation and tautomerization/hydrolysis to the corresponding meso-formylcorrolato complexes 92 and 95 in almost quantitative yields. The reaction of Co(OMC)(PPh3) with trimethylorthoformate/TFA does not afford the expected meso-formyl derivatives but gives a mixture of the corresponding β-formyl complexes 96 and 97 (Scheme 16).279 Oxidation of Co(OMC)(PPh3) with DDQ also affords the diformylcorrole 97.100 Paolesse and co-workers258 considered the formylation of H3TPC using the Vilsmeier protocol. The expected 3-formylH3TPC was obtained as the major product (58% yield), along with the inner core ethane bridged derivative 98 (15% yield) and the 2-formyl derivative (in trace amounts) (Figure 22). Compound 98 became the major reaction product (60% yield) if a higher amount of DMF is used. This protocol was later extended to the formylation of other free-base mesotriarylcorroles.14,280 Gross and co-workers261 applied the formylation protocol to the Ga(III)(TPFC)py complex. The protection of corrole inner core by the metal ion allowed the 3-formyl (99) or the 2,17diformyl (100) derivatives to be isolated as the main products depending on the corrole:Vilsmeier reagent molar ratio. The 2formyl corrole 101 was later reported by Cavaleiro and coworkers281 during their studies concerning the use of these derivatives in 1,3-dipolar cycloaddition reactions (see section 4.5).

P(3,17-Br2TTC)(OMe)2, along with some unreacted starting material. 3.1.4. Iodination. The access to the partially iodinated Al and Ga corrole complexes 89 and 90 (Scheme 14) has been Scheme 14. Iodination of Al(III) and Ga(III) Corroles

reported by Gross and co-workers. These compounds were obtained selectively by treating Al(III)(TPFC)py2 or Ga(III)(TPFC)py2 with N-iodosuccinimide (NIS) or iodine, respectively.277,278 The reaction of Al(TPFC)py2 with 1,3-diiodo-5,5dimethylhydantoin also gave the tetraiodo derivative Al89, but in lower yield.277 The authors concluded from the crystallographic data that the iodinated corroles maintain the planarity and the emission properties are more sensitive to the presence of additional iodine atoms than the absorption ones. All the iodinated corroles display prompt fluorescence, phosphorescence, and thermally activated delayed fluorescence at room temperature.278 The reaction of Ag(TTC) with NIS gives the 3,17-diiodo derivative in 11% yield.266

3.3. Carboxylation

Two different protocols giving access to corroles bearing a carboxylic group directly linked to a β-pyrrolic position were reported. In one of them, Gross and co-workers were able to obtain selectively the corrole-3-carboxylic acid 102 in 58% yield from the reaction of the Ga(TPFC)py with phosgene in

3.2. Formylation

Corroles bearing a formyl group directly linked to a meso- or βpyrrolic position can be obtained by the Vilsmeier−Haack Scheme 15. Formylation of H3OMC and 10-Ph-H3OMC

R


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Scheme 16. Oxidation of Co(OMC)(PPh3) to 3-Formyl- and 3,17-Diformylcorrole

Figure 22. β-Formylcorroles obtained by the Vilsmeier−Haack formylation of H3TPC and Ga(TPFC)py.

Figure 23. β-Carboxylated corroles and inner core-substituted corroles (103 and 104) obtained from the reaction of H3TPFC with phosgene in pyridine or in N,N-dimethylaniline.

pyridine (Figure 23).259 However, while atempting the carboxylation of H3TPFC with phosgene in pyridine, or in N,N-dimethylaniline, the authors obtained, respectively, the carbamide 103 (in 96% yield) or the N(21)-(4dimethylaminobenzoyl)corrole 104 (in 32% yield). The other protocol, reported by Giribabu and co-workers, involves the reaction of free-base 5,10,15-triarylcorrole-3-carbaldehydes, or the corresponding copper complexes, with hydroxylamine hydrochloride and phthalic anhydride in dry MeCN at reflux.280

This method allowed the access to a series of free-base corrole3-carboxylic acids and the corresponding copper complexes (105−108 and Cu105−Cu108). This protocol was extended to the synthesis of free-base and copper complexes of triarylamine-functionalized corrole-3-carboxylic acids 109 to be used as sensitizers in dye sensitized solar cells (DSSCs).14,282 A recent study on the cellular uptake and anticancer activity of four Ga(III) corroles has shown that the carboxylated derivatives are promising chemotherapeutic agents with the S


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Figure 24. β-Nitro corroles obtained by nitration of Ga(III)(TPFC) with NaNO2/tris(4-bromophenyl)aminium hexachloroantimonate.

silver ion and the key step of the reaction is the attack of NO2− to the π-cation radical of the Ag(III)corrole (formed by reaction with excess Ag+ ion). It was observed that, although not being necessary for the success of nitration, addition of I2 increases the rate of the reaction, but the products are the same. Further studies revealed that the reaction of H3TTC with the AgNO2/NaNO2 system was regioselective and gave mono- and dinitrocorrole derivatives when the stoichiometry was carefully controlled.285 Using the ratio of 1:1:9 for corrole/AgNO2/ NaNO2 the free-base 3-nitrocorrole 114 was obtained as the main product (52% yield) and 2-nitrocorrole 115 as a minor one (Figure 26). Changing the molar ratio to 1:2:8 (corrole/

advantage that they also can be used for tumor imaging.21 The carboxylated corrole 102, and the aminocaproate derivative 160 (Figure 43), display high efficacy toward prostate, melanoma, breast, and ovarian cancer cells. The corrole-3-carboxylic acid 102 showed the highest uptake by all human cancer cells studied. 3.4. Nitration

Various protocols were developed to give access to corroles bearing one or more nitro groups directly linked to β-pyrrolic positions.283 In 2002, Gross and co-workers261 explored the efficiency of NaNO2 in the presence of tris(4-bromophenyl)aminium hexachloroantimonate, a one electron oxidant, to nitrate Ga(III)(TPFC). Depending on the amount of oxidant used, the reaction gives the 3-nitro- (110), the 3,17-dinitro(111), and the 2,3,17-trinitrocorrole 112 as the main products (Figure 24). The full consumption of the initial corrole, and the formation of the mono (84% yield) and dinitro (9% yield) corroles when the hexachloroantimonate salt is used in 75 mol %, in the presence of an excess of NaNO2, is indicative of a chain reaction. The authors concluded that under these conditions the NO2− is oxidized to NO2 while the Ga(III) complex is not oxidized to the corresponding π-cation radical. The studies showed that nitrating reagents like HNO3/H2SO4, N2O4 or AgNO2/I2, successfully used in the case of mesotetraarylporphyrins,284 led to significant decomposition of the corrole ring and to mixtures of polynitrated derivatives. Relevant contributions by Paolesse and co-workers on the efficiency of various nitrating agents for the nitration of freebase corroles and metallocorroles started in 2007. This group reported the direct conversion of free-base corroles into the corresponding 3-nitrocorrole silver complexes 113a−c (Figure 25) using AgNO2 as the nitrating agent.173 These nitrocorroles

Figure 26. Products obtained from the reaction of H3TTC with AgNO2/NaNO2.

AgNO2/NaNO2), the 3,17-dinitrocorrole 116 becomes the main product (20% yield). The 3-NO2-5-OH-isocorrole 117 (9% yield) and minor amounts of 2,3-dinitro- and 3,12dinitrocorrole derivatives 118 and 119 were also isolated from this reaction. The authors observed that the β-nitro substituents have a strong influence on the spectroscopic and redox properties of the corroles (a positive shift of the E1/2 on the redox processes) and on their optical absorption spectra (the number of bands increased accompanied by significant red shifts). Theoretical studies enabled them to conclude that these features are related with the conjugation of the β-NO2 groups with the π-aromatic system of the corrole. Paolesse and co-workers reported that the nitration of H3TPC with TFA/NaNO2 or HCl/NaNO2 leads to the initial formation of β-nitroisocorroles that can be converted into the corresponding β-nitrocorroles by treatment with Co(OAc)2 and PPh3.286 The same group verified that the in situ formed copper complex of 5,10,15-tris(4-t-butylphenyl)corrole can be converted selectively into mono- or dinitrocorroles using

Figure 25. Silver 3-nitro-5,10,15-triarylcorrole complexes.

were isolated as the main products accompanied by the Ag(III) complex of the starting corrole. The authors found that electron-releasing meso-aryl groups favored the nitration while electron-withdrawing aryl groups (e.g., Ar = C6F5 or 3,5F2C6H3) promoted the decomposition of the corrole ring, affording ring-opened derivatives. These studies have shown that the success of the reaction requires the presence of the T


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NaNO2 as the primary source of NO2− coupled with AgNO2 used as oxidant.287 By selecting the appropriate molar ratio of the reagents it was possible to direct the product distribution toward 3-nitro- and 3,17-dinitro derivatives. A two-step protocol involving the metalation of the free-base corroles with FeCl2 followed by treatment of the isolated Fe(III) complexes with NaNO2 enabled the isolation of the Fe(III) nitrosyl complexes of 3-nitro- and 3,17-dinitro-5,10,15tris(X-phenyl)corroles (X = H, 4-Me, 4-MeO, 4-NO2, 2,6-Cl2 and F5) in moderate yields (11−17% for the 3-nitro- and 27− 33% for the 3,17-dinitrocorroles).90 In this procedure the Fe(III) nitrosyl complexes of the β-unsubstituted corroles were also isolated (5−14% yield). A one-pot variation of this protocol allowed nearly exclusive formation of the iron nitrosyl 3,17-dinitrocorroles. Best yields (28−33%) were obtained with corroles bearing electron-releasing substituents on the meso-aryl groups), probably due to the more facile formation and stabilization of the intermediate iron corrole π-cation radical. The axial nitrosyl group was found to be more labile in corroles bearing electron-withdrawing meso-aryl groups. For example, the 3,17-dinitro derivative with meso-pentafluorophenyl groups readily dissociated the nitrosyl ligand leading to the direct formation of the iron μ-oxo dimer complex (ca. 9% yield). Electrochemical and spectroelectrochemical studies of iron βnitrocorroles elucidated the site of electron transfer and the influence of the peripheral nitro groups. Paolesse and co-workers also studied the nitration of germanium(IV) 5,10,15-triphenylcorrolates using nitrate salts.288 In the presence of the mild nitrating system LiNO3/ Ac2O/AcOH, the Ge(IV) 3-nitrocorrole monomer 120 and the corresponding Ge(IV) 3-nitrocorrole μ-oxo dimer 121 were obtained (Figure 27). In contrast, using the more severe nitrating mixture NaNO3/Ac2O/AcOH, the 3,17-dinitro compounds 122 and 123 were isolated. The substitution was highly regioselective in each case, giving only 3-nitro or 3,17dinitro derivatives. The dimers 121 and 123 were converted into the monomeric derivatives 120a,b and 122a,b with dilute

HCl. Crystallization of the monomeric corroles from CH2Cl2/ MeOH gave the methoxy derivatives 120c and 122c. Paolesse and co-workers also studied the nitration of the copper complex of meso-tris(p-(t-butylphenyl))corrole with AgNO2/NaNO2 system under different experimental conditions. Using a 5-fold excess of both silver and sodium nitrites with respect to corrole, the main product was the 2,3,17(NO2)3-substituded corrole complex (25% yield), together with traces of the isomeric 3,8,17-(NO2)3-corrole derivative.289 The same group extended these studies to the phosphorus(V) complex P(TTC)(OMe)2. 276 While the AgNO2 /NaNO 2 system failed to give any nitrated product, both the LiNO3/ AcOH/Ac2O and the NaNO3/AcOH/Ac2O systems afforded mixtures of the same mononitro and dinitro corroles P(3-NO2TTC)(OMe)2 and P(3,17-(NO2)2-TTC)(OH)(OMe), albeit in different ratios: the method using LiNO3 gave mainly the mononitro derivative, while the method using NaNO3 gave predominantly the dinitrocorrole. 3.5. Sulfonation/Chlorosulfonation

The chlorosulfonation of corroles was first reported by Gross and co-workers.290 Addition of chlorosulfonic acid (HSO3Cl) to H3TPFC, at 25 °C for 5 min, resulted in an almost quantitative formation of the 2,17-bis(chlorosulfonyl) corrole 124a (Figure 28). This compound could be converted into the

Figure 28. Bis(sulfonyl) derivatives of H3TPFC.

corresponding bis(sulfonamide) 124b (by addition of piperidine) or the bis(sulfonic acid) 124c (by hydrolysis). Metalation of 124b by Co(OAc)2, followed by addition of PPh3, afforded the cobalt(III)(corrole)(PPh3) complex 125. A minor amount (3% yield) of the isomeric 3,17-bis(sulfonamide) was also isolated.290,291 Sulfonation of H3TPFC with concentrated sulfuric acid at room temperature gave a quantitative yield of a 9:1 mixture of the 2,17-bis(sulfonic acid) 124c and the 3,17- isomer.291 Compound 124c was used for the synthesis of several metallocorroles, namely Fe(III), Ga(III), Mn(III), Al(III), Cu(III), Rh(III), Co(III), Sn(IV) and Sb(V).292 The two sulfonato groups induced not only water-solubility but also amphipolarity to corroles, being responsible for the extremely strong affinity of the corresponding metal complexes to a variety of proteins.201−205,208,293−295 Some of these complexes acted as catalysts for the decomposition of reactive oxygen and nitrogen species.296,297 The 2,17-bis(chlorosulfonyl)corrole 124a and its Ga(III) and Al(III) complexes were used to decorate covalently the surface of TiO2 nanoparticles (NPs). The resulting fluorescent Al(III)- and Ga(III)corrole−TiO2 NP conjugates were used as contrast agents for noninvasive optical imaging.196,298 Recently, Gross and co-workers reported a plausible explanation for the selectivity observed in the chlorosulfonation

Figure 27. Ge(IV) β-nitrocorroles obtained by nitration of Ge(IV)(TPC)X complexes with nitrate salts. U


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of corroles.234 They found that the chlorosulfonation of the phosphorus corrole 126a yielded only the 2,17-bis(chlorosulfonyl) phosphorus corrole 127a while the complex 126b provided the analogous compound 127b as the major product, but with much lower selectivity (Figure 29). These

Figure 31. Isocorroles obtained from the reaction of mesotriarylcorroles with DDQ in methanol.

withdrawing groups in the meso-aryl substituents makes this reaction difficult (or may prevent it). For instance, 10-(4methoxycarbonylphenyl)-5,15-bis(pentafluorophenyl)corrole was converted to the corresponding 10-methoxyisocorrole in 35% yield167 and meso-tris(4-nitrophenyl)corrole gives only traces of the isocorroles, along with a mixture of decomposition products.300 A possible pathway involving the initial formation of a cation radical was put forward. The oxidation of the corrole nucleus to an isocorrole species may also occur during the demetalation of silver(III) corrolates under acidic conditions. This transformation is particularly relevant in the case of 3-nitro substituted complexes, which are quantitatively converted into the corresponding 3-nitro-5hydroxyisocorroles upon silver ion removal.170 Ni(II), Cu(II), Fe(III), Mn(III), and Rh(III) complexes of a 10,10-dimethyl-5,15-diphenylisocorrole were synthesized by metalation of the corresponding free-base meso-triarylisocorrole.302 The crystal structures of some of these complexes were reported. The metal-templated oxidative macrocyclization of tetrapyrrolic 5,15-biladiene precursors was used to prepare metal complexes of 10,10-dimethylisocorrole ligands with full β-pyrrolic substitution.303,304 Copper and nickel meso-triarylisocorroles were prepared by metalation of the free-base mesotriarylisocorroles.167,301 The coordination behavior of the 5and 10-methoxyisocorrole derivatives (structures 132 and 133) with different metal ions has shown that the stability of the final isocorrole complex depends upon the coordinated metal ion. In fact, if a formal oxidation state higher than +2 is accessible to the coordinated metal ion, such as in the case of Co and Mn, a metal-to-ligand charge transfer occurs, with the consequent aromatization of the isocorrole to the corresponding corrole complex.301 Stable isocorrole complexes have been obtained in the case of Cu and Ni ions, and the redox behavior of these species was characterized by electrochemistry and spectroelectrochemistry.

Figure 29. Phosphorus meso-triarylcorroles and the corresponding 2,17-bis(chlorosulfonyl) derivatives.

experimental results and theoretical calculations reveal that the initial monochlorosulfonation (at C3) is under kinetic control while the reaction leading to the bis-chlorosulfonated corrole is probably under thermodynamic control, leading to the lower symmetry 2,17-bis-substituted corrole and not to the 3,17isomer predicted by DFT calculations. H3TPC reacts with HSO3Cl, under a modified clorosulfonation protocol, to yield mono- and disubstituted sulfonated corroles.299 This reaction shows an unprecedented regioselectivity for corrole derivatives, affording the corrole-2-sulfonic acid 128 as the main product (24% yield) (Figure 30). Corrole 129 (the first example of a monosubstituted corrole on pyrrole B), the 2,12-disubstituted corrole 130, and the 8,12disubstituted isocorrole 131 are also formed. 3.6. Formation of Isocorroles

A protocol for the conversion of corroles into isocorroles was reported by Paolesse and co-workers.300,301 They found that meso-triarylcorroles react with DDQ in methanol to give a mixture of the two isomeric isocorroles 132 and 133 in overall yields of ca. 75% (Figure 31). The presence of electron-

3.7. Oligomerization

Heating a solution of H3TPFC in 1,2,4-trichlorobenzene at 200 °C leads to the formation of three corrole dimers: dimers 134 and 135, linked respectively by the 2,3′ and 3,3′ carbons, and also the eight-membered ring dimer 136, with a double linkage by the 2,2′,18,18′ carbons (Figure 32).305 These compound were obtained in 10%, 3%, and 18% yield, respectively, with 32% of H3TPFC being recovered, in a 200 mg scale reaction. A higher yielding process for the oligomerization of H3TPFC was reported by Osuka and co-workers in 2012.103 They found that heating a solution of H3TPFC with p-chloranil in CHCl3 at reflux triggers a regioselective oxidative coupling reaction leading to the formation of the 3,3′-linked dimer 135 and the 3,3′,17′,3″-corrole trimer 137 in 62% and 19% yield, respectively. These two corrole derivatives had already been

Figure 30. Corrole-β-sulfonic acids (128 and 129), corrole-2,12disulfonic acid (130), and isocorrole-8,12-disulfonic acid (131) obtained by clorosulfonation of H3TPC. V


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Figure 32. Corrole oligomers obtained by oxidation of H3TPFC.

Scheme 17. Iridium-Catalyzed Borylation of Corroles

Oxidation of dimer 135 with p-fluoranil in refluxing CHCl3 gives tetramer 138 and a corrole hexamer in 26% and 7% yield, respectively.103 The crystal structures of the dimer 135 and of the cobalt(III) complexes of the trimer and tetramer were reported. Other examples of corrole dimers are discussed in section 5.1.

identified as the main products of the photodegradation of H3TPFC in halogenated solvents (CHCl3 and CH2Cl2) at ambient light and under air.306 The use of DDQ in the synthesis of corroles from dipyrromethanes and aldehydes may induce the formation of 3,3′-linked corrole dimers.269 Corrole dimers with the 3,3′-connection are also produced upon metalation of H3TPFC with either Co102 or Cu307 ions. A 2,2′-linked corrole dimer (an isomer of dimers 134 and 135) was obtained from the monoborylated corrole 139 by a palladium-catalyzed oxidative homocoupling (see Scheme 32, in section 5.1).308

3.8. Other Functionalizations at the Peripheral Positions

3.8.1. Borylation. In 2005 Osuka and co-workers developed a protocol for the selective borylation of corroles under iridium catalysis.309 The reaction was highly selective, affording only the 2-boryl-substituted corrole. The experimental W


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procedure consisted in heating a dioxane solution of H3TPFC, bis(pinacolato)diborane (1.1 equiv), and catalytic amounts of [Ir(cod)OMe]2 (1.5 mol %) and 4,4′-di-t-butyl-2,2′-bipyridyl (dtbpy; 3.0 mol %), at 100 °C for 24 h (Scheme 17). After purification by preparative GPC-HPLC the corrole 139 was obtained in 91% yield. The structure of the 2-borylcorrole was unambiguously confirmed by X-ray diffraction analysis. The new corrole derivative was used in Suzuki−Miyaura crosscoupling reactions with a diversity of bromoaryl compounds (see Figure 75). 3.8.2. Fluoroalkylation. H3TPFC reacts with I(CF2)4Cl (2.0 equiv) and Na2S2O4 (1.5 equiv) in DMSO at room temperature to give a 1:1 mixture of the mono(fluoroalkyl)corroles 140a and 140b in 46% yield (Figure 33). The two

Scheme 18. Selective Hydrogenation of H3TPFC to a 7,8Dihydrocorrole

3.8.5. Corroles as 2π Components in Cycloaddition Reactions. The ability of corroles to act either as the 2π or the 4π component in cycloaddition reactions was revealed by Cavaleiro and co-workers,311 as an extension of their studies in the porphyrin field.312 In this pioneering study, it was found that H3TPFC reacts with pentacene in 1,2,4-trichlorobenzene at 110 °C affording the corresponding dehydrogenated Diels− Alder cycloadducts 145 and 146 in 23% and 6% yield, respectively (Figure 35). Heating the reaction mixture at 200 °C, the [4 + 4] cycloadduct 147 is also formed (13% yield).311

Figure 33. β-Fluoroalkylcorroles.

isomers can be separated by flash column chromatography, although they are not very stable during this process.268 Under similar conditions, H3TPFC reacts with α,ω-diiodoperfluoroalkanes I(CF2)nI (n = 3 or 4) to give the five- and sixmembered fluorinated ring-fused corroles 141a or 141b in 25% and 35% yield, respectively. 3.8.3. Aminomethylation. Heating a solution of Ga(III)(TPFC)py, paraformaldehyde and N-methylglycine (in a 1:1:1 ratio) in toluene at 80 °C for 2 h leads to the formation of a mixture of the two isomeric aminomethyl corroles 142 (15% yield) and 143 (17% yield) (Figure 34); 65% of the starting metallocorrole is recovered.310

4. POST-FUNCTIONALIZATION OF CORROLES The presence of appropriate substituents at the peripheral positions of the corrole macrocycle allows their conversion into other corrole derivatives just by applying “simple” organic reactions such as, for instance, the nucleophilic substitution of the para-fluorine atom of meso-pentafluorophenyl groups by S-, O-, or N-nucleophiles. The successive transformation of selected functional groups is also a versatile method to produce new corrole derivatives. A typical example is the reduction of a nitrocorrole to an aminocorrole and its subsequent transformation into amide-functionalized corroles. Another classic method involves the use of formylcorroles as key compounds in the postfunctionalization of corroles. As discussed below, they have been used for the formation of imines, in Wittig reactions, to generate azomethine ylides, or to produce corrole−BODIPY conjugates. Appropriately substituted corroles can also be modified by cycloaddition reactions or via metal catalyzed reactions. All these types of reactions have been used for the postfunctionalization of corroles and are discussed in this section. 4.1. Nucleophilic Aromatic Substitutions

Nucleophilic aromatic substitutions (SNAr) are of great significance in organic synthesis. In fact, certain organic compounds can efficiently be obtained by following SNAr procedures. Strong electron-withdrawing substituents directly attached to the aromatic entity play a key role in such reactions. In the case of corroles, such substituents may be at the mesoaryl groups or directly attached to the β-positions of the macrocycle. The former cases, involving the meso-aryl substituents, have been special targets for several research groups. Reports of corrole SNAr reactions were first published in 1999. Since the p-fluorine atoms of meso(pentafluorophenyl)porphyrins can be substituted by nucleophiles,313,314 the same approach has been applied to corroles

Figure 34. Dimethylaminomethyl corroles obtained from the reaction of Ga(TPFC)py with paraformaldehyde and N-methylglycine.

3.8.4. Hydrogenation. H3TPFC reacts with p-toluenesulfonylhydrazide, a diimide precursor, in pyridine at 110 °C to give selectively the 7,8-dihydrocorrole 144 in 75% yield (Scheme 18).195 The isomeric 2,3-dihydrocorrole is not formed in this reaction. To the best of our knowledge, this reaction is the only peripheral corrole functionalization that is selective for the B ring of the corrole. The aluminum complex of the dihydrocorrole 144 is formed in quantitative yield by reaction with AlMe3. X


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Figure 35. Dehydrogenated cycloadducts obtained from the reaction of H3TPFC with pentacene.

A few years later, Osuka and co-workers316 used the SNAr method for the synthesis of 5,10,15-tris(4-amino-2,3,5,6tetrafluorophenyl)corroles (149a−h) (Figure 37). They obtained these meso-tris(aminoaryl)corroles in good yields (57−95%) from the reaction of H3TPFC with an excess of primary or less sterically hindered secondary amines. However, amines containing steric hindrance, such as diisopropylamine and dibenzylamine, did not afford the expected aminosubstituted corroles. Monoaminoaryl derivatives were obtained using 2.2 equiv of amine and milder reaction conditions: the isomeric derivatives 150a−h and 151a−h have been obtained in ratios of 3.4−4.5:1. This method has been applied by the same authors to the synthesis of ABC-type meso-triarylcorroles, such as 152 and 153a,b (Figure 38).316 The higher reactivity of the 5-(pentafluorophenyl) group of H3TPFC has been put forward by Drain and co-workers317 and explained by Osuka and co-workers by DFT calculations:316 the LUMO coefficient at the para-carbon atom in the 5-(pentafluorophenyl) group is higher than at the 10-substituent. Osuka extended the SNAr method to the synthesis of corrolebased organogels (154a−d) (Figure 39) by treating H3TPFC

and, as a result, synthetic methods for these corrole derivatives are now established. Gross and co-workers6 have shown that the reaction of H3TPFC with 2-pyridyllithium, followed by addition of iodomethane, gives rise to the tricationic corrole derivative 148 (Figure 36). This compound demonstrated to be more efficient in the inhibition of endothelial cell proliferation, tumor progression, and metastasis than a large variety of porphyrins.315

Figure 36. Tricationic meso-triarylcorrole derivative.

Figure 37. Mono- and tri(aminoaryl)corroles obtained from the reaction of H3TPFC with amines. Y


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M HCl) afforded the corresponding free-base corroles 158a−j (Figure 41). Using the appropriate corrole/nucleophile ratio, it was possible to obtain mono-, di-, and trifunctionalized corroles. Barata et al. reported the synthesis of one of the first examples of corrole−silica hybrid nanoparticles320 which were obtained by treating Ga(III)(TPFC) with silica particles prefunctionalized with 3-aminopropyltriethoxysilane. Although the singlet oxygen produced by the corrole−silica nanoparticles 159 (Figure 42) was not high, it compares with other hybrid materials that have been considered as potential photosensitizers for photodynamic therapy. A recent example of the versatility of the SNAr reactions for the functionalization of corroles involved the synthesis of the biologicaly active aminocaproate-substituted corrole 160 (Figure 43).21 It was obtained in 37% yield from the reaction of Ga(TPFC) with a substoichiometric amount of 6-aminocaproic acid in anhydrous DMSO with 1% anhydrous pyridine at 100 °C for 18 h. This compound demonstrated high cytotoxic activity against all cell lines derived from nine tumor types (including prostate, breast, and ovarian cancer cells) and very high toxicity against melanoma cells. Faustino and co-workers321 used the SNAr approach to synthesize corrole derivatives containing galactose residues in the p-position of aryl rings of meso-triarylcorroles. The reaction between H3TPFC and 1,2:3,4-di-O-isopropylidene-α-D-galactopyranose, in the presence of a base, gave rise to the expected monogalactose−corrole conjugates 161a,b (Figure 44); the ratio of 5- to 10-substituted derivatives has been 2:1. Biological studies with glycocorrole 161a showed a higher cell uptake when compared with H3TPFC. However, both compounds exhibited low PDT efficacy on Jurkat cells, probably due to their intracellular localization.321 This approach was extended to the synthesis of corrole−β-cyclodextrin conjugates 161c,d.322 Their photosensitizing efficiency on HeLa cells were evaluated. The results showed that compound 161d was cytotoxic in the dark at the PDT effective concentration (0.1 mM) and the noncytotoxic compound 161c accumulated preferentially in a perinuclear region of the cells, on the Golgi apparatus. Recently, Reissig, Wiehe, and co-workers published results from nucleophilic aromatic substitution reactions of porphyrinoids with alcohols.323 H3TPFC was used in SNAr reactions with butane-1,4-diol, prop-2-yn-1-ol, and β-cholestanol to generate the corresponding ether. The trisubstituted corrole derivatives 162a−c (Figure 45) were obtained in 61−87% yield. The same group extended their studies to A2B corroles bearing pentafluorosulfanyl (SF5)-substituted phenyl groups.324 Such corrole derivatives reacted with propargyl alcohol affording compounds of type 163 (Figure 46) that were then used in 1,3-dipolar cycloaddition reactions with azides. Dyads containing corrole and BODIPY (BODIPY = boron dipyrromethene) moieties linked by 1,2,3-triazole linkers, such as 164, were reported.324,325 Barata and co-workers have shown that corrole−chitosan films (165) (Figure 47) can be obtained by grafting H3TPFC and chitosan. In this process the p-fluorine atoms of the corrole were replaced by amino groups of chitosan. Such fluorescent films exibited good thermomechanical properties and thermal stability, and showed bacteriostatic effects against S. aureus.16 All the previous publications confirm that the nucleophilic substitution of p-fluorine atoms of pentafluorophenyl groups is

Figure 38. ABC-type meso-triarylcorroles obtained by SNAr with amines.

Figure 39. Corrole-based organogels.

with amines containing long alkyloxy chains connected through an amide bond. It was demonstrated that the methoxy-, octyloxy-, dodecyloxy-, or hexadecyloxy-substituted corroles 154 do not form gels in several solvents (aromatic or halogenated solvents, alcohols, and esters) but they give rise to dark-green gels in hydrocarbon solvents.316 Schoefberger and co-workers318 reported the synthesis of water-soluble A2B and A3-corrole−amino acid conjugates using a procedure that consists of heating a DMSO solution of 10-(4bromophenyl)-5,15-bis(pentafluorophenyl)corrole (or H3TPFC), NaH, and an amino acid (glycine ethyl ester, cysteine and taurine) at 100 °C for 6 h. The corrole derivatives 155−157 (Figure 40) were obtained in 56−68% yield. Their Mn(III) and Ga(III) complexes were also obtained and their photophysical properties studied. The same group reported studies on the substitution of the p-fluorine atoms of the Bi(TPFC) complex using a range of S-, O-, and N-nucleophiles.319 The reactions with the nucleophiles were carried out at room temperature in anhydrous DMSO in the presence of NaH. Immediate demetalation of the functionalized Bi complexes (by dropwise addition of aq. 0.15 Z


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Figure 40. Water-soluble A2B and A3-corrole−amino acid conjugates.

Figure 43. Aminocaproate-substituted corrole obtained from Ga(III)(TPFC) by a SNAr reaction.

potential important applications in several fields. However, other approaches have been considered for the nucleophilic substitution of appropriate substituents present in meso-aryl groups of corrole. As an example, A2B-corroles bearing a 10(4,6-dichloropyrimidinyl) group were used in SNAr studies involving the 4,6-dichloropyrimidinyl substituent.200,326 Since the free-bases of these corroles are not very stable, the corresponding copper complexes were used as the starting macrocycles. Phenolic compounds were used as nucleophiles and, depending on the nucleophile and conditions used, monoand disubstituted products such as 166a,b (Figure 48) were obtained. Paolesse and Smith groups found that corroles participate in SNAr reactions at free β-positions of the macrocycle if appropriate β-substituents are attached to nearest positions.327 The NO2 group is a very useful substituent in organic synthesis and it has been widely used in the functionalization of porphyrins.284 While it is well documented that β-nitroporphyrins can undergo nucleophilic addition and substitution reactions, only a few publications can be found on the functionalization of β-nitrocorroles, especially by SNAr procedures.

Figure 41. Functionalized corroles obtained from the reaction of Bi(TPFC) with S-, O-, and N-nucleophiles.

Figure 42. Corrole−silica hybrid nanoparticles.

an excellent procedure to functionalize corroles. It allows simple access to a wide variety of corrole derivatives with AA


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Figure 47. Corrole−chitosan conjugates.

Figure 44. Glycosylated corroles.

Figure 48. 10-(Pyrimidin-5-yl)-substituted corroles obtained by SNAr reaction with 4-t-butylphenol.

4H-1,2,4-triazole gives rise to β-amino-β′-nitro derivatives: a 3nitro-corrolatocopper gives the corresponding 2-amino-3-nitro derivative 167 while a 3,17-dinitrocorrolatocopper gives the corresponding 2,18-diamino-3,17-dinitrocorrole copper complex 168 (Figure 49). A similar reaction profile is observed with

Figure 45. Corrole derivatives containing alkyloxy groups obtained from the reaction of H3TPFC with alcohols. Figure 49. Corroles bearing amino and nitro groups in adjacent positions.

Paolesse, Smith and Kadish groups have studied the reactivity of the copper and germanium complexes of β-nitrocorroles.287 They found that the reaction of such complexes with 4-amino-

Figure 46. Propargyl-substituted corrole and a corrole−BODIPY conjugate. AB


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Scheme 19. Synthetic Routes to Pyrazino-Fused Corroles

a Ge(IV) 3-nitrocorrole, giving the 2-amino-3-nitrocorrolate in 50% yield. This has been considered to be the first example of a vicarious nucleophilic substitution taking place on corrole derivatives. Recently, compounds 167 and 168 were used as precursors to pyrazino- and bispyrazino-fused corroles. Compounds 170, 171, and 173 were prepared in a one-pot procedure that involved the reduction of the β-amino-β′-nitrocorroles to the corresponding diamino or tetraamino derivatives 169 or 172 followed by condensation with 1,2-diketones (Scheme 19).328 Compound 167 was also used as precursor of the novel πextended β,β′-pyrrolo[1,2-a]pyrazino-fused corroles 176 and 177 (Scheme 20).329 The synthetic route involved the initial formation of the 2-(pyrrol-1-yl) unit by reacting 167 with 2,5dimethoxytetrahydrofuran in a refluxing mixture of acetic acid and toluene. The resulting 2-(pyrrol-1-yl)-3-(NO2)-corrole complex, obtained in 90% yield, was then reduced with 10% Pd/C-NaBH4 to give 174 in 83% yield. The reaction of this compound with substituted benzaldehydes, at room temperature and in the presence of dodecylbenzenesulfonic acid, afforded mixtures of copper(II) β,β′-fused pyrrolo[1,2-a]pyrazinocorroles 176a−c (21−32% yield) and 177 (9−19% yield). Formation of 177 probably occurred by a competing elimination of XC6H5 for the aromatization of the cyclic adduct 175. Paolesse and Smith groups,327 following the SNAr method, were able to obtain the interesting 2,3-difunctionalized corrole derivatives 178 and 179 (Figure 50).327 These compounds were obtained in acceptable yields (ca. 30%) from the reaction of copper complexes of 3-nitro-meso-triarylcorrole with methylene active carbanions (from diethyl malonate and diethyl 2-chloromalonate).

Scheme 20. Synthesis of Pyrrolo[1,2-a]pyrazino-Fused Corroles

4.2. Amines, Amides, and Imines

The insertion of an amino group at the corrole periphery (both at β-pyrrolic positions or at the meso-aryl groups) may be accomplished by direct amination (as discussed above) or, most frequently, by reduction of a nitro or azido group. In most of the publications concerning the synthesis of aminocorroles their transformation into the corresponding amides is also reported. Thus, these two transformations are discussed together. AC


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Figure 51. Picket fence corrole and a monoamide functionalized corrole. Figure 50. Copper corroles functionalized with nitro and diethyl malonate groups in adjacent positions.

Paolesse and co-workers reported the reduction of 3-nitro5,10,15-tris(t-butylphenyl)corrole to the corresponding amino derivative 180 as a key step to obtain the 3-functionalized amide corroles 181a−d (Scheme 21).330 The reduction, performed with an excess of NaBH4 in the presence of palladium supported on carbon, gave 180 in 35% yield. Acylation with anhydrides or formic acid, either performed in situ or in a two step procedure after amine isolation, afforded the target amides 181a−d in 56−71% yield. In the same publication, the authors explored the reactivity of 181c with sodium azide to obtain a corrole bearing a peripheral azido group which was further used in the Huisgen 1,3-dipolar cycloaddition with 1-ethynyl-4-methylbenzene. Corroles bearing amino groups attached to meso-phenyl substituents were also used in the design of some sophisticated derivatives. Collman and Decréau,331 reported a synthetic route to picket fence corroles, such as 182 (Figure 51), that involved the reduction of the nitro groups of 5,10,15-tris(2-nitrophenyl)corrole with SnCl2 in HCl and the acylation of the triamine (the ααα atropisomer is obtained by enrichment of the statistical mixture of atropisomers) with the appropriate acyl chloride. A similar strategy was reported by the same group to prepare the A2B metallocorroles 183 that were used as catalysts in the electroreduction of O2.332 Strapped (184)333 and capped (185)331 corroles (Figure 52) were prepared by similar procedures. Picket fence corroles were also prepared via the Buchwald−Hartwig reaction (see Figure 81). Barbe and co-workers334 have carried out studies on the synthetic design of organic−inorganic hybrid materials. One of the targets, the aminocorrole 186 (Figure 53), was obtained from 5,15-dimesityl-10-(4-nitrophenyl)corrole by the following steps: reduction of the p-nitro group with H2/Pd/C, followed by functionalization with (3-isocyanatopropyl)triethoxysilane affording compound 186. This compound was then metalated with cobalt(II) acetylacetonate to give the corresponding cobalt

Figure 52. Strapped (184) and capped (185) corroles.

complex Co186. Following well-established sol−gel process conditions, the hybrid materials were obtained by further reaction of 186 and Co186 with tetraethoxysilane (TEOS) and methyltriethoxysilane (MTEOS). The Co(III)corrole-based materials showed a very high affinity for carbon monoxide when used as gas sensors. The same research group extended their studies to the synthesis of other organic−inorganic hybrid materials using corroles 187a,b.335 Preparation of these corroles involved the reduction of an azidomethyl group with PPh3 and reaction with (3-isocyanatopropyl)triethoxysilane. It was remarked that the reactions leading to the silane derivatives 187a,b are much faster than the one leading to corrole 186 (12 h versus 4 days). The synthesis of the ruthenium(II)tris(bipyridine)-functionalized corrole 188 involved the initial reduction of 10-(4nitrophenyl)-5,15-ditolylcorrole with SnCl2 in HCl/AcOH and the reaction of the resulting aniline with a ruthenium(II)tris(bipyridine) acyl chloride.336 The electrochemical and photophysical properties of this photoactive dyad suggest that an

Scheme 21. Synthesis of 3-Amino and 3-Amido-Substituted Corroles

AD


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Figure 53. Corroles used in the construction of organic−inorganic hybrid materials.

Figure 54. Corroles functionalized via amide bonds.

Figure 55. Corrole−azafullerene dyad and biscorroles linked via amide bonds. AE


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fragmentation of the resulting imines was studied by mass spectrometry.343 Considering that systems constituted by electron donor− acceptor units play a vital role in the design of photovoltaic materials and devices, Giribabu and co-workers344 reported a simple access to triad 196 (Figure 57) where two corrole units

electron transfer from the copper corrole unit to a photogenerated ruthenium species is thermodynamically possible. The ditopic corroles 189 and 190, used as lanthanidebinding motifs, were obtained in ca. 90% yield by acylation of 10-(4-aminophenyl)-5,15-bis(pentafluophenyl)corrole with the dianhydrides of EDTA and DTPA, respectively (Figure 54).337 These corrole derivatives react with LnCl3 (were Ln = Nd, Er, Yb, and Lu) to give complexes that are dual-range emitters, in the visible and near-IR, in ambient conditions.337,338 The synthesis of a structurally related Cu corrole covalently linked to a folate and a Gd DOTA units was reported.339 The heterobimetallic complex addresses the requirements of a potential contrast agent for bimodal imaging. Gryko and co-workers340 also used the 10-(4-aminophenyl)5,15-bis(pentafluophenyl)corrole in the construction of the first corrole−azafullerene dyad 191 (Figure 55). The reaction between the corrole and an azafullerene carboxylic acid was performed in the presence of EDCI−HOBt in dichloromethane at room temperature. The photophysical and electrochemical properties of the dyad were examined and have revealed that the photoexcitation of the corrole unit leads to the formation of a charge separated state (corrole)•+−C59N•− from the corrole singlet excited state. The Co(III) 10-(3-aminophenyl)-5,15-diphenylcorrole, obtained by reduction of the corresponding (3-nitrophenyl)corrole followed by metalation, was treated with diacyl chlorides to give the jaw-like biscorroles 192 and 193.341 The host ability of these two derivatives with different cavity sizes toward C60 and C70 was evaluated and it was found that 192 has a more powerful binding ability for C60 than 193 while compound 193 tends to be more effective than 192 toward C70. These biscorroles have larger binding constants for fullerenes than corrole monomers, probably by a cooperative effect between the two corrole units. 10-(4-Aminophenyl)-5,15bis(pentafluophenyl)corrole was also used as precursor to a series of corrole−amino acid conjugates.342 Compounds 194 and 195 (Figure 56) are examples of such conjugates. Their

Figure 57. Structure of a corrole−anthraquinone−corrole triad.

are linked to anthraquinone by azomethine bridges. This triad was synthesized from 5,10,15-tritolylcorrole-3-carbaldehyde and 1,2-diaminoanthraquinone in dry toluene at reflux for 24 h. Although the photophysical and electrochemical studies showed that there is negligible electronic communication in the ground state between the donor (tritolylcorrole) and acceptor (anthraquinone) units, significant quenching of corrole fluorescence was observed compared to the monomeric units; this result was attributed to an intramolecular photoinduced electron transfer from the excited corrole to the ground state of the anthraquinone. The trimethoxysilane-functionalized corrole 197 (Scheme 22) was used as a key intermediate to incorporate the highly emissive corrole unit onto optically transparent silica nanoparticles.345 The synthesis of 197 involved the reaction of 3formylcorrole 99 with ethylenediamine in absolute ethanol. The resulting imine was then treated with (3isocyanatopropyl)trimethoxysilane to give compound 197. Grafting 197 with LUDOX was achieved by magnetic stirring at 80 °C for 2 days. The metal sensing ability of the silica nanoparticles coated with 197 was evaluated in the presence of Cu2+, Hg2+, and Ag+. Groups of satellite AgNPs were formed around the silica NPs upon the addition of Ag+ and, at the same time, a color change from green to yellow was observed. 4.3. Wittig Reaction

The Wittig reaction was applied for the first time in corrole chemistry by Neves and co-workers211 for the conversion of 3formylcorrole 99 into the 3-vinylcorrole 198 (Scheme 23). This new compound resulted from the reaction of 99 with the ylide generated from methyltriphenylphosphonium bromide and sodium hydride. Corrole 198 was successfully used as either the diene or dienophile in Diels−Alder reactions (see Figure 61 and Scheme 24). A complementary strategy was reported by Giribabu and coworkers,346,347 that selected the corrole derivative 199 as the phosphonium salt partner (Figure 58). The condensation of this compound with aldehydes allowed the synthesis of the donor−acceptor dyads 200−203. Wittig reactions were performed in DMF at room temperature in the presence of K 2 CO 3 and 18-crown-6. All dyads exhibit significant

Figure 56. Corrole−amino acid conjugates.

synthesis involved the initial formation of a maleimidefunctionalized corrole followed by reaction with L-cysteineOMe, or the reaction with cyanuric chloride followed by addition of L-phenylalanyl-L-phenylalanine-OMe. These conjugates form spherical aggregates that are characterized by the impressive absorption of light over nearly the whole visible range. The copper complex of meso-triphenylcorrole-3-carbaldehyde was reacted with several amino acid esters and the AF


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Scheme 22. Synthesis of a Trimethoxysilane-Functionalized Corrole

Scheme 23. Synthesis of a Gallium(III) 3-Vinylcorrole

fluorescence emission quenching (higher than 88%) of the corrole emission when compared to the free-base corrole monomer. The authors remarked that depending on the moiety attached to the corrole the corrole unit may act as electron donor or as electron acceptor.347 4.4. Reactions with Methylene Active Compounds and with Pyrroles

The Knoevenagel condensation of formyl groups with methylene active compounds has been extensively used to functionalize the periphery of tetrapyrrolic compounds (mainly porphyrins and phthalocyanines), especially to give access to acrylic acid derivatives for DSSCs. Conversely, this type of reaction has scarcely been used in the corrole field. Gross and co-workers reported the condensation of the Ga(III) 2,17diformylcorrole 100 with malononitrile, affording the corresponding 2,17-bis(2,2-dicyanovinyl)corrole.348 Later, the same

Figure 58. Synthesis of 3-(2-arylvinyl)corroles from a corrole-3phosphonium salt and various aldehydes.

Scheme 24. Synthesis of Corrole−Coumarin Conjugates via Hetero-Diels−Alder Reactions

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corrole. Kräutler and co-workers reported the use of the tetra(sulfoleno-fused) corrole 206 (Figure 60) as a diene precursor in Diels−Alder reaction with C60.352 The thermal extrusion of SO2 (in 1,2-dichlorobenzene at 140 °C) in the presence of an excess of C60 (30 equiv) afforded the difullereno-corrole 207 in 83% yield. The structure of the resulting adduct allows to infer that corrole 206 shows a remarkable preference for the loss of SO2 from the two sulfoleno groups fused to the bipyrrole unit. The 3-vinyl-corrolatogallium(III)pyridine 198 was used as the diene in Diels−Alder reactions with 1,4-benzoquinone, 1,4naphthoquinone, and dimethyl acetylenedicarboxylate (DMAD) in refluxing toluene.211,212 The major reaction products were the dehydrogenated cycloadducts 208, 209 and 210, respectively (Figure 61). In the reaction with DMAD,

group prepared the amphiphilic complexes Ga204 and Al204 (Figure 59) by treating the corresponding complexes of the 3-

Figure 59. Amphiphilic metallocorrole complexes, prepared via Knoevenagel condensation, and a Ga(III)corrole−BODIPY triad.

formylcorrole with 2-cyanoacetic acid, in refluxing DMF.349 These corroles exhibit electronic absorption and fluorescence at lower energies than their hydrophobic analogs. The observed red-shift in the absorption and emission spectra was rationalized by the electron-withdrawing effect of the CH C(CN)(CO2H) group. Recently, Sankar and co-workers214 reported the synthesis of the gallium(III)corrole−BODIPY triad 205 in 9% yield starting from the Ga(III) 2,17-diformylcorrole 100. The synthesis involved the reaction of 100 with 2,4-dimethylpyrrole in the presence of a catalytic amount of trifluoroacetic acid, followed by oxidation of the resulting bis(dipyrromethane) with DDQ and subsequent addition of ethyldiisopropylamine and BF3· OEt2. This triad exhibits unprecedented solid-state emission, which had not previously been demonstrated for corrole derivatives. 4.5. Corroles as 4π Components in Cycloaddition Reactions

The postfunctionalization of corroles via cycloaddition reactions has been considered in synthetic routes aiming to obtain derivatives with specialized functionalities for various applications. Studies show that corroles,283,350 like porphyrins,312,351 can act as dienophiles, dienes, dipolarophiles, or 1,3dipoles in cycloaddition reactions, although, in some cases, peculiar reactivity is observed due to unique properties of

Figure 61. Compounds obtained from the reaction of 3-vinylcorrole 198 with 1,4-benzoquinone, 1,4-naphthoquinone, and DMAD.

Figure 60. Tetra(sulfoleno-fused) corrole 206 and the difullereno-corrole 207 obtained from the reaction of 206 with C60. AH


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Scheme 25. meso-Linked Corrole−Fullerene Dyads via [4 + 2] Cycloadditions

corrole 211 was also formed in 16% yield.212 Cycloadduct 208 was used as chemosensor for anions and amines, showing good affinity for fluoride, nicotine, and caffeine in water samples. Adducts 210 and 211 showed particular affinity to cyanide anions. The 3-vinylcorrole 198 was also used as the 2π component in hetero-Diels−Alder reactions. It reacted with o-quinone methides (generated in situ from a 4-hydroxycoumarin and paraformaldehyde) in refluxing dioxane to give corroles 212a,b in high yields (Scheme 24).353 These corrole−coumarin conjugates showed ability for anion sensing (fluoride, cyanide, and acetate).353 The anthracene-functionalized corroles 213 react with C60 in 1,2-dichlorobenzene at room temperature, under darkness, to give the corresponding meso-linked corrole−fullerene dyads 214 in 35−46% yield (Scheme 25).354 These [4 + 2] cycloadducts are not stable in solution at room temperature and decompose slowly (10 days) back to 213 by a retrocycloaddition. A retro-Diels−Alder reaction was the key step in the conversion of the copper bicyclo[2.2.2]octadiene-fused corrole 215 (Figure 62) into the Cu-tetrabenzocorrole 216.355 Corrole 215 was obtained from the condensation of 4,7-dihydro-4,7ethano-2H-isoindole with benzaldehyde, followed by aromatization with tetrachloro-p-benzoquinone and metalation with Cu(OAc)2. Heating corrole 215 at 250 °C under vacuum (2 mmHg) for 20 min leads to the extrusion of four ethylene

molecules and the formation of Cu-tetrabenzocorrole 216 in quantitative yield. Sublimation of 215 onto an Au(111) substrate under ultrahigh vacuum conditions at ca. 300 °C also lead quantitatively to 216. The authors demonstrated the ability of the adsorbed π-extended corrole molecules to modulate spin states for possible molecular spintronics applications. A synthetic strategy based in 1,3-dipolar cycloadditions where corroles act as the dipole was explored for the synthesis of the adducts 218−225 (Figure 63).281,356 The procedure involves the in situ generation of azomethine ylide 217 (from the reaction of the gallium(III) complex of 5,10,15-tris(pentafluorophenyl)corrole-3-carbaldehyde (99) with N-methylglycine in refluxing toluene) in the presence of the

Figure 62. Copper bicyclo[2.2.2]octadiene-fused corrole and the resulting copper tetrabenzocorrole obtained by a retro-Diels−Alder reaction.

Figure 63. Compounds obtained from a Ga(III) 3-formylcorrole via the corresponding azomethine ylide 217. AI


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dipolarophile.356 The best yields (ca. 90%) were obtained for reactions involving 1,4-benzoquinone, C60, and dimethyl fumarate. DMAD was not an efficient dipolarophile, affording the adduct 218 in 33% yield.281,356 Peculiar compounds were unexpectedly obtained in the reactions with 1,4-anthraquinone and 1,4-naphthoquinone. In these reactions, the expected dehydrogenated cycloadducts 221 and 223 were accompanied by the π-extended quinone-fused corroles 222 and 224, which were isolated in 18% and 46% yield, respectively.356 A similar azomethine ylide generated from a 2-formylcorrole and Nmethylglycine was also trapped with 1,4-naphthoquinone, yielding the expected dehydrogenated 1,3-dipolar cycloadduct and the quinone-fused corrole derivative 224.281 A similar approach was followed by D’Souza and co-workers for the preparation of the free-base corrole−C60 dyads 226a,b (Figure 64).357 The intermediate azomethine ylides were

Scheme 26. Corrole−C60 Dyads Bearing Rigid and Semirigid Spacers

Figure 64. Corrole−C60 dyads prepared by 1,3-dipolar cycloaddition reactions with C60.

generated in situ from the corresponding 3-formylcorroles and N-methylglycine in the presence of C60. The cycloadducts, obtained in ca. 60% yield, showed ultrafast formation of a highenergy charge separated state in toluene; this property may be of particular utility in solar-electricity and solar-fuel conversion schemes. Gryko and co-workers also used the azomethine ylide approach to synthesize corrole−C60 dyads bearing rigid and semirigid spacers.358,359 Dyads 227−229 were synthesized from corroles bearing an aldehyde moiety, N-methylglycine and C60 (Scheme 26). Photophysical and theoretical studies performed in polar and nonpolar solvents demonstrated that these corrole−C60 dyads undergo efficient photoinduced electron transfer from the corrole singlet state to fullerene and a slow charge recombination process in nonpolar solvents. A similar approach was used for the synthesis of a corrole−Sc3N@C80 dyad,360 which was obtained from the reaction of Sc3N@C80 with six equivalents of 10-(4-formylphenyl)-5,15-bis(pentafluorophenyl)corrole and N-methylglycine in o-dichlorobenzene at 130 °C for 5 h. The dyad showed a fast and quantitative electron transfer deactivation of the photoexcited corrole to form the charge-separated species [corrole]•+Sc3N@ C80]•−. The Ga(III) complex of 5,10,15-tris(pentafluorophenyl)corrole-3-carbaldehyde (99) reacts with N-methylhydroxylamine hydrochloride in toluene at 60 °C to give the corrolyl nitrone 230 in 77% yield (Figure 65).361 This species gives the expected isoxazolidine-substituted corrole 231 upon treatment

Figure 65. Corrolyl nitrone 230 and the products resulting from its reaction with dimethyl fumarate and DMAD.

with dimethyl fumarate but, surprisingly, the reaction with DMAD leads to the formation of the amide derivative 232. In recent years, great efforts have been devoted to the design and synthesis of BODIPYs. Various types of pyrrolic macrocycles (namely porphyrins and corroles) have been linked covalently to BODIPY units aiming to “fill” the blue-green region of the visible spectra of the dyads, i.e., to complement the weak absorption of the tetrapyrrolic macrocycles in region between the Soret and the Q bands. Panchromatic light harvesting is an important feature for the design of photovoltaic materials and devices. Dyads incorporating corrole and BODIPY units are particularly interesting compounds with potential application in light-harvesting systems but also in photodynamic therapy. A myriad of corrole−BODIPY AJ


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conjugates have been prepared by the Huisgen azide−alkyne cycloaddition (AAC), and these derivatives are concisely discussed bellow. These azide−alkyne cycloadditions are typically carried out under Cu(I)-catalyzed conditions (CuAAC, a well-known “click” reaction). The CuAAC approach is being used with success in the preparation of corrole derivatives containing units of other chromophores, namely porphyrins,362 BODIPYs,324,363,364 or other corrole units.365 An interesting example of the versatility of this approach was reported by Harvey and co-workers who prepared Ga(III)corrole−BODIPY conjugates bridged by triazole linkers.363 Conjugates 233−236 (Figure 66) were

Figure 67. Cu(corrole)−BODIPY conjugates.

sodium ascorbate, while the best yield (32%) for 237c was obtained when the reaction was carried out in DMSO at room temperature. Systems bearing two BODIPY units (238a,b) were prepared in a similar manner using opp-A2B-corroles with two propargyl groups. These systems are potentially useful for light-harvesting applications and electron transfer studies.324 Gryko and co-workers applied the CuAAC protocol to convert propargyl- and azido-functionalized corroles into new A2B type corroles.365 As an example, the biscorroles 239a,b were obtained by treating an azido-functionalized Cu(corrole) with propargyl-functionalized (Cu or Fe)corroles (Figure 68).365

Figure 66. Ga(III)corrole−BODIPY conjugates.

obtained in near quantitative yield (>96%) by treating the gallium(III) complex of 10-(4-azidomethylphenyl)-5,15-dimesitylcorrole with BODIPYs functionalized with a terminal alkyne in the presence of CuI/DIPEA in THF at room temperature. It was verified that the direction of the singlet energy transfer (i.e., Ga(III)corrole* → BODIPY or BODIPY* → Ga(III)corrole) depends on the number of styryl groups attached to the BODIPY unit.363 The Cu(corrole)−BODIPY conjugates 237a−c (Figure 67) were also synthesized via the CuAAC method, but in this case the corroles were alkyne-functionalized while the BODIPY unit was azide-functionalized.324 Dyads 237a,b were obtained in 44% and 58% yield, respectively, when the reactions were performed in THF at 80 °C in the presence of CuSO4 and

Figure 68. Bis-corroles prepared by “click” reactions.

Ngo et al. explored the versatility of the CuAAC approach to prepare a variety of corrole−porphyrin conjugates, with Cu and Zn ion centers, respectively, in order to evaluate the photoinduced electron transfer within the two moieties.364 Dyad 240, triad 241, and pentad 242 (Figure 69) were obtained by coupling an azido-corrole with porphyrins bearing one, two, or four terminal alkynes. The best yields were obtained when tetrakis(acetonitrile)copper(I) hexafluorophosphate was used as catalyst. The analysis of the photophysical properties of these conjugates demonstrated the occurrence of AK


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4.6.1. Suzuki−Miyaura Cross-Coupling Reaction. The utility of the Suzuki−Miyaura reaction, also named as Suzuki reaction, to access highly substituted corroles was reported by Scrivanti et al. in 2004,366 and by Ghosh and co-workers in 2012.367 Both research groups used the fully brominated copper complex 243 as a platform for preparing undecaarylcorroles 244a−d (Figure 70). The first group was able to

Figure 70. β-Octabromocorrole and the products obtained via Suzuki−Miyaura cross-coupling.

4.6. Metal Catalyzed Reactions

obtain corrole 244a in 55% yield by carrying out the coupling reaction with p-chlorophenylboronic acid in toluene at 90 °C, using the iminophosphine-palladium(0) complex 245 in the presence of K2CO3. A 40 000:1 corrole:catalyst ratio was used. The Ghosh group prepared undecaphenylcorrole derivatives 244b−d by performing the reaction between 243 and a variety of arylboronic acids in toluene at reflux and using Pd2(dba)3 (dba = dibenzylidene acetone) as the catalyst.367 The singlecrystal X-ray structure of 244b allowed to conclude that the introduction of the β-aryl groups accentuates the degree of saddling when compared with the β-unsubstituted derivatives, albeit, to a lesser extent than for the β-Br groups in 243. Nevertheless, the Soret maxima of such undecaphenylcorroles are similar to that of corrole 243. Based on the electrochemical studies, it was found that the para substituents on the β-phenyl groups tune the redox potentials of copper corroles more effectively than those on meso-phenyl substituents. Canard and co-workers synthesized a series of copper βoctaaryl-meso-triarylcorroles with different aryl groups in meso and β positions (246, Figure 71) using a similar protocol.368−370 Several crystal structures were obtained and it was

Synthetic strategies based on reactions catalyzed by transition metals, namely Suzuki−Miyaura, Sonogashira, Heck, Buchwald−Hartwig, Liebeskind−Srogl, and Migita−Kosugi−Stille reactions, are frequently used to construct new functionalized corroles. In most of these studies, the reactions are catalyzed by palladium(0) complexes and the corroles bear bromine atoms in β-pyrrolic positions or at the meso-aryl substituents. A few systems were constructed using a complementary approach with boronic ester corroles. Corroles of A3 and A2B types with meso-4,6-dichloropyrimidin-5-yl moieties are also efficient templates in functionalization reactions using metal catalyzed protocols. This section is organized by reaction types.

Figure 71. β-Octaaryl-meso-triarylcorroles with different aryl groups in meso and β positions.

Figure 69. Corrole−porphyrin conjugates prepared by CuAAC.

charge separation states where the porphyrin is the electrondonor and the corrole is the electron-acceptor. The authors highlighted the potential of copper corroles as electronacceptors in model photosynthetic systems.

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tuned by the insertion of a high number of groups on their periphery. The penta-, hexa- and heptaaryl substituted free-base corroles 249−251 (Figure 72) were obtained from the reaction of the

found that the saddling dihedrals increase with the electrondonating efficiency of the meso substituents. Macrocyclic deformation is reinforced by the steric hindrance of the β-aryl groups, an effect similar to that produced by β-octabromination.368 It was verified from the crystal structure of the free-base of the undecaarylcorrole 246 (Ar1 = C6H4-4-CO2Me, Ar2 = 4FC6H4) (obtained by reductive demetalation) that the 11 bulky aryl groups do not produce unusual saddling distortions in the free-base macrocycle. The same authors reported369 the isolation of a copper decaarylcorrole during the Suzuki crosscoupling synthesis of the corresponding undecaaryl derivative. The remaining β-pyrrole position, which did not undergo Suzuki coupling, bears a proton that resulted from a competing debromination reaction. It was possible to establish by X-ray diffraction that the β-hydrogen atom is placed on the carbon atom C-13. The copper undeca(4-methoxyphenyl)corrole 246a (Ar1 = 4MeOC6H4) was converted into the liquid crystalline corrole 247 by following the synthetic strategy summarized in Scheme 27.370 It was shown that at room temperature this corrole has a hexagonal columnar organization combined with a H-arrangement of the discotic chromophores producing a blue-shifted light absorption. The authors concluded that the aggregation mode and the hole-transport properties of corroles can be

Figure 72. Partially brominated corroles 248a−c and β-arylsubstituted corroles 249−251.

partially brominated corroles 248a−c with phenyl- or pmethoxyphenylboronic acid in 62−89% yield.268 These reactions were performed in refluxing toluene in the presence of Pd2(dba)3 and K2CO3. A series of copper and bismuth opp-A2B-corrole complexes were synthesized under Suzuki−Miyaura cross-coupling conditions from the corresponding Cu371 and Bi240 complexes of 10-(4-bromophenyl)-5,15-bis(pentafluorophenyl)corrole and a variety of boronic acids or esters (Figure 73). The reactions with the Cu(corrole) were performed in toluene at 80 °C using Pd2dba3 as the catalyst, SPhos (2-dicyclohexylphosphino-2′,6′dimethoxybiphenyl) as the ligand, and K3PO4 as the base.371 The coupling products 252 were obtained in high yields (∼80%) in most cases. For less stable boron reagents, better yields were obtained using the precatalyst 253 (developed by Buchwald and co-workers372) in THF/H2O at 45 °C.371 The cross-coupling reactions of the Bi(III) (corrole) with boronic acids were carried out in a THF/H2O mixture, using a combination of SPhos (5 mol %), Pd(OAc)2 (2.5 mol %), and K3PO4.240 The expected Bi252 derivatives were obtained in 69−87% yield. Treatment of these resulting complexes with HCl/THF enabled isolation of the corresponding free-base corroles 252. The electrochemical characterization of some Bi corroles by cyclic voltammetry and spectroelectrochemistry revealed reversible ring-centered oxidation and reduction processes. Maes, Dehaen and co-workers selected corroles bearing one, two or three meso-pyrimidinyl substituents as attractive scaffolds for postfunctionalization via Pd-catalyzed crosscoupling reactions.200,326,373 Starting from the copper complex 254 or the free-bases 255, 256, and using the Suzuki−Miyaura protocol, the authors were able to prepare sterically encumbered arylcorroles 257−259 (Figure 74). The reactions with phenylboronic acid were performed in refluxing toluene in the presence of Pd(PPh3)4 and Na2CO3 and gave the expected products in excellent yield (>75%).200,326,373 As a complementary variation of the previous protocols, Osuka and co-workers used corrole 139309 as the organoboron

Scheme 27. Liquid Crystalline Copper Corrole

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presence of Pd(dba)2, PPh3, Cs2CO3 and CsF in toluene/ DMF/H2O, afforded compounds 260−263 in 56−94% yield (Figure 75). A 2,2′-linked corrole dimer was obtained from 139 by a palladium-catalyzed oxidative homocoupling using chloroacetone as the oxidant (see Scheme 32).308 4.6.2. Sonogashira Cross-Coupling Reaction. The Sonogashira reaction has been used in the postfunctionalization of corroles by some researchers, aiming, in some cases, the synthesis of model compounds for the study of light energy collection and conversion. An example of such work was reported by Gryko and co-workers374 who applied this reaction in the final step of a six-step preparation of the multichromophoric system 264 (Figure 76). An A2B-corrole was used as the halide component and 4-ethynyl-N,N-diphenylaniline as the alkynyl component. The reaction was carried out in anhydrous DMSO in the presence of Pd(PCy)2Cl2 and Cs2CO3, in a dry Schlenk tube at 120 °C for 32 h affording 264 in 10% yield. The electrochemical and spectroscopic properties of 264 confirmed weak conjugation between the different chromophores of the triad. Additionally, the photophysical measurements showed that efficient energy occurs from the diphenylaminophenylacetylene groups and perylene bisimide to the corrole unit, which is followed by a subsequent electron transfer from the corrole to the perylene bisimide. Gryko and co-workers also synthesized compound 265 (Figure 77), which incorporates a corrole unit, a 1,8naphthaleneimide, and a Zn(II) porphyrin. The synthetic strategy involved the preparation of an A2B-corrole bearing an imide unit with a bromine atom that was coupled with an A3Bporphyrin bearing a 4-ethynylphenyl group.375 The reaction was performed in DMSO at 80 °C using Pd(OAc)2, PPh3, and Cs2CO3 as the catalyst system. Compound 265 was obtained in 55% yield. The photophysical properties of this triad indicate that the corrole is the final acceptor of energy absorbed by the assembly. The donor−acceptor carbazole−corrole dyad 266 (Figure 78) was prepared by coupling 10-(4-iodophenyl)-5,15-bis(pentafluorophenyl)corrole with N-butyl-3-ethynylcarbazole in dichloromethane at room temperature, using PdCl2(PPh3)2 as the catalyst and triethylamine as the base.376 The corresponding copper and cobalt complexes were obtained by metalation

Figure 73. Copper and bismuth complexes obtained by Suzuki− Miyaura cross-coupling reactions with the boronic acids and esters shown.

component in the Suzuki−Miyaura reaction.309 The condensation of 139 with a variety of bromoaryl derivatives (5bromoanthracene, 1-bromo-2,5-dimethoxybenzene, 1,4-dibromobenzene and a meso-bromoporphyrin), performed in the

Figure 74. Corroles bearing 4,6-dichloropyrimidin-5-yl groups and products obtained under Suzuki−Miyaura conditions. AN


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Figure 75. 2-Substituted corrole derivatives obtained from the monoborylated corrole 139 via the Suzuki−Miyaura reaction.

Figure 76. Multichromophoric system prepared via Sonogashira cross-coupling.

of the free-base with the appropriate metal salts. The photophysical characterization of the dyads demonstrated that the two monomeric units retain their individual identities. Fluorescence studies with dyad 266 revealed efficient energy transfer from the carbazole donor to the corrole acceptor. The palladium-catalyzed reaction of 10-(4-iodophenyl)-5,15-bis(pentafluorophenyl)corrole (as free-base, copper, and manganese complexes) with ethynylbenzene under different catalytic conditions was also reported.377 4.6.3. Heck Cross-Coupling Reaction. The functionalization of corroles via Heck reaction has been the subject of a few reports.105,378,379 A remarkable example is the reaction of the octabromo-meso-triarylcorroles 267a,b with methyl acrylate.105 These reactions were performed in toluene/DMF at 125 °C, for 3 days, in the presence of Pd(OAc)2, PPh3 and K2CO3. The

Figure 77. Corrole−naphthaleneimide−porphyrin triad obtained by Sonogashira coupling.

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procedure involved the reaction of the octabrominated copper corrole 267b with various terminal alkenes bearing different functional groups. The resulting 5,10,15-triaryltetrabenzocorroles 270a−d were obtained in 21−24% yield. The Heck protocol was also used to couple copper 10-(4bromophenyl)-5,15-bis(pentafluorophenyl)corrole with 4-vinylpyridine, giving access to a corrole bearing a peripheral pyridyl moiety (271, Scheme 28).378 A similar reaction with the Scheme 28. Synthesis of the Pyridine-Substituted Corrole 271 and Corrole−Pt(II)(bpy) Conjugates Used in Binding Studies with DNA and HSA

Figure 78. Corrole−carbazole dyad.

products of the reactions were not the “expected” octaacrylatesubstituted corroles but the meso-triaryltetrabenzocorroles Cu268 and Cu269 (Figure 79), in 13% and 16% yield,

free-base corrole gave only minute amouns of the expected freebase corrole 271 but this compound could be obtained by an alternative route that involved the reaction of 4-bromobenzaldehyde with 4-vinylpyridine, followed by the condensation of the resulting 4-(2-(pyridin-4-yl)vinyl)benzaldehyde with 5(pentafluorophenyl)dipyrromethane. This last approach was also used for a similar functionalization of 3-bromobenzaldehyde. The coordination of both free-base isomers with cisdichloro(2,2′-bipyridine)platinum(II) afforded complexes 272 and 273 in 75% and 79% yield, respectively. It was demonstrated that these corrole−Pt(II)(bpy) conjugates are able to establish noncovalent interactions with calf-thymus DNA and human serum albumin. Additionally, gel electrophoresis experiments demonstrated that corrole−Pt(II)(bpy) conjugates 272 and 273 are able to bind plasmid pMT123 DNA, inducing alterations on its secondary structure. 4.6.4. Buchwald−Hartwig Cross-Coupling Reaction. The formation of C−N bonds mediated by palladium catalysts was also explored for the functionalization of corroles. In the known examples of Buchwald−Hartwig reactions with corroles, the corrole was always the halide partner. An interesting example of this type of reaction involved the cross-coupling of the Bi(III) complex of 10-(4-bromophenyl)-5,15-bis(pentafluorophenyl)corrole with heterocyclic and aryl amines (Figure 80).240 The coupling reactions were performed in tBuOH at 80 °C in the presence of Pd2(dba)3 (2.5 mol %), Xphos (5 mol %) and Cs2CO3 (2 equiv). The amine-

Figure 79. Octabromocorroles used in Heck reactions and the resulting tetrabenzocorroles.

respectively. The demetalation of Cu269 with a mixture of H2SO4 in CHCl3 gave the corresponding free-base 269 that was metalated with SnCl2 or Co(OAc)2 to the corresponding complexes Sn269 and Co269 in 51% and 63% yield, respectively. Later the same group reported the synthesis of the aluminum(III), gallium(III), germanium(IV), and phosphorus(V) tetrabenzocorrolates from the free-base 269.197 Soon afterward, the same group extended the synthetic Heck cross-coupling protocol to the synthesis of mesotriaryltetrabenzocorroles functionalized with different electron-withdrawing groups on the benzo-fused rings.379 The AP


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2,2-dimethylcyclopropane-1-carboxamide has afforded the diamido and tetraamido chiral corroles 279a and 279b in 21% and 41% yield, respectively. The iron nitrosyl complex of 278a (obtained by reaction of the free-base with FeCl2, followed by addition of NaNO2) catalyzes the cyclopropanation of styrene with ethyl diazoacetate in a diastereoselective fashion.91 Alternative routes to the bis-picket fence corroles were also evaluated, namely the introduction of the pivaloylamide moieties at the o-bromophenyl groups at the dipyrromethane stage (before the oxidative macrocyclization) but without success. Sterical reasons based on the stretched conformation adopted by the precursors might originate such failure. It was concluded that the palladium-catalyzed amidation of the o-bromophenyl groups must be performed at the corrole stage.380 Manganese(III) and iron(III) complexes of a chiral corrole similar to 279b (where Ar = C6F5) were obtained under a similar amidation protocol.381 The potential of these complexes as enantioselective catalysts in sulfoxidation of thioanisole by PhI and H2O2 was evaluated by the Gross group. However, due to the low chemical stability of the catalysts under the oxidative conditions, even in the best experimental conditions, the corresponding sulfoxide was formed in a 40% yield with a 27% enantiomeric excess.381 4.6.5. Other Cross-Coupling Reactions. 4.6.5.1. Liebeskind−Srogl Cross-Coupling Reaction. The less conventional Pd-catalyzed Liebeskind−Srogl cross-coupling reaction was utilized for the functionalization of corrole 280.326 Treatment of 280 with 4-cyanophenylboronic acid in the presence of Pd(PPh3)4 and copper(I) thiophene-2-carboxylate (2 equiv) afforded corrole 281 in 61% yield (Scheme 29). Corrole 281 can also be obtained directly from the free-base of 280 but 3 equiv of copper(I) thiophene-2-carboxylate are required (1 equiv is first consumed on the metalation of the corrole). The authors remarked that, surprisingly, in this reaction no product from a Suzuki-type reaction at the chlorine substituents was detected. 4.6.5.2. Migita−Kosugi−Stille Reaction. Maes, Dehaen, and co-workers selected the Migita−Kosugi−Stille protocol (usually designed as Stille reaction) to prepare corrole 282 (Figure 82).200,326 This compound was obtained in 73% yield from the reaction of the copper corrole 254 (Ar = Mes) with 2(tributylstannyl)thiophene in refluxing toluene in the presence of Pd(PPh3)4 and CsF. Paolesse and co-workers253 used the Stille reaction to prepare the 3,17-bis(2-phenylethynyl)corroles 283a,b from the Cu and Ag complexes of 3,17-dibromo-5,10,15-tris(4-tbutylphenyl)corrole (Figure 82). The reaction of the Cu or Ag corrole with tributyl(phenylethynyl)tin was performed in dioxane at 90 °C in the presence of a catalytic amount of Pd(PPh3)4. Compounds 283a and 283b were obtained in 64% and 89% yield, respectively. The corresponding free-base 283 (M = 3H) was obtained in 48% from the silver complex using the DBU/THF demetalation protocol (see section 2.2). 4.6.5.3. Trifluoromethylation. Copper β-octabromo-mesotriarylcorrole derivatives react with methyl 2,2-difluoro-2(fluorosulfonyl)acetate (FO2SCF2CO2Me) to provide βoctakis(trifluoromethyl)-meso-triarylcorrole complexes 284a−d in moderate yields (14−40%) (Figure 83).382 The trifluoromethylation of corroles is typically performed in DMF at 100 °C in the presence of Pd2(dba)3·CHCl3 (5 mol %), AsPh3 (40 mol %), and CuI (42 equiv). For corrole 284a, the presence of Pd and As is not necessary. The remarkable effects of the eight

Figure 80. Amine-functionalized corroles obtained by the Buchwald− Hartwig protocol.

functionalized corroles 274 were obtained in 27−54% yield. The corresponding free-bases were obtained in 67−94% yield (except 274c, which was isolated in trace amounts) after demetalation with HCl in THF. The Buchwald−Hartwig protocol was also applied to the functionalization of 5,15-bis(2,6-dibromophenyl)corroles 275a−e, but using amides instead of amines (Figure 81).91

Figure 81. meso-5,15-Bis(2,6-dibromophenyl)corroles 275a−e and the products obtained from them by coupling reactions with amides.

Corroles 275a−e were coupled with pivalamide at 125 °C in the presence of Pd(OAc)2 (10 mol %), Xantphos, and Cs2CO3 and in all the cases, di-, tri-, and tetra-substituted products 276a−e, 277a−e, and 278a−e were formed in comparable amounts, except for the cyano derivative 275d, which produced larger amounts of the di- and trisubstituted products 276d and 277d. Under similar conditions, the reaction of 275a with (S)AQ


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Scheme 29. Functionalization of a Corrole via the Liebeskind−Srogl Reaction

5.1. Oxidation

The stability of corroles is an important issue considering their potential applications in different areas, namely gas, metal, and anion sensing, catalysis, light energy conversion, and singlet oxygen generation. The studies carried out have shown that the oxidative pathway is strongly dependent on the corrole structural features and reaction conditions. Isocorroles, biliverdins, dimers, and trimers are frequently formed. In some studies, the oxidative processes have shown to be a versatile tool for corrole postfunctionalization, as outlined below. It is well established that corroles, especially the free-bases, are photosensitive. In fact, due to the reduced aromaticity and planarity of the corrole macrocycle, they are less stable in solution toward light and air than their porphyrin counterparts.383,384 Guilard and co-workers, in 1998,385 were the first to report that corroles, at ambient light and under air, are converted into ring-opened derivatives. During their studies concerning the synthesis of corrole 285, these authors found that this corrole, when dissolved in dichloromethane, at ambient light and under air, is slowly transformed into the open-chain biliverdin derivative 286 in 24% yield (Scheme 30). This transformation was explained as a reaction with dioxygen at the pyrrole−pyrrole bond, with the formation of a dioxetane intermediate, followed by the cleavage of that bond. Soon afterward, Paolesse and co-workers reported the photodegradation of the porphyrin−corrole dyad 287 in solution to the corresponding porphyrin−biliverdin 288.386 Considering that a symmetrical corrole−corrole dyad did not show a similar behavior, it was remarked that significant interactions between the porphyrin and corrole macrocycles are probably responsible by the activation of the corrole nucleus to the oxygenation reaction, with subsequent ring opening. The formation of a new type of ring-opened corrole derivative was observed during the oxidative degradation of the free-base 5,15-dimesityl-10-(4-nitrophenyl)corrole.387 This compound slowly converts into porphyrin 289 and the biliverdin-type compound 290 (Scheme 31). A plausible mechanism for the formation of these two compounds involves an initial dimerization of the corrole (via a 2π + 2π cycloaddition) leading to a spirocyclobutane intermediate, which is then split after oxidation into the porphyrin and the biliverdin derivative 290. In this process, the nitrophenyl substituent is lost. The ring-opened corrole derivative 291 (Figure 84) was reported as a byproduct isolated from the reaction of

Figure 82. Corroles obtained by the Stille protocol.

Figure 83. Compounds obtained by trifluoromethylation of copper βoctabromo-meso-triarylcorroles.

highly electron-withdrawing trifluoromethyl groups on the ground- and excited-state properties of the complexes were discussed with respect to the electronic absorption, 1H and 19F NMR spectroscopies, electrochemistry, and DFT calculations.

5. OTHER REACTIONS A few chemical transformations of corroles that were not included in the previous sections, namely oxidation reactions leading to ring opening and ring expansion, are discussed in this section. AR


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Scheme 30. Photodegradation of Corrole Derivatives

Figure 84. Ring-opened corrole derivative 291 and 292a−d.

electron-withdrawing, electron-donating or sterically hindered substituents at the meso-aryl groups allowed the identification of isocorroles and ring-opened tetrapyrroles as the main photochemical decomposition products.389 The authors found that the rate of corrole decomposition was solvent dependent, being higher in acetonitrile than in dichloromethane, hexane, methanol or ethyl acetate. Based on the relative intensities of peaks corresponding to the corrole and its oxidation products as a function of time, it was found that corroles bearing electron-withdrawing groups are the most stable. These results are consistent with those previously reported for the photostability of corroles in solution.383 After exposing an acetonitrile solution of corrole 293 (in a preparative scale) to sunlight for 60 h, the authors isolated and characterized 10-hydroxyisocorrole 294 and two biliverdin derivatives 295 and 296 (differing only on the orientation of the terminal pyrrolone) as a consequence of dioxygen attack at the meso-C10 carbon (Figure 85). Scheme 31. Oxidative Conversion of a Corrole into a Porphyrin and a Biliverdin

Figure 85. Corrole 293 and its photooxidation products: the isocorrole 294 and the biliverdins 295 and 296.

The photooxidation of unhindered A3-type corroles, namely H3TPC and meso-tris(4-methoxyphenyl)corrole, in dichloromethane, at ambient light and under air, gives, in both cases, mixtures of isocorroles 297 and 298 and biliverdins 299 and 300 as the primary degradation products (Figure 86). The biliverdins, which contain a direct pyrrole−pyrrole bond, are the main products.390 Osuka and co-workers reported a series of new corrole derivatives starting fom the monoborylated corrole 139309 (see Scheme 17, in section 3.6). Oxidation of 139 with oxone in

germanium(IV)TPC with Br2 in CHCl3/pyridine.215 The source of the coordinated Zn was probably the silica gel used in the chromatographic separation of the reaction mixture. Biliverdin analogues 292a−d were also identified as byproducts formed during the oxidative expansion of meso-triarylcorroles with 4-amino-4H-1,2,4-triazole.388 A systematic evaluation by mass spectrometry on the stability and decomposition pathways of meso-triarylcorroles bearing AS


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chemical reactivity of the biscorrole 303, which exists as a stable singlet biradical species in the ground state.391 They verified that 303 reacts with pyridine and pyridines with electrondonating groups exclusively at the 3 and 3′ positions affording the stable zwitterions 304 and 305 together with the reduced form 136 (Scheme 33). The mono and double nucleophilic addition products are obtained in 24−61% and 5−10% yield, respectively. The zwitterionic biscorrole 305a has selective sensing ability toward fluoride ion, as confirmed by spectroscopic studies (absorption, fluorescence, emission, 1H NMR, and high-resolution cryospray ionization mass spectrometry). The Osuka group reported the oxidative meso−meso coupling reactions of the meso-free corroles 306 and 308. Both reactions proceeded in very mild conditions to give regioselectively the 5,5′-linked dimer 307 and the 10,10′-linked dimer 309 (Scheme 34).264 The absorption and fluorescence spectra of the two dimers revealed a larger electronic interaction in dimer 307 than in 309. The Osuka group also reported that the oxidation of the singly 5,5′-linked corrole dimer 310 with DDQ affords the nonaromatic 2H-corrole dimer 311 (it bears only two NH protons in each corrole unit) in 63% yield (Scheme 35).392 Treatment of 311 with NaBH4 at 0 °C gave the aromatic 3Hcorrole dimer 312 in 83% yield; this dimer reverted to 311 under aerobic conditions. Single crystals of the doubly protonated dimer 313 were obtained from slow vapor diffusion of pentane into a 1,2-dichlorobenzene solution of 312 containing a trace amount of TFA. In a recent publication, Osuka and co-workers189 reported the synthesis of the triply linked 2H-corrole dimers 315a−c from the oxidation of singly 10,10′-linked corrole dimers 314a−c with DDQ in CHCl3 under highly diluted conditions (ca. 0.03 mM; Scheme 36). The new compounds, isolated in 39−94% yield, showed 1H NMR and absorption spectra typical

Figure 86. Isocorroles and ring-opened compounds obtained from the photooxidation of unhindered A3-type corroles.

THF/H2O resulted in the efficient (62% yield) formation of the 2-hydroxycorrole 301, which exists as a mixture of keto (predominant in CHCl3 solutions) and enol (predominant in DMSO solutions) tautomers (Scheme 32).309 The palladium-catalyzed oxidative homocoupling308 of 139 using chloroacetone as the oxidant afforded the 2,2′-linked corrole dimer 302 that, by oxidation with DDQ in toluene at 50 °C, gave the doubly linked corrole dimer 303 in 83% yield. Finally, reduction of 303 with NaBH4 in THF/MeOH at room temperature produced 136 in 67% yield (based on the amount of 302). Other examples of corrole dimers and oligomers are discussed in section 3.7. Later, the group of Osuka studied the

Scheme 32. Oxidation and Palladium-Catalyzed Oxidative Homocoupling of the Monoborylated Corrole 139

AT


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Scheme 33. Reaction of Biscorrole 303 with Pyridines

Scheme 34. Oxidative Dimerization of Corroles 306 and 308

corrole to a porphyrin or other tetrapyrrole macrocycle (Schemes 38−45). The ring contraction of the highly electron deficient mesotetrakis(trifluoromethyl)porphyrin was observed when it was refluxed with [Re2(CO)10] in benzonitrile for 1 h. The diamagnetic oxorhenium(V) corrole was obtained in ca. 9% yield after chromatographic separation (Scheme 37).72 The conversion of a corrole derivative to a porphyrin was first reported by Johnson and co-workers in 1969 and involved the thermal rearrangement of nickel(II) tetradehydrocorrin salts.398,399 As an example, heating a o-dichlorobenzene solution of iodide 316 at reflux affords the meso-unsubstituted porphyrin

of nonaromatic electronic networks. The aromatic 3H-corrole dimers obtained by reduction of 2H-corrole dimers 315 with NaBH4 are unstable and easily oxidize back to the 2H-corrole dimers upon exposure to air. Bis(Zn(II)) complexes of the 2Hcorrole dimers were synthesized and characterized as rare examples of nonaromatic zinc(II) corrole complexes (see Scheme 10, in section 2.1.10). 5.2. Ring Expansion

Pyrrolic macrocycles undergo ring contraction or expansion reactions.393−395 Thus, it is possible to both contract a porphyrin to a corrole (Scheme 37)72,396,397 and expand a AU


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Scheme 35. Oxidation of the 5,5′-Linked Corrole Dimer 310 to the Nonaromatic 2H-Corrole Dimer 311

octamethylporphyrin and the corresponding porphyrinatoruthenium(II) complex (Scheme 40).60 The expansion of the corrole macrocycle to the porphyrin involves the insertion of a carbonyl group (from the DMF or from the Ru reagent) into the pyrrole−pyrrole bond, and subsequent rearrangements. Palmer and co-workers reported a corrole to monoazaporphyrin ring expansion where the nitrogen atom was incorporated selectively between the two directly linked pyrrolic units.152 They found that the iridium(III) corrole 321 reacted rapidly with NBS, in the presence of ammonia, to afford a mixture of the monoazaporphyrins 322a−c in 4%, 8% and 35%, respectively (Scheme 41). Longer reaction periods, with excess NBS in solution, did not increase the relative yield of the dibromo compound 322c, indicating that the azaporphyrins are resistant to bromination. This is an indication that bromination must occur prior to nitrogen insertion. The use of 15N-labeled ammonia confirmed that it was the source of the nitrogen inserted into the corrole framework. It also enabled the positions of the bromine atoms on the azaporphyrin to be assigned. The 15N−1H HMBC NMR spectrum, that showed a strong signal corresponding to 3-bond 15 N−1H coupling, confirmed that there are protons linked to the C2 and C18 atoms; the bromine atoms were, thus, assigned to the 3 and 17 positions of the azaporphyrin. The expansion of a corrole to a hemiporphycene system was reported by Paolesse and co-workers.401 The reaction of H3TPC with tetraiodomethane in CH2Cl2/DMF at reflux afforded 5-iodo-6,11,16-triphenylhemiporphycene (323) as the major product (30% yield) and a trace amount of 5-iodo10,15,20-triphenylporphyrin (324) (Scheme 42). A similar corrole to hemiporphycene expansion was observed for the reaction of 5,10,15-tris(4-t-butylphenyl)corrole with 2,3-bis(bromomethyl)-5,6-dicyanopyrazine in 1,2,4-trichlorobenzene at 150 °C, in the presence of KI (Scheme 43).402 The resulting 5-substituted hemiporphycene 325 (23% yield) was accompanied by the corresponding 2-bromocorrole and a mixture of monobromo-substituted hemiporphycenes. An unprecedented corrole to 6-azahemiporphycene expansion reaction was observed during the attempted amination of silver(III) 3-nitro-5,10,15-tris(4-t-butylphenyl)corrole (326a) with 4-amino-4H-1,2,4-triazole (Scheme 44).403 Instead of the expected 2-amino-3-nitrocorrole, two green fractions were

Scheme 36. Triply Linked 2H-Corrole Dimers Obtained by Oxidation of the 10,10′-Linked Corrole Dimers 314 with DDQ

Scheme 37. Porphyrin to Corrole Ring Contraction

Scheme 38. Thermal Rearrangement of Nickel(II) Tetradehydrocorrin Iodide 316 to Porphyrin 317

317 in 43% yield (Scheme 38). Thermolysis of (1,19dimethyloctadehydrocorrinato)nickel(II) chloride 318 gives secochlorin 319 as the major product (34% yield) in addition to nickel porphyrin 320 (15% yield) (Scheme 39).400 Compound 319 was previously assigned as an epoxy-chlorin,399 but its structure was later unequivocally established by X-ray crystallography.400 H3OMC reacts with RuCl3 in refluxing DMF or with [Ru3(CO)12] in refluxing toluene to give both free-base AV


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Scheme 39. Thermolysis of (1,19-Dimethyloctadehydrocorrinato)nickel(II) Chloride 318 to a Secochlorin (319) and a Porphyrin (320)

crystallography. The structure of the minor isomer was not identified.

Scheme 40. Corrole to Porphyrin Ring Expansion by Metallation with RuCl3 or [Ru3(CO)12]

6. FINAL REMARKS Research groups all over the world have devoted their scientific attention to the chemical, physical, and biological features and potential applications of corroles, a “relatively” new group of tetrapyrrolic macrocycles. Their studies allowed the establishment of new synthetic methods for the functionalization and postfunctionalization of a corrole macrocycle. As it is shown in this review, such derivatization processes can take place at the macrocycle inner core, at the β-positions, at free meso-positions or at meso-substituents, by considering well-established organic chemistry procedures and using simple corroles as starting materials. New types of metallocorroles, with unusual structural features and properties, are being reported every year. Metalation/demetalation of corroles are now “trivial” steps in their functionalization routes, allowing the access to compounds substituted at specific positions both in the free-base and metal complex forms. Corroles with functional groups at peripheral positions can be obtained from a wide variety of transformations (e.g., halogenation, formylation, carboxylation, nitration, sulfonation, chlorosulfonation, borylation, etc.). Such corrole derivatives can be used in studies concerning their properties and applications or being used as starting reagents in postfunctionalization reactions that may include simply the formation of amines, imines, amides, sulfonamides, etc. but also being used in “modern” metal-catalyzed reactions (Suzuki, Sonogashira, Heck, Buchwald, and Stille). Knowing that this impressive progress in corrole chemistry occurred only since the last 15 years it can be anticipated that corroles will be “golden targets” for future studies. On the chemical side, there are challenges to meet. Certainly metalation studies will appear looking for metallocorroles not yet reported and also for their particular reactivities. The derivatization area also includes matters to be further elucidated, mainly the regioselectivity of these reactions. Based on the information available, the hydrogenation is the

Scheme 41. Corrole to Monoazaporphyrin Ring Expansion

formed: the first fraction, representing the major product of the reaction (44% yield), was identified as the free-base 6azahemiporphycene 327a. The more polar fraction, obtained in a minor amount, was the free-base corrole. Similarly, the 6azahemiporphycene 327b was obtained in 56% and 53% yield from 326b and 326c, respectively, under the same experimental conditions. These results suggest that the reaction does not depend on the presence of the nitro group nor the metalation of the corrole. This reaction was successfully applied to other free-base meso-triarylcorroles.388 Gross and co-workers reported a new method to convert corroles into 6-azahemiporphycenes.63 The oxidation of Mn(III) 5,10,15-tris(4-nitrophenyl)corrole with sodium hypochlorite and ammonia at room temperature afforded a mixture of two neutral manganese(V) nitrido complexes of isomeric azahemiporphycenes, of which the 6-aza isomer 228 (Scheme 45) was characterized by X-ray Scheme 42. Corrole to Hemiporphycene Ring Expansion

AW


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Scheme 43. Corrole to Hemiporphycene Ring Expansion

ORCID

Scheme 44. Corrole to 6-Azahemiporphycene Ring Expansion

M. Amparo F. Faustino: 0000-0003-4423-3802 José A. S. Cavaleiro: 0000-0001-5495-5126 Notes

The authors declare no competing financial interest. Biographies Joana F. B. Barata received her B.Sc. degree in Chemistry and Food Chemistry in 2001 and her M.Sc. degree in Chemistry of Natural Products in 2004 from the University of Aveiro, Portugal. She was awarded in 2009 a Ph.D. degree in Chemistry, at the same university, under the supervision of Profs. José Cavaleiro and Maria da Graça Neves. She was awarded the ‘“Dow Portugal Prize”’ (2001) and PYCA (Portuguese young chemists award) prize (2010). She has been a postdoctoral research fellow with a working program focused on the synthesis of new corrole-inorganic hybrid nanomaterials with biological applications.

Scheme 45. Synthesis of a Manganese(V) Nitride 6Azahemiporphycene

Maria da Graça P. M. S. Neves is Associate Professor with Habilitation at the Department of Chemistry, University of Aveiro, Portugal. She obtained her Habilitation and Ph.D. degree both at the University of Aveiro, her M.Sc. degree at UMIST, Manchester, Great-Britain, and her B.Sc. degree in Chemistry at the University of Lourenço Marques, Mozambique. Her research interests are centered on the synthesis, functionalization, and potential applications of tetrapyrrolic macrocycles like porphyrins, corroles, and phthalocyanines.

only known site selective corrole transformation, occurring, surprisingly, at positions 7/8. There is no clue yet for that. The most reactive β-positions of the corrole macrocycle are the 2, 3, 17, and 18 positions. However, for a few reactions (bromination, sulfonation, and nitration) the functionalization can also take place at other carbon centers. Theoretical and detailed studies on those reaction mechanisms may lead to the development of selective methods in the near future. In such a way the reactivity of all β carbons of the corrole macrocycle may be controlled, allowing the selective synthesis of any isomer. (We thank a reviewer for having rised this matter to us). Corroles are a group of compounds with impressive potentialities. A quick search in databases reveals their antitumor and medical imaging properties and their use in catalysis, in nonlinear optics, and as gas, anion and metal ion sensors. Their potential medicinal applications in cancer therapy and in the photoinactivation of microorganisms (e.g., antibiotic resistant bacteria) deserve a special highlight in such context. Then it can be foreseen that, in the near future, corroles will be found in real applications. Overall, corroles will continue to take part in the process to solve everyday problems and in the search for better conditions of life in Nature.

Maria Amparo F. Faustino received her Ph.D. in Chemistry at the University of Aveiro in 1999. Since 2001 she has been an Assistant Professor in the Department of Chemistry, University of Aveiro. Her current area of research includes synthesis and transformation of tetrapyrrolic derivatives with structural features to be used in DSSCs, photodynamic therapy (PDT) of neoplastic tissues, and microorganisms photoinactivation (PDI) aiming at medical and environmental applications. Augusto C. Tomé is an Associate Professor with Habilitation at the Department of Chemistry, University of Aveiro, Portugal. He got his B.Sc. and Ph.D. degrees at the University of Aveiro. His research interests are centered on the synthesis of pyrrolic compounds (porphyrins, corroles, phthalocyanines, calixpyrroles, etc.) and fullerene derivatives for application in PDT, PDI, chemical sensing, or solar cells. José A. S. Cavaleiro got his B.Sc. degree at the University of Coimbra, Portugal and his Ph.D. degree at the Robert Robinson Laboratories (supervision of Profs. George W. Kenner and Kevin M. Smith), University of Liverpool, U.K. His academic career as a staff member started at Coimbra University and later continued at Lourenço Marques (Mozambique) and Aveiro (Portugal) Universities. Since 1986 he has been Professor of Chemistry at the Aveiro University. He is the recipient of several prizes (e.g., Parke-Davis prize, Liverpool University, 1973; Ferreira da Silva prize, Portuguese Chemical Society,

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. AX


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(4) Harrison, H. R.; Hodder, O. J. R.; Hodgkin, D. C. Crystal and Molecular Structure of 8,12-Diethyl-2,3,7,13,17,18-Hexamethylcorrole. J. Chem. Soc. B 1971, 640−645. (5) Paolesse, R.Syntheses of Corroles. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp 201−232. (6) Gross, Z.; Galili, N.; Saltsman, I. The First Direct Synthesis of Corroles from Pyrrole. Angew. Chem., Int. Ed. 1999, 38, 1427−1429. (7) Paolesse, R.; Mini, S.; Sagone, F.; Boschi, T.; Jaquinod, L.; Nurco, D. J.; Smith, K. M. 5,10,15-Triphenylcorrole: A Product from a Modified Rothemund Reaction. Chem. Commun. 1999, 1307−1308. (8) Gryko, D. T.; Koszarna, B. Refined Methods for the Synthesis of meso-Substituted A3- and Trans-A2B-Corroles. Org. Biomol. Chem. 2003, 1, 350−357. (9) Nardis, S.; Monti, D.; Paolesse, R. Novel Aspects of Corrole Chemistry. Mini-Rev. Org. Chem. 2005, 2, 355−374. (10) Koszarna, B.; Gryko, D. T. Efficient Synthesis of mesoSubstituted Corroles in a H2O−MeOH Mixture. J. Org. Chem. 2006, 71, 3707−3717. (11) Aviv, I.; Gross, Z. Corrole-Based Applications. Chem. Commun. 2007, 1987−1999. (12) Flamigni, L. Functional Arrays for Light Energy Capture and Charge Separation. Chem. Rec. 2016, 16, 1067−1081. (13) Srikanth, M.; Sastry, G. N.; Soujanya, Y. Molecular Design of Corrole-Based D-π-A Sensitizers for Dye-Sensitized Solar Cell Applications. Int. J. Quantum Chem. 2015, 115, 745−752. (14) Sudhakar, K.; Giribabu, L.; Salvatori, P.; De Angelis, F. Triphenylamine-Functionalized Corrole Sensitizers for Solar-Cell Applications. Phys. Status Solidi A 2015, 212, 194−202. (15) Robert, C.; Ohkawara, T.; Nozaki, K. Manganese-Corrole Complexes as Versatile Catalysts for the Ring-Opening Homo- and Co-Polymerization of Epoxide. Chem. - Eur. J. 2014, 20, 4789−4795. (16) Barata, J. F. B.; Pinto, R. J. B.; Vaz Serra, V. I. R. C.; Silvestre, A. J. D.; Trindade, T.; Neves, M. G. P. M. S.; Cavaleiro, J. A. S.; Daina, S.; Sadocco, P.; Freire, C. S. R. Fluorescent Bioactive Corrole GraftedChitosan Films. Biomacromolecules 2016, 17, 1395−1403. (17) Pohl, J.; Saltsman, I.; Mahammed, A.; Gross, Z.; Röder, B. Inhibition of Green Algae Growth by Corrole-Based Photosensitizers. J. Appl. Microbiol. 2015, 118, 305−312. (18) Sims, J. D.; Hwang, J. Y.; Wagner, S.; Alonso-Valenteen, F.; Hanson, C.; Taguiam, J. M.; Polo, R.; Harutyunyan, I.; Karapetyan, G.; Sorasaenee, K.; et al. A Corrole Nanobiologic Elicits Tissue-Activated MRI Contrast Enhancement and Tumor-Targeted Toxicity. J. Controlled Release 2015, 217, 92−101. (19) Samaroo, D.; Perez, E.; Aggarwal, A.; Wills, A.; O’Connor, N. Strategies for Delivering Porphyrinoid-Based Photosensitizers in Therapeutic Applications. Ther. Delivery 2014, 5, 859−872. (20) Preuβ, A.; Saltsman, I.; Mahammed, A.; Pfitzner, M.; Goldberg, I.; Gross, Z.; Röder, B. Photodynamic Inactivation of Mold Fungi Spores by Newly Developed Charged Corroles. J. Photochem. Photobiol., B 2014, 133, 39−46. (21) Pribisko, M.; Palmer, J.; Grubbs, R. H.; Gray, H. B.; Termini, J.; Lim, P. Cellular Uptake and Anticancer Activity of Carboxylated Gallium Corroles. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, E2258− E2266. (22) Aviv-Harel, I.; Gross, Z. Aura of Corroles. Chem. - Eur. J. 2009, 15, 8382−8394. (23) Haber, A.; Angel, I.; Mahammed, A.; Gross, Z. Combating Diabetes Complications by 1-Fe, a Corrole-Based Catalytic Antioxidant. J. Diabetes Complications 2013, 27, 316−321. (24) Flamigni, L.; Gryko, D. T. Photoactive Corrole-Based Arrays. Chem. Soc. Rev. 2009, 38, 1635−1646. (25) Paolesse, R.; Nardis, S.; Monti, D.; Stefanelli, M.; Di Natale, C. Porphyrinoids for Chemical Sensor Applications. Chem. Rev. 2016, DOI: 10.1021/acs.chemrev.6b00361. (26) Teo, R. D.; Hwang, J. Y.; Termini, J.; Gross, Z.; Gray, H. B. Fighting Cancer with Corroles. Chem. Rev. 2016, DOI: 10.1021/ acs.chemrev.6b00400.

2004; Spanish-Portuguese prize, Royal Spanish Chemical Society, 2010). His research interests are centered on the synthesis, reactivity, and applications (medicinal, catalytic, and others) of porphyrins and related compounds. He is the author of 496 scientific publications in major journals of organic chemistry.

ACKNOWLEDGMENTS Thanks are due to the University of Aveiro and FCT (Fundaçaõ para a Ciência e a Tecnologia) for the financial support to the QOPNA research project (FCT UID/QUI/ 00062/2013) and the project PTDC/QEQ-QOR/6160/2014 through national funds and where applicable cofinanced by FEDER under the PT2020 Partnership Agreement, and also to the Portuguese NMR Network. J.F.B.B. thanks FCT for her postdoctoral grant (SFRH/BPD/63237/2009). ABBREVIATIONS BODIPY = boron dipyrromethene bpy = 2,2′-bipyridine cod = cycloocta-1,5-diene dba = dibenzylideneacetone DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene DDQ = 2,3-dichloro-5,6-dicyanobenzoquinone DEGME = diethylene glycol monomethyl ether DMAD = dimethyl acetylenedicarboxylate DMAP = 4-dimethylaminopyridine DMF = N,N-dimethylformamide DMSO = dimethyl sulfoxide DSSC = dye-sensitized solar cell HSA = human serum albumin Mes = mesityl (= 2,4,6-trimethylphenyl) NBS = N-bromosuccinimide NCS = N-chlorosuccinimide NIS = N-iodosuccinimide NP = nanoparticle OEC = trianion of 2,3,7,8,12,13,17,18-octaethylcorrole OMC = trianion of 2,3,7,8,12,13,17,18-octamethylcorrole OSC = organic solar cells PDT = photodynamic therapy PIFA = [bis(trifluoroacetoxy)iodo]benzene py = pyridine SPhos = 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl TFA = trifluoroacetic acid THF = tetrahydrofuran TPC = trianion of 5,10,15-triphenylcorrole TPFC = trianion of 5,10,15-tris(pentafluorophenyl)corrole TTC = trianion of 5,10,15-tritolylcorrole (tolyl = 4methylphenyl) XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl REFERENCES (1) Erben, C.; Will, S.; Kadish, K. M.Metallocorroles: Molecular Structure, Spectroscopy and Electronic States. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp 233−300. (2) Bendix, J.; Dmochowski, I. J.; Gray, H. B.; Mahammed, A.; Simkhovich, L.; Gross, Z. Structural, Electrochemical, and Photophysical Properties of Gallium(III) 5,10,15-Tris(pentafluorophenyl)corrole. Angew. Chem., Int. Ed. 2000, 39, 4048−4051. (3) Sheldon, R. A.Metalloporphyrins in Catalytic Oxidations; Marcel Dekker: New York, 1994. AY


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