Carbaporphyrinoid Systems - ACS Publications - American Chemical

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Carbaporphyrinoid Systems Timothy D. Lash* Department of Chemistry, Illinois State University, Normal, Illinois 61790-4160, United States ABSTRACT: Following immediately after the serendipitous discovery of N-confused porphyrins, remarkably diverse carbaporphyrinoid systems have been synthesized and investigated. By replacing a pyrrolic unit within the porphyrin framework with cyclopentadiene, indene, azulene, cycloheptatriene, or benzene, new families of porphyrin-like macrocycles were produced. True carbaporphyrins are fully aromatic structures, while benziporphyrins are essentially devoid of macrocyclic aromatic character, and azuliporphyrins fall midway between the two extremes. Monocarbaporphyrinoids are superior organometallic ligands and form stable complexes with copper(III), silver(III), gold(III), nickel(II), palladium(II), platinum(II), rhodium(III), iridium(III), and ruthenium(II). Unusual oxidation reactions have also been discovered, commonly leading to derivatization of the internal carbon atom. In addition, structural rearrangements have been uncovered that allow the conversion of azuliporphyrins into benzocarbaporphyrins, and benziporphyrins into carbaporphyrins. Although less well studied, many examples of dicarbaporphyrinoids have been reported, and these show equally intriguing characteristics. Furthermore, contracted and expanded carbaporphyrinoids have been investigated. Studies in this area provide fundamental insights into the aromatic properties, tautomerization, and reactivity of porphyrins and related macrocyclic systems.

CONTENTS 1. 2. 3. 4. 5.

Introduction Aromaticity in Porphyrinoid Systems History of Carbaporphyrins Synthetic Strategies True Carbaporphyrins, Carbachlorins, and Related Systems 5.1. Synthesis and Spectroscopic Properties 5.2. Protonation of Carbaporphyrins 5.3. Reactivity of Carbaporphyrins 5.4. Synthesis and Reactions of Carbachlorins and a Carbaporphyrin with an Unsubstituted Cyclopentadiene Ring 5.5. Synthesis and Reactivity of Heterocarbaporphyrins 6. N-Confused Porphyrins and X-Confused Heteroporphyrins 6.1. Metalated Derivatives of N-Confused Porphyrins 6.2. Meso-Unsubstituted N-Confused Porphyrins 6.3. Heteroanalogues of N-Confused Porphyrins 6.4. X-Confused Heteroporphyrins 7. Pyrazole-Containing Porphyrinoids 8. Neo-Confused Porphyrins 9. Azuliporphyrins and Heteroazuliporphyrins 9.1. Synthesis and Spectroscopic Properties of Azuliporphyrins 9.2. Reactivity of Azuliporphyrins: Ring Contractions, Oxidations, and Metalations 9.3. Heteroazuliporphyrins 10. Tropiporphyrins

© XXXX American Chemical Society

11. Benziporphyrins, Heterobenziporphyrins, and Related Systems 11.1. meso-Unsubstituted Benziporphyrins and Naphthiporphyrins 11.2. meso-Substituted Benziporphyrins 11.3. Dimethoxybenziporphyrins 11.4. Heterobenziporphyrins 11.5. Oxybenziporphyrins, Oxynaphthiporphyrins, and Related Hetero-Analogues 11.6. Further Oxidized Benziporphyrins and Heterobenziporphyrins 11.7. 22-Hydroxybenziporphyrins 11.8. N-Confused Pyriporphyrins 11.9. Benziporphyrins with Exocyclic Double Bonds 11.10. Benziphthalocyanines 11.11. p-Benziporphyrins, 1,4-Naphthiporphyrins, and Related Systems 12. Contracted Carbaporphyrinoids 13. Dicarbaporphyrinoid Systems 13.1. True Dicarbaporphyrins 13.2. Doubly N-Confused Porphyrins 13.3. Related Dicarbaporphyrinoid Systems 14. Tri- and Tetracarbaporphyrinoids 15. Expanded Carbaporphyrinoid Systems 15.1. Expanded Carbaporphyrins with Cyclopentadiene, Indene, and Azulene Subunits 15.2. Expanded Benziporphyrins

B B C D E E I J

M P U U V Y AA AE AG AK AK AP AX BA

BC BC BD BH BK BN BS BW BX CA CD CG CL CQ CQ CU CV CZ DD DD DG

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

A

DOI: 10.1021/acs.chemrev.6b00326 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 15.3. Expanded Norroles 15.4. Expanded Porphyrinoids with Inverted Pyrrole Rings that Resemble Carbaporphyrins 16. Porphyrin Earrings and Related Systems That Enclose Carbaporphyrin-like Binding Pockets 17. Conclusions and Future Prospects Author Information Corresponding Author Notes Biography Acknowledgments Abbreviations Used References

Review

defines them as carbaporphyrinoids. The synthesis, aromaticity, spectroscopy, and reactivity of these important families of porphyrin analogues are the subject of this review. However, meso-substituted N-confused porphyrins will not be described in detail, because these are the topic of a separate review in this special issue of Chemical Reviews. meso-Unsubstituted NCPs will be covered as these compounds are structurally closely related to many of the carbaporphyrinoids described below.

DL

DL DQ DS DS DS DS DS DT DT DT

2. AROMATICITY IN PORPHYRINOID SYSTEMS Porphyrin (11) possesses an 18π-electron substructure (shown in bold) that resembles [18]annulene, and it has been argued that this feature is responsible for the aromatic properties observed in porphyrinoids.4−7 Certainly, there is evidence for the 7,8- and 17,18-carbon−carbon bonds having more double bond character,28,29 and hydroporphyrins such as chlorin (12) and bacteriochlorin (13) (Figure 3), which retain the [18]annulene-type substructure, also exhibit macrocyclic aromaticity.30 Nevertheless, porphyrins appear to be far more than just examples of bridged [18]annulenes. Porphyrins readily protonate to give dications 11H22+, but these are better described as [20]annulene dications, as illustrated in structure 11′H22+. Metalloporphyrins (11M) also take on additional symmetry and might be considered to be derivatives of a bridged tetraaza[16]annulene dianion (structure 11M′). In truth, all of these descriptions are oversimplifications, but they nevertheless provide convenient insights into the properties of porphyrintype structures.6 These models can also take into account resonance contributors, such as 11″, that provide alternative [18]annulene pathways. Some theoretical studies have contradicted the bridged [18]annulene model and indicate that the aromatic properties are due to the individual 6π-electron contributions of the pyrrolic subunits.31−38 Unfortunately, this viewpoint does not explain many of the properties of porphyrins. After all, open-chain conjugated tetrapyrroles or cyclic structures such as phlorins (14) with interrupted conjugation pathways are relatively unstable and have very different reactivity and spectroscopic properties when compared to porphyrins.6 Of course, macrocyclic conjugation is obligatory if the system is going to exhibit the global diamagnetic ring current effects that are associated with aromatic systems. In a fairly recent paper, Schleyer and co-workers concluded that the thermodynamic stabilization of porphyrins is mostly due to the individual pyrrole subunits, but the spectroscopic properties are determined to a greater extent by the presence of 18π-electron delocalization pathways.39 Other interpretations have been made, and it is probably fair to say that the aromatic properties of porphyrinoid systems remain open to further discussion.40 In an earlier paper, on the basis of bond length analyses and theoretical studies, Schleyer suggested that porphyrins are better described as 22π-electron systems (structure 11x).41 In this model, two of the double bonds were not considered to be significantly involved in determining the aromaticity of the system, but the nitrogen lone pair electrons were thought to play an important role. Subsequent to this proposal, a deazaporphyrin named vacataporphyrin (15) was described that lacks one of the pyrrole-type nitrogens but nevertheless retains strongly aromatic characteristics and a porphyrin-like UV−vis spectrum.42 In order to gain a further understanding of the function of the nitrogen atoms in determining the aromatic character of porphyrins, a dideazaporphyrin (16) that lacks both of the nitrogens involved in Schleyer’s 22π-electron model was targeted for synthesis.43 This was accomplished by McMurry coupling of a pyrrole bis-

1. INTRODUCTION The porphyrins are a remarkably versatile family of tetrapyrrolic macrocycles, and in metalated form they are responsible for many critical biological functions, including oxygen transportation (hemoglobin), redox processes (cytochromes), and photosynthesis (chlorophylls) to name just a few.1 Nearly all metals form coordination complexes with the porphyrin nucleus and in some cases these exhibit valuable catalytic properties.2 Porphyrins and related macrocycles are also examples of nonbenzenoid aromatic systems, and this has led to numerous investigations in order to gain an understanding of these important characteristics.3−8 One approach to better understanding the aromatic properties of porphyrins and related macrocycles involves the synthesis of analogous ring systems that have been modified in one way or another.9−13 In this regard, a particularly fruitful area of research has been into carbaporphyrinoid systems.14−20 Carbaporphyrins are porphyrin analogues in which one or more of the pyrrolic nitrogens have been replaced by carbon atoms. In principle, this can result in monocarbaporphyrins 1, opp-dicarbaporphyrins 2, adj-dicarbaporphyrins 3, tricarbaporphyrins 4, and tetracarbaporphyrinoid structures such as quatyrin (5) and isoquatyrin (6) (Figure 1).15,21 A large

Figure 1. Carbaporphyrins.

number of related carbaporphyrinoid systems can also be considered, such as N-confused porphyrins (NCPs) 7,22−24 azuliporphyrins 8,25 benziporphyrins 9,26 and tropiporphyrins 1027 (Figure 2). Although the structures and properties of these species vary considerably, they all have at least one carbon atom placed inside of the porphyrinoid cavity, and it is this feature that B


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Figure 2. Selected carbaporphyrinoid systems.

401 nm. Addition of trifluoroacetic acid resulted in the formation of the related dication, and this gave an intensified Soret band and several Q-like bands in its UV−vis spectrum, while the proton NMR spectrum showed that this species has a substantially enhanced diatropic ring current. These results demonstrate that the spectroscopic properties of porphyrins are not reliant upon the presence of the pyrrole NH units, although these clearly do serve a function. The replacement of nitrogens by carbon atoms also allows the aromatic properties of porphyrinoid systems to be probed and has enhanced our understanding of this phenomenon.6,43,44

3. HISTORY OF CARBAPORPHYRINS Although investigations into carbaporphyrins only took off a little over 20 years ago, the first speculations about structures of this type can be traced back to the 1940s. Rothemund had reported that porphyrins could be obtained by heating mixtures of aldehydes and pyrrole in solvents such as methanol or pyridine (sealed tube) to give low yields of porphyrins.45−48 For instance, pyrrole and benzaldehyde reacted to give meso-tetraphenylporphyrin. However, additional porphyrin-like byproducts also appeared to have been formed in these reactions. Aronoff and Calvin separated a series of fractions from this type of reaction, each of which gave porphyrin-like UV−vis spectra.49 The authors speculated about the identities of these products and proposed a number of possible structures, including compounds 19 and 20a−e (Figure 4) with one or two inverted pyrrolic subunits.49 These structures, which all possess one or two carbon atoms within the porphyrin cavity, were said to belong to a class of “carboporphines”. Today, we recognize structure 19 as an Nconfused porphyrin, while 20a−e are doubly N-confused porphyrins, and it is noteworthy that none of the UV−vis spectra reported by Aronoff and Calvin correspond to these types of structures (in other words, none of the fractions were in actual fact N-confused porphyrins). Nevertheless, these speculations were remarkably prescient. In subsequent work, Calvin and coworkers reported that one of the porphyrin-like fractions was in actuality meso-tetraphenylchlorin.50,51 In a recent essay, Senge reported that Linus Pauling had also speculated about the possibility of synthesizing porphyrins with “extroverted” (i.e.,

Figure 3. Aromatic delocalization pathways in porphyrinoids.

acrylaldehyde (17) with Zn−TiCl4 (Scheme 1). A dihydro intermediate (18) is presumably formed initially, but this undergoes a spontaneous oxidation to give dideazaporphyrin 16. The NMR spectrum for 16 showed that this system retained a strong diatropic ring current, and the internal CH protons were observed upfield at −2.52 ppm, while the external protons showed up downfield between 9.8 and 10 ppm.43 The UV−vis spectrum for 16 was also porphyrin-like, showing a Soret band at Scheme 1. Synthesis of a Dideazaporphyrin

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Scheme 2. MacDonald 2 + 2 and 3 + 1 Syntheses of Porphyrins and Heteroporphyrins

Figure 4. Potential porphyrinoid byproducts from the Rothemund reaction.

inverted) pyrrole rings.52 Pauling referred to these structures as “isoporphines” and extended his analysis to porphyrin isomers with three or four inverted rings. He concluded that porphyrin analogues with three inverted rings would be very unstable, while structures with four inverted rings would be “impossible”,52 and computational studies have confirmed this analysis.53,54 Few speculations on the synthesis of carbaporphyrins 1−6 appeared in the literature until the first syntheses of monocarbaporphyrins were reported in the mid-1990s. In his analyses of porphyrin analogues such as porphycene (21) and the tetraoxaporphyrin dication 22 (Figure 5), Vogel et al. related tripyrranes 29a or 29b with furan or thiophene dialdehydes 30 in a “3 + 1” variant on the MacDonald condensation.61 The 3 + 1 route has many advantages, and while one of the reactants must still possess a plane of symmetry, structures that are not accessible by the “2 + 2” approach can easily be prepared in this fashion.59,62 Initially, the “3 + 1” MacDonald condensation was abandoned for more than 20 years, in part due to difficulties in preparing the required tripyrrane intermediates.62 However, following the introduction of a convenient method for preparing tripyrranes, the route became widely used in the preparation of porphyrins63−68 and porphyrin analogues such as carbaporphyrinoid systems.14,15 The versatility of this route is largely due to the fact that an exotic ring can often be introduced by reacting a dialdehyde with the tripyrrane intermediate. As was the case for the original “2 + 2” route, an oxidation step is generally required to furnish the final macrocyclic products. Alternative routes to porphyrin analogues make use of reactions that are similar to the Rothemund porphyrin synthesis,69 or the Alder−Longo70 and Lindsey71 modifications of this approach. NCPs 19 were originally observed as byproducts from the acid-catalyzed reaction of pyrrole with aromatic aldehydes (Scheme 3). Furuta et al. reacted pyrrole and benzaldehyde with 1 equiv of HBr in a 1:1 mixture of tert-butyl alcohol and dichloromethane, and following oxidation with pchloranil, tetraphenylNCP (19a) was isolated in 7% yield.72 Contemporaneously, Latos-Grażyński and co-workers obtained tetra-p-tolylNCP (19b) in 4% yield by condensing a 7:4 ratio of pyrrole and p-tolualdehyde in the presence of boron trifluoride etherate in dichloromethane, followed by oxidation with pchloranil.73 In both cases, the major product was mesotetraphenylporphyrin. Subsequently, Geier et al. developed a superior procedure for synthesizing NCPs that involved reacting pyrrole and benzaldehyde in the presence of methanesulfonic

Figure 5. Synthetic and hypothetical porphyrin analogues.

these systems to the hypothetical bridged annulene structure 5,4,5,55 which is now known as quatyrin.15 However, these discussions used 5 to illustrate the presence of an [18]annulene subunit, and this system was not presented as a synthetic goal in its own right. Hoffmann suggested that a tetracarbaporphyrinoid framework could be used to stabilize a planar tetracoordinate carbon within the fenestrane structure 23,56,57 but this species would be expected to be antiaromatic and highly unstable.

4. SYNTHETIC STRATEGIES Syntheses of porphyrins and analogous systems commonly make use of the ability of pyrrole to readily undergo electrophilic substitutions at the α-positions. In the MacDonald condensation, a dipyrrylmethane dialdehyde (24) is reacted with a dipyrrylmethane (25a), or the related carboxylic acid 25b, in the presence of an acid catalyst to give a porphodimethene intermediate (26) (Scheme 2).58−60 Subsequent oxidation then affords the fully aromatic porphyrin product. This approach provides a very versatile route to porphyrin-type macrocycles, and its main limitation is that one of the reactants must possess a plane of symmetry to avoid the formation of two isomeric products. In order to develop syntheses of heteroporphyrins 27, Johnson and co-workers reacted tripyrrane 28 or heteroD


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Scheme 3. One-Pot Syntheses of Tetraarylporphyrin and NConfused Porphyrin

Figure 6. True carbaporphyrins.

Scheme 5. Synthesis of Carbaporphyrin Aldehydes

acid.74,75 Under optimized conditions, NCP was isolated in 39% yield and regular porphyrin was only present as a minor byproduct. meso-Tetrasubstituted heteroporphyrinoids can be prepared by reacting suitably substituted dicarbinols under Adler−Longo or Lindsey reaction conditions. For instance, thiophene dicarbinol 31 reacted with pyrrole in refluxing propionic acid to give dithiaporphyrin (32) (Scheme 4).76,77 In addition, condensation of 31 with pyrrole and aromatic aldehydes in the presence of boron trifluoride etherate, followed by oxidation with DDQ, afforded the thiaporphyrins (33).78,79 The same strategies can be applied to the preparation of oxa-, selena-, and telluraporphyrins.80−87 Furthermore, this approach has been successfully utilized in the preparation of carbaporphyrinoid systems such as benziporphyrins.88,89 Mixed condensations of pyrrole, azulene, and aromatic aldehydes have also been applied to the synthesis of tetraarylazuliporphyrins.90−92

(37b) was prepared similarly from 35b and 36a in 6.5% yield. In the original study, the NMR spectra for 37a and 37b showed the presence of additional peaks that were attributed to tautomeric species. However, these signals did not coalesce, even when solutions of 37 in deuterionaphthalene were run in “hightemperature NMR experiments”. In independent work, Lash and Hayes reacted diformylindene 38 with tripyrrane 36 in the presence of TFA, and following neutralization with triethylamine and oxidation with DDQ, benzocarbaporphyrin 39 was isolated in 43% yield (Scheme 6).93 Carbaporphyrin 39 proved to be a Scheme 6. Synthesis of Benzocarbaporphyrins

5. TRUE CARBAPORPHYRINS, CARBACHLORINS, AND RELATED SYSTEMS 5.1. Synthesis and Spectroscopic Properties

Porphyrin analogues with a cyclopentadiene moiety in place of a pyrrole subunit are known as carbaporphyrins, and this family includes ring-fused structures such as 34 (Figure 6).93 As related macrocycles are also sometimes referred to a carbaporphyrins, the term “true carbaporphyrins” was introduced to distinguish porphyrinoids like 1 and 34 from more modified systems such as NCPs 7 and azuliporphyrins 8.14,94 Berlin reported that triformylcyclopentadiene 35a reacted with tripyrrane 36 in the presence of HBr to give a formylcarbaporphyrin (37a) in 7.8% yield (Scheme 5).95 A related 3-methyl-2-formylcarbaporphyrin

strongly aromatic system, and the proton NMR spectrum showed the internal CH and NH resonances upfield near −7 and −4 ppm, respectively, while the external meso-protons appeared far downfield as two 2H singlets at 9.82 and 10.10 ppm. True carbaporphyrins possess 18π-electron delocalization pathways,

Scheme 4. Synthesis of Thia- and Dithiaporphyrins

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and the reported results are consistent with the [18]annulene model for porphyrinoid aromaticity.93,96 However, no indications of tautomeric species were evident in these spectra, and the proton NMR results were consistent with either a single tautomeric species (39) being present that possessed a plane of symmetry or mixed tautomers that interconverted rapidly on the NMR time scale. In order to gain a better understanding of this system, carbaporphyrins 39b and 39c were obtained by reacting the corresponding tripyrranes 36b and 36c with dialdehyde 38.93,96 It has been established that the 4JHH coupling constants for allylic compounds are substantially larger than those observed for alkyl aromatic compounds, and this can be correlated to the degree of aromatic character.28,29 If the [18]annulene model holds true, then the 12,13-bond should be more like a double bond, whereas the 7,8- and 17,18-bonds should be part of the aromatic delocalization pathway. Hence, it would be predicted that the 4JMeH coupling for pyrrolic hydrogens adjacent to methyl substituents would be larger in 39c than in 39b because the intermediary carbon−carbon bonds have more double-bond character in the former species. This was confirmed experimentally, as the 4JMeH coupling constant for 39c was 1.3−1.4 Hz compared to 0.9−1.0 Hz for 39b (Figure 7).97

Figure 8. Aromatic tautomers of benzocarbaporphyrin showing the calculated relative energies.98

spectra for samples prepared under these conditions.93,96 It was suggested that the original samples were not pure, and this was borne out to a certain extent because the UV−vis molar absorptivities for the new preparations were increased by a factor of approximately 2.5 compared to the original report. Tripyrranes are not very stable structures and are likely to be susceptible to acidolytic cleavage. This can lead to scrambling of the alkyl substituents due to a fragmentation−recombination mechanism (Scheme 7).99 The extent to which this side reaction occurs will be conditions-dependent, and so long as macrocycle formation occurs relatively rapidly, isomerically pure samples will be obtained. DFT calculations were performed on unsubstituted carbaporphyrin 1 and these confirm that this tautomer is the most favored (Figure 9).21 The related tautomer 1′ with two adjacent NHs was shown to be 6.01 kcal/mol higher in energy, but tautomer 1″ with an internal methylene unit was only 2.98 kcal/mol higher in energy. Tautomer 1‴, which places the NH next to the methylene group, was calculated to be 13.33 kcal/mol higher in energy. NICS calculations show that all of these structures have strong diamagnetic ring currents and provide support for the presence of 18π-electron delocalization pathways.21 The UV−vis spectra of carbaporphyrins 37a and 37b show the presence of strong Soret bands at 436 and 438 nm, respectively.93,96 For 37a (Figure 10), a moderately strong absorption was noted at 364 nm, and a series of Q bands appeared at 524, 566, 640, and 710 nm. The absorptions were slightly red-shifted for 37b. Benzocarbaporphyrin 39a gave a more intense Soret band at 424 nm, a smaller absorption at 376 nm, and Q bands at 510, 544, 602, and 662 nm (Figure 10). The proton NMR spectra for these carbaporphyrins demonstrate that they are highly diatropic. For instance, the internal CH for 39a gave an upfield resonance at −6.74 ppm, while the NH protons afforded a broad 2H peak at −4.00 ppm. As would be expected, the external meso-protons were shifted downfield and gave rise to two 2H singlets at 9.82 and 10.10 ppm. The two methyl groups that are directly connected to the macrocycle were deshielded to give a 6H singlet at 3.68 ppm, while the CH2 units of the ethyl substituents appeared between 3.97 and 4.07 ppm. A number of benzocarbaporphyrins 39a−i have been prepared, and these all show similar spectroscopic properties.93,96,100−102 The X-ray crystal structure of diphenyl benzocarbaporphyrin 39d was obtained, and this showed that the indene subunit was tilted by

Figure 7. 4J coupling constants for alkyl-substituted benzocarbaporphyrins.

Unlike 39a and 39b, 39c does not possess a plane of symmetry. The proton NMR spectrum of 39c at room temperature in CDCl3 gave rise to a broad peak near −4 ppm for the NH protons, but as the temperature was lowered this resolved into two separate resonances. This result indicates that NH exchange occurs moderately rapidly at room temperature but can be frozen out at −50 °C. However, as the rest of the spectrum is not affected to any significant extent, the results confirm that only one tautomer is present at significant concentrations.97 A number of tautomers can be envisaged for the benzocarbaporphyrin system that retain 18π-electron delocalization pathways: 40, 40′, 40″, and 40‴ (Figure 8).21,98 Density functional theory (DFT) calculations were performed on these unsubstituted structures. Tautomer 40′ was shown to be 5.36 kcal/mol higher in energy than 40, while 40″ and 40‴ were 10.15 and 17.11 kcal/ mol higher in energy. The decreased stability of 40′ was attributed to increased steric interactions and reduced hydrogenbonding interactions. Tautomers 40″ and 40‴, where the [18]annulene substructure runs through the benzene ring, were consistently found to have decreased stability, even though steric crowding within the macrocyclic cavity had been reduced. NICS calculations showed that all four tautomers have strong diatropic ring currents and confirmed the significance of the 18π-electron delocalization pathway.21 Condensation of trialdehydes 35a and 35b with 36a under the TFA-catalyzed reaction conditions also gave carbaporphyrin aldehydes 37a and 37b in modest yields, but no extra peaks attributable to tautomeric species were present in the NMR F


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Scheme 7. Mechanism for the Acidolytic Fragmentation−Recombination of Tripyrranes Resulting in Isomer Formation

Figure 9. Aromatic tautomers of carbaporphyrin showing the calculated relative energies.21

15.5° relative to the mean macrocyclic plane.96 This was due to the steric congestion arising from the presence of three hydrogens within the porphyrinoid cavity. The data also confirmed that the NH protons were present on N22 and N24, i.e., that the expected tautomer 39 was observed. Benzocarbaporphyrins with additional fused aromatic rings have also been prepared.96 Reaction of butanotripyrrane 36j with 38 under standard 3 + 1 conditions (Scheme 6) gave the tetrahydrobenzocarbaporphyrin 39j, and this was dehydrogenated with DDQ in refluxing toluene to give dibenzocarbaporphyrin 41 in 46% yield (Scheme 8). Acenaphthylene-fused tripyrrane di-tert-butyl ester 4287 was treated with TFA to remove the protective groups and then condensed with indene dialdehyde 38 in 5% TFA−CH2Cl2. Following oxidation with DDQ, acenaphthobenzocarbaporphyrin 43 was generated in 29% yield.96 Phenanthrene-fused tripyrrane 44 similarly reacted with 38 to give the phenanthrobenzocarbaporphyrin 45 in 28% yield (Scheme 8).96 Recently, the preparation of a naphtho[2,3b]carbaporphyrin 46 from benzoindene dialdehyde 47 has been noted (Scheme 9).103

Figure 10. UV−vis spectra of carbaporphyrin 37a (upper spectrum) and benzocarbaporphyrin 39a (lower spectrum) in dichloromethane.96

Ring fusion has little or no effect on the diatropic properties of carbaporphyrins 41, 43, 45, and 46. The UV−vis spectrum for dibenzocarbaporphyrin 41 showed minor bathochromic shifts, and the absorption bands appeared at 386, 432 (Soret band), G


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was treated with TFA to cleave the protective groups and condensed with indene dialdehyde 38, followed by oxidation with DDQ, to give carbaporphyrin 50 in 12% yield as a mixture of stereoisomers. Upon heating to 200 °C, 50 underwent three retro-Diels−Alder reactions with extrusion of ethylene to give 48 in quantitative yield. Unfortunately, tetrabenzocarbaporphyrin 48 proved to be poorly soluble, and no NMR data for this compound was provided.107 An alternative synthesis of benzocarbaporphyrins from a carbatripyrrin 51 was recently reported (Scheme 11).108 Fulvene

Scheme 8. Synthesis of Benzocarbaporphyrins with Additional Fused Rings

Scheme 11. Synthesis of a Carbatripyrrin

52 was prepared in high yields by reacting pyrrole-2carbaldehyde with indene in the presence of KOH in refluxing ethanol. Reduction with lithium aluminum hydride afforded the corresponding dihydro derivative 53, and this was further reacted with pyrrole-2-carbaldehyde and KOH in refluxing ethanol in an attempt to prepare E-carbatripyrrene 54. Under dilute conditions, a mixture of E- and Z-isomers of 54 was formed, but at higher concentrations, carbatripyrrin 51 precipitated from the reaction mixture in 75% yield. Although this product had not been anticipated, it is structurally preorganized to facilitate the formation of porphyrinoid systems. Reaction of 51 with pyrrole dialdehyde 55a in the presence of TFA in CH2Cl2 gave benzocarbaporphyrin 56a in 51% yield (Scheme 12). Dialdehydes 55b and 55c similarly reacted with 51 to give carbaporphyrins 56b and 56c, while phenanthropyrrole dialdehyde 57 condensed with the carbatripyrrin to afford phenanthrocarbaporphyrin 58. An advantage of this strategy is that no oxidation step is required to generate the fully aromatic macrocyclic products. Interestingly, although most benzocarbaporphyrins give brown solutions, 56a gave green-colored solutions. The UV−vis spectrum of 56a gave a Soret band at 419 nm and Q bands at 500, 603, and 663 nm. As would be expected, the new carbaporphyrin structures gave NMR spectra that were consistent with highly diatropic compounds.108

Scheme 9. Synthesis of a Naphthocarbaporphyrin

518, 552, 610, and 672 nm.96 Larger shifts were observed for the phenanthrene-fused carbaporphyrin 45, which gave absorptions at 390, 442 (Soret band), 496, 529, 568, 618, and 677 nm.96 Acenaphthoporphyrinoid 43 gave the largest shift for the Soret band, but the remaining absorptions were less affected. Specifically, 43 showed absorptions at 343, 395, 454 (Soret band), 509, 544, 587, and 624 nm.96 Acenaphthylene-fused porphyrins have been shown to give rise to exceptionally large red shifts,65,66,104−106 but the effects of acenaphthylene fusion in the carbaporphyrin series are much reduced. The same synthetic strategy was used to prepare a tetrabenzocarbaporphyrin (48) (Scheme 10).107 A tripyrrane di-tert-butyl ester (49) with three fused bicyclooctadiene units Scheme 10. Synthesis of Tetrabenzocarbaporphyrin

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to two 2H multiplets at 7.71−7.74 and 8.68−8.71 ppm, while the meso-protons afforded two 2H singlets at 10.06 and 10.33 ppm. Further addition of TFA led to a broad complex spectra that could not be interpreted, but when the acid concentration was raised to 50% TFA−CDCl3, a new species attributable to Cprotonated dication 39aH22+ was observed. This species also exhibits a strong diamagnetic ring current, although the 18πelectron delocalization pathway has been relocated through the fused benzene ring.96 The internal methylene unit afforded a 2H singlet at −5.21 ppm, while the NHs gave rise to broad resonances near −1.6 ppm. The external meso-protons were drastically shifted downfield to give two 2H singlets at 10.46 and 11.07 ppm, and importantly, the benzo-unit now gave rise to two 2H multiplets at 8.94−8.96 and 10.14−10.17 ppm. The latter results confirmed that the macrocyclic ring current runs though the benzene ring, as these peaks are much further downfield than the equivalent resonances in the NMR spectra of 39a or monocation 39aH+. Dication 39aH22+ can be considered to be an example of a bridged benzo[18]annulene, although this species has considerably more aromatic character than the neutral hydrocarbon 59 (Figure 11).109 The UV−vis spectrum for

Scheme 12. Synthesis of Carbaporphyrins from a Carbatripyrrin

5.2. Protonation of Carbaporphyrins

Addition of trace amounts of acid to carbaporphyrins leads to the formation of monoprotonated cations, but in the presence of a large excess of acid, diprotonated dications are generated.93,96 This is best illustrated for benzocarbaporphyrin 39a (Scheme 13). Addition of trace amounts of TFA to solutions of 39a gave rise to the monocation 39aH+. The UV−vis spectrum for this species in 0.01% TFA−CH2Cl2 gave absorptions at 307, 399, 437 (Soret band), 473, 550, 586, and 611 nm. The proton NMR spectrum for 39aH+ demonstrated that the monocation has comparable diatropicity to the free-base carbaporphyrin, and the interior protons gave rise to resonances at −6.75 (CH), −4.61 (NH), and −3.22 ppm (2 × NH). The benzo-protons gave rise

Figure 11. Structure of Benzo[18]annulene.

Scheme 13. Protonation of Benzocarbaporphyrins

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dication 39aH22+ in 50% TFA−CH2Cl2 gave a strong Soret band at 426 nm (ε = 1.86 × 105) and weaker absorptions at 348, 614, and 662 nm. Addition of 50% TFA-d to solutions of 39 in CDCl3 resulted in rapid exchange for the NH and 21-CH protons, as would be expected, but slow exchange was also noted at the mesopositions.96 Exchange at the C-10 and C-15 positions occurred approximately 50% faster than at the C-5 and C20 positions. These results imply that minor C-protonated tautomers, such as 60a−d or the related dications, are in equilibrium with 39H+ and 39aH22+ (Scheme 13). Similar results were obtained for ringfused carbaporphyrins 41, 43, and 45, as well as for formylcarbaporphyrins 37a and 37b. For instance, 37a formed a monocation 37H+ in the presence of trace amounts of TFA (Scheme 14), and this exhibited strongly diatropic character-

rise to three separate resonances at −3.38, −2.31, and −2.23 ppm. In 50% TFA, a C-protonated dication 37aH22+ was generated that exhibited an enhanced diatropic ring current. The interior CH2 gave a singlet at −7.42 ppm, while the meso-protons gave rise to four downfield singlets between 10.99 and 12 ppm. The UV−vis spectrum for the dication in 50% TFA−CHCl3 gave a Soret band at 410 nm and weaker absorptions at 548, 600, and 664 nm. The proton NMR spectra for 37a in 50% TFA-d− CDCl3 showed rapid exchange for the NH and 21-CH protons and slow exchange for the 10-CH and 15-CH protons. The latter observation indicates that meso-C-protonated species such as 61a and 61b are in equilibrium with 37aH+ and 37aH22+ (Scheme 14).96

Scheme 14. Protonation of Formylcarbaporphyrins

Attempts to metalate carbaporphyrins were initially unsuccessful, and metal cations such as nickel(II), palladium(II), and copper(II) failed to give isolatable derivatives.110,111 However, benzocarbaporphyrin 39a reacted with silver(I) acetate at room temperature to give a stable organometallic silver(III) derivative 62a (Scheme 15).94,101 The complex proved to be nonpolar and eluted from a grade 3 alumina column as an orange-colored solution. The proton NMR spectrum in CDCl3 showed the mesoprotons downfield at 9.89 amd 10.06 ppm, confirming that the silver complex retains the diatropic nature of the carbaporphyrin system. The UV−vis spectrum showed a Soret band at 437 nm, followed by a series of Q bands at 482, 518, 555, and 593 nm. The mass spectrum for 62a showed the characteristic isotope pattern for 107Ag and 109Ag, which are present at natural abundance in an approximately 1:1 ratio. Diphenylcarbaporphyrin 39d reacted similarly to give silver(III) complex 62b, and this was further characterized by single-crystal X-ray diffraction. Unlike the carbaporphyrin free-base 39d, the silver complex was near planar and the indene unit was only tilted by 5.09° relative to the mean macrocyclic plane. The organometallic derivatives were also analyzed by cyclic voltammetry and these results were consistent with the formation of a silver(III) derivative. As excess silver(I) acetate was used in the preparation of the silver(III) complexes, it was suggested that the oxidation of the Ag(I) to Ag(III) was due

5.3. Reactivity of Carbaporphyrins

istics. The proton NMR spectrum showed three of the mesoprotons as singlets between 9.7 and 10.1 ppm, while the fourth resonance (20-CH) appeared further downfield at 10.54 ppm due to the proximity of this proton to the aldehyde moiety. In addition, the interior protons were strongly shifted upfield, and the 21-CH afforded a singlet at −5.83 ppm, while the NHs gave Scheme 15. Metalation of Benzocarbaporphyrins

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Scheme 16. Silver(III) and Gold(III) Complexes of Tetraarylbenzocarbaporphyrins

Scheme 17. Alkylation of a Benzocarbaporphyrin

rhodium complex. Again, the macrocycle maintained strongly aromatic characteristics, and the meso-protons were shifted downfield to give two 2H singlets at 9.40 and 9.83 ppm. The pyridine protons were shielded by the aromatic ring current and gave resonances at 1.44, 4.84, and 5.81 ppm.112 meso-Tetraarylbenzocarbaporphyrins 66 were also available from the ring contraction of azuliporphyrins (see section 9).90,91 Tetraphenyl and tetrakis(4-chlorophenyl) derivatives 66a and 66b reacted with silver(I) acetate in pyridine to give 80−97% yields of the related silver(III) complexes 67 (Scheme 16).101,113 Gold(III) acetate also reacted with 66a and 66b under these conditions to give the gold(III) complexes 68 in 67−83% yield. The UV−vis spectrum for silver complex 67a gave a Soret band at 450 nm and weaker absorptions at 396, 525, and 566 nm, while the related gold(III) derivative 68a afforded a Soret band at 437 nm and smaller absorptions at 395, 519, 561, and 608 nm. Both the silver(III) and gold(III) tetraarylcarbaporphyrins possessed strongly diatropic characteristics, and the proton NMR spectra for 67 and 68 showed the pyrrolic protons between 8.6 and 8.8 ppm. Transannular coupling to the pyrrolic protons in 67 from the NMR-active 107/109Ag nuclei was also observed.101 Carbaporphyrins have three internal protons and commonly act as trianionic ligands. In an attempt to modify this behavior, Nalkylation of benzocarbaporphyrin 39a was investigated.114 Reaction of 39a with excess methyl iodide and potassium carbonate in refluxing acetone gave a mixture of two monoalkylated products that were easily separated by column chromatography (Scheme 17). The major product was the Nmethyl derivative 69a (62%) and the minor byproduct was identified as a C-methylcarbaporphyrin (70a) (10%). The alternative, more symmetrical, N-methyl product 71a was not observed. Similar results were obtained with ethyl iodide, although longer reaction times were required, and the related ethylated derivatives 69b and 70b were isolated in 69% and 10% yields, respectively. The presence of an internal alkyl substituent did not have a significant effect on the aromatic properties of these carbaporphyrins. The proton NMR spectrum for 69a in CDCl3 showed the presence of the 21-CH at −6.20 ppm, while the N-methyl group gave a 3H singlet at −4.10 ppm and the NH gave rise to a broad peak at −2.45 ppm. The external mesoprotons afforded four 1H singlets due to the loss of symmetry,

to the simultaneous reduction of two silver(I) ions to produce silver metal according to the following equation: 3Ag + → Ag 3 + + 2Ag 0

Small deposits of silver are observed in these reactions. Carbaporphyrins 39e, 39f, and 39g all reacted with silver(I) acetate to give the related silver(III) derivatives 62e−g (Scheme 15).94,101 Attempts were made to react carbaporphyrins with gold salts to form the related gold(III) complexes. Reactions of carbaporphyrins 39 with gold(I) iodide gave very poor results. However, 39d reacted with gold(III) acetate in pyridine to afford the related gold(III) complex 63 in 7% yield.101 This derivative retained strongly diatropic character and the UV−vis spectrum was similar to those obtained for silver(III) derivatives 62. Recently, rhodium and iridium complexes of benzocarbaporphyrin 39a have been reported (Scheme 15).112 Reaction of 39a with 1 equiv of di-μ-chlorotetracarbonyldirhodium(I) in refluxing dichloromethane gave the rhodium(I) dicarbonyl complex 64 in 90% yield. The proton NMR spectrum for 64 demonstrated that the porphyrinoid retained strongly diatropic properties, and the internal CH and NH protons gave rise to resonances at −5.52 and −2.54 ppm, respectively. The X-ray structure for 64 showed that the macrocycle is fairly planar, although the rhodium is pivoted to one side of the system. Upon refluxing 64 in pyridine, a hexacoordinate rhodium(III) complex (65a) was generated in 55% yield. This complex proved to be poorly soluble in organic solvents, and the proton NMR spectrum showed the meso-protons as two broad singlets at 9.33 and 9.74 ppm. These values are consistent with the presence of a strong diamagnetic ring current. As would be expected, the axial pyridine units were strongly shielded and afforded resonances for the α-, β-, and γ-protons at 1.17, 4.80, and 5.75 ppm, respectively. The X-ray crystal structure for 65a showed that the porphyrinoid system was flat with the pyridine ligands orientated at right angles to the macrocyclic plane. Benzocarbaporphyrin 39a failed to react with [Ir(COD)Cl]2 in refluxing pyridine. However, when these reactants were heated in refluxing p-xylene, iridium(III) derivative 65b was formed in 22% yield. XRD analysis showed that 65a and 65b were structurally virtually identical. The iridium complex had superior solubility and the proton NMR spectrum gave much better resolution than the K


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Scheme 18. Formation of a Palladium Complexes of Benzocarbaporphyrins

and these appeared strongly downfield at 9.52, 9.67, 9.92, and 9.94 ppm. C-Methyl carbaporphyrin 70a gave comparable results, showing the interior methyl resonance at −5.16 ppm and a broad NH peak (2H) at −4.0 ppm, while the meso-protons appeared as two 2H singlets at 9.62 and 9.78 ppm. Unlike 69a,b, C-alkylated carbaporphyrins 70a,b retain a plane of symmetry. Reaction of 69a with palladium(II) acetate in refluxing acetonitrile for 30 min gave rise to a palladium(II) complex 72a, where the internal methyl substituent had migrated from the nitrogen to C21, rather than the expected N-methyl palladium(II) complex 73a (Scheme 18).114 N-Ethyl carbaporphyrin 69b reacted similarly to produce the C-ethyl palladium(II) complex 72b. When 69a was reacted with palladium(II) acetate for 5 min, the NMR spectrum for the crude product appeared to mostly consist of 73a, implying that alkyl group migration occurred following metalation. However, attempts to purify 73a were unsuccessful, and a substantial amount of the material was converted into 72a during column chromatography. It is possible that this migration involves a concerted [1,5]sigmatropic shift or a stepwise mechanism involving a palladium alkyl species. The 18π-electron delocalization pathway for 72a and 72b runs through the fused benzo-unit, but these complexes still show strongly aromatic properties. The proton NMR spectrum for 72a in CDCl3 confirmed that the product had regained a plane of symmetry and showed the internal methyl resonance at −3.21 ppm, while the external meso-protons gave rise to two 2H singlets at 9.56 and 10.27 ppm. The benzo-protons were also shifted further downfield, reflecting the relocation of the aromatic ring current, giving two 2H multiplets at 8.22−8.26 and 9.40−9.43 ppm. However, the UV−vis spectrum for 72a was more modified and gave no porphyrin-like Soret band, producing moderatesized peaks at 337, 421, and 697 nm instead. Related complexes were independently obtained by the ring contraction of pbenziporphyrins (see section 11.11).115 Reactions of benzocarbaporphyrins 39 with ferric chloride did not lead to metalation but instead gave rise to regioselective oxidation reactions.116,117 When 39a was refluxed with 500 equiv of ferric chloride in chloroform−methanol, a dimethyl ketal structure 74a was generated in 94% yield (Scheme 19). This derivative was isolated as a monoprotonated species 74H+, generally as the hydrochloride salt 74·HCl, but can be further protonated in the presence of TFA to give the related dication 74aH22+. When reactions were performed in the presence of ethanol, isopropyl alcohol, or ethylene glycol, the related carbaporphyrin ketals (CPKs) 74b−d were generated in good yields, although longer reaction times were necessary.116,117 Diphenylcarbaporphyrin 39d reacted similarly with methanol or ethanol in the presence of 500−600 equiv of FeCl3 to give CPKs 74e and 74f in 79% and 42% yields, respectively.117 The proton and carbon-13 NMR spectra for the carbaporphyrin ketals demonstrated that these derivatives have a plane of symmetry and clearly possess substantial diatropic characteristics. The proton NMR spectrum for 74aH+ gave a 6H singlet for the

Scheme 19. Preparation of Carbaporphyrin Ketals

methoxy substituents at −1.34 ppm, while the meso-protons were strongly shifted downfield to give two 2H singlets at 9.68 and 10.93 ppm. The 18π-electron delocalization pathways for CPKs must run through the fused benzene rings, and these structures can be considered to be further examples of bridged [18]annulenes. For this reason, the benzene protons are also highly deshielded and give rise to two 2H multiplets at 8.91−8.95 and 10.47−10.50 ppm. This can be attributed to 18π- or 22π-electron pathways shown in bold for structures 74H+ and 74′H+. The NH protons appeared as a broad resonance at 2.2 ppm, somewhat downfield from the values commonly associated with aromatic porphyrinoids, but this shift can be explained as being partially due to the delocalized positive charge over the macrocycle and more importantly because the protons fall into an environment that facilitates strong hydrogen-bonding interactions. The UV− vis spectrum of 74a·HCl in CHCl3 gave a Soret band at 422 nm and two strong absorptions in the far red at 751 and 832 nm (Figure 12). Addition of TFA led to the formation of the related dication 74aH22+ and this gave a Soret band at 434 nm and Q-like bands at 633, 683, and 748 nm. The NMR spectrum of dication 74aH22+ showed the meso-protons at 9.99 and 10.64 ppm, while the methoxy protons appeared at −0.96 ppm, suggesting that the diamagnetic ring current is slightly reduced compared to that of monocation 74H+. However, as the benzo-protons appeared downfield as two 2H multiplets at 9.02−9.05 and 10.05−10.09 ppm, the macrocyclic delocalization pathways must still involve the fused arene unit. Similar results were obtained for carbaporphyrin ketals 74b−f. Addition of 5% DBU to solutions of 74H+ led to the formation of a new chromophore that was L


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As reactions carried out in the presence of alcohols led to the formation of ketals, it was anticipated that reactions with aqueous ferric chloride would afford the corresponding dihydroxy products 79 or the related ketone (Scheme 22).117 However, when a solution of 37a in chloroform or dichloromethane was reacted with FeCl3 in water under reflux for 1−3 h, a chlorobenzocarbaporphyrin (80a) was generated in up to 84% yield. The X-ray crystal structure for 80a showed that the indene subunit was tilted from the mean macrocyclic plane by 29.6° due to the presence of the large internal chlorine atom. However, the NMR spectrum for 80 showed that it possessed a diatropic ring current comparable to that of 39a, although the CH resonance was no longer present. In addition, the UV−vis spectrum was similar, showing a split Soret band at 421 and 434 nm and Q bands at 521, 560, 608, and 665 nm. When the reaction was carried out with ferric bromide under the same reaction conditions, the corresponding 21-bromo derivative 80b was isolated in 7% yield. Even though the larger bromine atom will undoubtedly further distort the macrocycle, the proton NMR spectrum for 80b showed that the macrocycle retained nearly all of its diatropic characteristics. When reactions with aqueous ferric chloride were carried out for 16 h, a polar green byproduct (81) could be isolated in 22% yield (Scheme 22). The proton NMR spectrum for this diketone showed that this species was nonaromatic. The two carbonyl units gave rise to resonances at 173.6 and 197.6 ppm in the carbon-13 NMR spectrum, but the IR spectrum showed the CO stretching peaks at unusually low wavenumber values of 1574 and 1591 cm−1. The low frequencies can be attributed to the vinylogous amide characteristics of these bonds, which greatly reduce the bond strength, although the internal carbonyl group will also be involved in strong intramolecular hydrogen-bonding interactions (Scheme 23). A mechanism to explain the formation of these products has been proposed (Scheme 24).117

Figure 12. UV−vis spectra of carbaporphyrin ketal 74a in dichloromethane (free base, purple line) and 5% TFA−dichloromethane (red line, dication 74aH22+).117

tentatively assigned as the free-base form 74, but this species was unstable and degraded on standing.117 The far-red absorptions exhibited by CPKs indicate that this system could find applications, for instance, as photosensitizers in photodynamic therapy. In fact, CPK 74a·HCl has been shown to be an effective agent in the treatment of leishmaniasis.118−120 In the presence of light, they act as photosensitizers capable of producing reactive oxygen species119 and inducing in vitro NO formation.120 As fused acenaphthylene rings are known to induce substantial red shifts in porphyrinoid systems, the reaction of acenaphthocarbaporphyrin 43 with ferric chloride in chloroform−methanol was investigated (Scheme 20).117 The resulting ketal 75·HCl gave a Soret band at 475 nm and longer wavelength absorptions at 606, 668, 752, and 833 nm. Although the Soret band undergoes a significant bathochromic shift, the effect on the longer wavelength absorptions is relatively small. In the presence of TFA, the corresponding dication 75H22+ gave a Soret band at 492 nm and a longer wavelength band at 749 nm. Red shifts are indeed observed in this case; these are not particularly valuable given the relative inaccessibility of this CPK.117 The formation of CPKs from benzocarbaporphyrins is surprisingly regioselective. A mechanism to explain this oxidation was proposed that involves a series of one-electron transfers from the iron(III) cation (Scheme 21). In a two-step process, 39 is oxidized to give the potentially antiaromatic dication 76, which may be prone to nucleophilic attack from an alcohol. Subsequent loss of a proton would afford alkoxycarbaporphyrin 77. Protonation would result in a species that is electron-deficient at C21, and further reaction with an alcohol would give a dialkoxy derivative 78. A further two-step two-electron oxidation and loss of a proton would afford CPK 74H+.117

5.4. Synthesis and Reactions of Carbachlorins and a Carbaporphyrin with an Unsubstituted Cyclopentadiene Ring

2,3-Dihydroporphyrins or chlorins (12) are important systems that provide the structural nucleus for many of the chlorophylls.121−123 Attempts to prepare a chlorin analogue (82) in the carbaporphyrin series by condensing cis-1,3cyclopentanedicarbaldehyde (83) with tripyrrane 36a under standard conditions failed to give any macrocyclic product (Scheme 25).124 However, bicyclo[3.3.0]octane dialdehyde 84a and the related alkene 84b reacted with 36a to give, following oxidation with DDQ in refluxing toluene, the ring-fused carbachlorins 85a and 85b, respectively, in 11−15% yield (Scheme 25).124 Carbachlorins 85 gave porphyrin-like UV−vis spectra and proved to be highly diatropic. The proton NMR spectrum for 85a showed the internal CH resonance at −6.93 ppm, while the meso-protons gave two downfield 2H singlets at

Scheme 20. Synthesis of Carbaporphyrin Ketals with a Fused Acenaphthylene Unit

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Scheme 21. Proposed Mechanism for the Formation of Carbaporphyrin Ketals

monoprotonated cations 85H+. The UV−vis spectrum of 85aH+ gave a split Soret band at 410 and 424 nm and a series of weaker absorptions at 534, 546, 580, and 634 nm. The proton NMR spectrum of 85aH + in TFA−CDCl 3 showed enhanced diatropicity, with the 21-CH appearing upfield at −7.06 ppm and the NH protons resonating at −5.57 (1H) and −4.2 (2H) ppm, while the meso-protons were strongly deshielded to give two 2H singlets at 9.58 and 10.09 ppm. Although chlorins are easily oxidized to porphyrins, carbachlorins 85 could not be converted into the corresponding carbaporphyrins.124 In most MacDonald-type “3 + 1” condensations, the dialdehyde component corresponds to an unsaturated or aromatic system. Aliphatic dialdehydes would not be expected to have equivalent reactivities, and this may contribute to the relatively low yields of carbachlorins generated from bicyclooctane dialdehydes 84. It was speculated that a conjugated cyclopentene dialdehyde (86a) (Scheme 26), which more closely resembles indene dialdehyde 38, might give superior results in MacDonald-type “3 + 1” condensation reactions.125 Hydrolysis of acetal 87 with TFA in a biphasic water−chloroform mixture afforded the enol ether 86b rather than the expected dialdehyde 86a (Scheme 26). However, as these structures are synthetically equivalent, 86b was taken on in a “3 + 1” MacDonald-type reaction with tripyrrane 36a. Following purification by column chromatography and recrystallization, carbachlorin 82 was isolated in 11−16% yield.125 Carbachlorin 82 gave a UV−vis spectrum similar to that of 85a, showing a Soret band at 401 nm and Q bands at 495, 593, and 651 nm (Figure 13). Addition of TFA afforded the corresponding cation 82H+, and this gave a split Soret band at 409 and 426 nm. Two aromatic tautomers can be considered for carbachlorins, 88a and 88b (Scheme 27), but DFT calculations indicate that 88a is 4.79−5.00 kcal/mol for stable than 88b (ΔG = 4.59 kcal/ mol).125 The proton NMR spectrum of 82 demonstrates that the carbachlorin is strongly diatropic, and this is confirmed by NICS calculations that are also consistent with the presence of a preferred 18π-electron delocalization pathway. Unlike carbachlorins 85, 82 was dehydrogenated with DDQ in refluxing toluene to give the corresponding carbaporphyrin 89 in up to 71% yield (Scheme 28). The UV−vis spectrum for 89 gave a weakened Soret band at 421 nm and Q bands at 510 and 585 nm (Figure 14). The aromatic ring current for 89 was evident in the proton NMR spectrum, which showed the external meso-protons as two 2H singlets at 9.77 and 8.83 ppm, while the interior NH and CH protons afforded resonances at −3.92 and −6.91 ppm, respectively. The external cyclopentadiene protons gave rise to a

Scheme 22. Reaction of a Benzocarbaporphyrin with Aqueous Ferric Chloride

Scheme 23. Dipolar Resonance Contributors for a Carbaporphyrin Diketone

9.15 and 9.81 ppm. The UV−vis spectrum for 85a afforded a strong Soret band at 404 nm (ε = 2.09 × 105) and a series of Q bands at 496, 592, and 650 nm. The longest wavelength band is slightly intensified and is reminiscent of the spectra seen for true chlorins.122,123 Addition of TFA afforded the corresponding N


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Scheme 24. Proposed Mechanism for the Formation of a Chlorocarbaporphyrin and a Nonaromatic Diketone from Benzocarbaporphyrin

Scheme 25. Synthesis of Carbachlorins with Fused FiveMembered Rings

Scheme 26. 3 + 1 Synthesis of a Carbachlorin

2H doublet (J = 1.6 Hz), due to coupling to the 21-H, at 8.15 ppm. The reduced downfield shift for these protons indicates that the C2−C3 double bond does not significantly contribute to the aromatic delocalization pathways. Addition of trace amounts of TFA afforded a related monocation (89H+) that gave a Soret band at 385 nm, but further addition of TFA produced a new species that corresponded to dication 89H22+. The UV−vis spectrum of 89H22+ gave a strong split Soret band at 400 and 410 nm, together with weaker absorptions between 500 and 600 nm. The proton NMR spectrum for 89H22+ in TFA−CDCl3 clearly showed that the aromatic characteristics of the dication have

Figure 13. UV−vis spectra of carbachlorin 82 in 1% triethylamine− dichloromethane (free base, purple line) and 1% TFA−dichloromethane (cation 82H+, red line).125

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Scheme 27. Carbachlorin Tautomers

been enhanced compared to the free-base form 89. The mesoprotons appeared downfield as two 2H singlets at 11.00 and 11.45 ppm, while the internal CH gave a strongly upfield singlet at −8.27 ppm. The external cyclopentadiene protons were also strongly shifted downfield to 11.11 ppm, because the 18πelectron delocalization pathway now passes through the outside of the five-membered carbon ring.125 Carbachlorin 82 reacted slowly with silver(I) acetate to give a 31% yield of the corresponding silver(III) complex 90 (Scheme 29).125 When a larger excess of silver acetate was used, a small amount of impure silver(III) carbaporphyrin 91 was noted, but attempts to prepare 91 directly from carbaporphyrin 89 were unsuccessful. Silver(III) carbachlorin gave a strong Soret band at 411 nm and a series of Q bands at 492, 524, 553, and 599 nm. As would be expected, the metalloporphyrinoid retained highly diatropic characteristics, and the proton NMR spectrum showed the external meso-protons at 9.23 and 9.90 ppm. Carbachlorin 82 was heated with methyl iodide and potassium carbonate in acetone and afforded the related N-methylchlorin 92 in 34% yield (Scheme 30). The N-alkylated chlorin retained a porphyrin-like UV−vis spectrum, and the proton NMR spectrum demonstrated that the structure is highly aromatic. The internal N-methyl group gave a 3H singlet at −4.27 ppm, while the 21-H was observed further downfield at −6.31 ppm. As the methyl group has been introduced at N-22 rather than N-23, the macrocycle no longer has a plane of symmetry, and as a consequence, the meso-protons show up as four separate singlets between 8.89 and 9.58 ppm. Addition of TFA gave cation 92H+, and the resulting UV−vis spectrum showed a split Soret band at 415 and 428 nm. When 92 was reacted with palladium(II) acetate in refluxing acetonitrile, a palladium(II) carbaporphyrin (93) was generated, together with the formation of impure palladium(II) chlorin 94 (Scheme 30). It is noteworthy that the formation of 93 involves a metalation, an oxidation, and the migration of the methyl substituent from N22 to C21. In initial experiments, 93 was isolated in 17% yield, but when the crude reaction mixture was treated with aqueous ferric chloride solution, the yield was raised to 34%. It is likely that metalation to form 94 occurs first, followed by oxidation to carbaporphyrin and then migration of the alkyl substituent. The UV−vis spectrum of palladium complex 93 did not resemble the spectra for porphyrins but instead gave moderately sized absorption bands at 394, 440, 524, and 612 nm. The proton NMR spectrum

Figure 14. UV−vis spectra of carbaporphyrin 89 in 1% triethylamine− dichloromethane (free base, purple line) and 10% TFA−dichloromethane (dication 89H22+, red line).125

Scheme 29. Synthesis of a Silver(III) Carbachlorin

demonstrated that the system is strongly aromatic and the 21methyl group was shifted upfield to −4.46 ppm. Due to the methyl group migration, the complex has regained a plane of symmetry, and for this reason, only two meso-proton resonances were observed, which showed up at 10.00 and 10.42 ppm. The 18π-electron delocalization pathway for the complex must pass through the outside of the cyclopentadiene ring, and this leads to the 2,3-protons appearing at a comparatively downfield value of 9.60 ppm.125 5.5. Synthesis and Reactivity of Heterocarbaporphyrins

Heteroanalogues of the carbaporphyrins have also been reported.126,127 Prior to those studies, oxa- and thiacarbaporphyrins 95−98 (Scheme 31) were investigated computationally.128 DFT studies demonstrated that 22-oxa-21-carbaporphyrin 96a was favored over tautomer 96b by 1 kcal/mol, but both structures are 7−8 kcal/mol less stable than the isomeric 23oxocarbaporphyrin 95. For thiacarbaporphyrins 97 and 98, 97

Scheme 28. Dehydrogenation of a Carbachlorin To Give a Carbaporphyrin

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Soret band at 428 nm, a secondary absorption at 371 nm, and a series of Q bands at 521, 619, 677, and 709 nm.127,129 Although the spectrum was similar to those obtained for benzocarbaporphyrins, the Soret band is weaker and somewhat broadened. The UV−vis spectrum for thiacarbaporphyrin gave a Soret band at 431 nm, a weaker absorption at 387 nm, and Q bands at 527, 623, and 683 nm. The related selenacarbaporphyrin 100c (Scheme 33) retained fully aromatic properties, and the proton NMR spectrum showed the internal CH resonance at −5.90 ppm, while the external meso-protons appeared downfield near 10 ppm. The UV−vis spectrum of 100c gave a broad Soret band at 436 nm and Q bands at 533, 565, and 630 nm. The proton NMR spectrum for the free-base form of 100a was obtained in pyridine-d5, and this gave a broad peak for the internal CH at −5 ppm, while the external meso-protons afforded two 2H singlets at 10.35 and 10.57 ppm, and these data confirm that this porphyrinoid is highly diatropic. Similarly, 100b and 100c in CDCl3 gave the internal CH resonances at −5.49 and −5.38 ppm, respectively, while the meso-protons were shifted downfield to give two singlets between 10.0 and 10.5 ppm. Addition of TFA to solutions of 100b and 100c gave the related monocations 100H+ (Schemes 32 and 33). The proton NMR spectrum for carbaporphyrin hydrochloride 100a·HCl in CDCl3 gave an upfield singlet corresponding to the 21-H at −6.45 ppm, while the interior NHs yielded a 2H resonance at −1.53 ppm, and the outer meso-protons afforded two 2H singlets at 10.08 and 10.32 ppm. Even when a large excess of TFA was added, 100b and 100c still favored the monoprotonated forms. In TFA−CDCl3, monocations 100b and 100c gave internal CH resonances at −7.45 and −5.57 ppm, together with broad NH signals at −5.86 and −4.31 ppm, respectively. The meso-protons for 100b were observed at 10.21 and 10.71 ppm, while these resonances appeared at 10.42 and 10.99 ppm for 100c. Addition of TFA-d to solutions of 100a−c in CDCl3 showed immediate deuterium exchange of the 21-H and NH protons, but unlike benzocarbaporphyrins 39, no exchange was noted for 100a and 100b at the meso-protons. However, selenacarbaporphyrin 100c did show slow exhange at the meso-positions. The results indicate that C-protonated dications 100H22+ are in equilibrium with the monoprotonated forms 100H + , but only the selenacarbaporphyrin system is in equilibrium with minor meso-protonated species. Although carbaporphyrin 39 is completely converted to the corresponding dication 39H22+ in 50% TFA−CDCl3, oxacarbaporphyrin 100a gave an NMR spectrum that was consistent with a mixture of different species, while the NMR spectra for 100b and 100c appeared to correspond to the monocations 100H+. The presence of an electronegative oxygen atom in 100a is likely to destabilize the related dication, while the dications of 100b and 100c are

Scheme 30. Alkylation and Metalation of a Carbachlorin To Give a Palladium(II) Carbaporphyrin

Scheme 31. Heterocarbaporphyrins

was predicted to be 3 kcal/mol more stable than the favored tautomer for 98. Tautomer 98a was calculated to be 3 kcal/mol more stable than tautomer 98b. Calculations for the bond lengths and angles were also reported.128 Oxa- and thiacarbaporphyrins were initially prepared by reacting indene dialdehyde 38 with oxa- or thiatripyrranes 99 in the presence of TFA, followed by oxidation with DDQ (Scheme 32).127,129 23-Oxa-21-carbaporphyrin 100a was isolated in 41% yield, while the thia-analogue 100b was obtained in 31% yield. Related oxa-, thia-, and selenaporphyrins have also been obtained from the ring contraction of heteroazuliporphyrins (see section 9).126,130 The high basicity of 100a led to its isolation in a monoprotonated form as the hydrochloride derivative 100a·HCl. The UV−vis spectrum of free-base 100a was obtained in 5% triethylamine−chloroform, and this showed a Scheme 32. 3 + 1 Syntheses of Heterocarbaporphyrins

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Scheme 33. Protonation of a Selenacarbaporphyrin

ppm. Addition of TFA gave a protonated species that was tentatively attributed to 102H+ (Scheme 34).124 An alternative synthesis of 23-heterocarbaporphyrins from carbatripyrrin 51 has been reported (Scheme 35).108 Furan

probably disfavored because the crowded porphyrinoid cavity cannot easily accommodate the presence of a methylene unit.122 Oxacarbaporphyrin 100a acted as a dianionic ligand and reacted with nickel(II) acetate in refluxing DMF to give the corresponding nickel(II) complex 101a in 53% yield (Scheme 34). Palladium(II) acetate reacted similarly to afford the

Scheme 35. Synthesis of Heterocarbaporphyrins from a Carbatripyrrin

Scheme 34. Metalation of Heterocarbaporphyrins

dialdehyde 30a reacted with 51 in the presence of trifluoroacetic acid to give unsubstituted benzo-oxacarbaporphyrin 103a in 43% yield. As expected, 103a was strongly diatropic, and the proton NMR spectrum showed the inner CH at −4.86 ppm, while the meso-protons were observed downfield at 9.89 and 10.20 ppm. However, attempts to react 51 with thiophene dialdehyde 30b failed to give any of the related thiacarbaporphyrin 103b. Nevertheless, carbatripyrrin 51 was used to prepare a series of diphenyl heterocarbaporphyrins (104) (Scheme 35). Condensation of 51 with furan, thiophene, and selenophene dicarbinols 105a−c in the presence of boron trifluoride etherate, followed by oxidation with DDQ, gave oxa-, thia-, and selenacarbaporphyrins 104a−c in 24−25% yield. This represents the first rational synthesis of a selenacarbaporphyrin, although this system had been obtained from the ring contraction of heteroazuliporphyrins (see section 9).130 While all three heterocarbaporphyrins are fully aromatic compounds, the proton NMR spectra indicate that the diatropicity increases in the order 104a < 104c < 104b. It was suggested that the electronegativity of the oxygen atom was responsible for the slight reduction in diatropicity. The larger selenium in 104c may also be responsible for distorting the macrocycle, causing a reduced diatropic ring current compared to that of thiacarbaporphyrin 104b. The UV− vis spectra of 104a, 104b, and 104c showed Soret bands at 422, 439, and 444 nm, respectively, together with a series of Q bands at higher wavelengths.108 The synthesis of a 22-oxacarbaporphyrin (106a) was developed using a fulvene dialdehyde (107a) as a key intermediate (Scheme 36).132 Reaction of furan-2-carbaldehydes

palladium(II) complex 101b in 70% yield, although reaction with platinum(II) chloride gave the related platinum(II) derivative 101c in only 5% yield.127,129 Palladium(II) complex 101b was characterized by single-crystal X-ray diffraction analysis, and this showed that the system is near-planar with a centrally bound palladium ion. The UV−vis spectrum for nickel complex 101a gave a Soret band at 392 nm and two broader absorptions at 470 and 580 nm. Palladium(II) complex 101b afforded a very different spectrum with multiple absorption bands including relatively strong absoptions at 378 and 473 nm. The platinum derivative 101c was quite different, again showing a Soret band at 380 nm and minor absorptions at 419, 454, and 569 nm. All three of these organometallic derivatives showed strong diatropic ring currents, although the downfield shifts for the meso-protons were largest for the palladium complex and smallest for the nickel complex. The covalent radius for nickel(II) is slightly smaller than the size of the macrocyclic cavity, and this may lead to distortion or ruffling of the porphyrinoid to accommodate this ion, thereby slightly reducing the aromatic character. When thiacarbaporphyrin 100b was heated with palladium(II) acetate in acetonitrile, palladium(II) complex 102 was generated in 92% yield (Scheme 34).131 The UV−vis spectrum of 102 showed two strong absorptions at 403 and 501 nm and a weaker band at 628 nm. The proton NMR spectrum confirmed that this complex is strongly aromatic, and the mesoprotons were observed as two 2H singlets at 10.04 and 10.46 R


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Scheme 36. Synthesis of a 23-Oxacarbaporphyrin

between the sulfur atom and the indene’s methylene unit, thereby frustrating macrocycle formation.132 Oxacarbaporphyrin 106a is highly diatropic porphyrinoid, and the proton NMR spectrum shows the internal CH and NH protons upfield at −5.73 and −3.04 ppm, respectively.132 The external meso-protons give rise to four 1H singlets at 9.65, 9.68, 9.96, and 10.12 ppm. The UV−vis spectrum in chloroform gave a Soret band at 430 nm and weaker absorptions at 371, 509, and 621 nm. In the presence of TFA, a monoprotonated species (106aH+) was generated, and this continued to display strongly diatropic characteristics. Under strongly acidic conditions, such as 1% HCl in TFA, a C-protonated dication (106aH22+) was formed, where the 18π-electron delocalization pathways run through the fused benzene ring (Scheme 38). The UV−vis spectrum showed a strong Soret band at 422 nm and Q bands at 579, 625, and 680 nm. The proton NMR spectrum for this species in HCl−TFA showed the 21-CH2 as a singlet at −5.67 ppm, and the NHs appeared as two broad upfield resonances at −2.57 (1H) and −2.00 (1H) ppm. The meso-protons were strongly shifted downfield to give 1H singlets at 11.13, 11.36, 11.90, and 11.92 ppm. Importantly, the furan protons afforded two 1H doublets at 10.52 and 10.92 ppm, while the benzo-unit gave rise to two 2H multiplets at 8.90−9.60 and 10.61−10.70 ppm. These data confirm that the diamagnetic ring current for the macrocycle runs through both the external furan carbons and the fused benzo-unit. Reaction of 106a with palladium(II) acetate in refluxing DMF gave the corresponding organometallic derivative 115 in 80% yield (Scheme 38).132 The proton NMR spectrum for 115 indicated a slightly reduced ring current in this species compared to free-base 106a, but the meso-protons were still observed downfield between 9.42 and 9.78 ppm. The UV− vis spectrum gave two strong absorption bands at 405 and 462 nm and weaker absorptions between 520 and 657 nm. X-ray crystal structures of 106a and 115 were obtained. Free base 106a was fairly planar, and the indene unit was only tilted from the mean macrocyclic plane by 1.37°. The data were also consistent with the tautomer shown for 106 rather than 106′ (Scheme 36). The X-ray structure for 115 showed that metalated porphyrinoid is also near planar and the indene unit is tilted by 2.4°.132 Sessler and co-workers attempted to prepare a carbatripyrrane (116) by reacting the cyclopentadienyl anion with acetoxymethylpyrroles 117 (Scheme 39).133 Acetoxymethylpyrrole 117 reacted with sodium cyclopentadienide to give isomeric

108a−c with indene enamine 109 in the presence of dibutylboron triflate, followed by hydrolysis with aqueous sodium acetate, gave fulvene monoaldehydes 110a−c. Iodofulvene 110c was protected as the dimethyl acetal 111c and then treated with n-Bu3MgLi at −100 °C. Following reaction with DMF and acid-catalyzed hydrolysis, the corresponding dialdehyde 107a was isolated in 46% yield. When the reaction was performed at −78 °C, the yield of dialdehyde 107a dropped to 23% and an α,β-unsaturated aldehyde 112 was formed as a byproduct. This presumably arises from a crossed-aldol condensation with valeraldehyde, which in turn may be generated by the reaction of n-butyllithium with DMF. Starting with thiophene-2-carbaldehydes 108d−f, a series of thiophenecontaining fulvene monoaldehydes 110d−f were also prepared, and iodofulvene 110f was similarly converted into the related fulvene dialdehyde 107b (Scheme 36). In this case, similar results were obtained at −78 and −100 °C. Acid-catalyzed condensation of dialdehyde 107a with dipyrrylmethane 113 in a “2 + 2” MacDonald-type condensation gave oxacarbaporphyrin 106a in 79% yield (Scheme 35). Surprisingly, thiophene-appended fulvene dialdehyde 107b failed to generate the corresponding thiacarbaporphyrin 106b (Scheme 37). However, an unstable open-chain product (114) was observed instead, which failed to undergo macrocyclization. X-ray crystallography confirmed that fulvene 107b has the correct geometry to afford porphyrinoid products, and it was suggested that the intermediary bilin structure 114 twists out of alignment due to steric interactions Scheme 37. Attempted Synthesis of a Thiacarbaporphyrin

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Scheme 38. Protonation and Metalation of a 23-Oxacarbaporphyrin

Scheme 39. Attempted Synthesis of Carbatripyrranes

Scheme 40. Synthesis of a Tripyrrane Analogue from a Fulvene Derivative

Scheme 41. Synthesis and Metalation of a Thiacarbachlorin and a Related Thiacarbaporphyrin

cyclopentadienylmethylpyrroles 118a and 118b. Treatment with sodium hydride gave the related dianions 119, and these were further reacted with 117 to produce dipyrrolic products. Unfortunately, tripyrrane analogue 116 was not formed, and isomeric 1,1- and 1,2-disubstituted cyclopentadienes 120a−c were generated instead.133 In an independent study, fulvene 121 was prepared by reacting pyrrole-2-carbaldehyde (122) with cyclopentadiene in the presence of pyrrolidine (Scheme 40).134 Reduction with lithium aluminum hydride afforded the dihydrofulvene 123, and this reacted with 122 and pyrrolidine to produce the tripyrrene analogue 124 as a mixture of isomers. Further reduction with lithium aluminum hydride gave

carbatripyrrane analogues 125, again as a mixture of isomers. In the original study, 125 was used in the preparation of ansacyclopentadienyl pyrrolyl titanium complexes.134 Recently, intermediate 125 was used to synthesize an example of a thiacarbachlorin (Scheme 41).135 Condensation of 125 with thiophene dicarbinol 126 in the presence of boron trifluoride etherate in chloroform containing 3% ethanol gave, following neutralization with triethylamine and oxidation with chloranil, thiacarbachlorin 127 in 5.2% yield. An internally bridged thiacarbaporphyrin 128 arising from substitution onto chloranil was also identified as a byproduct. XRD analysis demonstrated that the thiacarbachlorin system is near-planar. The proton NMR T


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spectrum of 127 showed the inner CH and NH resonances upfield at −5.16 and −3.70 ppm, respectively, while the mesoprotons gave a downfield singlet at 9.49 ppm. The UV−vis spectrum of 127 showed a split Soret band at 415 and 441 nm and Q bands at 525, 621, and 682 nm. Oxidation of 127 with 1 equiv of DDQ gave the related thiacarbaporphyrin 129 in 25% yield. The UV−vis spectrum for this compound gave a broad Soret band at 417 nm and some poorly defined Q bands at higher wavelengths. The proton NMR spectrum for 129 again confirmed the expected diatropic character for this system, showing the shielded 21-H and NH resonances at −4.79 and −2.95 ppm and the deshielded meso-protons downfield at 9.89 ppm. The external cyclopentadiene protons gave a doublet at 8.21 ppm, a value that is consistent with the C2−C3 bond lying outside of the main macrocyclic delocalization pathways. The NMR spectrum for 128 was more complex but still indicated that the porphyrinoid system retained its diatropic character. Porphyrinoids 127 and 129 reacted with palladium(II) acetate in toluene to give the palladium(II) complexes 130 and 131, respectively. DFT calculations indicated that the macrocycle must distort to accommodate the palladium ion. Specifically, the thiophene unit is substantially tilted relative to the macrocyclic plane.135

Scheme 42. Tautomers of N-Confused Porphyrin

Scheme 43. Nickel Complexes of N-Confused Porphyrins

6. N-CONFUSED PORPHYRINS AND X-CONFUSED HETEROPORPHYRINS N-Confused porphyrins (NCPs) can be considered to be 2azacarbaporphyrins and for this reason may show reactivity analogous to that of true carbaporphyrins.22−24 meso-Unsubstituted NCPs will not be discussed in detail, but the types of coordination complexes formed by the system are briefly presented136,137 so that these can be contrasted with the metalation of other carbaporphyrinoid structures. In addition, meso-unsubstituted NCPs are described because these are commonly not included in reviews of N-confused porphyrins. Nevertheless, these porphyrinoids provide a valuable contrast to other meso-unsubstituted carbaporphyrinoid systems. In addition, closely related hetero-NCPs and X-confused heteroporphyrins are presented. 6.1. Metalated Derivatives of N-Confused Porphyrins

Tetraaryl N-confused porphyrins are generally isolated as the fully aromatic tautomers 19a, but these are in equilibrium with a crossed-conjugated form (19b) with an external NH (Scheme 42). In fact, in polar aprotic solvents such as DMF, the crossconjugated form predominates due to favorable hydrogenbonding interactions, and this tautomer can be crystallized from DMF−methanol.138 DFT calculations indicate that 19b is only 3.4−5.7 kcal/mol higher in energy than 19a.128,139 In addition, a tautomer 19c with an internal methylene unit was found to have comparable stability (3.0−8.2 kcal/mol higher in energy than 19a), although this form has not been observed directly.139 Reaction of NCPs with nickel(II) chloride afforded nickel(II) complexes 132a (M = Ni, Scheme 43) that are essentially derived from tautomer 19b.73 The fully aromatic form 19a has three internal protons and is likely to act as a trianionic ligand,140,141 but in 19b one of the protons has been relocated onto the external nitrogen and this behaves as a dianionic ligand. Complexes of structural type 132 have also been obtained with Pd(II),134 Pt(II),142−144 Cu(II),145,146 Mn(III),147 Co(II),148 Rh(IV),149,150 and Mo(II)151 (Figure 15). Silver(I) trifluoroacetate was found to react with tetraaryl NCPs to give silver(III) derivatives 133a (M = Ag), corresponding to tautomer 19a

Figure 15. Selected coordination motifs for N-confused porphyrins.

(Scheme 44).140 This same coordination pattern is observed for Cu(III),145 Co(III),148 Rh(III),149,150 and Sb(V)152 complexes 133 (Figure 15). Tetraphenyl NCP 19 reacted with Nbromosuccinimide to give the 21-bromo derivative 134, and this could be metalated with AuCl·SMe2 to give the related gold(III) complex 135 (Scheme 44).153 Reaction of Ni(II) NCP 132a with methyl iodide gave a C-methyl derivative (136a) and a dialkylated derivative (137) (Scheme 43).154 The monoalkylated U


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reacted with dialdehyde 144 in the presence of HCl in acetic acid. Following air oxidation, NCP 145 was isolated in 25% yield.159 Unfortunately, the method could not be scaled up and this yield could only be obtained for very small scale reactions. A more general route was subsequently developed using a MacDonald-type “3 + 1” strategy (Scheme 46).160 2,4Pyrroledicarbaldehydes 146 were condensed with tripyrrane 36a in the presence of trifluoroacetic acid, and following an oxidation step, hexa- or heptaalkyl NCPs 147 were generated. Oxidation with DDQ gave poor results, but it was found that washing the crude reaction solutions with 0.1% aqueous ferric chloride solutions afforded good yields of the targeted NCPs. Recrystallization of the corresponding hydrochloride salts afforded pure samples of NCPs 147a−c in 28−61% yield when the confused pyrrole unit possessed a 3-alkyl substituent (i.e., for 147a and 147b), although the yield was only 16% when this substituent was absent (i.e., for 147c).160 This methodology can easily be scaled up and therefore provides a convenient route to meso-unsubstituted NCPs. The UV−vis spectrum for 147a gives a Soret band at 422 nm and a series of Q bands at 516, 554, 614, and 678 nm. The proton NMR spectrum confirmed that the isolated meso-unsubstituted NCPs corresponded to the aromatic tautomer 147 rather than the cross-conjugated form 147′. The internal CH for 147a afforded a resonance at −6.3 ppm, and the NHs appeared near −3.8 ppm, confirming the presence of a strongly shielded environment, whereas the external mesoprotons gave rise to four 1H singlets at 9.54, 9.64. 9.70 and 10.01 ppm. Even at room temperature, two separate peaks could be seen for the NHs, and this indicated that a single tautomer predominates in CDCl3. In DMF-d7, two tautomeric forms corresponding to 147 and 147′ were observed in an approximately 1:1 ratio. The internal CH for the crossconjugated form gave a resonance at −0.07 ppm, while the meso-protons appeared at 8.53 (2H), 8.80 (1H), and 9.12 (1H) ppm. These results indicated that this form retained a degree of aromatic character that was attributed to dipolar resonance contributors that possess 18π-electron delocalization pathways. The external NH appeared at 13.57 ppm due to hydrogen bonding to the DMF solvent. Addition of 1 equiv of TFA to solutions of 147 resulted in the formation of a new species, assigned as monocation 147H+, which showed a strong Soret band at 421 nm (Scheme 47). Further addition of TFA resulted in the formation of a dication 147H22+ that had a slightly redshifted Soret band at 425 nm. The proton NMR spectrum of 147H22+ in TFA−CDCl3 showed the inner CH at −4.0 ppm

Scheme 44. Silver(III) and Gold(III) Complexes of NConfused Porphyrins

product corresponds to the metallo-derivative of tautomer 15c, providing a third organometallic binding motif (136) (Figure 15) for this system. It is noteworthy, therefore, that true carbaporphyrins form organometallic derivatives related to 133 and 136, while other carbaporphyrinoid systems (vide infra) act as dianionic ligands, forming complexes similar to 132.110,111 Metallo derivatives of type 138, where only the nitrogen atoms are coordinated to the metal cation, are also known, specifically for zinc,155 manganese(II), and iron(II) derivatives.156−158 The Mn(II) and Fe(II) complexes 138 reacted with molecular oxygen to produce oxygen-bridged structures 139, and these can be demetalated with perchloric acid to give 21-hydroxyNCP 140.158 All of these processes are mirrored for other carbaporphyrinoid systems. However, NCPs can also coordinate at the external nitrogen, and this allows for further complexity, including the formation of dimeric complexes.23 6.2. Meso-Unsubstituted N-Confused Porphyrins

Rational syntheses of meso-unsubstituted NCPs have been developed that more closely resemble the carbaporphyrin structures discussed in section 5. The first approach to be reported made use of a “2 + 2” MacDonald-type condensation (Scheme 45).159 Acetoxymethylpyrrole 141 was condensed with benzyl 5-methylpyrrole-2-carboxylate (142) in aqueous acetic acid to give the 2,3′-dipyrrylmethane 143a. The benzyl ester protective groups were removed by hydrogenolysis over Pd/C to generate the related dicarboxylic acid 143b, and this was further

Scheme 45. 2 + 2 Synthesis of a Heptaalkyl N-Confused Porphyrin

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Scheme 46. 3 + 1 Synthesis of N-Confused Porphyrins

Scheme 47. Protonation of Meso-Unsubstituted N-Confused Porphyrin

together with two upfield resonances near −1.0 ppm and a broad peak at −1.7 ppm for the three NHs. The meso-protons appear as four downfield singlets at 9.49, 9.54, 9.90, and 10.00 ppm, confirming the highly diatropic nature of this species. The external NH showed up at 12.79 ppm. Addition of other acids, such as HCl, HBr, and HI afforded significantly different shifts, particularly for the external NH resonance, and these observations were attributed to strong ion pairing interactions. Addition of TFA-d to a solution of 147a in CDCl3 showed the rapid loss of the 21-H resonance, and this indicated that a cationic species such as 148 with an internal CH2 was present in equilibrium with the major diprotonated species 147H22+ (Scheme 47).160 Reaction of 147a with nickel(II) acetate in DMF at 145 °C gave nickel(II) complex 149 in 46% yield (Scheme 48).160 This derivative proved to be less stable than the related nickel(II) complex of tetraphenyl NCP. The UV−vis spectrum gave a relatively weak Soret band at 405 nm, secondary absorptions at 337 and 453 nm, and Q-like bands at 557, 682, and 750 nm. The

proton NMR spectrum in CDCl3 showed a substantially reduced aromatic ring current compared to the free-base form, and the meso-protons appeared as four broad singlets at 7.83, 8.53, 8.65, and 8.68 ppm, while the NH showed up at 9.09 ppm. However, addition of TFA to the solution gave rise to a protonated species that had a strong diatropic ring current. The meso-protons shifted downfield to give four 1H singlets at 9.37, 9.43, 9.75, and 10.01 ppm, and the NH gave an additional downfield resonance at 14.77 ppm. Importantly, a broad 1H doublet was evident at −4.93 ppm. The methyl groups also showed a strong downfield shift going from three 3H singlets at 3.02, 3.07, and 3.44 ppm in 149 to values of 3.16, 3.22, and 3.77 in the protonated species. The new species was identified as the C-protonated cation 149H + (Scheme 48),160 and similar protonations were subsequently noted for nickel(II) and copper(II) complexes of tetraphenyl NCP.145 The aromatic cation slowly underwent demetalation over a period of hours to give the NCP dication 147aH22+.160 The MacDonald-type “3 + 1” approach was also used to prepared N-substituted NCPs 150 (Scheme 49).161 N-Methyl and N-phenyl-2,4-pyrroledicarbaldehydes 151 were reacted with tripyrrane 36a in the presence of trifluoroacetic acid and oxidized with aqueous ferric chloride to give 2-substituted NCPs. The 2methyl derivative 150a was isolated in 39% yield, but phenyl NCP 150b was obtained in a lower yield of 17% together with the oxidation product 152b (12%). It was speculated that lactam 152 was formed by addition of water to 150b to form a hemiaminal that underwent oxidation to generate the carbonyl group (Scheme 50). NCPs 150a and 150b are frozen into the crossconjugated tautomeric form of NCPs and as a result show reduced diatropic character compared to 147. For instance, in the proton NMR spectrum for 150a, the internal CH was observed near 1.5 ppm and the NH gave a broad resonance at 2.75 ppm. For 150b, these signals appeared at 2.06 and 3.9 ppm, indicating that 150b has a slightly reduced macrocyclic ring current compared to 150a. The diatropic character of these structures has been attributed to dipolar resonance structures such as 150′ that possess [18]annulene substructures. The presence of a methyl group on the nitrogen would be expected to better stabilize the positive charge than a phenyl unit, and this rationalizes the observed results. In the presence of TFA,

Scheme 48. Synthesis of a Nickel(II) Complex of a MesoUnsubstituted N-Confused Porphyrin

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Scheme 49. Synthesis of N-Methyl and N-Phenyl N-Confused Porphyrins

Scheme 50. Mechanism for the Formation of N-Confused Porpholactones

Scheme 52. Metalation of N-Substituted N-Confused Porphyrins

dications 150H22+ were generated that showed greatly enhanced diatropic properties (Scheme 51). In TFA−CDCl3, the proton NMR spectrum for 150aH22+ showed that the 21-H resonance had shifted upfield to give a doublet at −2.05 ppm (J = 1 Hz), and the NHs gave rise to two broad resonances at 0.9 (1H) and 1.9 (2H) ppm. In addition, the meso-protons shifted downfield to give four singlets between 9.06 and 9.72 ppm. As would be expected, the diamagnetic ring current for the phenyl-substituted dication 150bH22+ was reduced, and while the 21-H resonance appeared at −2.07 ppm, three of the meso-protons were observed between 8.94 and 9.06 ppm. Only the meso-proton at C20 was shifted further downfield to 9.70 ppm, because it was adjacent to the phenyl substituent. The UV−vis spectrum of 150a in 1% triethylamine−chloroform showed two moderate absorptions at 409 and 420 nm and weak broadened bands in the visible region. In 1% TFA−CHCl3, dication 150bH22+ gave a stronger Soret band at 427 nm and a series of Q bands between 500 and 800 nm.161 NCPs 150a and 150b were metalated with nickel(II) acetate in refluxing DMF or palladium(II) acetate in refluxing acetonitrile.161 Nickel(II) complexes 153a and 153b were generated in 76−90% yield, while palladium(II) derivatives 154a and 154b were isolated in 58−63% yield (Scheme 52).161 All four organometallic complexes were stable and showed moderate diatropic character, although the palladium(II)

derivatives exhibited slightly larger downfield shifts for the meso-protons. Addition of TFA to a solution of 153a in CDCl3 resulted in the formation of the C-protonated aromatic cation 153H+, the proton NMR spectrum of which showed the pyrrole CH and meso-protons as five downfield singlets at 9.41, 9.45, 9.61, 9.98, and 10.19 ppm. The internal CH gave rise to a broad peak near −4.0 ppm. Related cationic species were generated for 153b and 154, although a higher concentration of TFA was required. The results demonstrated that the N-phenyl metalloporphyrinoids were more difficult to protonate than the related N-methyl complexes and that the nickel(II) derivatives reacted more easily than the palladium(II) complexes. Under these acidic conditions, the protonated nickel complexes 153H+ slowly underwent demetalation, but the protonated palladium complexes proved to be quite stable. The NMR data also indicated

Scheme 51. Protonation of N-Substituted N-Confused Porphyrins

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that palladium(II) cations 154H+ were more diatropic than their nickel(II) counterparts 153H+.161 Reaction of 150a with 3 equiv of silver(I) acetate in dichloromethane at room temperature gave the silver(III) NCP-lactam 155a in 65% yield (Scheme 52).161 Oxidation at the C3 position allowed the macrocycle to become fully aromatic by facilitating the introduction of an 18π-electron delocalization pathway. The proton NMR spectrum showed that the four mesoprotons were shifted downfield to give four 1H singlets at 9.10, 9.65, 9.74, and 9.86 ppm. In addition, the UV−vis spectrum was porphyrin-like, giving a strong Soret band at 432 nm and Q bands at 529, 547, 568, and 603 nm. The presence of a carbonyl moiety was also confirmed by the presence of a strong absorption in the IR spectrum at 1677 cm−1. N-Phenyl NCP 150b reacted similarly with silver(I) acetate to give silver(III) derivative 155b in 53% yield. NCPs 150 were also reacted with gold(III) acetate, but these experiments gave complex results and led to decomposition. However, when 150a was reacted with gold(III) acetate in pyridine, the corresponding gold(III) complex 156 could be isolated in 8% yield (Scheme 52). The proton NMR spectrum of 156 was similar to that of the silver complex 155a, but the UV spectrum showed a split Soret band at 419 and 436 nm.161 When tripyrrane 36a was reacted with pyrrole dialdehyde 151a under standard “3 + 1” conditions and silver(I) acetate was added to the acidic solution, the intermediates were oxidized to lactam 152a in 11% yield (Scheme 49).161 As expected, the oxoNCP was highly diatropic and the proton NMR spectrum showed the inner CH as a singlet at −6.63 ppm together with a broad 2H resonance at −3.9 ppm for the NHs. The meso-proton appeared downfield as four 1H singlets at 9.06, 9.64, 9.73, and 9.83 ppm. The UV−vis spectrum was also porphyrin-like, with two Soret bands at 421 and 435 nm and Q bands at 517, 556, 607, and 667 nm. Similar results were obtained for the related Nphenyl NCP-lactam 152b.161 The reaction of pyrrole dialdehyde 146c with tripyrrane 36a was reinvestigated using p-toluenesulfonic acid as a catalyst in ethanol solution, followed by oxidation with o-chloranil (Scheme 53).162 Under these conditions, three products were formed, NCP 147c (3%), 3-ethoxy NCP 157 (1%), and lactam 158 (6%).

When the reaction was carried out in rigorously dried ethanol, the yield of 157 was raised to 9% and the yield of NCP lactam 158 was reduced to 0.5%. Porphyrinoid lactam 158 gave a porphyrin-like UV−vis spectrum with a Soret band at 415 nm and Q bands at 513, 549, 603, and 664 nm. In the proton NMR spectrum, the diatropic ring current for 158 shifted the inner CH resonance upfield to −6.87 ppm and the NHs appeared as a broad 2H singlet near −4.0 ppm, while the meso-protons shifted downfield to give four 1H singlets between 9.12 and 9.86 ppm. The amide NH appeared as a broad peak at 9.22 ppm, and the carbon-13 NMR spectrum confirmed the presence of the carbonyl unit, giving rise to a resonance at 171.1 ppm. In CDCl3, the position of the NH resonance was temperature- and concentration-dependent, and this was attributed to an equilibrium between 158 and the hydrogen-bonded dimer 159. Interestingly, the protonated form of 158 was reported to bind fluoride and chloride anions through the outer NH and inner pyrrolic NH groups, and this gave rise to a dimer in dichloromethane solution. The structure of the chloride-bridged dimer was confirmed by XRD analysis.162 A synthesis of unsubstituted NCP 160 (N-confused porphine) using an alternative “3 + 1” strategy has been reported (Scheme 54).163 Reaction of N-tert-butyl N-confused tripyrrane 161 with 2,5-bis(hydroxymethyl)pyrrole (162) in the presence of boron trifluoride etherate, followed by oxidation with DDQ, gave tertbutyl-NCP 163 in 6.2% yield. The tert-butyl group was then cleaved by heating 163 with aqueous sulfuric acid at 165 °C to give the parent porphyrinoid 160 in 55% yield. The UV−vis spectrum for 160 in dichloromethane gave a Soret band at 413 nm, a secondary band at 362 nm, and Q bands at 506 and 680 nm. In DMF, the spectrum was considerably altered, with a weakened Soret band at 417 nm and Q bands at 565, 608, and 653 nm. These changes were attributed to the cross-conjugated form 160′ being favored in DMF. The proton NMR spectrum for 160 in CDCl3 confirmed that the system is fully aromatic, as the inner CH resonance appeared at −6.42 ppm and the NH protons gave a broad peak at −3.7 ppm. In addition, the external mesoand pyrrolic protons gave rise to a series of downfield resonances between 9.2 and 10.3 ppm. The X-ray crystal structure for 160 showed that the structure is quite planar, unlike mesotetrasubstituted NCPs, where the confused pyrrole ring is tilted relative to the mean macrocyclic plane by 20°−30°.163

Scheme 53. Synthesis of N-Confused Porphyrins and a NConfused Porpholactam under Modified Reaction Conditions

6.3. Heteroanalogues of N-Confused Porphyrins

Heteroanalogues of NCPs that incorporate furan or thiophene rings in addition to a “confused” pyrrolic unit have also been prepared. Lee and co-workers reported the synthesis of oxa- and thia-NCPs from a pyrrole dicarbinol (164) (Scheme 55).164−168 Pyrrole and N-alkylpyrroles 165 were diacylated with benzoyl chloride and aluminum chloride to afford 2,4-dibenzoylpyrroles 166, and subsequent reduction with lithium aluminum hydride gave the required dialcohols 164.164,165 Reaction of 164a with oxatripyrrane 167a in the presence of boron trifluoride etherate in chloroform, followed by oxidation with DDQ, gave 10,15diphenyloxa-NCP 168a in 5.5% yield.164,165 As there is no Nalkyl group on the confused ring, the oxa-NCP favors the fully aromatic form 168′ (Scheme 55). The proton NMR spectrum showed the internal CH at −3.20 ppm, while the external proton on the confused pyrrole ring and the two meso-protons appeared as three downfield singlets at 9.20, 9.82, and 10.19 ppm. The UV−vis spectrum afforded two Soret bands at 409 and 427 nm together with Q bands at 470, 538, 564, and 691 nm. A series of related oxa- and thia-NCPs 168b−j were prepared by the same Y


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Scheme 54. Synthesis of N-Confused Porphine

Scheme 55. Synthesis of Hetero-N-Confused Porphyrins

Scheme 56. 23-Oxa-, Thia-, and Selena-N-Confused Porphyrins

“3 + 1” strategy and yields of up to 30% were obtained for the thia-NCPs.166−168 Oxa-NCP 168b also exhibited strongly aromatic characteristics and appears to exist as tautomer 168′ rather than the alternative aromatic form 168″ (Scheme 55). However, N-methyl oxa-NCP 168c can only exist as the crossconjugated tautomer 168.166 Thia-NCPs gave more complicated results, and mixtures of tautomers were observed for 168d and 168i. For 168d, the major form present in chloroform solution was 168 and the aromatic structure 168′ appeared as a minor tautomer. The proton NMR spectrum for the major tautomer in

CDCl3 showed the internal CH at 2.6 ppm and the NH as a broad peak at 9.64 ppm. In contrast, the minor tautomer showed these resonances at −2.97 and −1.01 ppm, respectively. In a variable-temperature study, a third tautomer 168″ was noted at lower temperatures. At 223 K, the three forms were reported to be present in a ratio of 2:2:1. Thia-NCPs 168e−h were all trapped in the cross-conjugated form 168 and as a result showed relatively weak macrocyclic ring currents.167 Addition of TFA to 168e−h afforded the corresponding dications 168e−hH22+, and these exhibited greatly enhanced diatropicity due to resonance Z


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contributors such as 168′H22+ that possess 18π-electron delocalization pathways. An alternative synthesis of oxa-, thia- and selena-NCPs was reported by Chandrashekar and co-workers.169 Pyrrole diols 164a,f were reacted with excess pyrrole in the presence of trifluoroacetic acid to give N-confused tripyrranes 169 (Scheme 56). These were condensed with furan, thiophene, and selenophene dicarbinols 170−172 in the presence of catalytic p-toluenesulfonic acid, and following oxidation with p-chloranil, a series of oxa-, thia-, and selena-NCPs 168b,i,k−n were isolated in 19−30% yield.169 The X-ray crystal structure for thia-NCP 168k showed that the system existed in a highly ruffled conformation, and the N-confused pyrrole ring had a dihedral angle of 21.1°, while the remaining pyrrole units were tilted by 15.8° and 31.2°. In contrast to the observations made in the earlier work, the results from this study indicated that solutions of thia-NCP 168i favored the aromatic tautomer 168′ at room temperature, although a mixture of tautomers was noted at lower temperatures. Reaction of selenophene dicarbinol 172a with tripyrrane 173 and boron trifluoride etherate, followed by oxidation with pchloranil, gave selenaporphyrin 174 in 19% yield, together with the N-confused isomer 175 in 1% yield (Scheme 57).170 The

spectrum for 175 in CD2Cl2 showed the internal CH at 2.45 ppm and the NH as a broad peak at 9.6 ppm, although a poor-quality spectrum of 175 in CDCl3 gave the 21-CH resonance at −0.7 ppm. In pyridine-d5, the 21-H and NH resonances appeared at 2.82 and 14.33 ppm. The NMR data indicated that the crossconjugated tautomer 175″ was favored in pyridine-d5, while equilibrating mixtures of tautomers 175, 175′, and 175″ were present in other solvents. Condensation of thiophene dicarbinol 171b with 1 equiv of pyrrole in the presence of catalytic boron trifluoride etherate or methanesulfonic acid afforded dithia-NCP 176 in 4.7% yield (Scheme 58).171 Reactions using methanesulfonic acid afforded a dithiasapphyrin as a byproduct. The UV− vis spectrum of dithia-NCP 176 was porphyrin-like, showing a Soret band at 461 nm. Addition of acid led to the sequential formation of an externally protonated monocation 176H+, followed by dication 176H22+. The proton NMR spectrum of 176 in CDCl3 confirmed the aromatic character of this system. The internal CH appeared upfield at −1.6 ppm, while the external thiophene resonances appeared downfield between 9.26 and 9.53 ppm. The monocationic and dicationic forms also retained macrocyclic diatropicity.171 6.4. X-Confused Heteroporphyrins

Related porphyrinoids with inverted or confused furan or thiophene subunits have also been investigated.172 3-Thiophenecarbaldehyde was reacted with N-methylpiperazine (NMP), followed by metalation with sec-butyllithium-TMEDA, to give a 2-lithiated derivative (177) (Scheme 59).166 Subsequent addition of benzaldehyde afforded hydroxyaldehyde 178, and this reacted with phenylmagnesium bromide to give the 2,4thiophenedicarbinol 179. Condensation of 179 with 3 equiv of pyrrole and 2 equiv of benzaldehyde in the presence of boron trifluoride etherate, followed by oxidation with p-chloranil, gave S-confused thiaporphyrin 180 in 4−10% yield.173,174 In some experiments, the related phlorin 181 was also isolated. Porphyrinoid 180 was also obtained by reacting dicarbinol 179 with tripyrrane 173a and boron trifluoride etherate, followed by oxidation with p-chloranil, although the yield was only 2% in this case. Treatment of 180 with excess p-chloranil led to further oxidation, giving thiolactone 182 in 30% yield.174 The proton NMR spectrum for 180 was consistent with a weakly diatropic cross-conjugated species. The internal and external thiophene protons gave rise to two doublets (J = 1.1 Hz) at 4.76 and 8.25 ppm, respectively, and the NH afforded a broad peak at 5.81 ppm.173 However, the 3-oxo derivative 182 proved to be strongly aromatic, and the internal CH proton appeared at −5.31 ppm, while the NHs gave rise to two 1H resonances at −3.37 and −2.93 ppm. Zinc and cadmium complexes of 180 were also prepared.174 Furan dicarbinol 183 was similarly reacted with pyrrole (3 equiv) and aromatic aldehydes (2 equiv) in the presence of boron trifluoride etherate and then oxidized with DDQ in an attempt to

Scheme 57. Synthesis of a 22-Selena-N-Confused Porphyrin

UV−vis spectrum of selena-NCP 175 gave a Soret band at 451 nm and Q bands at 541, 578, 618, and 666 nm. The proton NMR Scheme 58. Synthesis of a Dithia-N-Confused Porphyrin

AA


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Scheme 59. Syntheses of S-Confused Thiaporphyrin

Scheme 60. Synthesis of a Pyrrole-Appended O-Confused Oxachlorin

Scheme 61. Metalation of a Pyrrole-Appended O-Confused Oxachlorin

prepare O-confused oxaporphyrin 184 (Scheme 60).175 However, a pyrrole-appended porphyrinoid 185 was generated instead. The structure has an 18π-electron delocalization pathway and as a result exhibits a porphyrin-like UV−vis spectrum with a Soret band at 437 nm and Q bands at 533, 567, 622, and 679 nm. The proton NMR spectrum of 185 in CD2Cl2 confirmed that this is a highly diatropic compound, as the internal CH appeared at −5.11 ppm and the external pyrrolic protons showed up between 8.29 and 8.49 ppm. At 188 K, the rate of NH exchange was slowed down to the point where two separate NH resonances resolved at −2.47 and −2.79 ppm.

Addition of TFA resulted in the formation of a monocation 185H+ and the UV−vis spectrum for this species showed a broad Soret band at 463 nm. The proton NMR spectrum for the protonated species shows that it has comparable diatropicity to the free-base form, and the inner CH appeared near −5.0 ppm, while the three NHs gave resonances at −3.69, −0.82, and −0.49 ppm.175 Metalation of 185 with nickel(II) or palladium(II) chloride and anhydrous potassium carbonate in refluxing acetonitrile gave the cross-conjugated metal complexes 186a and 186b in 40% and 68% yield, respectively (Scheme 61).175 The UV−vis and AB


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Scheme 62. Synthesis and Reactions of a Copper(II) Pyrrole-Appended O-Confused Oxachlorin

Scheme 63. O-Confused Oxaporphyrin Derivatives

spectrum indicated that 189 was less aromatic than 187a, as the pyrrolic protons were not shifted as far downfield, showing up between 7.65 and 8.01 ppm. It was suggested that the reduced aromatic character was due to contributions from a paramagnetic canonical form, (P2−•)Cu(II). In the presence of molecular oxygen, 189 was converted into the paramagnetic copper(II) complex 190 (Scheme 62). Addition of bromine to 190 oxidized the system to an aromatic copper(III) cation (191). The UV−vis spectrum for this species gave an intense Soret band at 496 nm, together with two comparable absorptions at 662 and 750 nm. Exposure of 189 to O2 over several hours resulted in oxidative cleavage to afford copper(II) complex 192. The original copper(II) organometallic complex 189 reacted with hydrogen peroxide and potassium hydroxide in water to give a quantitative yield of copper(II) complex 193. This could be demetalated with hydrochloric acid to afford 21-hydroxy-O-confused porphyrin 194. The hydroxy compound appeared to have very little aromatic character, and the proton NMR spectrum for 194 showed the pyrrolic proton resonances between 6.59 and 7.25 ppm. Reaction of 194 with copper(II) salts converted it back into 193. Cadmium(II) and zinc(II) complexes of 194 have also been prepared.177 Condensation of dicarbinol 183 with pyrrole, benzaldehyde, and boron trifluoride in the presence of ethanol, followed by oxidation with DDQ, gave the ethoxy O-confused porphyrinoid 195 in 5% yield (Scheme 63).178 Addition of ethanol to 184 competes with pyrrole, giving 195 instead of 185. As was the case for 185, ethoxy derivative 195 is aromatic and gave a porphyrin-

NMR spectra for these complexes confirmed that they have considerably reduced aromatic character compared to 185. OConfused porphyrinoid 185 also reacted with silver(I) acetate in acetonitrile to give silver(III) complex 187a, but in this case, the macrocycle retains its diatropic character (Scheme 61).175 When the reaction was carried out in the presence of ethanol, the closely related ethoxy derivative 187b was formed. The UV−vis spectrum for the latter complex showed a Soret band at 443 nm and Q bands at 508, 524, 560, and 607 nm. The proton NMR spectrum of 187b in CD2Cl2 also showed the external pyrrolic protons downfield between 8.34 and 8.72 ppm. Interestingly, the protons at positions 7, 8, 17, and 18 coupled with the NMRactive 107Ag and 109Ag isotopes, both of which have spin I values of 1/2, and gave doublets (4JAgH = ca. 1.5 Hz). The X-ray structure for 187b showed that the macrocycle is near-planar, although the ethoxy and pyrrolyl substituents are orientated on opposite sides of the porphyrinoid plane. Protonation of 187b with trifluoroacetic acid led to loss of ethanol and the formation of cation 188. This species gave a porphyrin-like UV−vis spectrum, and the proton NMR spectrum showed the pyrrolic proton resonances downfield between 8.44 and 8.73 ppm. In addition, the appended pyrrole unit appears to be strongly interacting with the macrocycle, suggesting that the observed aromatic characteristics are due to resonance contributors such as 188′. Reaction of 185 with copper(II) acetate in refluxing THF gave the analogous copper(III) derivative 189 (Scheme 62).176 The UV−vis spectrum of 189 produced a strong Soret band at 433 nm and Q bands at higher wavelengths. However, the proton NMR AC


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Scheme 64. Formation of a Carbaporpholactone and the Related Silver(III) Complex

Scheme 65. Carbaporpholactones with Internally Fused Rings

Scheme 66. Phosphanyl and Phosphoryl Derivatives of Carbaporpholactones

demetalation, addition of water and oxidation occurred to generate carbaporpholactone 200. The oxidation step was presumably mediated by the silver(III). The lactone derivative had strongly aromatic characteristics and gave a UV−vis spectrum with an intense Soret band at 438 nm and a series of Q bands at 534, 574, 627, and 690 nm. Reaction of 200 with silver(I) acetate in acetonitrile−chloroform gave the related silver(III) complex 201. This complex could also be obtained more directly by reacting 197 with silver(I) acetate. The UV−vis spectrum for 201 gave a strong Soret band at 437 nm and Q bands at 522, 558, and 605 nm, and the X-ray crystal structure showed only a small degree of distortion from planarity. The proton NMR spectrum was also consistent with a highly diatropic structure.178 Reaction of silver(III) complex 201 with methylamine or dimethylamine resulted in demetalation and the formation of 21amino derivatives 202a and 202b in 60% and 95% yields, respectively (Scheme 65).179 The addition, which takes place with loss of silver(I), does not occur with the parent carbaporpholactone 200. It was proposed that axial coordination to the silver ion occurs initially, followed by reductive elimination and loss of Ag+ to give the amino derivatives. The presence of the internal amino substituent does not significantly disrupt the

like UV−vis spectrum with a Soret band at 433 nm. In the proton NMR spectrum, the internal CH was shifted upfield to afford a singlet at −5.49 ppm, while the external pyrrolic protons appeared downfield between 8.41 and 8.55 ppm. Addition of trifluoroacetic acid or dichloroacetic acid to solutions of 195 gave a dicationic species 196 due to elimination of ethanol. The dication showed only weak absorptions in the UV−vis spectrum, and proton NMR spectroscopy demonstrated that the diatropic ring current for this species was greatly reduced compared to that of 195. Addition of sodium ethoxide re-formed 195, while reaction with water generated the hydroxy derivative 197. As would be expected, pyrrole reacted with 196 to give the pyrroleappended porphyrinoid 185.178 Ethoxyporphyrinoid 195 reacted with silver(I) acetate in chloroform−acetonitrile to give the silver(III) organometallic complex 198 in 75% yield (Scheme 64).178 The complex exhibited a porphyrin-like UV−vis spectrum with a Soret band at 431 nm and a series of Q bands between 500 and 800 nm. The proton NMR spectrum confirmed the aromatic nature of the silver complex, as the pyrrolic protons showed up downfield between 8.57 and 8.67 ppm. Addition of trifluoroacetic acid led to protonation and elimination of ethanol to produce a cation 199 that exhibited reduced diatropicity. Over a period of 12 h, AD


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in the presence of trifluoroacetic acid to give phlorin 214 in 40% yield. Although the phlorin was somewhat unstable, it could be purified by column chromatography and recrystallized. Attempts to oxidize 214a with DDQ or aqueous ferric chloride were unsuccessful. Dialdehydes 212b and 212c were similarly reacted with 36a to afford unstable phlorin analogues 214b and 214c, but in these cases, oxidation with 0.2% aqueous ferric chloride gave the fully conjugated porphyrinoids 213b and 213c in 16−21% yield.181,182 N-Phenylphlorin 214a was subsequently shown to oxidize with silver(I) acetate to afford the related pyrazoloporphyrin 213a. When the “3 + 1” procedure was carried out between dialdehyde 212a and tripyrrane 36a and the crude product was oxidized with silver(I) acetate, N-phenylporphyrinoid 213a was isolated in 18% yield.182 The UV−vis spectrum of 213a in 1% triethylamine−chloroform gave a weak Soret band at 395 nm and broad absorptions between 500 and 750 nm. In the presence of 5−10 equiv of TFA, a monoprotonated cation 213aH+ was generated, but in the presence of a larger excess of TFA, dication 213aH22+ was produced (Scheme 69). The UV− vis spectrum of 213H22+ in 1% TFA−chloroform gave a Soret band at 416 nm, a secondary absorption at 322 nm, and Q-like bands at 550, 594, 738, and 804 nm. Similar results were obtained for the N-alkylpyrazoloporphyrins 213b and 213c. The proton NMR spectrum for the free-base form of 213a in CDCl3 indicated that this system is only weakly diatropic. The mesoprotons were observed at 6.84, 6.89, 7.39, and 7.87 ppm, while the inner CH and NH resonances were seen at 5.87 and 7.19 ppm, respectively. It was hypothesized that the weak diatropicity of pyrazoloporphyrins 213 derived from dipolar resonance contributors such as 213′. As this canonical form places a positive charge next to an electronegative nitrogen atom, contributions of this type are severely limited. N-Alkyl derivatives 213b and 213c show slightly enhanced diatropicity, and this can be rationalized as being due to the comparative electon-donating abilities of methyl and ethyl substituents. Although the observed shifts are small, the diatropic character increased in the sequence 213a < 213b < 213c, in line with expectations. Addition of 1−10 equiv of TFA gave poor-quality NMR spectra for monocations 213H+, but these nevertheless demonstrated increased diatropicity. In excess TFA−CDCl3, phenyl-substituted dication 213H22+ showed the meso-protons at 7.45, 7.49, 7.94, and 8.83 ppm, while the 21-H appeared upfield at 3.20 ppm. These results also showed that 213H22+ had a larger diamagnetic ring current compared to the free-base form, and this property was slightly enhanced for the N-methyl and N-ethyl forms 213bH22+ and 213cH22+. Resonance contributors such as 213′H22+ are relatively favored for the protonated species, as they aid in charge delocalization, although placing the positive charge next to a nitrogen is not favored, and these rationalizations help to explain the observations.181,182 When phlorin analogue 214a was oxidized with silver(I) acetate, oxophlorin 215a was generated as a byproduct in 9% yield.182 The related N-methyl- and N-ethyl-substituted compounds 215b and 215c were prepared similarly (Scheme 70). In the most convenient procedure, the crude reaction mixtures from the “3 + 1” reactions were taken prior to oxidation and treated with silver(I) acetate. The N-methyl version 215b was only obtained in 3% yield, but the ethyl series oxidized to give 215c as the main product in 14% yield. In addition, an isomeric species 216 was noted in the latter case and was isolated as a blue prefraction in 1% yield. The IR spectra of oxophlorin analogues 215 showed a peak for carbonyl stretching at 1625 cm−1. The UV−vis spectra for these oxo-derivatives gave a weak Soret-like

aromatic character of these porphyrinoids. For 202b, the dimethylamino group gave a 6H singlet at −0.18 ppm, while the external pyrrolic protons were observed downfield between 7.87 and 8.38 ppm. Oxidation of 202b with 1 equiv of DDQ at room temperature afforded a derivative (203) with an internally fused seven-membered ring in 85% yield. Trace amounts of regioisomer 204 were also observed. When the reaction was carried out with 10 equiv of DDQ, the doubly fused ring species 205 was generated instead in 77% yield. Reaction of 203 with 10 equiv of DDQ also gave 205. Derivatives 203−205 all retained aromatic characteristics. For instance, the proton NMR spectrum for 203 showed the pyrrolic protons downfield between 7.4 and 8.7 ppm, and the internal N-methyl appeared upfield at −2.45 ppm. The diastereotopic inner methylene unit afforded two 1H doublets at −0.83 and −2.28 ppm. Similar reactions were noted for silver N-confused porphyrins. Silver(III) complex 201 also reacted with potassium diphenylphosphide to afford a mixture of diphenylphosphanyl carbaporpholactone 206 and the related diphenylphosphoryl structure 207 (Scheme 66).180 When a mixture of these two products was oxidized with DDQ, 207 was generated in 65% yield. Diphenylphosphoryl derivative 207 reacted with copper(II) acetate in THF to give a nonaromatic copper(II) complex (208). However, when 207 was reacted with silver(I) acetate, the phosphoryl group was lost and the silver(III) complex 201 was re-formed.180

7. PYRAZOLE-CONTAINING PORPHYRINOIDS Pyrazole analogues 209 of the porphyrins (Scheme 67) share structural features with N-confused porphyrins but are far less Scheme 67. Tautomers of a Pyrazole Analogue of the Porphyrins

well studied.181,182 To date, only N-substituted pyrazoloporphyrins have been described,181,182 although several examples of expanded porphyrinoids with pyrazole subunits have been reported.183−185 Initially, a synthesis of pyrazoloporphyrin 210 was attempted by reacting 3,5-pyrazoledicarbaldehyde (211) with tripyrrane 36a using the “3 + 1” variant on the MacDonald condensation (Scheme 68). However, no trace of this macrocycle could be identified in these studies. On the other hand, Nsubstituted pyrazole dialdehydes 212a−c could be reacted with 36a to generate pyrazoloporphyrins 213 (Scheme 69).181,182 NPhenylpyrazole dialdehyde 212a condensed with tripyrrane 36a Scheme 68. Attempted Synthesis of a Pyrazole-Containing Porphyrinoid

AE


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Scheme 69. 3 + 1 Syntheses of N-Substituted Pyrazoloporphyrins

Scheme 70. Pyrazole Oxophlorin Analogues

Scheme 71. Metalation of Pyrazoloporphyrins

band near 380 nm and a weak broad absorption between 600 and 700 nm. The proton NMR spectra for 215a−c showed the mesoprotons between 5.5 and 7.0 ppm, confirming the absence of a macrocyclic ring current in these cross-conjugated structures. Addition of acid to solutions of 215 led to a color change from dark blue to green and resulted in the formation of the corresponding dications 215H22+. The profound color change was tentatively ascribed to the formation of the hydroxyporphyrinoid 217 (Scheme 70). As had been expected, the protonated species showed no indication of diatropic character.182 Reaction of pyrazoloporphyrins 213a−c or phlorin analogue 214a with nickel(II) acetate in refluxing DMF or palladium(II) acetate in refluxing acetonitrile led to the formation of organometallic derivatives 218 and 219 in 41−61% yield (Scheme 71).181,182 Nickel complexes 218 gave green solutions, whereas the palladium complexes produced red-brown solutions. The UV−vis spectra for nickel complexes 218 showed two Soretlike bands near 340 and 390 nm and several broad Q-like bands between 500 and 700 nm. However, palladium complexes 219 afforded completely different UV−vis spectra, with a Soret band at 414−416 nm and numerous weaker absorptions between 500 and 800 nm. In contrast, the proton NMR spectra for the

nickel(II) and palladium(II) series were very similar. The mesoprotons were shifted downfield compared to free-bases 213, giving a series of singlets between 7.38 and 8.02 ppm. The slightly increased diamagnetic ring current was attributed to resonance contributors such as 218′ and 219′. An X-ray crystal structure was obtained for palladium(II) complex 219a, and this showed that the macrocycle was essentially planar. Addition of TFA to solutions of 218 or 219 gave rise to N-protonated species 218H+ and 219H+. These showed simplified UV−vis spectra with Soretlike bands between 368 and 377 nm for nickel derivatives 218a− c and between 387 and 396 nm for the palladium complexes 219a−c. The diatropic characteristics of these metalloporphyrinoids were completely lost in the protonated structures, and the proton NMR spectra for 218H+ and 219H+ in TFA−CDCl3 showed the meso-protons comparatively upfield between 6.33 and 6.86 ppm. This is not surprising, as canonical forms such as 218′H+ and 219′H+ that possess 18π-electron delocalization pathways are disfavored because two positive charges would be placed next to one another.182 AF


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In a recent paper, a TFA-promoted condensation of Nbenzylpyrazole dialdehyde 212d with tripyrrane 36a in methanol−dichloromethane was reported.186 Following neutralization with triethylamine and oxidation with DDQ, oxophlorin analogue 215d was isolated in 12% yield (Scheme 72). Cleavage

Scheme 73. Attempted Synthesis of a NBenzylpyrazoloporphyrin from a Pyrazole Dialcohol

Scheme 72. Synthesis of a Oxopyrazolophlorin That Forms a Hydrogen-Bonded Dimer

Figure 16. Structural comparison of porphyrin, N-confused porphyrin and neo-confused porphyrin.

named neo-confused porphyrin (221, Figure 16).187−190 As is the case for NCPs, neo-confused porphyrins place a carbon atom within the porphyrinoid cavity and can therefore be considered to be carbaporphyrinoid systems. Initially, neo-confused porphyrins were prepared with a fused benzo-unit.187 Indole-3carbaldehyde was reacted with sodium hydride and acetoxymethylpyrrole 117c to give the neo-confused dipyrrylmethane 222 in 85% yield (Scheme 74). The tert-butyl ester was cleaved with TFA and the resulting α-unsubstituted compound 223 was reacted with trimethyl orthoformate to give the related dialdehyde 224a.187 This method gave variable yields of 224, and for that reason, a more direct route to these types of dialdehyde was subsequently developed. In this route, indole-3carbaldehyde was reacted with sodium hydride and acetoxymethylpyrrole carbaldehyde 225a to generate the required dialdehyde 224b in 55% yield.189 MacDonald reaction of 224a with dipyrrylmethanes 113a and 113b in the presence of ptoluenesulfonic acid, followed by oxidation of the phlorin intermediates 226 with DDQ, gave neo-confused porphyrins 227a and 227b in 24−25% yield.187 In a later paper, the yield was raised to 40% by using 0.2% aqueous ferric chloride as the oxidant.189 Dialdehyde 224b condensed with dipyrrylmethane 113a under the latter conditions to give neo-confused porphyrin 227c in 44% yield.189 The UV−vis spectrum of 227a was remarkably similar to the spectra for true porphyrins, showing a strong Soret band at 407 nm and Q bands at 503, 537, 567, and 615 nm. Addition of TFA resulted in the formation of dication 227aH22+ (Scheme 75), and this species showed a split Soret band at 402 and 428 nm and a series of Q bands that extended to beyond 700 nm. Neo-confused porphyrins are aromatic

of the benzyl moiety with AlCl3 in benzene afforded the related N-unsubstituted oxophlorin 215e in 64% yield. In principle, 215e could exist in a number of tautomeric forms. DFT calculations indicated that 215e, which places the external NH next to the carbonyl unit, was 4.4 kcal/mol more stable than tautomer 215e′. Although hydroxypyrazoloporphyrin tautomers can also be considered, these are >8 kcal/mol higher in energy than 215e. The proton NMR spectrum for 215e showed that the macrocycle is nonaromatic, and the inner NHs and external pyrazole NH showed up at 9.14 and 11.3 ppm, respectively. Single-crystal XRD analysis showed that the porphyrinoid is planar and also demonstrated that a “head-to-head” dimer structure was present. Evidence for the formation of the dimer in solution was also presented.186 N-Benzylpyrazole dicarbinol 216 was also reacted with diphenyltripyrrane 217 in the presence of trichloroacetic acid, followed by oxidation with DDQ, in an attempt to prepare the N-benzylpyrazoloporphyrin 218 (Scheme 73).186 However, in this case, triphenylcorrole 219 was isolated instead together with a 0.2% yield of the unusual nonaromatic tetrapyrrolic macrocycle 220. This unprecedented structure incorporated a ring-opened DDQ unit and exists in a highly twisted conformation.

8. NEO-CONFUSED PORPHYRINS N-Confused porphyrins differ from true porphyrins by relocating the pyrrolic nitrogen from position 21 to position 2 (Figure 16). Recently, a new class of porphyrin isomers was described that places the nitrogen at position 1 instead, and this system has been AG


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Scheme 74. Synthesis of Benzo-Neo-Confused Porphyrins

diamagnetic ring current, this is much reduced compared to true porphyrins or carbaporphyrins. In TFA−CDCl3, the corresponding dication 227aH22+ showed greatly increased diatropic character, and the meso-protons moved downfield to give four 1H singlets at 9.77, 9.80, 10.52, and 10.92 ppm, while the internal CH shifted upfield to give a resonance at −3.85 ppm. Resonance contributors such as 227′H22+ are likely to be far more significant for the dication (Scheme 75), as they aid in charge delocalization, and this factor may be responsible for the observed changes. Neo-confused porphyrins without fused benzo-units have also been prepared.188,189 Pyrrole-3-carbaldehyde (228a) was treated with sodium hydride and then reacted with acetoxymethylpyrrole 117c in refluxing THF to give 229a in up to 36% yield, together with byproducts such as 230 (Scheme 76). However, the regioselectivity of the reaction was greatly improved in DMF at 100 °C, and under these conditions, 229a could be isolated in 80% yield. Methyl 4-formylpyrrole-2-carboxylate (228b) similarly reacted with 117c to give the related 1,2′-dipyrrylmethane 229b in 77% yield (Scheme 76). However, attempts to convert 229a or 229b into the corresponding dialdehydes were not successful and treatment of these dipyrroles with TFA to remove

Scheme 75. Protonation of Benzo-Neo-Confused Porphyrin

compounds, and this property has been attributed to a 17-atom 18π-electron delocalization pathway highlighted in bold for structure 227.187,189 It is possible that dipolar canonical forms such as 227′ also contribute, as these have more conventional 18π-electron pathways. The proton NMR spectrum for 227a in CDCl3 showed the meso-protons as four downfield singlets at 8.91, 8.96, 9.68, and 9.99 ppm. In addition, the internal CH and NH resonances appeared upfield at −0.74 and −0.33 ppm, respectively. While these values indicate that there is a significant

Scheme 76. Preparation of Neo-Confused Dipyrrylmethane Dialdehydes

AH


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Scheme 77. Synthesis of a Neo-Confused Tripyrrane

the tert-butyl ester moiety resulted in decomposition.189 In order to circumvent this problem, pyrrole aldehyde 228b was reacted with sodium hydride and acetoxymethylpyrrole-2-carbaldehydes 226a and 226b in DMF. This reaction proved to be very sensitive to temperature, but at 30 °C, dialdehydes 231a and 231b could be isolated in 75% and 45% yield, respectively. Unfortunately, reaction of pyrrole-3-carbaldehyde (228a) with 226a under these conditions only gave very poor yields of dipyrrylmethane 231c.189 Due to the difficulties encountered in the initial investigations, alternative routes to neo-confused porphyrins were considered.189 1,2′-Dipyrrylmethane monoaldehydes 229 were reduced with sodium borohydride to give the related alcohols 232 (Scheme 77). These were reacted with α-unsubstituted pyrrole 233 under acidic conditions in an attempt to prepare neoconfused tripyrrane 234. However, extensive decomposition was noted, and it was not possible to isolate the targeted tripyrrolic system. Reduction of 222 with sodium borohydride similarly afforded carbinol 235, but in this case, further condensation with 233 in refluxing ethyl acetate containing 5% acetic acid afforded the tripyrrane analogue 236 in 93% yield. However, attempts to cleave the tert-butyl esters with TFA led to decomposition, thereby thwarting attempts to develop a “3 + 1” route to neoconfused porphyrins. Attempts to purify 235 on silica gel led to a self-condensation reaction that afforded bilane analogue 237 (Scheme 78). It was speculated that protonation and elimination of water would give azafulvene cation 238, and this could react

with 235 to give 239. Elimination of formaldehyde would then furnish the observed product. Although tetrapyrrole 237 is structurally a potential precursor of doubly neo-confused porphyrins, its instability under acidic conditions prevented its use in the synthesis of further modified porphyrin isomers.189 The development of an efficient route to 1,2′-dipyrrylmethane dialdehydes 231a and 231b allowed “2 + 2” MacDonald-type syntheses of neo-confused porphyrins 240 to be investigated (Scheme 79).188,189 Dialdehyde 231a was condensed with dipyrrylmethane 113a in the presence of p-toluenesulfonic acid. In the absence of an oxidation step, the bright blue neoconfused phlorin 241 could be isolated. Attempts to oxidize the intermediate with DDQ led to decomposition, but reaction with 0.2% aqueous ferric chloride afforded neo-confused porphyrin 240 in 55% yield. In addition to the purple fraction corresponding to 240, a green byproduct (242) was isolated in 26% yield.188,189 This hexapyrrolic structure is derived from 2 equiv of 231a reacting with dipyrrylmethane 113a (Scheme 80). Initial reaction between 231a and 113a would be expected to give b-bilene 243a, and subsequent cyclization would lead to phlorin 241a. If 243a reacted instead with 231a, an open-chain hexapyrrolic structure 244 would be produced (Scheme 80). Following deprotonation, cyclization could occur to give tetrahydroporphyrin 245, and subsequent oxidation would give the observed product 242.189 The stereochemistry of this byproduct, which is racemic, was demonstrated by X-ray crystallography. Diethyl neo-confused dipyrrylmethane dialdehyde 231b similarly reacted with 113a, and following oxidation with 0.2% aqueous ferric chloride, porphyrin isomer 240b was isolated in 44% yield. In this case, two green bands were observed. The proton NMR spectrum for one of these compounds was consistent with intermediate 245, although it was too unstable to purify. The second band corresponded to 242b. MacDonald-type “2 + 2” condensation of 231c with 113a was also attempted, but no trace of porphyrinoid product was observed. This may be due to the intermediates taking on a conformation that could not lead to macrocyclic products, although it may also relate to the stability of the intermediates or the neo-confused porphyrin product.189 Neo-confused porphyrins 240 displayed aromatic properties, and the proton NMR spectrum for 240a showed the interior CH and NH resonances at upfield values of 1.23 and 1.69 ppm, respectively, while the meso-proton appeared downfield as four 1H singlets at 8.20, 8.30, 8.74, and 10.57 ppm (the latter peak is deshielded, in part due to its proximity to the carbonyl residue).188,189 Nevertheless, the diatropic character has been

Scheme 78. Formation of a Doubly Neo-Confused Bilane

AI


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Scheme 79. Synthesis of Neo-Confused Porphyrins and Unusual Dihydroporphyrin Byproducts

Scheme 80. Mechanism for the Formation of the Hexapyrrolic Byproducts

slightly reduced compared to benzo-neo-confused porphyrins 227. This effect was attributed to dipolar resonance contributors such as 240′ that interrupt the π-conjugation pathway. DFT calculations were performed on unsubstituted neo-confused porphyrin tautomers 246 and benzo-fused neo-confused porphyrin tautomers 247 (Figure 17).190 The results showed that although tautomer 246a is favored, this structure is 27.04 kcal/mol higher in energy than porphyrin. Tautomer 246b is over 10 kcal/mol higher in energy than 246a, while 246c is 6.63 kcal/mol higher in energy. The preference for tautomer 246a is due to reduced lone pair−lone pair repulsion and enhanced hydrogen-bonding interactions. Similar trends can be seen for the benzo-neo-confused porphyrin series.190 Tautomer 247a is 26.40 kcal/mol higher in energy than benzoporphyrin, but it is favored over tautomer 247b and 247c by 10.82 and 6.84 kcal/ mol, respectively. NICS calculations confirmed the aromatic character for these species, and 246a and 247a gave NICS(0) values of −11.41 and −12.21 ppm. These results are consistent

with strongly diatropic species, although the values are reduced compared to those of porphyrins and carbaporphyrins, in agreement with observations.190 The UV−vis spectrum for neo-confused porphyrins 240a and 240b showed a Soret band at 390 nm and Q bands at higher wavelengths. Addition of TFA led to the formation of the related dications 240H22+. Dication 240aH22+ in 1% TFA−dichloromethane showed the presence of a Soret band at 409 nm and Q bands at 528, 571, and 669 nm. The proton NMR spectra for these protonated neo-confused porphyrins indicated that the diatropic character had been significantly increased. Dication 240a in TFA−CDCl3 showed that the inner CH resonance had shifted downfield to −1.21 ppm, while the external meso-protons now appeared at 8.84, 8.89, 9.58, and 11.14 ppm.189 Metalation of neo-confused porphyrins 227a and 240a was investigated. When 227a was heated with nickel(II) acetate in acetonitrile, the corresponding nickel(II) complex 248a was formed in 90% yield (Scheme 81).187 Neo-confused porphyrin AJ


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decomposition occurred. However, when the chromatography was conducted on grade 3 alumina, 248b could be isolated in 92% yield.189 Reaction of palladium(II) acetate with 240a similarly afforded the palladium organometallic complex 249b in 77% yield. X-ray crystal structures were obtained for all four of these complexes, and these results demonstrated that they are all essentially planar.189 The proton NMR spectrum for nickel(II) complex 248a showed the meso-protons relatively far downfield at 8.98, 9.22, 9.71, and 9.85 ppm, while the palladium complex 248b gave these peaks at 9.09, 9.33, 9.83, and 9.91 ppm. These data indicate that the organometallic derivatives are slightly more diatropic than free-base neo-confused porphyrin 227a, and the diamagnetic ring current for the palladium(II) complex is enhanced compared to that of the nickel(II) derivatives. The same trend was seen for neo-confused porphyrin 240a and its metalated derivatives 249a and 249b.189 A DFT study has explored the possibility of obtaining further modified structures of this type (Figure 18).191 Five doubly neoconfused porphyrin tautomers 250a−e were considered, but fully conjugated structures of this type can only exist as dipolar species. All five versions were found to be high-energy structures that were 55.30−75.42 kcal/mol higher in energy than porphyrin. Nevertheless, NICS calculations show that all five doubly neo-confused porphyrins have diatropic properties comparable to that of neo-confused porphyrin 221. Structures 251a−f with one neo-confused and one N-confused ring were also considered (only the most stable tautomers are shown in Figure 18). These were still calculated to be relatively high in energy, but in some cases these porphyrin isomers were less than 40 kcal/mol higher in energy than porphyrin. These values are similar to those calculated for doubly N-confused porphyrins (N2CP),53 and as examples of N2CP are known, the results indicate that suitably substituted neo-confused−N-confused porphyrins may be synthetically accessible.191

Figure 17. Tautomers of neo-confused porphyrins showing the calculated energies relative to that of 21H,23H-porphyrin or 21H,23H-benzo[b]porphyrin.190

Scheme 81. Metalation of Neo-Confused Porphyrins

9. AZULIPORPHYRINS AND HETEROAZULIPORPHYRINS 9.1. Synthesis and Spectroscopic Properties of Azuliporphyrins

240a similarly reacted with nickel(II) acetate to give 249a in 68% yield.189 Benzoporphyrinoid 227a was heated with palladium(II) acetate in acetonitrile to generate the palladium(II) complex 248b. When purification was attempted on silica, extensive

Azuliporphyrins are porphyrin analogues in which an azulene moiety has replaced one of the pyrrolic subunits.25 The first example of this class of carbaporphyrinoid was reported in 1997

Figure 18. Doubly neo-confused and neo-confused−N-confused porphyrins showing the calculated energies in kcal/mol relative to that of 21H,23Hporphyrin.191 AK


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Scheme 82. Synthesis of Azuliporphyrins

by Lash and Chaney.192 In this work, 1,3-azulenedicarbaldehyde (252a) was reacted with tripyrrane 36a in the presence of trifluoroacetic acid, followed by neutralization with triethylamine and oxidation with DDQ (Scheme 82). The initially reported yield of 253a was 28%, but subsequently, azuliporphyrins have been prepared by this methodology in up to 77% yield.102 Aqueous ferric chloride has also been shown to be an effective oxidant for these preparations. Azuliporphyrins are crossconjuated structures, but this system nevertheless retains some overall aromatic character.192 Azuliporphyrin 253a was sparingly soluble in organic solvents and only poor-quality NMR spectra could be obtained in CDCl3. These spectra showed the mesoprotons as two singlets near 8 and 9 ppm, but it was not possible to identify the internal CH resonance. Azuliporphyrins 253 were subsequently prepared from 6-tert-butyl- and 6-phenylazulene dialdehydes 252b and 252c, and these proved to have much better solubility characteristics.193 These properties allowed the proton NMR resonances for the internal protons to be identified. For tert-butylazuliporphyrin 253f in CDCl3, the internal CH and NH protons afforded broad resonances at 2.8 and 2.9 ppm, respectively, while the meso-protons produced two 2H singlets at 8.15 and 9.05 ppm.193 The methyl substituents were also indirectly affected by the diatropic ring current and for 253f gave rise to a 6H singlet at 3.02 ppm. This compares to values of 3.5− 3.6 ppm for fully aromatic porphyrinoids and resonances near 2.4 ppm for nonaromatic macrocycles such as benziporphyrins. The corresponding phenylazuliporphyrin 253g showed the CH and NH resonances slightly upfield compared to those of 223f at 3.23 and 3.30 ppm, and the proton NMR spectra indicated that 253f has a slightly larger aromatic ring current than 253g.193 The aromatic characteristics of azuliporphyrins were initially attributed to dipolar resonance contributors such as 253′ that possess 18π-electron delocalization pathways.130,192 These are favored in part due to the presence of a tropylium moiety, although the effect is mitigated by the requirement for charge delocalization. The presence of an electron-donating tert-butyl group on the seven-membered ring would be expected to stabilize the positive charge and increase the contribution from this type of canonical structure, thereby enhancing the aromatic character. On the other hand, the decreased electron-donating nature of a phenyl group would not have this effect. However, the presence of phenyl substituents at the 12,13-positions led to a slight increase in the observed ring current. For instance, 12,13diphenyl tert-butylazuliporphyrin 253h gave the meso-proton resonances at 9.17 and 8.33 ppm, or 0.1−0.2 ppm further downfield than the values observed for the related 12,13-diethyl version 253f. The replacement of electron-donating ethyl

substituents with phenyl groups would help to stabilize the negative charge that has been proposed to be present on the macrocyclic component. Proton NMR spectra run in polar solvents such as acetone-d6, DMSO-d6, and DMF-d7 indicated that the macrocycles possessed enhanced diatropic properties, as the external protons shifted significantly downfield (unfortunately, the internal protons could not be observed in these spectra).130,193 For instance, the meso-protons for 253f in acetone-d6 appeared at 8.32 and 9.30 ppm and showed up still further downfield at 8.33 and 9.48 ppm in DMSO-d6. As polar solvents would be expected to stabilize dipolar resonance contributors such as 253′, these data are consistent with the proposed model. However, an analysis of the bond lengths from single-crystal X-ray diffraction suggested an alternative dipolar species (253h) that possesses a 17-atom 18π-electron delocalization pathway.193 As this species also fits the spectroscopic observations, this provides a more realistic explanation for the intermediary aromatic properties of azuliporphyrins. The X-ray structure showed that the system is nearly planar, although the azulene moiety tilts by 7.4° relative to the mean macrocyclic plane.193 The UV−vis spectrum of azuliporphyrins are very characteristic, showing four moderately strong bands between 350 and 500 nm, followed by a broad absorption running between 500 and 800 nm (Figure 19).130,193 Addition of TFA resulted in the formation of dicationic species 253h (Scheme 83) that gave more porphyrin-like UV−vis spectra. For instance, 253aH22+ in 1%

Figure 19. UV−vis spectra of azuliporphyrin 253f in 1% triethylamine− chloroform (free base, green line) and 1% TFA−chloroform (dication 253fH22+, red line).193 AL


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Scheme 83. Protonation of Azuliporphyrins

Scheme 84. Back-to-Front 3 + 1 Syntheses of Azuliporphyrins

Scheme 85. 2 + 2 Syntheses of an Azuliporphyrin

Scheme 86. Synthesis of Tribenzoazuliporphyrin

A back-to-front “3 + 1” route to azuliporphyrins was developed that made use of azulene’s propensity to undergo electrophilic substitution at the 1,3-positions (Scheme 84).126 Hence, azulene (254a) was reacted with acetoxymethylpyrrole 117c in refluxing isopropyl alcohol containing acetic acid to give azulitripyrrane 255a in 59% yield.126,130 6-tert-Butyl- and 6-phenylazulene 254b,c similarly reacted with 117c to give the related tripyrrane analogues 255b and 255c.193 Treatment of azulitripyrranes 255 with TFA to remove the tert-butyl ester protective groups, followed by dilution with dichloromethane, condensation with pyrrole dialdehyde 55a, and oxidation with DDQ or aqueous ferric chloride, afforded azuliporphyrins 256a−c in 36−51% yield.126,130,193 A “2 + 2” method for preparing azuliporphyrins has also been reported (Scheme 85).194 Reaction of azulene with one

TFA−CDCl3 gave two strong bands at 364 and 460 nm, followed by several Q-like bands at higher wavelengths. The proton NMR spectra for these dications showed that they possessed greatly increased aromatic ring currents. The proton NMR spectrum of 253aH22+ in TFA−CDCl3 showed that the internal CH had shifted upfield to −2.56 ppm, while the external meso-protons had moved downfield to give two 2H singlets at 9.42 and 10.32 ppm.130 The enhanced aromatic properties can be attributed to resonance contributors such as 253′H22+ that possess tropylium and porphyrin-like conjugation pathways. This type of structure is far more favorable than 253′, because it facilitates charge delocalization. However, contributions from species such as 253hH22+ with 19-atom 18π-electron delocalization pathways provide an alternative explanation for the observed results.193 AM


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Scheme 87. Syntheses of Di- and Tribenzo-tert-butylazuliporphyrins

Scheme 88. Syntheses of Mono- and Dibenzo-tert-butylazuliporphyrins

The back-to-front “3 + 1” strategy was adapted for the synthesis of benzoazuliporphyrins.195 In an initial study, azulene dialdehyde 252a was reacted with a tripyrrane (260) bearing three fused bicyclooctadiene moieties in a conventional “3 + 1” MacDonald-type condensation to generate azuliporphyrin 261 as a mixture of diastereomers (Scheme 86).107 On heating, 261 eliminated 3 equiv of ethylene via a retro-Diels−Alder reaction to give tribenzoazuliporphyrin 262.107 Unfortunately, this derivative was virtually insoluble in organic solvents, and no NMR data were reported. Greatly improved results were subsequently obtained by making use of 6-tert-butylazulene using the aforementioned back-to-front methodology.195 Hence, 254b

equivalent of 117c in refluxing acetic acid−isopropyl alcohol gave pyrrolylmethylazulene 257 as the major product. Cleavage of the tert-butyl ester with TFA, followed by Vilsmeier−Haack formylation, gave the related dialdehyde in 61% yield. MacDonald “2 + 2” condensation of 258 with dipyrrylmethane 113a in the presence of HCl in acetic acid gave, following oxidation with aqueous ferric chloride, the asymmetrical azuliporphyrin 259 in 13% yield. Interestingly, this azuliporphyrin was relatively soluble in CDCl3 compared to 253a, although the internal CH and NH resonances could still not be identified in the proton NMR spectrum for this compound. AN


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Scheme 89. Synthesis of meso-Tetraarylazuliporphyrins

was reacted with 2 equiv of acetoxymethylpyrrole 263 in the presence of K10 montmorillonite clay to give an azulitripyrrane (264) with two fused cyclooctadiene units in 79% yield (Scheme 87). Treatment with TFA and subsequent condensation with pyrrole dialdehyde 265 afforded azuliporphyrin 266, and this underwent thermal extrusion of 3 equiv of ethylene at 200 °C to give tribenzoazuliporphyrin 267. Tripyrrane 264 similarly reacted with dialdehyde 55a to give azuliporphyrin 268, and subsequent retro-Diels−Alder loss of ethylene at 200 °C afforded dibenzoazuliporphyrin 269. 6-tert-Butylazulene was also reacted with 2 equiv of acetoxymethylpyrrole 270 in the presence of montmorillonite clay to generate azulitripyrrane 271 in 78% yield, while reaction with 1 equiv of 270 gave pyrrolylmethylazulene 272 in 60% yield (Scheme 88). Further treatment of 272 with 263 and montmorillonite clay afforded the asymmetrical azulitripyrrane 273. Using the same procedures, azulitripyrranes 271 and 273 were used to prepare monobenzoazuliporphyrins 274 and 275 and dibenzoazuliporphyrin 276. Although the UV− vis spectra of the free-base benzoazuliporphyrins were only slightly modified, significant bathochromic shifts were noted for the corresponding dications in TFA−CH2Cl2. In particular, the longer wavelength bands were red-shifted and intensified, and the dication for tribenzoazuliporphyrin gave a particularly strong absorption at 736 nm. The proton NMR spectra for 269, 274, and 275 in CDCl3 indicated that these derivatives had slightly enhanced diatropicity based on the downfield shifts for the mesoprotons and the upfield shifts of the internal CH. These results indicated that the slight increase in diatropicity was largest for dibenzoazuliporphyrin 269 (269 > 275 > 274), and the internal CH was observed at 1.59 ppm in this case. As was the case for alkyl-substituted azuliporphyrins 253, the dications for 269, 274, and 275 exhibited much larger macrocyclic ring currents, although the differences between the benzo-fused azuliporphyrins and 253 were not significant. The poor solubilities of 262 and 276 prevented their diatropic characteristics from being assessed by proton NMR spectroscopy. However, NICS calculations indicated that the macrocyclic aromaticity of 262 and 276 for both the free-base and diprotonated forms was reduced compared to the values obtained for 253, 269, 274, and 275 (calculations were performed on structures without the alkyl

substituents). The X-ray crystal structures for 269, 274, and 275 showed that these macrocycles are essentially planar. The bond lengths for the fused benzene units were consistent with localized 6π arene moieties.195 A more direct route to meso-tetraaryl-substituted azuliporphyrins 277 has been developed using a modified Rothemund reaction.90 Lindsey et al. had reported superior syntheses of mesotetrasubstituted porphyrins 278, where pyrrole and aromatic aldehydes were reacted under dilute conditions in dichloromethane using an acid catalyst such as boron trifluoride etherate.71 This resulted in the formation of porphyrinogen intermediates 279 as a mixture of diastereomers. Subsequent oxidation then afforded the porphyrin products. As azulene (254a) favors electrophilic substitution at the 1,3-positions, which are structurally equivalent to the α-positions in pyrrole, it was hypothesized that 254a might be able to take part in Lindsey−Rothemund-like reactions.90 In fact, azulene was shown to react with aldehydes to afford calix[4]azulenes196 (see section 14) and in mixed condensations with pyrrole generated azuliporphyrins 277 (Scheme 89).90,91 Initial attempts to react azulene, pyrrole, and benzaldehyde in a 1:3:4 ratio in dichloromethane in the presence of boron trifluoride etherate gave very poor results. However, when the solvent was switched to chloroform and the condensation step was carried out for 16 h at room temperature, tetraphenylazuliporphyrin 277a could be isolated in 10−13% yield, following an oxidation step with DDQ.90,91 When 0.29 mmol of azulene was reacted in 120 mL of chloroform, 10−11% yields of 277a were obtained, and this was raised to 13% when the volume was increased to 480 mL. Chloroform contains a small amount of ethanol as a stabilizer, and this presumably modifies the activity of the Lewis acid catalyst.90,91 In fact, this effect was first noted by Lindsey and Wagner during the development of a synthesis of mesotetramesitylporphyrin.197 Subsequently, it was reported that other sterically hindered porphyrins, such as tetraphenanthroporphyrin198 and tetraaryltetraacenaphthoporphyrins,104,105 could only be obtained in good yields using Lindsey’s conditions when chloroform was used as the solvent. When the reactions were terminated after 1 h, the only isolatable product was mesotetraphenylporphyrin, and this result indicates that porphyriAO


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Scheme 90. Protonation of meso-Tetraarylazuliporphyrins

Scheme 91. Reaction of Pyrrolidine with an Azuliporphyrin

a slight increase in the diatropic properties for 277b−d, while the slightly electron-donating p-tolyl group in 277e caused a small decrease in the ring current effect. The largest effects were seen for the more-electron-withdrawing p-nitrophenyl and pentafluorophenyl substituents in 277g and 277h, and these showed the internal CH resonance at 2.94 and 2.84, respectively. If the aromatic characteristics of azuliporphyrins 277 are due to dipolar canonical forms such as 277′, it would be expected that electronwithdrawing groups would stabilize the negative charge on the macrocycle and thereby increase the contribution from this type of species. The introduction of a tert-butyl group on the azulene ring further increased the aromatic properties of the system, as the electron-donating group helps to stabilize the tropylium cationic character found in the seven-membered ring for resonance contributor 277′. As expected, the corresponding dications 277H22+ showed far stronger diamagnetic ring currents, and the 21-CH resonance for 277aH22+ in TFA−CDCl3 at 50 °C appeared upfield at −0.33 ppm. The internal NH protons were also relatively upfield, appearing as a 1H singlet at 1.27 ppm and a 2H singlet at 2.71 ppm. Some variations in the observed resonances were noted at lower temperatures. The pyrrolic protons were also observed downfield between 8.0 and 8.4 ppm. The presence of a tert-butyl substituent in 277i,jH22+ led to a small but noticeable increase in the diatropic character, while 23phenyl groups in 277k,lH22+ induced a smaller increase in the ring current when compared to those of 277a,bH22+. The presence of electron-withdrawing meso-substituents, such as 4chlorophenyl groups, led to a small decrease in the diatropicity, and this was attributed to a slight destabilization of dications 277H22+.91

nogen formation occurs far more rapidly than the generation of azuliporphyrinogen 280 (Scheme 89). The steps leading to the formation of 279 are reversible, although once oxidation with DDQ has been carried out the porphyrinogen is irreversibly converted into porphyrin. Therefore, azuliporphyrinogen 280 must slowly accumulate over longer reaction times due to the less reactive nature of azulene compared to pyrrole. Although the yields of 279 are modest, the chemistry is nevertheless remarkable, as eight carbon−carbon bonds must be formed between three different reagents in a regiospecific fashion. Improved yields were obtained with 4-chloro-, 4-bromo-, and 4iodobenzaldehyde to give the related azuliporphyrins 277b−d in 17−21.5% yield.91 p-Tolualdehyde gave poorer results (under dilute conditions, a 9% yield of 277e could be obtained), while panisaldehyde gave poor yields of impure tetrakis(4methoxyphenyl)azuliporphyrin 227f, and 4-nitrobenzaldehyde afforded 90% yield. Although the porphyrin units retained aromatic characteristics, the proton NMR spectra and NICS calculations indicated that the side units were antiaromatic. In the proton NMR spectrum of 842a, the NH resonance was shifted downfield to 17.48 ppm, while the encapsulated meso-proton appeared at 16.51 ppm. For 844, the equivalent resonances were also strongly deshielded to 17.18 and 15.9 ppm. The UV−vis spectra for porphyrin earrings gave broadened Soret bands and bathochromically shifted Q bands that tailed into the nearinfrared (up to 900 nm). Palladium complexes 845 and 846 gave very different spectra, with Q bands extending into the nearinfrared between 700 and 1500 nm.396 Prior to the synthesis of porphyrin earrings, porphyrin dimers and trimers that encapsulated similar dicarbaporphyrinoid cavities had been described. Suzuki−Miyaura coupling of diborylporphyrin 847a with 2,5-dibromothiophene gave dimer 848a and trimer 849a in 29% and 10% yields, respectively (Scheme 269).397 Doubly thienylene-bridged diporphyrin 848 encloses a dithiadicarbaporphyrin-like cavity. Metalation of 848a and 849a with nickel(II) acetylacetonate or zinc acetate afforded the related nickel complexes 848b and 849b or zinc complexes 848c and 849c. Zinc bis(5-bromo-2-thienyl)porphyrin 850 similarly underwent palladium-catalyzed coupling with tetraborylated zinc porphyrin 851c to give a trimer 852 that possessed two dicarbaporphyrinoid cavities of this type. Using the same strategy, nickel(II) dipyridylporphyrin 853 underwent Suzuki−Miyaura coupling with 847b to give the pyridine-linked porphyrin dimer 854, while reaction of 853 with 851b afforded the related porphyrin trimer 855 (Scheme

Figure 31. Examples of vinylogous porphyrins.

trapesoidal analogue 835 of these systems was prepared in 32% yield by reacting stretched pyrrole dialdehyde 836 with tripyrrane 36a under standard MacDonald-type “3 + 1” conditions (Scheme 266).393 Protonation afforded the corresponding dication 835H22+. The proton NMR spectrum of this species showed the internal CH protons at −10.2 ppm, while the NHs appeared at −7.2 (2H), −5.8 (1H), and −5.4 ppm (1H). The peripheral CH protons were strongly shifted downfield to give two 2H doublets at 12.7 and 12.8 ppm and a singlet at 12.4 ppm, confirming the highly diatropic properties of this system. Reaction with palladium(II) chloride led to a conformational rearrangement to give the palladium(II) complex 837.393 Vacataporphyrin 15 has similar structural features and formed metal complexes 838 with Pd(II), Cd(II), Ni(II), and Zn(II) (Scheme 267). 394 Photochemical rearrangement of the palladium(II) complex gave the organometallic derivative 839, although further transformations led to a series of derivatives that in some cases possessed Mobius antiariomatic properties.395

16. PORPHYRIN EARRINGS AND RELATED SYSTEMS THAT ENCLOSE CARBAPORPHYRIN-LIKE BINDING POCKETS In a recent development, carbaporphyrin-like cavities have been constructed using the periphery of a porphyrin as a scaffold (Scheme 268).396 Diboryltripyrrane 840 was prepared using an iridium-catalyzed borylation reaction. Palladium-catalyzed Suzuki−Miyaura coupling of 840 with nickel(II) dibromoporphyrins 841a or 841b gave the porphyrin “earrings” 842a and 842b in Scheme 266. Synthesis and Metalation of [22]Porphyrin(3.1.1.3)

DQ


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Scheme 267. Metalation of Vacataporphyrin

Scheme 268. Synthesis of Porphyrin Earrings

Scheme 269. Porphyrin Dimers and Trimers Enclosing Dithiadicarbaporphyrin-like Cavities

DR


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Scheme 270. Porphyrin Dimers and Trimers That Enclose Dicarbaporphyrin-like Cavities

270).398 In these systems, the linkages enclose cavities that resemble dicarbaporphyrins, and it was found that this component could act as a dianionic ligand. Specifically, 854 and 855 reacted with palladium(II) acetate to give the mixed nickel(II)−palladium(II) derivatives 856 and 857, respectively. In these complexes, porphyrin meso-carbons form two bonds to each palladium cation. These molecular belts again have highly curved geometries. In 854, the two porphyrin units have a dihedral angle of 106.5°, although this decreased to 87.3° in palladium derivative 856. Closely related porphyrin dimers with carbazole, fluorene, and fluorenone bridges have also been reported.399

reactions have been reported, and carbaporphyrin ketals obtained by oxidizing benzocarbaporphyrins have been shown to have significant biological activity.118−120 Many examples of dicarbaporphyrinoid systems have also been prepared, and these demonstrate equally interesting properties. However, little progress has been made on the synthesis of tri- and tetracarbaporphyrins. On the other hand, many types of expanded carbaporphyrinoids have been discovered. These systems are providing important insights into the aromatic properties of porphyrinoid systems, including Möbius aromaticity in expanded benziporphyrins. New carbaporphyrinoid structures continue to be discovered, and many of these systems show promise in the development of catalytic systems.149,400 Furthermore, although carbaporphyrinoids have been little studied as components in supramolecular systems226 or in the development of fluorescence sensors,227,401 these systems do show promise for applications in these areas. In fact, it is likely that carbaporphyrinoids will remain at the forefront of discovery with regard to gaining fundamental insights into aromaticity, organometallic chemistry, and molecular rearrangements, and these investigations may also result in the discovery of unique applications for these intriguing macrocyclic structures.

17. CONCLUSIONS AND FUTURE PROSPECTS Carbaporphyrins have emerged over the last 20 years as an important class of porphyrin analogues that exhibit unusual reactivity and the ability to form stable organometallic derivatives. Replacement of one of the nitrogen atoms within the porphyrin core with a carbon produces carbaporphyrins, and this system retains strongly aromatic properties. Related porphyrinoids can be obtained with carbocyclic rings replacing a pyrrole unit to give benziporphyrins, azuliporphyrins, and tropiporphyrins, while heterocyclic units placing a carbon atom within the porphyrinoid core produce carbaporphyrin analogues with “confused” pyrrole, furan, thiophene, selenophene, pyrazole, and pyridine rings. These different families of monocarbaporphyrinoids exhibit a wide variety of different properties and may exist as aromatic, nonaromatic, or even antiaromatic systems. Carbaporphyrins can act as trianionic ligands and can stabilize unusual oxidation states such as silver(III), but other carbaporphyrinoids such as azuliporphyrins and benziporphyrins are dianionic ligands. Organometallic derivatives have been prepared that incorporate many different transition metal ions, such as Cu(III), Ag(III), Au(III), Ni(II), Pd(II), Pt(II), Rh(III), Ir(III), and Ru(II). Unusual oxidation

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The author declares no competing financial interest. Biography Timothy D. Lash obtained his B.Sc. (Hon.) from the University of Exeter (Exeter, UK) in 1975 and completed his Ph.D. degree in 1979 at the University of Wales, College of Cardiff (Cardiff, UK) under the direction of Prof. A. H. Jackson. He joined the Department of Chemistry DS


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Krygowski, T. M., Cyranski, M. K., Eds.; Springer, 2009; Vol. 19, pp 83− 154. (9) Johnson, A. W. Structural Analogs of Porphyrins. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed., Elsevier: Amsterdam, 1975; pp 729−754. (10) Latos-Grażyński, L. Core-Modified Heteroanalogues of Porphyrins and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp 361−416. (11) Matano, Y.; Imahori, H. Phosphole-Containing Calixpyrroles, Calixphyrins, and Porphyrins: Synthesis and Coordination Chemistry. Acc. Chem. Res. 2009, 42, 1193−1204. (12) Brückner, C.; Akhigbe, J.; Samankumara, L. P. Porphyrin Analogs Containing Non-Pyrrolic Heterocycles. In Handbook of Porphyrin ScienceWith Applications to Chemistry, Physics, Material Science, Engineering, Biology and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific Publishing: Singapore, 2014; Vol. 31, pp 1−275. (13) Brückner, C. The Breaking and Mending of meso-Tetraarylporphyrins: Transmuting the Pyrrole Building Blocks. Acc. Chem. Res. 2016, 49, 1080−1092. (14) Lash, T. D. Syntheses of Novel Porphyrinoid Chromophores. In The Porphyrin Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 2, pp 125−199. (15) Lash, T. D. Carbaporphyrinoids: Taking the Heterocycle Out of Nature’s [18]Annulene. Synlett 2000, 2000, 279−295. (16) Lash, T. D. Recent Advances on the Synthesis and Chemistry of Carbaporphyrins and Related Porphyrinoid Systems. Eur. J. Org. Chem. 2007, 2007, 5461−5481. (17) Pawlicki, M.; Latos-Grażyński, L. CarbaporphyrinoidsSynthesis and Coordination Properties. In Handbook of Porphyrin Science With Applications to Chemistry, Physics, Material Science, Engineering, Biology and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific: Singapore, 2010; Vol. 2, pp 104−192. (18) Lash, T. D. Carbaporphyrins and Related Systems. Synthesis, Characterization, Reactivity and Insights into the Nature of Porphyrinoid Aromaticity. In Handbook of Porphyrin ScienceWith Applications to Chemistry, Physics, Material Science, Engineering, Biology and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific Publishing: Singapore, 2012; Vol. 16, pp 1−329. (19) Szyszko, B.; Latos-Grażyński, L. Core Chemistry and Skeletal Rearrangements of Porphyrinoids and Metalloporphyrinoids. Chem. Soc. Rev. 2015, 44, 3588−3616. (20) Pawlicki, M.; Latos-Grażyński, L. Aromaticity Switching in Porphyrinoids. Chem. - Asian J. 2015, 10, 1438−1451. (21) AbuSalim, D. I.; Lash, T. D. Relative Stability and Diatropic Character of Carbaporphyrin, Dicarbaporphyrin, Tricarbaporphyrin and Quatyrin Tautomers. J. Org. Chem. 2013, 78, 11535−11548. (22) Srinivasan, A.; Furuta, H. Confusion Approach to Porphyrinoid Chemistry. Acc. Chem. Res. 2005, 38, 10−20. (23) Toganoh, M.; Furuta, H. Synthesis and Metal Coordination of NConfused and N-Fused Porphyrinoids. In Handbook of Porphyrin ScienceWith Applications to Chemistry, Physics, Material Science, Engineering, Biology and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific Publishing: Singapore, 2010; Vol. 2, pp 103−192. (24) Toganoh, M.; Furuta, H. Blooming of Confused Porphyrinoids − Fusion, Expansion, Contraction, and More Confusion. Chem. Commun. 2012, 48, 937−954. (25) Lash, T. D. Out of the Blue! Azuliporphyrins and Related Carbaporphyrinoid Systems. Acc. Chem. Res. 2016, 49, 471−482. (26) Lash, T. D. Benziporphyrins, a Unique Platform for Exploring the Aromatic Characteristics of Porphyrinoid Systems. Org. Biomol. Chem. 2015, 13, 7846−7878. (27) Bergman, K. M.; Ferrence, G. M.; Lash, T. D. Tropiporphyrins, Cycloheptatrienyl Analogues of the Porphyrins: Synthesis, Spectroscopy, Chemistry and Structural Characterization of a Silver(III) Derivative. J. Org. Chem. 2004, 69, 7888−7897.

at Illinois State University (Normal, IL) in 1984 as an assistant professor and was promoted to full professor in 1993. In 2000, he was awarded the title of distinguished professor. His research has focused on the synthesis, reactivity, biochemistry, geochemistry, and spectroscopy of porphyrins and related conjugated macrocycles. This work has resulted in over 200 publications, including 4 book chapters.

ACKNOWLEDGMENTS The author’s work in this area has received virtually continuous support from the National Science Foundation over the last 25 years, most recently under grant no. CHE-1465049. Additional support has been received from the Petroleum Research Fund, administered by the American Chemical Society. ABBREVIATIONS USED Ac acetyl acac acetylacetonate Ar aryl Bn benzyl t-Bu tert-butyl COD 1,5-cyclooctadiene DBU 1,8-diazabicyclo[5.4.0]undec-7-ene DDQ 2,3-dichloro-5,6-dicyano-1,4-benzoquinone DFT density functional theory DMAD dimethyl acetylenedicarboxylate DMF N,N-dimethylformamide DMSO dimethyl sulfoxide i-Bu isobutyl i-Pr isopropyl Mes mesityl NBS N-bromosuccinimide NCP N-confused porphyrin NICS nucleus independent chemical shifts NMP N-methylpiperazine NMR nuclear magnetic resonance PAH polycyclic aromatic hydrocarbon Tf trifluorosulfonyl (triflyl) TFA trifluoroacetic acid TIPS triisopropylsilyl TMEDA N,N,N′,N′-tetramethylethylenediamine Tol p-tolyl (p-MeC6H4) p-TSA p-toluenesulfonic acid XRD X-ray diffraction REFERENCES (1) Milgrom, L. R. The Colours of Life. An Introduction to the Chemistry of Porphyrins and Related Compounds; Oxford University Press: New York, 1997. (2) Handbook of Porphyrin ScienceWith Applications to Chemistry, Physics, Material Science, Engineering, Biology and Medicine; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific Publishing: Singapore, Vols. 1−35, pp 2010−2014. (3) Johnson, A. W. Aromaticity in Macrocyclic Polypyrrolic Systems. Pure Appl. Chem. 1971, 28, 195−218. (4) Vogel, E. The Porphyrins from the ‘Annulene Chemist’s’ Perspective. Pure Appl. Chem. 1993, 65, 143−152. (5) Vogel, E. Porphyrinoid Macrocycles: A Cornucopia of Novel Chromophores. Pure Appl. Chem. 1996, 68, 1355−1360. (6) Lash, T. D. Origin of Aromatic Character in Porphyrinoid Systems. J. Porphyrins Phthalocyanines 2011, 15, 1093−1115. (7) Lash, T. D. Carbaporphyrins, Porphyrin Isomers and the Legacy of Emanuel Vogel. J. Porphyrins Phthalocyanines 2012, 16, 423−433. (8) Stępień, M.; Latos-Grażyński, L. Aromaticity and Tautomerism in Porphyrins and Porphyrinoids. In Topics in Heterocyclic Chemistry; DT


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