Synthesis of Corroles and Their Heteroanalogs - Chemical Reviews

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Synthesis of Corroles and Their Heteroanalogs Rafał Orłowski, Dorota Gryko,* and Daniel T. Gryko* Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44-52, 01-224 Warsaw, Poland ABSTRACT: Corroles have come a long way from being a curiosity to being a mainstream research topic. They are now regularly synthesized in numerous research laboratories worldwide with diverse specific aims in mind. In this review we present a comprehensive description of corroles’ synthesis, developed both before and after 1999. To aid the investigator in developing synthetic strategies, some of the sections culminate in tables containing comparisons of various methodologies leading to meso-substituted corroles. The remaining challenges are delineated. In the second part of this review, we also describe the syntheses of isocorroles and heteroanalogs of corroles such as triazacorroles (corrolazines), 10-heterocorroles, 21-heterocorroles, 22-heterocorroles, N-confused corroles, as well as norcorroles. The review is complemented with a short outlook.

CONTENTS 1. Introduction 2. β-Substituted Corroles 2.1. Classical Methods 2.1.1. Pyrrole Tetramerization 2.1.2. Synthesis of β-Substituted Corroles from Tetrapyrrolic Intermediates 2.2. Nonclassical Methods 2.2.1. Synthesis from Derivatives of 2,2′-Bipyrrole and Dipyrranes 2.2.2. Contraction of Thiaphlorins 3. Meso-Substituted A3-Corroles 3.1. General Considerations 3.2. Reaction under Neat Conditions 3.3. Synthesis Using Al2O3 as a Solid Support 3.4. Synthesis in Acetic Acid 3.5. Synthesis in Organic Solvents 3.6. Synthesis in Water−Methanol−HCl System 3.7. Synthesis via Porphyrin Ring Contraction 3.8. N-Alkyl Corroles 3.9. Guidelines for the Preparation of A3-Corroles 4. Meso-Substituted trans-A2B-Corroles 4.1. General Considerations 4.2. Synthesis from Aldehydes and Dipyrranes 4.3. Synthesis from 2,2′-Bipyrrole and Dipyrrane− Diols 4.4. Synthesis from Alkyl Oxalyl Chlorides and Dipyrranes 4.5. Synthesis from Dipyrrane−Diols and Pyrrole 5. cis-A2B-Corroles 6. ABC-Corroles 7. Conjugated Corrole Dimers 8. Meso-β-Substituted Corroles 8.1. Synthesis of Mono-Corroles Substituted at Meso and β-Positions 8.2. Face-to-Face Porphyrin−Corrole Systems 8.3. Cofacial Bis(Corrole) Dyes 9. Isocorroles © XXXX American Chemical Society

10. Core-Modified Corroles 10.1. N-Confused Corroles 10.2. Heteroanalogues of Corroles 10.2.1. 10-Heterocorroles 10.2.2. Corrole Heteroanalogs Modified in Pyrrole Subunit 10.2.3. Triazacorroles (Corrolazines) 11. Norcorroles 12. Stability of Corroles 13. Summary and Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments References

A B B B C E E F F F I J J J J K K K K K L

Y Y Z Z AC AC AC AD AE AF AF AF AF AF AF

1. INTRODUCTION Corroles (Figure 1) were discovered by serendipity by Kay and Johnson in 1964.1 The formidable effort by the Johnson group2 was oriented toward synthesis of the corrin skeleton and resulted

P P Q Q Q R S

Figure 1. Structure and numbering of corrole.

S V W W

Special Issue: Expanded, Contracted, and Isomeric Porphyrins Received: July 6, 2016

A

DOI: 10.1021/acs.chemrev.6b00434 Chem. Rev. XXXX, XXX, XXX−XXX

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in the first synthesis of corroles via cyclization of a,c-biladienes. Corroles differ from many π-extended or π-expanded porphyrinoids in that corroles have been the subject of advanced studies rather than being compounds prepared just once and characterized. This fact is mainly due to the event that happened in 1999. Two research groups, one from Israel3 and one from Italy,4 independently published one-pot syntheses of mesosubstituted A3-corroles from pyrrole and aromatic aldehydes. Even though the yields were initially low, the preparation of these macrocycles in just one operation was an important advance. In a short time, the number of publications on this topic increased enormously, with ∼140 papers in 2015 alone. Meso-substituted corroles have slowly but steadily replaced β-substituted corroles in the literature. Needless to say, if corroles and their complexes did not possess fascinating properties, their straightforward synthesis would not have made such an impact. The possibility to prepare them in gram quantitates prompted more adventurous projects, resulting in expansion of the “corroles periodic table”5,6 and an increase in the range of applications in various fields of research.7,8 The synthesis of corroles has been previously reviewed.9−15 The present review aims to provide a comprehensive and updated overview of the topic.

Scheme 2. Synthesis of β-Substituted Corroles via Tetramerization of 3-Ethyl-4-methyl-2-formylpyrrole-5carboxylic Acid (4)

2. β-SUBSTITUTED CORROLES 2.1. Classical Methods

Most of the reported methodologies leading to assembly of the corrole core possessing eight substituents at the β-positions are Scheme 1. Two Routes toward Co(III) Complex of Octamethylcorrole 2

into two parts describing classical and nonclassical methods. Although this division is highly subjective, it will allow the reader to easily maneuver through this review. 2.1.1. Pyrrole Tetramerization. Paolesse et al. first reported the direct tetramerization of substituted pyrroles that resulted in formation of a corrole in 1994 (see section 8.1).16 The first synthesis of β-substituted meso-free corroles following this approach involved the tetramerization of 3,4-dimethyl-pyrrole2-carbaldehyde (1) and resulted in the formation of βsubstituted corrole complex 2 in 10% yield.17 Moreover, when

based on the stepwise formation of tetrapyrrolic intermediates. Starting from pyrrole derivatives or corresponding dipyrranes (dipyrromethanes), a number of β-substituted corroles can be prepared in moderate yields. Despite decades of research, this process is still the most convenient way for the direct βsubstituted corrole synthesis. For clarity, we divided this section B


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Scheme 3. Synthesis via a,c-Biladiene Intermediate

Scheme 4. Synthesis of Unsymmetrically β-Substituted Corrole 14

3,4-dimethyl-2-formylpyrrole-5-carboxylic acid (3) was used as a starting material, corrole 2 was obtained in similar yield, probably due to the rapid decarboxylation of the parent pyrrole in acidic media (Scheme 1). The mechanism of this reaction must involve a cascade of Friedel−Crafts reactions followed by dehydration, formation of a cobalt complex, cyclization, and, finally, extrusion of one of the meso-carbon atoms. However, details of this process have not been explained. The synthesis of unsymmetrically substituted corroles was investigated.17 Tetramerization of 3-ethyl-4-methyl-2-formylpyrrole-5-carboxylic acid (4) led to a mixture of symmetrically substituted corrole 5, two asymmetric corroles 6 and 7, and a cobalt etioporphyrin I 8 byproduct (Scheme 2) with 8% yield of combined corrole products. The complex mixture of products seriously limits the described methodology. 2.1.2. Synthesis of β-Substituted Corroles from Tetrapyrrolic Intermediates. The corrole synthesis from a,cbiladienes was the most commonly used procedure until 1999. The [2 + 1 + 1] approach18 involved acidic condensation of either 3,4-dialkylpyrroles with 5,5′-diformyldipyrrane 9 or 2formylpyrroles with dipyrrane dicarboxylic acid 10 (Scheme 3). Both routes provided the crystalline dihydrobromide salt, which underwent photochemical oxidative cyclization in methanolic ammonia or sodium acetate solution with up to 67% yield.18 The oxidation step was performed under light irradiation with a 200

W tungsten lamp or a medium-pressure mercury lamp. Notably, it was possible to induce the formation of a monoazaporphyrin byproduct (up to 5%) utilizing ammonia solution. Further reports indicated that irradiation could be successfully replaced by a range of oxidizing agents, such as potassium hexacyanoferrate(III), cerium(IV) sulfate, iron(III) chloride, benzoyl peroxide, or hydrogen peroxide, while maintaining basic reaction conditions.19 p-Chloranil was introduced as a versatile oxidant by Vogel and co-workers only in 1994, improving significantly a,c-biladiene cyclization yields.20 The utilization of potassium hexacyanoferrate(III) or benzoyl peroxide suggested a free-radical mechanism.19 However, the external addition of radical scavengers did not affect reaction yields, ruling out that hypothesis. Hence, considering the Woodward−Hoffmann rules, the most probable pathway is a two-electron oxidative conrotatory cyclization of a parent biladiene.21 Despite being the most commonly used method for the direct synthesis of β-substituted corroles, this method is far from flawless. Preparation of the corresponding building blocks was laborious, and consequently, overall yields were not astonishing. The alternative approach is based on the in situ preparation of dibromobiladienes, starting from the corresponding dipyrranes. High reactivity of terminal halogen atoms was the driving force C


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Scheme 5. Synthesis β-Substituted Corroles via Tripyrrane as an Intermediate

Scheme 6. [2 + 1 + 1] Approach toward Corroles Synthesis

Scheme 7. Synthesis of Corrole Utilizing tert-Butyl 5-Iodo3,4-dimethylpyrrole-2-carboxylate (24)

for the above-mentioned cyclization. It is noteworthy that omission of an oxidant in the cyclization step prevented partial product decomposition, thus lowering the yield. This synthesis was first realized by the condensation of dipyrrane 11 with 5bromodipyrrane 12 with an unsubstituted 5′-position. Subsequent thermal cyclization of obtained 1,19-dibromo-a,c-biladiene 13 in o-dichlorobenzene afforded unsymmetrical corrole 14 in a good yield (Scheme 4).22 The major drawback of this method was the multistep synthesis of bromodipyrrane precursors, thus limiting its potential use. Aiming to simplify this synthetic strategy, Smith and coworkers23 examined the possibility of the in situ 1,19-dibromoa,c-biladiene synthesis via tripyrrane 17 as an intermediate. Luckily, the stepwise reaction of dipyrrane dicarboxylic acid

derivative 15 with equivalents of 2-bromo-5-formyl-3,4dimethylpyrrole (16) in the presence of p-toluenesulfonic acid afforded corresponding a,c-biladiene 18 in 50% overall yield, which was smoothly transformed into the expected corrole 19 D


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Scheme 9. Synthesis of “Figure-Eight” Octaphyrin 33a

Scheme 8. Synthesis of β-Substituted Corroles via Condensation of Dipyrranes with Derivatives of 2,2′Bipyrrole

a

β-Substituents omitted for clarity.

The last modification of this synthetic route was reported in 1997.26 It was based on the utilization of more readily accessible tert-butyl 5-iodo-3,4-dimethylpyrrole-2-carboxylate (24) which after in situ decarboxylation was reacted with 5,5′-diformyldipyrromethane 26, yielding 1,19-diiodo-a,c-biladine dihydrobromide (Scheme 7). Subsequent thermal cyclization of the biladiene intermediate afforded final product 27 in 20% yield. It is worth noting that the alleged improvement of this protocol, which is based on the accessibility of the substrate, is balanced by the lower yield of the cyclization step. 2.2. Nonclassical Methods

Besides the aforementioned procedures, other pathways lead to similar products, albeit they are based on slightly different approaches. Moreover, they require the presence of a templating agent (which secures the correct geometry of intermediates) or the use of specific starting materials. Despite a number of drawbacks, these protocols must be considered. 2.2.1. Synthesis from Derivatives of 2,2′-Bipyrrole and Dipyrranes. An analysis of the structure of the corrole ring suggests that the most logical pathway to assemble the corrole core is via reaction of a suitably substituted derivative of dipyrrane with 2,2′-bipyrrole. The first successful example was reported by Conlon et al. in 1973.27 The reaction of dipyrrane 9 with bipyrrole 28 under acidic conditions led to a red precipitate which was converted by heating in the presence of triphenylphosphine and cobalt(II) acetate to the corresponding corrole complex 29 in a moderate yield (Scheme 8). It is worth noting that a different arrangement of formyl and carboxyl groups on bipyrrole 28 and dipyrrane 10 did not affect the reaction outcome. Moreover, the formation of the desired corrole was suppressed in the absence of cobalt(II) acetate, emphasizing its role in the reaction cycle. The lack of metal ions favored the formation of

(Scheme 5). This methodology proved to be effective for the preparation of both symmetrically and asymmetrically substituted corroles.24 As in all previously described cases, this procedure suffers from a few shortcomings (arduous substrate synthesis, moderate overall yields, etc.) that negatively influenced its future utilization. In various reactions, iodoarenes proved to be more reactive as compared to their bromo analogues due to the lower energy of their C−Hal bond. Thus, a promising alternative to the abovementioned strategy can employ the 1,19-diiodo-a,c-biladienes. Engler and Gossauer25 altered this approach by reacting 5-iodo2-formylpyrrole 20 (2 equiv) with dipyrrane 21. Obtained a,cbiladiene 22 was subsequently transformed into corresponding corrole 23 at an elevated temperature (Scheme 6). Unfortunately, the yield of the cyclization step was moderate, bringing no significant improvement to the field. E


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Scheme 10. Synthesis of β-Substituted Corrole 36 via Desulfuration of Thiaphlorin 35

more complex octapyrrolic macrocycles, like the golden-red “figure-eight” octaphyrin 33 (Scheme 9). The most probable role of the cobalt cation is to both stabilize the tetrapyrrolic intermediate and to act as the templating agent, ensuring the proximity of reactive sites.28 2.2.2. Contraction of Thiaphlorins. Ring contraction is a well-known strategy in the preparation of heterocyclic scaffolds.29,30 Johnson and co-workers31 applied this strategy to corrole chemistry, starting from the corresponding thiaphlorin 35, which was prepared in an acid-catalyzed condensation of bispyrrol-2-yl sulfide 34 and dipyrrane 10 (Scheme 10). The ring contraction was found to be most efficient in the presence of triphenylphosphine in o-dichlorobenzene, providing desired corrole 36 in 31% yield.32 The stability of the thiaphlorin intermediate was crucial in this transformation. Thus, contraction of thiaphlorin 35 led to the expected product with a 2-fold improved yield (Scheme 10). Additional experiments aiming to elucidate the reaction mechanism were performed. The external addition of radical scavengers, such as tert-butylcatechol or hydroquinone, did not affect the reaction outcome, indicating that the reaction proceeds via a concerted rather than a radical-type mechanism. Unfortunately, even though the described method affords the expected products in reasonable yields, it suffers from numerous drawbacks. The poor stability of the sulfur-based starting material was the main shortcoming which limited the interest in this methodology.

3. MESO-SUBSTITUTED A3-CORROLES 3.1. General Considerations

Although meso-substituted corroles can be synthesized using various strategies, one approach prevails in the literature. It relies on the formation of bilane (tetrapyrrane) and its macrocyclization via oxidation. Tetrapyrranes can be synthesized in at least three different ways: directly from pyrrole and aldehydes; from dipyrranes (dipyrromethanes) and aldehydes; and from dipyrrane−diols and pyrrole (Scheme 11). In analogy to the broadly accepted nomenclature for mesosubstituted porphyrins, meso-substituted corroles can be divided Scheme 11. Retrosynthetic Analysis of Strategies Leading to meso-Substituted Corroles via Bilanes as Intermediates

F


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Figure 2. Types of meso-substituted corroles.

Scheme 12. Reaction of Pyrrole with Aldehydes as a Cascade of Electrophilic Aromatic Substitutions

Scheme 13. One-Pot Synthesis of 5,10,15Tris(pentafluorophenyl)corrole (39) from Pyrrole (37) and Pentafluorobenzaldehyde (38)

Scheme 14. Synthesis of A3-Corroles from Pyrrole (37) and Aromatic Aldehydes

into four distinct groups (Figure 2). Corroles bearing three identical substituents at all three meso positions are called A3corroles. A3-Corroles can, in principle, be synthesized by any of the methods shown on Scheme 11. In practice, however, only direct condensation of pyrrole with aldehydes has been utilized. Consequently, we shall exclusively focus on this strategy in this section. The condensation of pyrrole with aldehydes is the simplest strategy with respect to the commercial availability of substrates. Still, this one-pot synthesis of corroles is actually a very complex process in which seven new C−C bonds are formed and initially formed bilane is oxidized with concomitant macrocyclization leading to corrole. A few points require special attention here.

(A) The first step in the synthesis of meso-substituted corroles differs fundamentally from the synthesis of mesosubstituted porphyrins. In porphyrins, a macrocyclic precursor (i.e., porphyrinogen) is formed, while in the G


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Table 1. Comparison of Optimized Reaction Conditions for the Synthesis of meso-Substituted A3-Corroles from Various Aldehydes under Neat Conditions

a

entry

experimental factor

highly reactive aldehydes

sterically hindered aldehydes

moderately reactive aldehydes

1 2 3 4 5

TFA (mmol)/aldehyde (mmol) pyrrole (mmol)/aldehyde (mmol) time of acid-catalyzed step CH2Cl2 per 1 mmol of aldehyde DDQ (mmol)/aldehyde (mmol)

0.012 1.5 10 min 10 mL 1.2

0.21 5 16 h a 1

0.023 3 1h a 1

The reaction mixture in CH2Cl2 and the solution of DDQ in CH2Cl2 were added simultaneously to CH2Cl2 during 10 min.

Table 2. Comparison of Yields (%) of Different A3-Corroles Obtained via Direct Condensation of Pyrrole with Aldehydes under Various Conditionsa reaction conditions and reference

a

aromatic aldehyde

Al2O33,48

AcOH52

C6H5CHO 4-CH3C6H4CHO 4-MeOC6H4CHO 2-NO2C6H4CHO 2-ClC6H4CHO 3-BrC6H4CHO 3-NO2C6H4CHO 3-CNC6H4CHO 4-BrC6H4CHO 4-FC6H4CHO 4-CNC6H4CHO 4-NO2C6H4CHO 4-CF3C6H4CHO 4-(CO2Me)C6H4CHO 2,6-F2C6H4CHO 2,6-(MeO)2C6H4CHO 2,6-Cl2C6H3CHO 2,4,6-Me3C6H2CHO C6F5CHO 4-N3C6F5CHO 3,5-(CF3)2C6H4CHO pyridine-4-carboxaldehyde

6−7 6−7 2−5

6 6 7 8 9 9 15

neat45

TFA, CH2Cl247

MeOH/H2O/HCl42

21

27 25 22

9 15

14 15

5 13 19 19 14 8 13b 9 7 21 19 17

22 7−10 6 1 6−10

0 0 0 4

19

14

23 22 25 21 10 16 17 0 8 0

9

Isolated yields. bHighly modified conditions were used.65

case of corroles, the intermediate bilane is nonmacrocyclic and its formation is not dependent on the concentration of reagents. The reaction between pyrrole and aldehydes is a cascade of electrophilic aromatic substitutions.33 Since the electron density of dipyrrane (the first isolable product) is higher than that of pyrrole itself, the next molecule of aldehyde reacts preferentially with dipyrrane than with pyrrole (Scheme 12). That reaction sequence inevitably leads to many linear (and macrocyclic) products. By using a large excess of pyrrole34−36 or by performing the reaction in water,37 this process can be shifted toward the formation of dipyrranes. The same trick, however, cannot be easily adapted to optimize the yields of the next oligocondensates, i.e., tripyrrane and tetrapyrrane (bilane). (B) Even though the synthesis of meso-substituted corroles has been investigated so many times, we have no proof of the actual order of steps during the oxidation−macrocyclization process. The more probable option is that bilane is oxidized first to bilene and then to biladiene followed by intramolecular oxidative aromatic coupling.38 The second possibility involving intramolecular oxidative coupling and subsequent oxidation of the macrocyclic ring with removal

of four electrons and four protons cannot be excluded since there is no definitive evidence, and intermolecular oxidative coupling is known for pyrroles. Whereas for simple pyrrole derivatives only one set of reagents (i.e., bis[(trifluoroacetoxy)iodo]benzene/trimethylsilyl bromide) proved to work efficiently,39 many other oxidants have been successfully used in more sophisticated examples. Iron(III) chloride has been used by Sessler and co-workers to oxidize 2,2′-bipyrrole into cyclo[8]pyrroles,40 while electron-deficient derivatives of pyrrole have been coupled with the use of FeCl3 and K2Cr2O7.41 (C) Macrocyclization, in general, is not a high-yielding reaction except under conditions of high dilution or if a template effect is used. Still, a confirmed yield for the transformation of bilanes into corroles is in the range 65−85%, although neither the template effect nor the highly dilute conditions are utilized. This phenomenon is not easy to explain. An additional aspect is the dependence of this transformation on concentration. Though a few studies have been performed, only one of them used pure bilane (as a mixture of diastereomers and enantiomers).42 These studies suggest that the degree of a dilution of the reaction mixture makes it possible to increase the yield of corrole H


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Scheme 17. Synthesis of N-21-Methylcorroles via Mixed Condensation of Aromatic Aldehydes with Pyrrole (37) and N-Methylpyrrole (44)

Scheme 15. Synthesis of Rhenium Corrole 41 via Porphyrin Ring Contraction

Scheme 16. Synthesis of π-Extended Corrole 43 via Ring Contraction of Ni(II)−Porphyrin 42 aliphatic aldehyde (i.e., CnH2n+1CHO) with pyrrole (or for that matter using any other methodology). This outcome is strongly connected with the inherent electron richness of the corrole core, which is supported by the plausible idea that the oxidation potential of the electron-donating aliphatic substituents is too low to permit the product to survive, i.e., even if it forms during the second step, it immediately decomposes in the presence of an excess of an oxidizing agent present in the reaction mixture. 3.2. Reaction under Neat Conditions

The reactivity of the carbonyl group in processes with nucleophiles depends on its partially positive charge on its carbon atom, which in turn depends on the substituents directly attached to it. The pentafluorophenyl (C6F5) group is a strong electron-withdrawing substituent, and the presence of this functionality accounts for why pentafluorobenzaldehyde (38) was the first compound to be used in the reaction with pyrrole (37) under neat conditions (Scheme 13).43 Its reaction with pyrrole (37) is strongly exothermic without the addition of any acid (although traces of pentafluorobenzoic acid may be present in the commercially available aldehyde 38), and the yield of 5,10,15-tris(pentafluorophenyl)corrole (39) depends not only on the reaction’s scale but also on the batch. This is easily understandable, based on the considerations presented in section 2.1. Adding 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to a mixture of oligocondensates, obtained on the small scale, gave corrole 39 in 13% yield (Scheme 13). The same procedure was subsequently applied to haptafluorobutyraldehyde hydrate to obtain the corresponding A3corrole in 1% yield.44 The methodology introduced by Gross has been subsequently explored by Gryko’s group (Scheme 14).45 Two key starting rationales for reinvestigation of this method were the following: (1) most of aldehydes were not reactive enough to react with pyrrole without an acid; (2) there were significant differences in reactivity of various aromatic aldehydes, and one could not expect the same conditions to work equally

even 2-fold. Details will be presented in the section concerning trans-A2B-corroles.42 (D) The formation of corroles from only aromatic aldehydes will be presented, simply because no one has successfully prepared the A3-corrole via condensation of a typical I


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Table 3. Guidelines for the Synthesis of A3-Corroles type of aromatic aldehyde

reaction conditions for the first step

ref

electron neutral, small electron deficient (except C6F5CHO and 4-NO2C6H4CHO), small C6F5CHO 4-NO2C6H4CHO sterically hindered aldehydes with big molecular mass, hydrophobic aldehydes with big molecular mass, hydrophilic

MeOH/H2O/HCl MeOH/H2O/HCl TFA, CH2Cl2, 15 °C HOAc TFA, no solvent TFA, CH2Cl2 MeOH/H2O/HCl

42 42 57 52 45, 47 47 42

consistency of the reaction mixture combined with difficulties in removal of propionic acid discouraged chemists from further exploration. As a consequence, it was only in 1999 when Paolesse and co-workers decided to change the ratio of pyrrole to an aldehyde from 1:1 to 3:1.4,52 They obtained porphyrin accompanied by a second macrocyclic product which turned out to be 5,10,15-triphenylcorrole. Chromatography allowed them to separate these two macrocycles, and A3-corrole was isolated in 9% yield. It was quickly found that these are general conditions, and numerous aldehydes reacted with pyrrole to give A3-corroles (Scheme 14). The highest yield was obtained for 4nitrobenzaldehyde (22%) (Table 2). The latter corrole (i.e., 5,10,15-tris(4-nitrophenyl)corrole) has been synthesized using this methodology by numerous researchers over the last 15 years.53,54 It is noteworthy to add that in 1994 Loim and coworkers prepared 5,10,15-tris(cymantrenil)corrole in 4% yield by heating Cp(CHO)Mn(CO)3 with pyrrole in AcOH (A4porphyrin was the main product).55

efficiently for all of them (efficiency here is understood as maximization of bilanes yield). Consequently, aldehydes were divided into three groups: (1) highly reactive (i.e., possessing electron-withdrawing groups, typically at the para position); (2) moderately reactive; and (3) sterically hindered aldehydes. Thorough examination of various reaction parameters (catalyst, solvent, concentration, time) eventually led to the development of three sets of conditions (Table 1). In 2000 Lee and co-workers reported two examples of condensation of aromatic aldehydes with pyrrole in neat conditions (benzaldehyde and methyl 4formylbenzoate), and in this case the corresponding bilanes were purified (technically speaking, this was the first case when isolation of pure bilanes was reported in the literature).46 Their oxidation reaction to afford the corresponding A3-corroles proceeded in higher yields in MeCN and EtCN than in CH2Cl2, which, from a time perspective, could be rationalized by minimization of isocorroles formation (see section 9). Identification of key factors influencing the formation of bilanes and their conversion into corroles provided A3-corroles in reasonable yields (i.e., at the date of publication (2003), these were the highest reported yields in most cases). As an average, the yields were ca. 17% for highly reactive aldehydes, ca. 13% for moderately reactive aldehydes, and 8% for sterically hindered aldehydes. Specific yields are presented in Table 2. At the same time, Paolesse and co-workers proposed more universal conditions, with a pyrrole:aldehyde ratio of 10:1, furnishing corroles in particularly good yields (10−15%) for electronneutral aldehydes.47

3.5. Synthesis in Organic Solvents

Capricious yields achieved when attempting to increase the scale of the synthesis of 5,10,15-tris(pentafluorophenyl)corrole (39) under neat conditions inspired Gross to perform the first step on a solid support, with Al2O3 giving the best results. Heating of a mixture of pentafluorobenzaldehyde (38) with pyrrole (37) on Al2O3 for 4 h, followed by suspending it in CH2Cl2 and adding DDQ, gave corrole 39 in 11% yield. The same method was applied to 2,6-dichlorobenzaldehyde and 2,6-difluorobenzaldehyde (Scheme 14, Table 2). Subsequently, it was also found that other aromatic aldehydes, including, for example, 4-methoxybenzaldehyde, reacted equally well under these conditions (Table 2).48 Yet another modification was described by Collman and Decréau. Using microwaves for heating of the reaction mixture, eight A3-corroles were obtained (mostly from fluorinated benzaldehydes), but yield improvements achieved by microwaves vs classical heating was rather small.49 An average yield of A3-corroles under these conditions was 12%.

Not all aldehydes can be mixed with or dissolved in pyrrole (present in a reasonable molar excess). This is especially true for planar aromatic aldehydes possessing a large molecular mass. The most lipophilic of them cannot be solubilized in boiling AcOH. Thus, investigators turned to the method in which the first step is performed in methylene chloride. Paolesse and coworkers found that in such cases a 10-fold molar excess of pyrrole must be present in the reaction mixture to ensure the highest yields of corroles. This method gave high yields for four exemplary A3-corroles (Scheme 14, Table 2). It is noteworthy to add that one of the pre-1999 serendipitous syntheses of A3corroles was also performed in CH2Cl2 with TFA as a catalyst.56 Very recently, the reaction of aldehyde 38 with pyrrole (37) was investigated again by the joint efforts of Virgil, Grubbs, and Gray.57 These authors reanalyzed both steps (i.e., the cascade of electrophilic aromatic substitution and macrocyclization). First, they purified tetrapyrrane and under optimized conditions were able to transform it into corrole 39 in 84% yield. Moreover, they found that (a) the three 2e− oxidations proceed as a cascade to corrole (i.e., an intermediate species (presumably bilene, biladiene) do not accumulate) and (b) evaporation of pyrrole before adding DDQ reduced the formation of polymeric byproducts and simplified purification. Optimizing a ratio of reagents and performing both steps at lower temperatures (15 and 5°C, respectively) allowed them to increase the yield of 5,10,15-tris(pentafluorophenyl)corrole (39) to 17% on the impressive scale of 4.6 g.

3.4. Synthesis in Acetic Acid

3.6. Synthesis in Water−Methanol−HCl System

Adler and Longo introduced the synthesis of tetraaryl-porphyrins in acetic acid in 1964,50 and for a long time the optimized procedure in propionic acid51 was exclusively used for the preparation of these valuable compounds. Possibly the

The concomitant formation of A4-porphyrins was the nuisance in the A3-corrole synthesis. This side product was hard to eliminate given that porphyrins form from pyrrole and aromatic aldehydes even in the gas phase.58 In 2006, however, inspired by Král’s work

3.3. Synthesis Using Al2O3 as a Solid Support

J


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on dipyrranes,59 Gryko and Koszarna proposed an elegant solution to this problem.42 The first step of the reaction was performed in a mixture of water and methanol, and the reaction was catalyzed by HCl. A proportion of both solvents was optimized so that both dipyrrane and tripyrrane remained in the solution, but more lipophilic tetrapyrrane precipitated from the reaction mixture. The selectivity of this precipitation was not ideal, but it led to an incomparable jump in yields of A3-corroles to reach, in some cases, almost 30% (Scheme 14, Table 2). No less important was the fact that the amount of porphyrin which had to be removed decreased to as little as ∼1%.

following guidelines in the form of a table (Table 3) to assist with choosing the best method. Although the vast majority of A3-corroles were synthesized from commercially available substituted benzaldehydes, there are some exceptions. One of the most complex A3-corroles was prepared by Vicente and co-workers. Condensation of dicarboranylaldehyde possessing two boron cages with pyrrole under Paolesse’s conditions gave the corresponding corrole (containing 60 atoms of boron) in 30% yield.66

3.7. Synthesis via Porphyrin Ring Contraction

4.1. General Considerations

A peculiar example of A3-corrole synthesis via porphyrin ring contraction was reported by Chan and co-workers.60 Metalation of porphyrin 40 with [Re2(CO)10] in refluxing benzonitrile and subsequent cooling in air afforded the corresponding oxorhenium(V) 5,10,15-tris(trifluoromethyl)corrolate (41) in 9% yield (Scheme 15). Despite the novelty of this approach, its applications are limited due to the utilization of the expensive rhenium complex and low overall reaction yield. During the studies of Friedel−Crafts acylation of porphyrins, Callot’s research group found many novel porphyrinoids. Part of this broad investigation, initially published in 2004, revealed that subjecting Ni(II) complexes of A4-porphyrins to benzoic anhydride, first in the presence of a Lewis acid and subsequently after suitable workup, in the presence of DMAP, resulted in the ring contraction leading to π-extended corrole 43 in 15% yield (Scheme 16).61 This compound, although perfectly planar and very stable, is nonaromatic. Since analogous transformation did not occur for copper(II) complex, it was hypothesized that ring contraction is triggered by the fact that small nickel cation fits much better in the corrole cavity. The mechanism of this tandem process is rather complex, and although initial steps are clear, the following part is rather puzzling.62

While aforementioned A3-corroles are sufficient for coordination studies as well as for other investigations such as further

4. MESO-SUBSTITUTED TRANS-A2B-CORROLES

Scheme 18. Scrambling−Acidolytic Breaking of C−C Bonds between Pyrrole Units and Benzylic Carbon Atoms Leading to Rearrangement of Tetrapyrranes

3.8. N-Alkyl Corroles

In 2003 Gryko and Koszarna discovered that reaction of aromatic aldehydes with a mixture of pyrrole (37) and N-methylpyrrole (44) afforded a mixture of A3-corrole and N-21-methylcorrole (Scheme 17).63 Only three examples were shown, and the yields were rather low (3−6%). This mixed condensation was performed both in neat conditions (TFA as catalyst) as well as under HCl/MeOH/H2O conditions. The most intriguing thing was the selectivitythe regioisomeric N-22-methylcorrole was not formed at all. Apparently, if the N-methylpyrrole moiety is located in the middle of a linear tetrapyrrane, the resulting steric hindrance hampers ring closure into the corrole. The analogous process was observed for mixed condensations with Nbenzylpyrrole but not with N-phenylpyrrole.63 It is worth mentioning that the same compounds can be prepared via Nalkylation of corroles.64

functionalization at the β-positions, etc., other inquiries require corroles possessing one specific substituent at a distinct site at the perimeter of the macrocycle. In agreement with the nomenclature typically used for porphyrins, corroles with two identical substituents at positions 5 and 15, and a different substituent at position 10 are called trans-A2B-corroles. Although both cis-A2Bcorroles and trans-A2B-corroles are equally possible, the trans variants are considerably more popular because of synthetic accessibility. No one has attempted to synthesize trans-A2Bcorroles directly by reacting a mixture of two aldehydes with pyrrole. The reduced symmetry of the corrole core (C2v) and hence the inevitable formation of two regioisomers would make the products hard (or impossible) to separate. The syntheses of trans-A2B-corroles borrow the key idea from Lindsey’s work on trans-A2B2-porphyrins from dipyrranes and aldehydes.34,67,68 In dealing with the synthesis of these compounds, the term “scrambling” has been introduced. Scrambling is the undesired rearrangement of substituted dipyrranes or already formed bilanes (or porphyrinogens) leading to a mixture of products bearing different types and/or patterns of substituents at the perimeter of the macrocycle. Scrambling is more pronounced if there is no steric hindrance at the benzylic carbon atom linking pyrrole units (Scheme 18). A comprehensive study by Lindsey has shown that whereas for sterically hindered dipyrranes

3.9. Guidelines for the Preparation of A3-Corroles

Condensation of pyrrole with aldehydes leading to mesosubstituted A3-corroles has attracted the attention of many researchers. As a consequence, various conditions coexist in the literature, and it is valuable to compare yields for the most important A3-corroles in the form of a table (Table 2). A significant effort has been devoted to optimization of a onepot synthesis of A3-corroles from pyrrole (37) and aldehydes. Practitioners in the field can feel a bit disoriented, and newcomers can be even lost. Therefore, we compiled the K


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Scheme 19. Synthesis of trans-A2B-Corroles from Dipyrranes and Aromatic Aldehydes Using Different Conditions

Scheme 20. Synthesis of Corrole 47 from Dipyrrane 45 and Pyridine-4-carboxaldehyde (46)

(dipyrromethanes) it is possible to find reasonable conditions for reactions leading to trans-A2B2-porphyrins without scrambling, the same is very hard to achieve for unhindered dipyrranes.67

liquid aldehydes as products of autoxidation, were, in fact, the catalysts of the reaction. Brückner and co-workers found that if unhindered dipyrranes are reacted with aromatic aldehydes (ratio 6:1) in the presence of TFA, after adding DDQ, trans-A2Bcorroles are formed in 20−40% (Scheme 19).70 This method suffers from a large excess of dipyrrane, which must be synthesized. Dehaen and co-workers focused their attention on sterically hindered 5-(2,6-dichlorophenyl)dipyrrane.71 Its reac-

4.2. Synthesis from Aldehydes and Dipyrranes

About the same time, i.e., between July 2000 and June 2001, a few methods for the synthesis of trans-A2B-corroles from dipyrranes and aldehydes were published. Historically, the first was the synthesis without an acidic catalyst proposed by Gryko (Scheme 19),69 although this method gave low yields (4−19%). Later, it was discovered that benzoic acids, present in varying amounts in

Table 4. Guidelines for the Synthesis of trans-A2B-Corroles from Various Types of Aldehydes and Dipyrranes dipyrrane type

aldehyde type

hindered

aromatic, lipophilic

unhindered electron withdrawing

aromatic, lipophilic aromatic

hindered unhindered

aromatic, small or hydrophilic aromatic, small or hydrophilic

hindered hindered unhindered hindered

dialdehydes bearing basic nitrogen atom bearing basic nitrogen atom bearing basic nitrogen atom, hindered electron withdrawing

hindered

unhindered, bearing nitrogen atom hindered, bearing basic nitrogen atom

ald. (mM)

time of acid-catalyzed step

acid (mM)

yields (%)

ref

3−30 23−32 6−10 9−21 20−53 27−31 13−56

72 45 72 76 79 42 42

TFA/6.3 TFA/50 TFA/17 BF3·Et2O/0.12

DDQ DDQ DDQ DDQ DDQ p-chloranil p-chloranil or DDQ DDQ DDQ DDQ p-chloranil

12−19 3−28 4−9 19

87 80 80 82

1h 24 he

BF3·Et2O/1.6

p-chloranil

22

37

17

1h

TFA/67

p-chloranil DDQ

17 2−11

80

1.52f

24 h

BF3·Et2O/0.07

p-chloranil

9−28

83

17 67 67 17 15 3.3 5

5h 10 min 5h 20 min 5h 2h 1h

TFA/1.3 TFA/13 TFA/0.26 TFA/13 BF3·Et2O/30 HCl/200 HCl/300

4.4a 17 17 2.84b

4h 1h 5h 1h

4.3c 11.1d

aromatic aromatic

oxidant

a

Dipyrrane:ald = 7:1. bDipyrrane:ald = 1:1. cDipyrrane:ald = 5:1. dDipyrrane:ald = 3.2:1. eFirst step was performed at 0°C. fA solution of aldehyde and BF3·Et2O in CH2Cl2 was added dropwise over 6−10 h to a solution of dipyrrane. L


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Scheme 21. Synthesis of trans-A2B-Corroles from 5(Carboxyethyl)dipyrrane (48)

Scheme 23. Synthesis of Hangman Corrole 54

Scheme 22. Synthesis of Bis(corrole) 50

second important factor was the solubility of the products. Although the solubility of meso-substituted corroles is generally higher than corresponding meso-substituted porphyrins, in accordance to what was found for porphyrins the difference between the solubility of 5,15-dimesityl-10-arylcorroles and 5,15diphenyl-10-arylcorroles is huge. While the 5,15-dimesityl analogs were often soluble in hexane, the 5,15-diphenyl variants were only soluble in sufficient quantity to perform NMR in THFd8. Still, the nonscrambling conditions developed for unhindered dipyrranes were used occasionally, as, for example, in the synthesis of 5,10,15-tri(3-thienyl)corrole.73 Later, it was found that an increase in the concentration of TFA to 13 mM did not cause scrambling for the sterically hindered dipyrranes. The advantage of this procedure lies not in the yields of corroles (they are only slightly higher) but in the considerable reduction of the reaction time required for the first step (from 5 h to 10 min, see Table 4).45 Higher concentrations of both reagents were also applied in this case. This brings us to the interesting phenomenon: sterically hindered dipyrranes react with aromatic aldehydes (present in ratio 1:1) in CH2Cl2 in the presence of TFA giving either exclusively trans-A2B2-porphyrins or almost exclusively trans-A2B-corroles depending on the concentration of reagents and acid. While a high concentration of TFA and a low concentration of reagents favors formation of porphyrin,67 the use of a high concentration of substrates and low concentration of TFA is essential to maximize the yield of corrole.45,72 It should be pointed out that in analogy to the condensation of pyrrole with aldehydes, the condensation of dipyrranes with aldehydes is also an unstoppable cascade. Higher

tion with reactive aldehydes catalyzed by BF3·OEt2 and followed by purification of bilanes and oxidation with DDQ gave the corresponding trans-A2B-corroles in good yields (22%). However, all attempts to apply these conditions to other substrates such as mesityldipyrrane, benzaldehyde, or 4-nitrobenzaldehyde failed. The most universal set of conditions was published by Gryko and Jadach, who found that reaction with sterically hindered dipyrranes can be performed without scrambling at much higher acid concentrations ([TFA] = 1.3 mM) than the reaction with unhindered dipyrranes ([TFA] = 0.26 mM).72 Significantly lower acid concentrations have their dark side; however, the yield of trans-A2B-corroles was only 5−7%, while the yield for analogous condensations from sterically hindered dipyrranes reached 30%. Still, the method optimized for unhindered dipyrranes remained the most universal procedure for a long time, while conditions optimized for sterically hindered dipyrranes were more often used by other research groups because of two important factors. The improved 5-fold higher yields of trans-A2B-corroles were a strong motivation, but a M


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Scheme 24. Improved Methodology for “Face-to-Face” Porphyrin−Corrole Systems

Scheme 25. Synthesis of Meso-Substituted Corroles from 2,2′Bipyrrole and Dipyrrane−Diols

Scheme 26. Synthesis of trans-A2B-Corroles from Ethyl Oxalyl Chloride

oligocondensates form as well, such as oxidation of the product of [3 + 2] condensation that leads to phlorin, which often accompanies trans-A2B-corroles.74 Many trans-A2B-corroles were prepared using this methodology. Perhaps the most complex example is Bröring’s bis-picketfence corrole synthesized from 5-(2,6-dibromophenyl)dipyrrane.75 The recognition of the importance of electron-withdrawing groups to the stability of corroles (see section 12) provided motivation to further optimize the reaction conditions for such dipyrranes as 5-(pentafluorophenyl)dipyrrane and 5-(4cyanophenyl)dipyrrane. It was possible to increase the concentration of TFA from 0.26 to 13 mM without inducing scrambling.76 At the same time, the conditions with a higher acid concentration improved the yields of trans-A2B-corroles from 5% to 18−21% (Table 4, Scheme 19). These results corroborate earlier observations showing that in the synthesis of trans-A2B2porphyrins from dipyrranes bearing an electron-withdrawing group at position 5 the reaction can be performed at higher concentrations of TFA without scrambling.77,78 The stabilizing effect of two C6F5 groups allowed, for the first time, the synthesis of corrole bearing a simple alkyl group (rather than perfluorinated alkyl group)44 at the meso-10 position, namely, 5,15-bis(pentafluorophenyl)-10-ethylcorrole, in 3% yield. Last but not least, according to this work, the 100-fold dilution of the reaction mixture prior to adding DDQ increases the yield of 5,15bis(pentafluorophenyl)-10-(4-cyanophenyl)corrole from 14% to 21%.

The breakthrough in the synthesis of trans-A2B-corroles came in 2006 when Gryko and Koszarna discovered that as long as aldehydes and dipyrranes were relatively small and/or hydrophilic, performing this reaction in a mixture of water and methanol in the presence of HCl increased the yields from 5% to 7% to ∼55% from unhindered dipyrranes (Scheme 19).42 For sterically hindered dipyrranes, reaction conditions must be modified so that the ratio of MeOH/H2O is 2:1, which affords yields in the range 27−31%. The most astonishing aspect of this methodology is that there is no scrambling at all, although the concentration of HCl is 300 mM. The importance of this discovery lies in the fact that with such high yields gram quantities of product became available either via simplified N


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Scheme 27. Synthesis of trans-A2B-Corroles from Dipyrrane− Diols and Pyrrole

substrates, they obtained six trans-A2B-corroles from 5(pentafluorophenyl)dipyrrane in 20−53% yields. A very high yield (53%) was also obtained when butyraldehyde was used as the aldehyde, and a single example of the use of phenyldipyrrane resulted in formation of 5,15-diphenyl-10-(pentafluorophenyl)corrole in 45% yield. Although yields were very high and reuse of ionic liquid was demonstrated, the price of the latter reagent diminished the attractiveness of this methodology. The synthesis of trans-A2B-corroles from substrates possessing basic nitrogen atom(s) is a very special case. The main problem is that both dipyrranes and aldehydes of this type can form salts which may precipitate from the reaction mixture. Another crucial question relates to the concentration of an acid. It is rational to expect that the addition of the amount of acid optimized for more typical aromatic aldehydes can lead to detrimental results since it will be fully neutralized by basic nitrogen atom(s). In other words, there will be no acid to catalyze condensation of dipyrrane with aldehyde. This problem has been comprehensively solved by Gryko and Piechota, who proposed three different sets of conditions for various cases (Scheme 20, Table 4).80 The most important case, i.e., reaction of sterically hindered dipyrranes with aldehydes bearing one basic nitrogen atom, requires 3 equiv of TFA vs aldehyde. In an exemplary synthesis, mesityldipyrrane (45) reacted with pyridine-4-carboxaldehyde (46) to give corrole 47 in 28% yield (Scheme 20). The conditions optimized for this reaction were successfully used for a diverse set of aldehydes that were also tertiary amines.80 They were also applied by Saltsman et al. to the synthesis of trans-A2B-corroles from pyrimidine-2carbaldehyde.81 Furthermore, Dehaen and co-workers studied condensation of sterically hindered dipyrranes with aldehydes bearing basic nitrogen atoms, namely, 4,6-dichloropyrimidine-5-carbaldehyde and its derivatives.82 The optimization of reaction conditions for this aldehyde with mesityldipyrrane (45) led to the conclusion that 0.043 equivs of BF3·OEt2 was optimal, furnishing the corresponding trans-A2B-corrole in 35% yield. At the same time, the authors noticed that applying the same conditions for other 4,6-dichloropyrimidine-5-carbaldehydes bearing an additional substituent at position 4 led to lower yields or required optimization of the amount of BF3·OEt2. An extension of this study aimed at using the pyrimidine moiety as a part of the dipyrrane. Thus, the corresponding dipyrrane from 4,6-dichloro-2-methylsulfanylpyrimidine-5-carbaldehyde was obtained and subjected to reaction with various 4substituted benzaldehydes.83 This reaction proved difficult, and only after devising a procedure in which an excess of dipyrrane was created in situ by slow addition of the aldehyde, activated with BF3·OEt2, into a dilute solution of dipyrrane in CH2Cl2, were the authors able to obtain the model corrole from 4cyanobenzaldehyde in 28% yield. Slightly lower yields were reached with other aldehydes. In both cases, the ratio of an acid to an aldehyde or dipyrrane possessing a basic nitrogen atom was well below 1:1, and condensation still occurred. These results can be rationalized by the specific structure of 4,6-dichloro-2methylsulfanylpyrimidine-5-carbaldehyde - the combined influence of steric hindrance and electron-withdrawing effect of the chlorine atoms markedly decreased the basicity of the nitrogen atoms. Little is still known about corroles possessing nonaromatic substituents at meso positions. As mentioned above, the introduction of CnH2n+1 alkyl chains is very difficult for stability reasons, and so recent attention has turned toward corroles possessing two ester groups at the meso positions 5 and 15.84

chromatography or in some cases via crystallization directly from the reaction mixture. Indeed, exemplary 10-(3-cyanophenyl)5,15-di(4-methylphenyl)corrole was prepared on a 1.2 g scale, and purification involved evaporation of the reaction mixture to dryness and then a first recrystallization from EtOH and then a second recrystallization from THF/cyclohexane. This work also established the important paradigms: (1) p-chloranil gives higher yields of corroles than DDQ in all cases except aldehydes with strong electron-withdrawing groups (where DDQ must be used to obtain corrole); (2) macrocyclization of tetrapyrranes is strongly solvent dependent; (3) hydrazine hydrate can be used to quench reaction after addition of oxidant, thereby decreasing the amount of isocorroles and increasing the yield of corrole; and (4) ∼2.2 mM is an optimal concentration of tetrapyrrane which assures the highest yields of macrocyclization. Jiang and co-workers proposed carrying out the condensation of dipyrranes with aldehydes in ionic liquid ([bmim][BF4]).79 Using BF3·OEt2 as a catalyst and a high concentration of O


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56 was utilized as a starting material to prevent protonation of the free base porphyrin by TFA. This methodology proved to be effective for dipyrranes substituted with electron-withdrawing and electron-donating groups, affording desired dimers in a convenient and efficient way. Since a variety of conditions exist for the preparation of transA2B-corroles from aldehydes and dipyrranes, for the convenience of the reader, they have been collected and compared in Table 4. This table may be used as a guideline for the synthesis of specific types of corroles from various types of aldehydes and dipyrranes. The methodologies presented in this section have proven to be universal. Numerous trans-A2B-corroles from small or polar dipyrranes have been synthesized using the H2O/MeOH/HCl system for studies related to self-assembly,90−92 corroles possessing an additional coordinating site,93 simple donor− acceptor bichromophoric architectures,94 models for photophysical studies,95 corroles for complexation of lanthanides,96 and building blocks for the construction of larger systems.97 On the other hand, for the studies of electron and energy transfer in dyads and triads containing corroles, the classical methodology (TFA, CH2Cl2) proved to be more efficient as corroles used in such studies were typically prepared from large lipophilic aldehydes.98−104 Nonetheless, it must be emphasized that there are limitations to currently available methodologies. If both aldehyde and dipyrrane possess large molecular mass and poor solubility (in typical organic solvents) their condensation fails to give corresponding trans-A2B-corrole.11 This is related to the fact that condensation of dipyrranes with aldehydes leads to bilanes only at relatively high concentrations. There have been also no reports on the synthesis of meso-substituted corroles from arylpropargyl aldehydes, although analogous reactions leading to porphyrins are well known. Such π-extended corroles could not be synthesized using an alternative methodology. Trimethylsilylpropargyl aldehyde reacts with mesityldipyrrane (45) to give the corresponding trans-A2B-corrole, but removal of the protecting group proved impossible.11

Canard, Balaban and co-workers found that among the many published conditions, the highest yields were obtained using a procedure optimized for dipyrranes bearing electron-withdrawing groups (Scheme 21).76 Although yields were low (2− 8%), the importance of this work lies in the following facts: (1) For the first time it was possible to obtain the meso-10-free corrole via direct condensation of a dipyrrane with formaldehyde. All previous attempts gave no corrole at all or formed a mesomeso-linked dimer since the oxidation potential for dimerization was lower than for any of the previous oxidations leading to the corrole core (see section 7). This transformation was possible with CO2Et groups because this substituent has a stronger electron-withdrawing effect than the C6F5 group (this is visible in the first oxidation potential).84 (2) Canard and co-workers obtained corroles possessing a long aliphatic chain at the meso-10 position. Since the electron-donating effect of alkyl substituents located at meso positions is very strong, only the strongest electron-withdrawing groups at positions 5 and 15 are able to keep the first oxidation potential high enough to allow for isolation of such corroles. A surprising effect has been observed for trans-A2B-corroles derived from 5-trifluoromethyldipyrrane. Although the CF3 group is also electron withdrawing, the attempts to condense these compounds with various aldehydes failed to give corroles in isolable yield. Very fast decomposition of the initially formed corrole has been observed.11 The only successful case in this regard is the condensation of 5-trifluoromethyldipyrrane with pentafluorobenzaldehyde (38) to give 5,15-bis(trifluoromethyl)10-pentafluorophenylcorrole in 5% yield.85 Guilard and co-workers86 reported a novel cofacial bis(corroles) synthesis which was based on the acid-mediated condensation72 of the corresponding dipyrrane 45 and dialdehyde 49, followed by the oxidative cyclization of the obtained intermediate. Unfortunately, the desired bis(corrole) 50 was obtained as the minor product, while the corrole 51 was the main one (Scheme 22). Further optimization of this protocol stands on the modification of the ratio of substrates (8-fold excess of dipyrrane) and lowered acid loading (to 1.4 equiv), which resulted in significant improvement of the reaction outcome, i.e., 20% yield of desired product 50.87 Recently, corroles bearing the hangman scaffold drew considerable attention due to their potential application in the activation of small molecules through proton-coupled electron transfer (PCET). Their first efficient synthesis was reported by Dogutan et al. in 2011.88 On the basis of the modified procedure for trans-A2B-corroles synthesis,76 TFA-catalyzed condensation of xanthene derivative 52 with an excess of 5(pentafluorophenyl)dipyrrane (53) followed by oxidative cyclization afforded corrole 54 in moderate yield (Scheme 23). Neither sterically hindered substituents nor electron-withdrawing or -donating groups on the dipyrrane influenced the reaction outcome. Interestingly, the hydrolysis/metalation sequence led to the expected Co-complex 55, which was capable of promoting the oxygen reduction reaction. The necessity to synthesize cofacial corrole−porphyrin dimers induced further modifications of reaction conditions. Thus, when a 3-step protocol ((1) TFA, CH2Cl2; (2) DDQ, CH2Cl2; (3) HCl, CH2Cl2)89 was applied for the reaction of the so-called hangman derivative 56 with the dipyrrane 45, desired dimer 58 was obtained with 11% yield (Scheme 24). That is nearly twice higher than in all previous cases, while the number of synthetic steps was significantly reduced. The corresponding zinc complex

4.3. Synthesis from 2,2′-Bipyrrole and Dipyrrane−Diols

Collman and Decréau were the first to attempt the synthesis of corroles from 2,2′-bipyrrole (59) (Scheme 25).105 Direct condensation of heterocycle 59 with dipyrrane−diols proved to be a very difficult reaction to drive toward the formation of a corrole. After long efforts at optimization, it was found that BF3· OEt2 as a catalyst in MeCN with the condensation step taking 24 h, in the presence of NH4Cl, gave the best yields of trans-A2Bcorrole 62 (8%). This reaction afforded corroles only if a large amount of a catalyst was used and long reaction times were applied. Nevertheless, it is worth emphasizing that trimesitylcorrole was obtained in 12% yieldthe highest reported yield to date. Shortly afterward, Geier and Grindrod investigated the same reaction in a more systematic manner.106 After changing a solvent, an acid catalyst, acid quantity, an oxidant, oxidant quantity, and the reaction time, they eventually found that only in THF the TFA-catalyzed reaction gives isolable yield (10%) of triphenylcorrole 63 (Scheme 25). All other conditions led to the formation of [34]octaphyrin(1.1.1.0.1.1.1.0) as a main macrocyclic product. 4.4. Synthesis from Alkyl Oxalyl Chlorides and Dipyrranes

In principle, it should be possible to perform the reaction of reactive acid chlorides with dipyrranes leading to 1-acyldipyrrane with a release of HCl. The HCl may catalyze the subsequent reaction of a ketone with the second molecule of dipyrrane P


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Scheme 28. Synthesis of cis-A2B-Corroles

leading to bilene (i.e., analog of bilane possessing one additional unsaturated double bond), which in turn can undergo macrocyclization to corrole under the influence of DDQ. The studies performed by Gryko and co-workers showed that only one type of acid chlorides reacts in this way, the alkyl oxalyl chlorides (Scheme 26).107 trans-A2B-Corroles were obtained in reasonable yields without scrambling. The same strategy failed for benzoyl chloride and its derivatives. 4.5. Synthesis from Dipyrrane−Diols and Pyrrole

Lindsey and co-workers reported that diols prepared by the reduction of diacyl−dipyrranes react with dipyrranes to form porphyrins with various patterns of substituents.108 The reaction of such dialcohols with pyrrole should lead to bilane, which without purification can be oxidized to the corresponding mesosubstituted corrole. This strategy toward trans-A2B-corroles was first realized by joint efforts of the Gryko and Guilard groups (Scheme 27).109,110 The conditions for bis-acylation of dipyranes were taken from Lindsey’s papers about porphyrins. Dipyrranes were reacted with EtMgBr, and the formed dianions were acylated with acid chlorides to give diketones in moderate yields. Subsequent reduction to dialcohols was followed by reaction with pyrrole (37) performed in neat conditions. Evaporation of pyrrole was followed by oxidation with DDQ. One of the problems with this strategy is the availability of the required diketones. Their preparation via Grignard reagents requires rather tedious purification;111 therefore, an alternative method for the synthesis of 1,9-diacyldipyrranes was developed (Scheme 27). This procedure gives slightly higher yields, but purification and scale-up are much easier. The diacyldipyrranes formation involves the acylation of dipyrranes with salts made in situ from POCl3 and tertiary amides.110 This modified Vilsmeier approach gives no concomitant formation of monoacyldipyrranes as compared with “the Grignard route”. Moreover, compounds possessing groups that were previously inaccessible (CN, NO2, etc.) can be synthesized. During optimization of the transformation of diacyldipyrranes into meso-substituted corroles, it was found that if the macrocyclization reaction mediated by DDQ was performed in the presence of a large excess of pyrrole, meso-substituted [22]pentaphyrins(1.1.1.0.0) could be obtained in a moderate yield. Independently, the same reaction was investigated by Geier and co-workers.112 More extensive examinations of various reaction parameters such as type and concentration of an acid, ratio of diol to pyrrole, etc., allowed them to propose the following optimized conditions: diol/pyrrole = 1:100, Dy(OTf)3 (or Yb(OTf)3), CH2Cl2, p-chloranil. Under these conditions four different corroles were prepared in yields ranging from 24% to 80% (Scheme 27).

Scheme 29. Synthesis of cis-A2-Corrole 73

An interesting extension of this method is the synthesis of the meso-15-free corrole. Ooi et al.114 reacted 5(pentafluorophenyl)dipyrrane-1-carbinol 67 and unsubstituted dipyrrane 68 under conditions developed for the preparation of the cis-A2B-corroles.113 Despite all efforts, the expected 5,10bis(pentafluorophenyl)corrole (70) was obtained in a very low yield (Scheme 28). The first cis-A2-corrole was however prepared via [3 + 1] condensation of tripyrrane 71 with pyrrole-2-carboxaldehyde (72) by Chandrashekar and co-workers in 2002 (Scheme 29).115 The reactions conditions were refined for 22-oxacorrole synthesis (see section 10.2.2). The yield of corrole 73 was only 2%.

5. CIS-A2B-CORROLES A facile protocol for cis-A2B-type corrole synthesis was reported by Osuka and co-workers.113 The procedure utilized an acidcatalyzed [2 + 2] condensation of the chosen carbinol 67 and dipyrrane 45, subsequent isolation of bilane intermediate, and its oxidation to corrole 69 with DDQ (Scheme 28). Notably, without prior purification of the bilane, the reaction yield was greatly diminished. This procedure was tested for a wide range of dipyrranes. Those bearing either electronwithdrawing or electron-donating substituents as well as sterically hindered compounds afforded the corresponding corroles in up to 12% yield.

6. ABC-CORROLES ABC-Corroles are the most difficult to access. Paolesse et al. synthesized one corrole of this type via mixed condensation of two different dipyrranes with one aldehyde in 10% yield.116 Such statistical condensation obviously led to formation of three different corroles and required a rather lengthy separation. Q


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7. CONJUGATED CORROLE DIMERS Given that π-extension of the porphyrin chromophore was always a major goal in porphyrin chemistry,118,119 it is not that surprising that the same aim was pursued in the corrole community. The pursuit of novel molecular materials with unique photophysical properties led to the directly linked corrole dimers. First observation of covalently bond dimer of corrole was prepared by Gross and co-workers in 2001. They found that under specific reaction conditions cobalt insertion occurred with the concomitant formation of 3,3′-linked dimer 82 (Scheme 31).120 A similar reaction has been later observed for copper insertion.121 These are elaborated examples of intermolecular oxidative aromatic coupling,38 which occur easily since corroles are electron rich. In both cases, the combination of a metal cation with an additional ligand plays a role of oxidant. The same process can be mediated by electron-deficient derivatives of 1,4benzoquinone.122 Osuka and co-workers found that refluxing of corrole 39 with p-chloranil gave both 3,3′-linked dimer 83 and corresponding corrole trimer 84 in good yields (Scheme 31). Subjecting dimer 83 to 2,3,5,6-tetrafluoro-1,4-benzoquinone induces the next intermolecukar oxidative aromatic couling reactions leading to both tetramer and hexamer, all containing exclusively 3,3′-linkages.122 Even simple heating of 5,10,15tris(pentafluorophenyl)corrole (39) in high-boiling solvent (1,2,4-trichlorobenzene at 215 °C) in the presence of oxygen leads to intermolecular oxidative aromatic coupling, although there is no regioselectivity in this case.123 Both 3,3′-linked dimer 83 and an analogous 2′,3-linked dimer were detected in this reaction, accompanied by planar 2,2′,18,18′-linked corrole dimer 81, which plausibly originates from 2,2′-linked dimer (Scheme 31). The first successful attempt to synthesize doubly linked (i.e., conjugated) corrole dimer 81 was reported by Osuka and coworkers.124 Palladium-catalyzed oxidative homocoupling of 2borylcorrole 78 in the presence of chloroacetone as an oxidant afforded 2,2′-linked corrole dimer 79 which, in the presence of DDQ, was converted into the planar 2,2′,18,18′-linked corrole dimer 80 possessing the formal planar cyclooctatetraene core. Finally, corrole 80 was reduced by sodium borohydride, affording desired aromatic corrole dimer 81 (Scheme 31). Interestingly, dimer 80 is characterized by higher stability in comparison to its reduced form 81. Moreover, when compound 80 was reacted with zinc acetate, the stable bimetallic complex was obtained. This complex exhibited unique biradical character, which was confirmed by ESR data, temperature-dependent magnetic susceptibility, as well as cyclic voltammetry, making it the first reported stable biradicaloid, based on porphyrinoids.124 The library of conjugated corroles dimers was expanded by Ooi et al.,125 who reported a new synthetic approach to provide nonaromatic 3,3′,5,5′-linked dimers in up to 3% yield. This strategy involved acid-catalyzed condensation of 1,1,2,2tetrapyrroloethane (85) and dipyrrane-1-carbinol 67, followed by the stepwise oxidation of bilane intermediate with DDQ and reduction of the obtained corrole 87 with NaBH4 to macrocycle 88 (Scheme 32). It is noteworthy that in order to obtain doubly linked species oxidation of a singly linked dimer should be performed under high-dilution conditions. In analogy to the previous case,124 corrole dimers 87 and 88 exhibited different stability, which was in favor of compound 87. Conversion to the more stable compound 87 can be easily achieved in the presence of air, which reflects the low oxidation potential of 88.

Scheme 30. Exemplary Synthesis of ABC-Corrole

Another option, proposed by Guilard, Gryko, and co-workers, relies on the regioselective acylation of dipyrrane 53 with thioester 74 furnishing monoketone 75 (Scheme 30).109 The subsequent second acylation gives rise to bisketone 76, which was transformed into corrole 77 using standard steps. The overall yield of this procedure starting from dipyrrane 53 was 21%. In the subsequent variation of this method, the second acylation of monoacylated dipyrrane was successfully performed under Vilsmeier conditions.110 This last strategy was successfully used by Churchill and co-workers in the synthesis of another ABCcorrole, namely, 5-(benzothien-2-yl)-15-(4-bromophenyl)-10(pentafluorophenyl)-corrole.117 In this case, however, the free base corrole purification was unsuccessful. Geier and co-workers also prepared exemplary ABC-corrole following diacyl−dipyrrane route and modifying the condition of the final step, i.e., condensation of dipyrromethanedicarbinols with pyrrole. The use of milder acids and/or lower concentration of TFA (see section 4.5) secured a higher yield of corrole (59%).112 R


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Scheme 31. Synthesis of Corrole Dimers 79−84

Inspired by Osuka’a tapes,126 Gryko and Koszarna attempted the synthesis of a corrole dimer triply linked at positions 8, 10, and 12. More elaborate methods for the synthesis of 10,10′linked corrole 89 failed, and this compound was finally obtained by direct condensation of mesityldipyrrane (45) with formaldehyde (Scheme 33).127 During this tandem reaction, the formation of meso-free corrole is followed by immediate intermolecular aromatic oxidative coupling, leading to the meso-meso dimer in 3% yield. Their attempts to convert singly linked dimer 89 into the triply linked dimers 90 or 91 were futile (Scheme 33). This work was a strong inspiration for Osuka and co-workers,128 who reported successful synthesis of desired conjugated corrole dimer 90 by means of careful oxidation of meso-meso-linked dimer 89 with DDQ under high dilution. The attempts to reduce the oxidized dimer 90 into reduced dimer 91 with NaBH4 were only partially successful. Although reaction proceeded smoothly, corrole 91 immediately reverted back to compound 90 under aerobic conditions. After replacing four mesityls with 4-octyloxy-2,3,5,6-tetrafluorophenyl substituents authors were able to record 1H NMR spectrum of the corresponding reduced corrole albeit only in situ. The analogous corrole dimers were also prepared with four C6F5 substituents.128 In this case, however, the different approach was employed for the synthesis of 10,10′-linked dimer 96 (Scheme 34). The reduction of bis(pentafluorobenzoyl)dipyrrane (92), subsequent acid-catalyzed condensation of the obtained dicarbinol 93 with pyrrole (37), followed by purification and oxidation of the bilane intermediate with DDQ yielded desired 5,15-bis(pentafluorophenyl)corrole (95). The overall yield of this procedure is 18%. Final oxidation with DDQ gave the triply linked corrole dimer 97. Oxidative coupling is one of the most desirable methods for formation of biaryl linkages; however, it is rarely used in the corrole chemistry due to the limited stability of these macrocycles toward various oxidants. The following example is

the perfect illustration of these difficulties. To achieve intermolecular oxidative aromatic coupling in the case of corrole 95 (with no substituent at the position 10), the utilization of [bis(trifluoroacetoxyl)iodo]benzene (PIFA) was required (Scheme 34).114 For regioisomeric corrole 70, it was proven, however, that 5,5′-linked corrole dimer 86 could be obtained in a reasonable yield only using a milder oxidation agent, i.e., AgNO2 (Scheme 35).114 It is noteworthy to add that historically the first example of 5,5′-linked corrole dimer was published by Verma and coworkers in 2007 for 22-oxacorroles.129 5,10-Dimesityl-22oxacorrole 98 (preparation of 22-oxacorroles is described in section 10.2.2) was subjected to reaction with either silver(I) triflate or iron(III) chloride. In both cases intermolecular oxidative coupling occurred smoothly, resulting in the formation of dimer 99 in 85−90% yield (Scheme 36).

8. MESO-β-SUBSTITUTED CORROLES 8.1. Synthesis of Mono-Corroles Substituted at Meso and β-Positions

The first example of tetramerization of substituted pyrroles, resulting in meso- and β-substituted corrole formation, was reported in 1994 by Paolesse and co-workers.16 In this case, condensation of four 2-(α-hydroxy-benzyl)pyrrole 100 units in acidic media, followed by the cyclization in the presence of cobalt(II) acetate and PPh3, led to expected product 101 (Scheme 37) in 25% yield. The synthesis required the presence of the cobalt(II) salt, which drives this reaction toward the formation of the desired product. All attempts to replace cobalt with other metal ions such as nickel(II), rhodium(III), iron(III), or manganese(III) led to the corresponding metalloporphyrin, porphyrin, or did not give any macrocyclic product. Limited availability and tedious preparation of the starting material are the major drawbacks of the above-mentioned methodology. S


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Scheme 32. Synthesis of 3,3′,5,5′-Linked Corrole Dimer 88

Scheme 33. Synthesis of 8,8′,10,10′,12,12′-Linked Corrole Dimer 90

Through the early work on meso-substituted corroles with βalkyl substituents, the stability problem was encountered. Such macrocycles are prone to oxidative ring opening, leading to their decomposition.130 More stable analogs were designed by Guilard et al.,131 who replaced alkyl substituents with aryl groups (Scheme 38). Bulky substituents at positions 2, 3, 17, and 18 prevented the corrole oxidation, which resulted from the direct pyrrole−pyrrole bond cleavage. Recently, Paolesse and co-workers132 reported the synthesis of β-alkyl-meso-aryl corroles bearing methyl substituents on the less reactive 7, 8, 12, and 13 carbon atoms, albeit the overall yield of the multistep synthesis was very low. In 1960 Johnson and co-workers claimed that the cyclization of the palladium derivative of bis(dipyrrin) 106 with formaldehyde in the presence of HCl led to a palladium complex of corrole 107 (Scheme 39).133 Interestingly, this claim has proven erroneous, and 3 years later the same research group found that the macrocyclic product turned out to be palladium(II)−10oxacorrole 105 (see section 10.2.1).134,156

Only 47 years later, Bröring and Hell135 performed oxidation of bis(dipyrrin) 108, replacing the palladium salt with manganese(II) acetate (Scheme 40). Obtained in a moderate yield, the manganese(III) complex 109 could be easily demetalated with HBr, giving the corresponding free base corrole. It is worth mentioning that the reaction outcome was T


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Scheme 34. Synthesis of 8,8′,10,10′,12,12′-Linked Corrole Dimer 97

Scheme 35. Synthesis of 5,5′-Linked Corrole Dimer 86

Scheme 36. Synthesis of 5,5′-Linked 22-Oxacorrole Dimer 99

Scheme 37. Tetramerization of Pyrrole Derivative Leading to Corrole 101

strongly influenced by the presence of CH2Cl2, probably due to the limited stability of manganese(III) corroles in chlorinated U


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Scheme 38. Synthesis of Corrole 104

Scheme 39. Attempted Synthesis of Corrole Skeleton via Cyclization of Bis(dipyrrins) in 1960

solvents. All in all, the multistep synthesis of the parent tetrapyrrolic substrate and moderate yields of the oxidation step resulted in restricted use of this protocol. Ghosh and co-workers, applying condensation on Al2O3 as the support (see section 3.3), were able to synthesize A3-corroles from a few aromatic aldehydes and 3,4-difluoropyrrole (110) in 6−8% yield (Scheme 41).136 Under these conditions, it was not possible to obtain the analogous corrole from pentafluorobenzaldehyde. The synthesis of 2,3,7,8,12,13,17,18-octafluoro5,10,15-tris(pentafluorophenyl)corrole was achieved 2 years later by Chang and co-workers137 with 5% yield by modifying the reaction conditions (Scheme 41). Interestingly, fully substituted corrole was obtained as an unexpected product from the reaction of triisopropylsilylpropynal with 3,4-diethylpyrrole in 7% yield.138 Finally, it is also worthwhile to mention that undeca-aryl-substituted corroles can also be synthesized via bromination of A3-corroles followed by the Suzuki coupling.139

Scheme 40. Formation of Manganese(III) Corrole via Cyclization of Linear Precursor 108

8.2. Face-to-Face Porphyrin−Corrole Systems

While the chemistry of cofacial porphyrins has attracted considerable attention in recent decades,141,142 their corrole counterparts remained unexplored until the late 20th century. Guilard and co-workers143 prepared first corrole−porphyrin dimers utilizing a modified stepwise approach established for face-to-face porphyrins by Fish and co-workers.144 In the exemplary case, a,c-biladiene 111, prepared in situ, was reacted with monoprotected diformylanthracene 112, affording corresponding porphyrin 113, which after deprotection of the second formyl group was subjected to the 6-step synthesis of the corrole

core leading to the final porphyrin−corrole dimers 118 and 119 (Scheme 42). V


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Scheme 41. Synthesis of Corroles from 3,4-Difluoropyrrole (110)

Scheme 42. Synthesis of Cofacial Corrole−Porphyrin Dimers

While cofacial dye 118 exhibited extremely low stability in the presence of air and light, the replacement of the alkyl with phenyl substituents provided a compound less prone to oxidation,130 hence, the more stable dye 119. It is also worth emphasizing that metalation of this dimer occurred first at the corrole ring, which allowed a straightforward synthesis of the heterobimetallic complexes.143 Later, Guilard et al.145 published a revised synthetic pathway where 1,8-diformylanthracene (120) was reacted with the corresponding a,c-biladiene 111 in the presence of ptoluenesulfonic acid (TsOH), omitting the deprotection step (Scheme 43). The obtained porphyrin 121 (with improved yield, 51% versus 25%143) was then reacted with 2-ethoxycarbonyl-3,4dimethylpyrrole (115) in the presence of TsOH leading to the corresponding α-ethyl ester, decarboxylation of which was performed with KOH in refluxing diethylene glycol (DEG). The remaining steps leading to cofacial corrole−porphyrin dimer 119 were performed according to the above-mentioned procedure. Moreover, further yield improvement (by a factor of 3) was achieved when sodium bicarbonate was used instead of sodium acetate in the final cyclization step.146 Despite the implemented changes, the synthesis of face-to-face dimers remained laborious and overall yields are modest. 8.3. Cofacial Bis(Corrole) Dyes

The family of face-to-face dimeric porphyrinoids was expanded by Guilard and co-workers, who were the first to report the synthesis of cofacial bis(corroles).147 Starting from the bis(dipyrrane) 122, possessing a suitable anthracene linker and an α-formyl pyrrole 117, corresponding intermediate 123 was obtained and then underwent in situ oxidative cyclization, leading to the bis(corrole) 125 in 6% yield (Scheme 44). Unfortunately, in the presence of air and light dimer 125 was prone to oxidative cleavage, leading to biliverdin-like structures. To address this issue, the more stable tetraphenyl derivative 126 was synthesized, wherein the bulky substituents prevented oxidation of the direct pyrrole−pyrrole bond.

9. ISOCORROLES In the past, the name isocorrole was assigned to the corrole(2.0.1.0) isomer, but Paolesse convinced the porphyrin community that it should be given to a corrole macrocycle in W


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Scheme 43. Revised Strategy toward Corrole−Porphyrin Dimers

Scheme 44. Synthesis of Bis(corroles) 125 and 126

Scheme 45. Synthesis of Spiro-di-isocorrole 128a

a

Ethyl groups were omitted for clarity.

3

which one of the meso-carbon atoms is sp hybridized. Consequently, isocorrole is a nonaromatic macrocycle that has two ionizable N-pyrrolic protons (Schemes 45−48). There are not many synthetic methods leading to isocorroles in which, depending on the location of the sp3-hybridized carbon atom, they can exist as 5-isocorroles or 10-isocorroles. Historically, the first paper describing a compound possessing isocorrole skeleton was published by Vogel and co-workers in 2003.148 During the studies on π-expanded porphyrins, they found that heating of dioxo derivative of octaphyrin(1.0.0.0.1.0.0.0) 127 with nickel(II) acetate led to the

formation of expected complex accompanied by spiro-diisocorrole 128 (Scheme 45). It was proved that the latter one is the product of rearrangement of compound 127 prior to complexation. In the subsequent work, Vogel and co-workers149 reported condensation of α-formyl pyrrole 117 with dipyrrane 129, followed up by the Ni(II)-assisted oxidative cyclization of the resulting linear intermediate 130 (Scheme 46). Unfortunately, this methodology led to the corresponding isocorrolato− nickel(II) 131 due to the presence of a templating agent. The X


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Scheme 46. Synthesis of 10-Isocorrole 131

Scheme 48. Synthesis of 10-Isocorrole

Alternatively, 5,5-dimethyldipyrrane-1-carbinol (137) and dipyrrane 53 can be utilized, albeit the yield does not exceed 2%. Direct preparation of ethyl- or methoxy-substituted triarylisocorroles from corroles may be performed by treatment of the parent macrocycle with DDQ in methanol152 or another nonnucleophilic solvent (followed by subsequent treatment with ethyl magnesium bromide).153 One of the most important aspects of hydroxy−isocorrole chemistry is their formation (as side-products) in various functionalization reactions. This phenomenon was observed, for example, during the bromination of 3-nitro-tris(tolyl)corrole.154 This result is strongly related to the fact that 5- and 10-hydroxyisocorroles are often the first products of oxidation of corroles, and many reagents used in electrophilic aromatic substitution are also oxidants.

Scheme 47. Synthesis of Free Base 10-Isocorrole 134

10. CORE-MODIFIED CORROLES 10.1. N-Confused Corroles

Although N-confused porphyrins have been known since 1994, the synthesis of unique N-confused corroles was developed by Furuta and co-workers only in 2011.155 This is mostly related to the fact that the preparation of the required linear precursors 138 and 141 is a tour de force by itself. Both 2-aza-21-carbabilane 138 and 7-aza-22-carbabilane 141, upon DDQ oxidation in refluxing acetonitrile, afforded N-confused corroles 139, 140, and 142 (Schemes 49 and 50). Those exceptional corrole analogues adopted the 3H form and preserved aromaticity, as proved by the NMR spectroscopy and X-ray crystallographic analyses. The confused pyrrole rings are significantly tilted from the plane composed by the remaining tripyrrole moiety, which is an indication of lower aromaticity, when compared to regular corroles. For macrocycles 139 and 140, reduced fluorescence quantum yields (1.4% and 5.7%, respectively, measured in CH2Cl2) were observed together with strong bathochromic shifts of both absorption and emission maxima. Relatively high

alternative approach, affording free base 10-isocorrole 134, was reported by Setsune et al., who reacted bipyrrole 133 with chosen dipyrrane 132 in metal-free conditions (Scheme 47).150 Flint et al.151 reported two synthetic pathways yielding regioisomeric 5-isocorrole 136. InCl3-catalyzed condensation of 5,5-dimethyldipyrrane (135) with 5-(pentafluorophenyl)dipyrrane-1-carbinol (67) followed by DDQ oxidation gave expected 5-isocorrole 136 in 31% yield (Scheme 48). Y


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Scheme 49. Synthesis of N-Confused Corroles 139 and 140

Scheme 51. α,α′-Dichlorodipyrrin 143 Cyclization into 10Azacorroles 144 and 145

Scheme 50. Synthesis of N-Confused Corrole 142

Scheme 52. Synthesis of Free Base 10-Azacorrole 147

Stokes shifts of 800−1500 cm−1 reflect the high flexibility of the new macrocycles. 10.2. Heteroanalogues of Corroles

With the heteroatom being incorporated directly into the corrole’s π-aromatic system, the coordination behavior of its macrocyclic cavity as well as spectroscopic properties significantly change. This can be achieved by exchanging carbon atoms either in pyrrolic rings or in meso positions. A considerable number of corrole heteroanalogues has been reported date that

are strongly related with fine tuning of their photophysical properties. 10.2.1. 10-Heterocorroles. The replacement of one of the meso-carbon atoms with a heteroatom such as oxygen, nitrogen, or sulfur is the simplest modification of the corrole core. While Z


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Scheme 53. Synthesis of 10-Oxa-, 10-Thia-, and 10Selenacorroles

Scheme 54. Synthesis of 10-Thiacorroles as Al(III) Complexes

Scheme 55. Synthesis of 10-Oxacorrole 159 via Oxidation of Norcorrole 158

10-heterocorroles have been known since the 1960s,31,156 regioisomeric 5/15-heterocorroles remained uncharted until now because the C2v symmetry induced synthetic difficulties. Despite having a relatively simple structure, the available procedures providing 10-heterocorroles are long and tedious. This issue reflected the limited interest in these compounds for over 50 years. The first convenient synthetic pathways were developed only a few years ago. Shinokubo and co-workers reported that when the α,α′dichlorodipyrrin Ni(II) complex 143 was reacted with benzylamine under the Buchwald−Hartwig amination conditions, Nbenzylazacorrole 144 and azacorrole 145, as nickel complexes, formed in 27% and 8% yield, respectively (Scheme 51).157 The same authors changed the starting material to α,α′-dibromodipyrrin and modified the catalytic system, which improved the yields of the desired azacorroles, even though the reaction sequence was prolonged with an additional step. The reaction proceeded via the cyclization of the aminodipyrrins in the presence of aluminum or nickel salts.158 The choice of the metal ion seemed to play the crucial role, since it acts as the templating

agent in the macrocyclization process. With two possible tautomers of the 10-azacorrole, the outer nitrogen appears to be an amine type with its lone pair participating in the aromatic 18-π[17]triazaannulene electron ring system. This behavior is reflected in the spectroscopic behavior of dye 144. Furthermore, AA


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Scheme 56. Synthesis of 22-Oxacorrole 162

Scheme 58. Synthesis of 21-Oxacorrole 169

Scheme 59. Synthesis of Dioxacorrole 173 Scheme 57. Synthesis of 22-Oxacorroles 162 and 165

alternative strategy toward these macrocycles. Very recently, Shinokubo and co-workers proposed a new method toward 10azacorroles (Scheme 52).159 Linear precursor 146 has been synthesized starting from dipyrrane 45, and macrocyclization of this zinc complex was achieved via the Yamamoto reaction. It is well known that removal of Zn2+ from porphyrinoids is considerably easier than Ni2+ or Cu2+. As expected, the initially formed macrocyclic complex was demetalated with TFA to obtain the free base azacorrole 147 in 98% yield (Scheme 52). Bröring and co-workers synthesized similar 10-heterocorroles bearing Se, S, and O in the core.160 For this purpose, α,α′dibromodipyrrin 148 was cyclized in the presence of copper(II) salts yielding the corresponding 10-oxacorrole copper complex 149 (Scheme 53). Subsequent demetalation with tin(II) chloride and HCl afforded the free amine 152. Reaction of compound 148 with potassium selenocyanate or sodium sulfide afforded 10selena- and 10-thiacorrole, respectively.

azacorrole 144 is planar and its oxidation potential is surprisingly low, in comparison to those of diazaporphyrins. Removal of Ni(II) from 10-azacorroles proved to be impossible, which provided motivation to search for an AB


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coupling of 16-oxatripyrrane 161 with the dipyrrane 160, which proceeds via acid-catalyzed fragmentation (scrambling) of compound 161, afforded not only the expected core-modified smaragdyrin 163 but also 22-oxacorrole 162 in 8% yield (Scheme 56).165,166 When compound 160 was replaced with pyrrole-2-carboxaldehyde (164), the synthesis of 22-oxacorrole 165 with a free meso-15 position was achieved (Scheme 57).115 It is noteworthy to add that subjecting compound 161 to TFA without the presence of aldehyde 164 yields 22-oxacorrole 162 bearing phenyl substituents at all three meso positions, as a result of scrambling (Scheme 57).167 Lee et al. developed another method that appeared to be different, but if one considers all the consequences of scrambling, the approach is analogous. Alcohol 167 was condensed with dipyrrane 166 to give oxacorrole 169 in 9% yield (Scheme 58).168,169 In view of the later results of Osuka and co-workers (Scheme 28), it is noteworthy that the analogous reaction of 5(p-tolyl)-2-[α-hydroxy-α-(p-tolyl)methyl]dipyrrane (168) with dipyrrane 166 did not afford the expected corrole 170 (leading instead only to the respective porphyrin) (Scheme 58).168 The first examples of stable 22-thiacorroles were prepared using thiophene−monocarbinols as key precursors. Condensation of thiophene−monocarbinol with 4-nitrobenzaldehyde and pyrrole in propionic acid led to 22-thiacorroles in 2−3% yield.170 Recently, 21-oxacorrole was also synthesized, for the first time, via ring contraction of the corresponding 21-oxaporphyrin subjected to POCl3.171 Dioxacorrole 173 synthesis was reported by Latos-Grażyński and co-workers, who performed acid-catalyzed condensation of 2,5-bis(phenylhydroxymethyl)furan (171) with 2-phenylhydroxymethylfuran (172) and pyrrole (37) (Scheme 59).172 It is worth mentioning that 22-oxacorroles and dioxacorroles retain their aromatic character. That is proved by the shape of the UV−vis spectra (characteristic Soret and Q bands) as well as by the presence of signals corresponding to protons that are highly shielded by the aromatic ring current in the 1H NMR spectra, corresponding to the inner protons. Complexes of oxacorroles with Cu(II), Ni(II), or Co(II) have already been prepared by reaction with CuCl2, NiCl2, or Co(OAc)2 in DMF.166 10.2.3. Triazacorroles (Corrolazines). If one replaces all three methine bridges in the corrole framework with nitrogen atoms, the new aromatic macrocycle is formed, termed corrolazine or triazacorrole. Goldberg and co-workers developed an efficient methodology toward triazacorroles via ring contraction of porphyrazinesporphyrinoids relatively easy to prepare in a multigram scale. The reaction, conducted with the large excess of PBr3, gave phosphorus corrolazine 175, which after treatment with Na/NH3 provided free base 176 (Scheme 60).173 This methodology has been used by Goldberg and co-workers in numerous studies related to the reactivity of corrolazine complexes in oxidation of various organic compounds.174−176

Scheme 60. Synthesis of Triazacorroles via PBr3-Induced Ring Contraction of Porphyrazines

Recently, Wachi et al. reported that 10-thiacorroles could be synthesized when 5,15-dithiaporphyrins copper complexes were heated in the presence of PPh3.161 Free base of 5,15dithiaporphyrin 156 could be prepared if a zinc(II) complex of α,α′-dichlorodipyrrin 155 was heated with sodium sulfate in DMF (Scheme 54). Attempted metalation of this compound with AlCl3 in pyridine followed by treatment with PPh3 led to the formation of Al(III) complex of 10-thiacorrole 157.140 A different approach toward 10-heterocorroles was reported by Shinokubo and co-workers.162 Oxidation of norcorrole 158 with O2 afforded the expected 10-oxacorrole 159, albeit the reaction proceeded exceedingly slowly (Scheme 55). Utilization of m-CPBA shortened the reaction time greatly and slightly increased the overall yield. Sakow et al. reported a novel 10-thiacorrole synthesis utilizing [2 + 2] MacDonald-type condensation of bis(formylpyrrole)sulfide with the corresponding 2,2′-bipyrrole; however, the tedious synthesis of the pyrrole precursor was a serious limitation of this methodology.163 Formation of 10-oxacorrole was observed also as a result of unpredicted loss of two methyl groups by 2,2′-bidipyrrins during the oxidative macrocyclization, although in unremarkable yield.164 10.2.2. Corrole Heteroanalogs Modified in Pyrrole Subunit. The development of synthetic methodologies for coremodified corroles coincided with the development of all-nitrogen corroles. In 1999, Chandrashekar and co-workers found that the

11. NORCORROLES Over the years of research a question has been raised: what are the limits of contraction? Would it be possible to remove the two meso-carbons from the core of the porphyrin? Interestingly, the existence of such a molecule was predicted theoretically in 2005; a few years before it was actually synthesized.177 The first synthesis of norcorroles was reported by Bröring and co-workers, who confirmed the formation of a new macrocycle by NMR and MALDI-TOF analyses of the crude reaction mixture from the AC


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absorption bands in the 600−1000 nm region, characteristic for antiaromatic porphyrinoids. Later studies showed that norcorrole 180 can be easily functionalized179 and that it can be used as an electrode-active material in rechargeable batteries, showing high discharge capacity even after 100 cycles when examined with a Li metal anode.180 As for now, all attempts to remove nickel and obtain free base norcorrole failed.

Scheme 61. Synthesis of Norcorrole 178

12. STABILITY OF CORROLES Corroles, regardless of the substitution pattern, are less stable than the corresponding porphyrins. Even though corroles have Scheme 62. Synthesis of Norcorrole 180 via the Yamamoto Reaction

Scheme 63. Light-Induced Transformation of trans-A2BCorrole 186 into AB2C-Porphyrin 187

oxidation of the iron complex of 2,2′-bidipyrrin 177 (Scheme 61). All attempts to isolate 178 failed due to a high instability and rapid dimerization.178 A few years later, Shinokubo and co-workers successfully synthesized NiII−norcorrole complex 180 via Ni(II)-templated reductive homocoupling of α,α′-dibromodipyrrin 179 (Scheme 62).162 The expected product 180 was isolated in a remarkable 90% yield as a stable green solid. This gram-scale synthesis allowed an unambiguous characterization of these elusive molecules. NMR and NICS studies confirmed that its 16 πelectron system is strongly antiaromatic. At the same time the skeleton of dye 180 is perfectly planar as shown by X-ray diffraction analysis. The UV−vis spectrum of norcorrole 180 exhibited a broad absorption band at 431 nm and weak

Figure 3. Exemplary products of decomposition and ring expansion of various corroles. AD


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Some truly unusual cases were described such as transformation of 5,15-di(mesityl)-10-(4-nitrophenyl)corrole (186) into the corresponding porphyrin 187 possessing one meso position free (Scheme 63).182 The formation of AB2C-porphyrin 187 was initially noticed when the NMR tube containing the solution of corrole 186 was incidentally left at ambient conditions for 1 month. Repeating this reaction under more controlled conditions gave porphyrin 187 in 19% yield after 48 h (Scheme 63). Guilard and co-workers proved that the 5-mesocarbon atom originates from the second molecule of corrole. They also proposed that 2π + 2π cycloaddition of the corrole is the first step of this elaborated chain of reactions, followed by oxidation, splitting of the resulting spirocyclobutane intermediate into porphyrin 187 and biliverdin 183 (presence of which has been confirmed) (Figure 3). In view of these results the stability of corrole 39 is remarkable, since in the presence of light and oxygen it undergoes transformation only into its β−β-linked dimers and trimers.184 Yet, the most typical pathway includes the hydroxylation to form hydroxyisocorrole and further ring opening to form biliverdintype structures.130,184,185 Typical decomposition pathways are shown on Scheme 64. The in-depth studies by Gryko, Danikiewicz, and co-workers166 proved that (1) decomposition of corroles occurs most rapidly in CH3CN and that this solvent should be avoided; (2) among the many mass spectrometry ionization techniques, field desorption (FD) is best for evaluation of corrole purity. The use of electrospray induces the formation of hydroxy−isocorrole and consequently is not reliable. The rate of corroles’s decomposition is strongly related to the influence of the substituents at both the β and the meso positions. The presence of electron-donating substituents (even ones as weak as alkyl groups) can decrease the stability so markedly that isolation of such corroles becomes impossible. The rationale behind the spectacular success of 5,10,15-tris(pentafluorophenyl)corrole (39)7 is the presence of three strongly electron-withdrawing groups at all meso positions. Other electron-withdrawing groups also have significant effects, and the strongest stabilization can be achieved with ester groups located directly at the meso positions.84 The uniqueness of the corrole scaffold is also visible in some remarkable ring expansions, which were discovered by Paolesse and co-workers. Subjecting triphenylcorrole to carbon tetraiodide in the mixture of DMF and CH2Cl2 led to the formation of 5-iodo-6,11,16-triphenylhemiporphycene (184) in 30% yield (Figure 3).186 According to authors the first step involved the formation of N-substituted derivative, which underwent skeletal rearrangement. Independently, another A3-corrole was subjected to 4-amino-1,2,4-triazole in toluene/MeOH with NaOH as a catalyst. 5,10,15-Tris(4-tert-butylphenyl)corrole underwent ring expansion with concomitant nitrogen insertion, which eventually led to 6-azahemiporphycene 185 in 53% yield (Figure 3).187 This discovery has been made during a broader study aiming at βamination of nitro-corroles.

Scheme 64. Decomposition of 10-(4-Cyanophenyl)-5,15dimesitylcorrole (188) in the Combined Presence of Light and Oxygen

the same 18π-electron system, they have one less carbon atom, and hence, the average electron density is higher. Therefore, the first oxidation potential of corroles is lower than that of porphyrins possessing the same substituents pattern. A lower stability of corroles has been described as early as in 1998 by Guilard and co-workers.130 Subsequently, various groups discovered that depending on the conditions and the particular pattern of substituents at the periphery of the macrocycle, decomposition of corroles can proceed in various directions.112,152,181−184 Exemplary structures are presented on Figure 3.

13. SUMMARY AND OUTLOOK The synthesis of corroles has undergone incredible changes. From multistep strategies that attracted only practitioners in the field, the procedure has been transformed into a one-pot process from commercially available reagents. While decades of development brought a number of synthetic approaches leading to βsubstituted corroles, all of them were fraught with serious limitations. The tedious synthesis of substrates, low yields, a AE


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synthesis of corroles, π-extended porphyrinoids, and other functional dyes as well as on two-photon absorption, artificial photosynthesis, excited-state intramolecular proton transfer, and fluorescence imaging.

number of byproducts, and the lack of a general and efficient methodology that allowed full control of the arrangement of the β-substituents caused the exploration of β-substituted corroles to become overshadowed by the flourishing chemistry of mesosubstituted analogs. In 1996, it would have been hard to imagine that 20 years later the chemistry of corroles would expand to create an independent field. The synthetic revolution made it possible to try risky ideas in diverse areas of materials chemistry and in various biology- and medicine-oriented applications.7 There is, however, another view of progress in this field since 1999. If one assumes (which is almost certain) that transformation of bilanes into corroles goes through biladienes as intermediates, we have only changed the preparation of these latter compounds, leaving the crucial processmacrocyclizationuntouched. Multiple challenges still remain in the preparation of corroles. Since corroles can form only if the concentration of reagents is high, the desired product cannot be formed if the two reactants (aldehyde and dipyrrane) with large molecular mass cannot be dissolved at required concentrations. Also, the peculiar instability of corroles possessing CF3 groups at meso positions has not yet been satisfactorily explained. Last but not least, the parent macrocycle, i.e., the corrole lacking any substituents, has not been synthesized yet. There is a double connection between the synthesis of corroles and their use in various research areas. The development of methodologies giving access to multigram quantities sparked riskier projects. On the other hand, optimization of performance of corroles stimulates further progress in the synthesis. It is hard to predict where this interaction will take us, but this analysis of the research performed over the last 20 years suggests that the future of corroles is only limited by our imagination.

ACKNOWLEDGMENTS We thank the National Science Centre, Poland (Grants MAESTRO-2012/06/A/ST5/00216 and SYMFONIA 2014/ 12/W/ST5/00589), for financial support. D.T.G. and D.G. thank Prof. Jonathan Lindsey of NCSU, with whose support and guidance they learned to negotiate the finer points and difficult passages of porphyrin chemistry. REFERENCES (1) Johnson, A. W.; Kay, I. T. The Pentadehydrocorrin (Corrole) Ring System. Proc. Chem. Soc. London 1964, 89−90. (2) Smith, K. M. Development of Porphyrin Syntheses. New J. Chem. 2016, 40, 5644−5649. (3) Gross, Z.; Galili, N.; Saltsman, I. The First Direct Synthesis of Corroles from Pyrrole. Angew. Chem., Int. Ed. 1999, 38, 1427−1429. (4) Paolesse, R.; Jaquinod, L.; Nurco, D. J.; Mini, S.; Sagone, F.; Boschi, T.; Smith, K. M. 5,10,15-Triphenylcorrole: a Product from a Modified Rothemund Reaction. Chem. Commun. 1999, 1307−1308. (5) Thomas, K. E.; Alemayehu, A. B.; Conradie, J.; Beavers, C. M.; Ghosh, A. The Structural Chemistry of Metallocorroles: Combined Xray Crystallography and Quantum Chemistry Studies Afford Unique Insights. Acc. Chem. Res. 2012, 45, 1203−1214. (6) Buckley, H. L.; Arnold, J. Recent Developments in Out-of-Plane Metallocorrole Chemistry Across the Periodic Table. Dalton Trans. 2015, 44, 30−36. (7) Aviv-Harel, I.; Gross, Z. Aura of Corroles. Chem. - Eur. J. 2009, 15, 8382−8394. (8) Lemon, C. M.; Dogutan, D. K.; Nocera, D. G. Handbook of Porphyrin Science; World Scientific Publishing Co. Pte. Ltd.: Singapore, 2012; Vol. 21, p 1. (9) Paolesse, R. The Porphyrin Handbook; Academic Press: New York, 2000; p 201. (10) Nardis, S.; Monti, D.; Paolesse, R. Novel Aspects of Corrole Chemistry. Mini-Rev. Org. Chem. 2005, 2, 355−374. (11) Gryko, D. T. Adventures in the Synthesis of meso-Substituted Corroles. J. Porphyrins Phthalocyanines 2008, 12, 906−917. (12) Gryko, D. T.; Fox, J. P.; Goldberg, D. P. Recent Advances in the Chemistry of Corroles and Core-Modified Corroles. J. Porphyrins Phthalocyanines 2004, 8, 1091−1105. (13) Lemon, C. M.; Brothers, P. J. The Synthesis, Reactivity, and Peripheral Functionalization of Corroles. J. Porphyrins Phthalocyanines 2011, 15, 809−834. (14) Gryko, D. T. Recent Advances in the Synthesis of Corroles and Core-Modified Corroles. Eur. J. Org. Chem. 2002, 2002, 1735−1743. (15) König, M.; Faschinger, F.; Reith, L. M.; Schöfberger, W. The Evolution of Corrole Synthesis  from Simple One-Pot Strategies to Sophisticated ABC-Corroles. J. Porphyrins Phthalocyanines 2016, 20, 96−107. (16) Paolesse, R.; Licoccia, S.; Bandoli, G.; Dolmella, A.; Boschi, T. First Direct Synthesis of a Corrole Ring From a Monopyrrolic Precursor. Crystal and Molecular Structure of (Triphenylphosphine)(5,10,15-triphenyl-2,3,7,8,12,13,17,18-octamethylcorrolato)cobalt(III)-Dichloromethane. Inorg. Chem. 1994, 33, 1171−1176. (17) Paolesse, R.; Tassoni, E.; Licoccia, S.; Paci, M.; Boschi, T. One-Pot Synthesis of Corrolates by Cobalt Catalyzed Cyclization of Formylpyrroles. Inorg. Chim. Acta 1996, 241, 55−60. (18) Johnson, A. W.; Kay, I. T. Corroles. Part I. Synthesis. J. Chem. Soc. 1965, 1620−1629. (19) Dolphin, D.; Johnson, A. W.; Leng, J.; van den Broek, P. The BaseCatalysed Cyclisations of 1,19-Dideoxybiladienes-ac. J. Chem. Soc. C 1966, 880−884.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Rafał Orłowski obtained his B.Eng. and M.Eng. degrees in Chemistry from Warsaw University of Technology (Warsaw, Poland) in 2014. He is currently pursuing his Ph.D. degree in Organic Chemistry at the Institute of Organic Chemistry of the Polish Academy of Sciences in Professor Daniel T. Gryko’s research group. His current research is focused on studying the self-assembly capability and photophysical properties of corroles. Dorota Gryko obtained her Ph.D. degree from the Institute of Organic Chemistry of the Polish Academy of Sciences in 1997 under the supervision of Professor J. Jurczak. After a postdoctoral stay with Professor J. Lindsey at North Carolina State University (1998−2000), she started her independent career in Poland. In 2009 she received the prestigious TEAM grant from the Foundation for Polish Science. She became Full Professor in 2015. Her current research interests are focused on photoredox catalysis and vitamin B12 chemistry. Daniel T. Gryko was born in Białystok (Poland) in 1970. He obtained his Ph.D. degree from the Institute of Organic Chemistry of the Polish Academy of Sciences in 1997 under the supervision of J. Jurczak. After postdoctoral studies with J. Lindsey at North Carolina State University (1998−2000) he began his independent career in Poland. He became Full Professor in 2008. His current research interests are focused on the AF


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Synthesis of Corroles and Their Heteroanalogs - Chemical Reviews

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