Conjugated macrocycles related to the porphyrins. 25. Proton NMR spectroscopic evidence for a preferred [18]annulene substructure in carbaporphyrins from the magnitude of selected 4J(H,H) CH=C-CH3 coupling constants

Organometallic Chemistry within the Structured Environment Provided by the Macrocyclic Cores of Carbaporphyrins and Related Systems

The unique environment within the core of carbaporphyrinoid systems provides a platform to explore unusual organometallic chemistry. The ability of these structures to form stable organometallic derivatives was first demonstrated for N-confused porphyrins but many other carbaporphyrin-type systems were subsequently shown to exhibit similar or complementary properties. Metalation commonly occurs with catalytically active transition metal cations and the resulting derivatives exhibit widely different physical, chemical and spectroscopic properties and range from strongly aromatic to nonaromatic and antiaromatic species. Metalation may trigger unusual, highly selective, oxidation reactions. Alkyl group migration has been observed within the cavity of metalated carbaporphyrins, and in some cases ring contraction of the carbocyclic subunit takes place. Over the past thirty years, studies in this area have led to multiple synthetic routes to carbaporphyrinoid ligands and remarkable organometallic chemistry has been reported. An overview of this important area is presented.

Synthesis of 2-bromo- and 2-phenyl-neo-confused porphyrins

New examples of neo-confused porphyrins are reported. These retain global diatropic characteristics but are relatively unstable in solution.

What’s in a name? The MacDonald condensation

In 1960, MacDonald and coworkers introduced a new method for porphyrin synthesis that involved the acid-catalyzed condensation of dipyrrylmethane dialdehydes with [Formula: see text],[Formula: see text]-diunsubstituted dipyrrylmethanes or the related dicarboxylic acids, followed by an air oxidation. The key bond forming steps entail electrophilic substitution at two pyrrole units with the aldehyde moieties to generate, following elimination of water, a 5,15-dihydroporphyrin or porphodimethene intermediate. Following addition of sodium acetate, or in later procedures zinc acetate, the dihydroporphyrins readily air oxidize to the fully aromatic porphyrin system. This strategy, which parallels chemistry contemporaneously developed by R. B. Woodward at Harvard for the total synthesis of chlorophyll [Formula: see text], demonstrated that dipyrrylmethanes were sufficiently stable to be utilized as intermediates in porphyrin syntheses and that porphodimethene formation circumvented acidolytic scrambling reactions that might lead to isomeric porphyrin products. This type of chemistry was later adapted by Johnson, Woodward and others to prepare porphyrin-type systems by a “3 + 1” strategy, or expanded porphyrins by “3 + 2” or other combinations of oligopyrrolic precursors. In recent years, the term “MacDonald condensation” has been increasingly used to describe other types of chemistry involving oligopyrrolic intermediates. Following on from a historical review of this area, guidelines for the identification of MacDonald-type reactions are proposed.

Carbaporphyrins, porphyrin isomers and the legacy of Emanuel Vogel

Vogel discovered a large number of bridged annulene structures, including constitutional porphyrin isomers, heteroporphyrinoid species such as the tetraoxaporphyrin dication, corrole isomers and novel expanded porphyrinoid systems. In this review, the impact of Vogel's work on the author's research program is discussed. Carbaporphyrinoid systems with one or two carbocyclic rings replacing the usual pyrrole subunits have been synthesized and their diatropic characteristics can be related to the presence of 18π electron delocalization pathways within these macrocycles. The preparation of a dideazaporphyrin also provides strong support for the concept, espoused by Vogel, that porphyrins are examples of bridged annulenes. Ring contractions of azuliporphyrins to benzocarbaporphyrins with tert-butyl hydroperoxide in the presence of base have been proposed to involve a Cope rearrangement where a cycloheptatriene unit rearranges to give a norcaradiene-fused carbaporphyrinoid, followed elimination of tert-butyl alcohol, and Vogel's investigations into these types of valence tautomerization processes provides insights into these unusual transformations. In addition, a new class of porphyrin isomers has been synthesized and these porphyrin analogs extend the observations previously made for N-confused porphyrins and Vogel's constitutional isomers.

Synthesis, Spectroscopy, and Reactivity of meso-Unsubstituted Azuliporphyrins and Their Heteroanalogues. Oxidative Ring Contractions to Carba-, Oxacarba-, Thiacarba-, and Selenacarbaporphyrins

This paper reports the first detailed study on meso-unsubstituted azuliporphyrins, an important family of porphyrin-like molecules where one of the usual pyrrole rings has been replaced by an azulene subunit. Although the azulene moiety introduces an element of cross-conjugation, zwitterionic resonance contributors with tropylium and carbaporphyrin substructures give azuliporphyrins diatropic character that falls midway between true carbaporphyrins and nonaromatic benziporphyrins. Protonation affords an aromatic dication where this type of resonance interaction is favored due to the associated charge delocalization. Two different "3 + 1" syntheses of meso-unsubstituted azuliporphyrins have been developed. Acid-catalyzed reaction of readily available tripyrrane dicarboxylic acids with 1,3-azulenedicarbaldehyde, followed by oxidation with DDQ or FeCl(3), affords good yields of azuliporphyrins. Alternatively, azulene reacted with acetoxymethylpyrroles (2 equiv) in refluxing acetic acid/2-propanol to give tripyrrane analogues, and following a deprotection step, condensation with a pyrrole dialdehyde in TFA-CH(2)Cl(2) gave the azuliporphyrin system. The latter approach was also used to prepare 23-thia- and 23-selenaazuliporphyrins. However, reaction of the azulitripyrrane with 2,5-furandicarbaldehyde produced a mixture of three oxacarbaporphyrins in moderate yield. The free base forms of thia- and selenaazuliporphyrins both showed intermediary aromatic character that was considerably enhanced upon protonation. The UV-vis spectra for azuliporphyrins and their heteroanalogues showed four bands between 350 and 500 nm and broad absorptions at higher wavelengths. Addition of TFA gave dications that showed porphyrin-like spectra with Soret bands between 460 and 500 nm. In the presence of pyrrolidine, azuliporphyrins and their heteroanalogues undergo nucleophilic attack on the seven-membered ring to give carbaporphyrin adducts. These systems also undergo oxidative rearrangements under basic conditions with t-BuOOH to give benzocarbaporphyrins. The selenaazuliporphyrin afforded two benzoselenacarbaporphyrins, a previously unknown core-modified carbaporphyrin system. The proton NMR spectra for these compounds showed strong diatropic ring currents with the internal CH resonance upfield above -5 ppm, while the meso-protons resonated downfield near 10 ppm. The UV-vis spectra were also porphyrin-like and gave strong Soret bands at ca. 440 nm.

Synthesis of Sapphyrins, Heterosapphyrins, and Carbasapphyrins by a “4 + 1” Approach

Sapphyrins are an important group of expanded porphyrins that show valuable anion binding characteristics. In this study, a "4 + 1" route to sapphyrin systems has been developed. Reaction of dialdehydes with a known tetrapyrrole intermediate 11b incorporating a bipyrrolic subunit afforded a wide range of sapphyrin-type products. The best conditions for these reactions involved carrying out the condensation of the dialdehydes with the tetrapyrrole in TFA-dichloromethane, followed by oxidation with dilute aqueous solutions of ferric chloride. A pyrrole dialdehyde reacted under these conditions to give sapphyrin in 50% yield, while furan and thiophene dialdehydes afforded the corresponding oxa- and thiasapphyrins in 66-90% yield. Pyrrole dialdehydes with fused phenanthrene or acenaphthylene rings also reacted with 11b to give the related phenanthro- and acenaphthosapphyrins in excellent yields. As was the case for acenaphthoporphyrins, the acenaphthosapphyrin gave longer wavelength absorptions than the corresponding phenanthrene fused structure, although the differences were not as marked as those seen in the porphyrin series. Reaction of 11b with 1,3-diformylindene gave a benzocarbasapphyrin in 38% yield, while a triformyl cyclopentadiene reacted with the tetrapyrrole to give a carbasapphyrin aldehyde in 7-12% yield. The free base carbasapphyrins were unstable but the monoprotonated hydrochloride salts could easily be isolated and characterized. Carbasapphyrins retain a strong diatropic ring current due to the presence of 22pi electron delocalization pathways. In the presence of trifluoroacetic acid, C-protonated dications are generated. Condensation of 1,3-azulenedicarbaldehyde with 11b gave an azulisapphyrin dihydrochloride salt in 35% yield, and this also showed a strong diatropic ring current. Addition of base gave the unstable free base form, while pyrrolidine formed an unstable adduct that showed an intense Soret band at 480 nm. These results demonstrate that many of the themes observed for modified porphyrins and carbaporphyrins also apply to the sapphyrin series, although in some cases reduced stability hampers these investigations.

Tropiporphyrins, Cycloheptatrienyl Analogues of the Porphyrins: Synthesis, Spectroscopy, Chemistry, and Structural Characterization of a Silver(III) Derivative

Tripyrranes were condensed with 1,3,5-cycloheptriene-1,6-dicarbaldehyde in TFA-CH(2)Cl(2) to give, following oxidation with 0.1% aqueous ferric chloride solutions, a series of tropiporphyrins 9. These cycloheptatrienyl analogues of the porphyrins show strong diatropic ring currents by proton NMR spectroscopy where the internal CH gives a resonance at -7.3 ppm, although the meso-protons are not shifted as far downfield as most aromatic porphyrinoid systems. These data indicate that the seven-membered ring distorts the porphyrinoid macrocycle and decreases the overall diatropicity in tropiporphyrins. Addition of trace amounts of TFA to solutions of 9 affords the corresponding aromatic monocations, and at higher acid concentrations a nonaromatic dication is generated. The dication has undergone C-protonation at one of the meso-bridges and has lost the plane of symmetry present in the parent system. This species shows significant downfield shifts to the cycloheptatrienyl protons, indicating that this unit has taken on tropylium character. Tropiporphyrin 9a underwent a Diels-Alder cycloaddition with dimethyl acetylenedicarboxylate in refluxing xylenes to give modest yields of the related adduct. The Diels-Alder adduct 17 showed an increased diatropic ring current where the internal proton shifted beyond -9 ppm, and this indicates that the [18]annulene substructure has flattened out compared to 9a. Diimide reduction of 9a afforded a dihydrotropiporphyrin that also showed a stronger ring current. Tropiporphyrins 9 were also shown to react with silver(I) acetate in the presence of DBU in refluxing pyridine to give the corresponding silver(III) organometallic derivatives. The meso-protons for these metal complexes give proton NMR chemical shift values similar to those for the parent tropiporphyrins, indicating that the macrocycle is still distorted, but the external olefinic protons are shifted downfield compared to 9. A diphenyl-substituted silver(III) derivative 18b was further characterized by X-ray crystallography. This shows that the cycloheptatriene unit takes on a highly twisted geometry that distorts the overall conformation of the porphyrinoid macrocycle.

Oxybenziporphyrins, Oxypyriporphyrins, Benzocarbaporphyrins, and Their 23-Oxa and 23-Thia Analogues: Synthesis, Spectroscopic Characterization, Metalation, and Structural Characterization of a Palladium(II) Organometallic Derivative

A series of nine porphyrin analogues have been synthesized using the "3 + 1" variant on the MacDonald condensation. Tripyrrane-type systems with a centrally unsubstituted pyrrole, furan, or thiophene ring were prepared using conventional methods, and these were condensed with indene-1,3-dicarbaldehyde, 5-formylsalicylaldehyde, or 3-hydroxy-2,6-pyridinedicarbaldehyde in the presence of TFA to generate benzocarba-, oxybenzi-, and oxypyriporphyrins, respectively. The furan-containing analogues proved to be highly basic and could only be isolated as the corresponding hydrochloride salts. All nine analogue systems showed porphyrin-like UV-vis spectra with one or two Soret absorptions near 400 nm and a series of weaker bands at longer wavelengths. These systems also showed large diatropic ring currents by proton NMR spectroscopy that were comparable to true porphyrins. In the presence of trace amounts of TFA, benzocarbaporphyrin 12 formed a monocation, and in 50% TFA a C-protonated dication was generated. The 23-oxacarbaporphyrin 14 gave a monocation in chloroform, although the free base was generated in 5% Et(3)N-chloroform. In 50% TFA-CHCl(3), 14 afforded a mixture of mono- and diprotonated species. Thiacarbaporphyrin 15 also formed a monocation in the presence of TFA, but C-protonation was relatively disfavored for this system. Nonetheless, in the presence of TFA-d, 12, 14, and 15 all showed rapid exchange of the internal NH and CH protons. Carbaporphyrin 12 also showed slow exchange at the meso-positions, but this process was not observed for its heteroanalogues 14 and 15. Protonation studies were also conducted for oxybenziporphyrins and oxypyriporphyrins 16-21. Oxacarbaporphyrin 14 was shown to be a superior organometallic ligand and afforded good yields of the related nickel(II) and palladium(II) derivatives under mild conditions. A low yield of the platinum(II) complex could also be isolated. All three complexes retained their aromatic character, although the Pd(II) derivative appeared to possess a slightly larger diatropic ring current. The palladium(II) complex 27 was further characterized by X-ray crystallography. The macrocyclic core was shown to be highly planar where the dihedral angles of the component pyrrole, furan and indene rings relative to the mean [18]annulene plane were all </=2.1 degrees.

Carbaporphyrinoid Chemistry Has a Silver Lining! Silver(III) Oxybenzi-, Oxynaphthi-, Tropi-, and Benzocarbaporphyrins

[structure: see text] Carbaporphyrinoid systems with semiquinone, cycloheptatriene, or indene subunits react under mild conditions with silver(I) acetate to give stable silver(III) organometallic derivatives.

Preparation of tripyrrane analogues from resorcinol and 2-methylresorcinol for applications in the synthesis of new benziporphyrin systems

Acid catalyzed condensation of resorcinol or 2-methylresorcinol with 2 equiv. of an acetoxymethylpyrrole gave bis(pyrrolylmethyl)benzene derivatives in moderate yields; these afforded a series of novel aromatic benziporphyrins using the MacDonald "3 + 1" methodology.

Regioselective Oxidations of Benzocarbaporphyrins with Ferric Chloride: A Facile Synthesis of Bridged [18]Annulene Ketals with Strong Absorptions in the Far Red and an Unexpected Halogenation Reaction at the Interior Carbon Atom

Benzocarbaporphyrins 4 were found to undergo regioselective oxidations with ferric chloride in methanol, ethanol, isopropyl alcohol, or ethylene glycol to give bridged benzo[18]annulene ketal derivatives 5 in excellent yields. These polar derivatives were generally isolated in a monoprotonated form and the corresponding free bases appeared to be relatively unstable. Addition of TFA resulted in the formation of spectroscopically distinct dications. The ketals 5 were highly diatropic in nature, showing the internal alkoxy substituents upfield beyond -1 ppm in their proton NMR spectra. The external meso-protons resonated near 10 ppm, confirming the presence of a strong aromatic ring current. The UV-vis spectra for 5 showed a Soret band at 422 nm, and two strong absorptions in the far red at 751 and 832 nm. A carbaporphyrin with a fused acenaphthylene ring was also oxidized with ferric chloride and this produced a ketal derivative with still further bathochromically shifted absorptions particularly for the Soret band. Also, the use of different alcohols in these reactions allows the overall polarity of these ketal products to be controlled and this could have merit in biomedical applications. Reaction of carbaporphyrin 4a with aqueous ferric chloride afforded the corresponding 21-chloro derivative 20 in good yields, and at longer reaction times a nonaromatic dione was isolated. Aqueous ferric bromide gave a 21-bromocarbaporphyrin product but in this case very poor yields (<10%) were noted. Mechanisms are proposed to explain the formation of these unusual oxidation products. The structure of 21-chlorocarbaporphyrin 20 was further demonstrated by X-ray crystallography. The presence of the interior chlorine atom was found to tilt the indene moiety by 29.59(4) degrees relative to the [18]annulene macrocyclic ring. The crystal packing for 20 shows offset face-to-face pi-stacking interactions that link the porphyrinoid molecules as closely paired dimers.

Organometallic Chemistry of Azuliporphyrins: Synthesis, Spectroscopy, Electrochemistry, and Structural Characterization of Nickel(II), Palladium(II), and Platinum(II) Complexes of Azuliporphyrins

Four azuliporphyrins, two meso-unsubstituted and two meso-tetraaryl substituted, were investigated in the synthesis of novel organometallic compounds. The meso-unsubstituted or "etio" series azuliporphyrins 8 reacted with nickel(II) acetate, palladium(II) acetate, and platinum(II) chloride in DMF to give the corresponding chelates 14-16, where the metal cation lies within the macrocyclic cavity and binds to all three nitrogens and the internal carbon atom. The newly available meso-tetraarylazuliporphyrins 13 similarly afforded the corresponding nickel(II), palladium(II), and platinum(II) complexes, 17-19, respectively. The new organometallic complexes are stable nonpolar compounds and were fully characterized spectroscopically and by mass spectrometry. The UV-vis data indicate that these complexes, in common with the parent azuliporphyrin system 8, do not possess porphyrin-type aromaticity. However, electron donation from the azulene unit can give rise to dipolar resonance contributors that provide a degree of carbaporphyrin-type aromatic character. The platinum(II) azuliporphyrins 16 gave noteworthy proton NMR spectra where the meso-protons showed satellite peaks due to transannular coupling to platinum-195. The pyrrolic protons of the platinum(II) meso-tetraarylazuliporphyrin 19b also showed similar satellite peaks due to coupling from the platinum-195 isotope. The electrochemistry of free base tetraphenylazuliporphyrin 13a and the related nickel(II) and palladium(II) complexes was investigated using cyclic voltammetry, and these data indicate that metal coordination improves the reversibility of the ligand-based oxidations. Nickel(II) azuliporphyrin 14a and palladium(II) tetrakis(4-chlorophenyl)azuliporphyrin 18b were also structurally characterized by X-ray crystallography. The macrocyclic core of the palladium(II) complex 18b was significantly more planar than the nickel(II) derivative 14b, and this difference was attributed to the better size match between the azuliporphyrin cavity and the larger palladium(II) ion. The straightforward synthesis of metalloazuliporphyrins under mild conditions, and their interesting spectroscopic, electrochemical, and structural features, demonstrates that the azuliporphyrin system holds great promise as a platform for organometallic chemistry.