Conjugated Macrocycles Related to the Porphyrins. 12.1 Oxybenzi- and Oxypyriporphyrins: Aromaticity and Conjugation in Highly Modified Porphyrinoid Structures

Serendipitous Discovery of Conjugated Dipyrromethene-Linked Nitronaphthoporphyrins

Acid-catalyzed condensation of a nitronaphthalene-fused dipyrrylmethane with dipyrrylmethane dialdehydes afforded unique dipyrromethene-naphthoporphyrin conjugates together with expected nitronaphthoporphyrins. The unusual conjugated system retained aromatic porphyrin-type characteristics but afforded highly modified UV-vis spectra with multiple absorptions throughout the visible region.

Synthesis, Spectroscopic Properties, and Metalation of 3-Alkoxybenziporphyrins

A series of 5-alkoxy-1,3-benzenedicarbaldehydes and related dimers were prepared in three steps from dimethyl 5-hydroxyisophthalate. Acid catalyzed condensation of the dialdehydes with a tripyrrane dicarboxylic acid, followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, afforded good yields of 3-alkoxybenziporphyrins, although dimeric tetraaldehydes failed to give isolatable porphyrinoid products. Proton NMR spectroscopy gave no indication of an aromatic ring current, but addition of trifluoroacetic acid resulted in the formation of dications that exhibited weakly diatropic characteristics. Spectroscopic titration with TFA demonstrated that stepwise protonation took place, generating monocationic and dicationic species. 3-Alkoxybenziporphyrins reacted with nickel(II) or palladium(II) acetate to give the related nickel(II) or palladium(II) complexes. These stable organometallic derivatives showed increased diatropic properties that were most pronounced for the palladium(II) complexes. These unique porphyrinoids provide further insights into the properties of benziporphyrins.

Di(p-dibenzi)[40]decaphyrin(1.0.0.0.0.1.0.0.0.0) PdII Complex: A Weakly Hückel 38π-Aromatic Macrocycle

Di(p-benzi)[40]decaphyrin(1.0.0.0.0.1.0.0.0.0) BF2 complex and tris(p-benzi)[60]pentadecaphyrin(1.0.0.0.0.1.0.0.0.0.1.0.0.0.0) BF2 complex were synthesized by Suzuki-Miyaura coupling of α,α'-diborylated tetrapyrrole BF2 -complex with 1,4-diiodobenzene. Bis-BF2 complex was converted to bis-PdII complex via its free base. Macrocycles bis-BF2 and tris-BF2 complex take Möbius topology but are nonaromatic, since the macrocyclic conjugation is disrupted by the locally aromatic 1,4-phenylene units. In contrast, bis-PdII complex is a weakly Hückel 38π-aromatic macrocycle as evinced by its red-shifted, enhanced, and structured Q-like bands and a small electrochemical HOMO-LUMO gap. Interestingly, one 1,4-pheylene part of bis-PdII complex takes a quinonoidal distorted structure and the other takes a usual benzene structure in a figure-eight conformation with Hückel topology.

Advent and features of pyriporphyrinoids: an overview of a pyridine-based porphyrin analogue

Pyriporphyrinoids have recently attracted a significant proliferation of attention due to their versatile characters, which stem from structural motifs in which the pyridine moiety is involved. The evolution of pyriporphyrin chemistry revealed the subtle modifications of the macrocyclic core that tweak the electronic structure as compared to the parental macrocycle. The amendment of π-electronic organization inside the core manifests exceptional photophysical and coordination properties that cover a vast range of seemingly contradictory fields. In fundamental chemistry, the pyridine unit acts as a modulator of π-conjugated porphyrinoid systems, resulting in aromaticity swapping. From the applied chemistry perspective, these macrocycles are primarily utilized as (i) sensors, (ii) NIR absorbing photoacoustic dyes, (iii) electrochemical catalysts, (iv) singlet biradicaloid generation and (v) contributors to generate metal complexes with intriguing binding modes. Surprisingly, despite their prominence, pyriporphyrinoids are inadequately investigated, while pyridine unit-embedded calixphyrin, calixpyridinopyrrole and calixpyridine are barely reported. This review article illustrates the controlled formation of specific porphyrinic scaffolds with pyridine unit(s) and diverse functionalized heterocyclic and/or carbocyclic building block(s), and demonstrates a substantial influence on the macrocyclic properties.

Synthesis and Properties of Nonaromatic meso-fused Heterobenzihomoporphyrin(2.1.1.1)s

Fluorene-based tripyrrane has been used as a fused precursor to synthesize three novel examples of nonaromatic meso-fused thia and selenabenzihomoporphyrin(2.1.1.1)s by condensing it with appropriate 2,5-bis(hydroxymethyl)aryl thiophene or selenophene under acid catalyzed conditions. The meso-fused heterobenzihomoporphyrins contain one fluorene unit, two pyrrole rings and one thiophene/selenophene ring connected via five meso-carbons in the macrocyclic framework. The macrocycles were thoroughly characterized by HR-MS, 1D and 2D NMR, absorption, cyclic voltammetry and DFT/TD-DFT studies. NMR, absorption, and DFT studies indicated the nonaromatic nature of meso-fused heterobenzihomoporphyrins. The macrocycles displayed one intense band at ∼380 nm along with a shoulder band at 450 nm and a broad band in the region of 590-850 nm which were bathochromically shifted in the monoprotonated derivatives and absorbed prominently in the NIR region with the peak maxima at ∼1035 nm. The electrochemical studies revealed that the macrocycles showed three well-defined oxidations and reductions, and TD-DFT studies corroborated experimental observations.

Aromatic Character and Relative Stability of Pyrazoloporphyrin Tautomers and Related Protonated Species: Insights into How Pyrazole Changes the Properties of Carbaporphyrinoid Systems

Pyrazoloporphyrins (PzPs), which are porphyrin analogues incorporating a pyrazole subunit, are examples of carbaporphyrin-type structures with a carbon atom within the macrocyclic cavity. DFT calculations were used to assess a series of 17 PzP tautomers, nine monoprotonated species and four related diprotonated PzP dications. The geometries of the structures were optimized using M06-2X/6-311++G(d,p), and the relative stabilities computed with the cc-PVTZ functional. Nucleus independent chemical shifts, both NICS(0) and NICS(1)_zz, were calculated, and the anisotropy of the induced current density (AICD) plots were generated for all of the species under investigation. The results for free base PzPs show that fully aromatic PzP tautomers are not significantly more stable than weakly aromatic cross-conjugated species. In addition, strongly aromatic structures with internal CH_2's are much less stable, a feature that is also seen for protonated PzPs. The degree of planarity for the individual macrocycles does not significantly correlate with the stability of these structures. The results allow significant aromatic conjugation pathways to be identified in many cases, and provide insights into the aromatic properties of this poorly studied system. These investigations also complement experimental results for PzPs and emphasize the need for further studies in this area.

m-Benziporphyrin(1.1.0.0)s as a Rare Example of Ring-Contracted Carbaporphyrins with Metal-Coordination Ability: Distorted Coordination Structures and Small HOMO-LUMO Gaps

m-Benziporphyrin(1.1.0.0) and m-pyreniporphyrin(1.1.0.0) were prepared as ring-contracted carbaporphyrins. While m-Benziporphyrin(1.1.0.0) was unstable, m-pyreniporphyrin(1.1.0.0) was fairly stable. Both of their Pd^II complexes showed distorted coordination structures with extremely short Pd-C bonds. As compared with the reported m-benziporphyrin Pd^II complexes, these Pd^II complexes showed considerably small HOMO-LUMO gaps, despite their smaller molecular size. Pd^II metalation of the m-pyreniporphyrin(1.1.0.0) dimer gave the corresponding Pd^II complex, which showed similar distorted coordination and a smaller HOMO-LUMO gap. Finally, Pd^II metalation of a pyrene-sharing formal p-benziporphyrin(1.1.1.1) dimer gave a nonaromatic Pd^II dimer, which rearranged to an aromatic Pd^II complex upon treatment with alumina.

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.

Two-photon absorption of 28-hetero-2,7-naphthiporphyrins: expanded carbaporphyrinoid macrocycles

The one- and two-photon absorption (1PA and 2PA) properties of three expanded aceneporphyrinoids, 28-thia-, 28-selena- and 28-tellura-2,7-naphthiporphyrin, have been studied. The open-aperture Z-scan technique was used to determine two-photon absorption cross-sections in the near infrared range using an amplified femtosecond laser system. The maximum values of the cross sections were found to be 99, 200 and 650 GM at 900 nm and 1, 13 and 31 GM at 1400 nm for the three investigated compounds, respectively. These results demonstrate enhanced 2PA properties compared with well-known porphyrin photosensitizers, such as Foscan®, showing the potential of porphyrin core modification for optimizing infrared nonlinear absorbers.

The one- and two-photon absorption (1PA and 2PA) properties of three expanded aceneporphyrinoids, 28-thia-, 28-selena- and 28-tellura-2,7-naphthiporphyrin, have been studied.

Metal Complexes of Porphyrinoids Containing Nonpyrrolic Heterocycles

The replacement of one or more pyrrolic building block(s) of a porphyrin by a nonpyrrolic heterocycle leads to the formation of so-called pyrrole-modified porphyrins (PMPs), porphyrinoids of broad structural variability. The wide range of coordination environments (type, number, charge, and architecture of the donor atoms) that the pyrrole-modified frameworks provide to the central metal ions, the frequent presence of donor atoms at their periphery, and their often observed nonplanarity or conformational flexibility distinguish the complexes of the PMPs clearly from those of the traditional square-planar, dianionic, N₄-coordinating (hydro)porphyrins. Their different coordination properties suggest their utilization in areas beyond which regular metalloporphyrins are suitable. Following a general introduction to the synthetic methodologies available to generate pyrrole-modified porphyrins, their general structure, history, coordination chemistry, and optical properties, this Review highlights the chemical, electronic (optical), and structural differences of specific classes of metalloporphyrinoids containing nonpyrrolic heterocycles. The focus is on macrocycles with similar "tetrapyrrolic" architectures as porphyrins, thusly excluding the majority of expanded porphyrins. We highlight the relevance and application of these metal complexes in biological and technical fields as chemosensors, catalysts, photochemotherapeutics, or imaging agents. This Review provides an introduction to the field of metallo-PMPs as well as a comprehensive snapshot of the current state of the art of their synthesis, structures, and properties. It also aims to provide encouragement for the further study of these intriguing and structurally versatile metalloporphyrinoids.

Synthesis of a Series of Tropone-Fused Porphyrinoids

A series of tropone-fused porphyrinoids with unique spectroscopic features has been prepared. A dimethyl tropone-fused pyrrole was reacted with lead tetraacetate to give a bis(acetoxymethyl) derivative that condensed with an α-unsubstituted pyrrole tert-butyl ester to form a tripyrrane intermediate. Cleavage of the tert-butyl ester protective groups, followed by condensation with a series of aromatic dialdehydes and oxidation with DDQ, afforded the tropone-fused porphyrinoid systems. Reactions with pyrrole, furan, thiophene, and selenophene dialdehydes gave tropone-fused porphyrins and related heteroporphyrins. In addition, indene, 4-hydroxybenzene, and 3-hydroxypyridine dicarbaldehydes generated examples of carba-, oxybenzi-, and oxypyriporphyrins. The electronic absorption spectra of the tropone-fused porphyrinoids were greatly altered, showing shifts to longer wavelengths and the appearance of strong Q bands between 600 and 800 nm. The proton nuclear magnetic resonance spectra were also very unusual, as the internal protons were strongly shifted upfield, in some cases giving rise to resonances that approached -10 ppm. However, the external protons showed reduced downfield shifts compared to porphyrinoids that do not have tropone ring fusion. The profound changes observed for these macrocycles demonstrate that the introduction of fused tropone units, together with other structural changes such as core modification, can provide the means by which porphyrinoids with unique spectroscopic properties can be accessed.

Synthesis, Characterization, and Spectroscopic Studies of Bis-(meso-4-methoxyphenyl)-Benziporphyrin and Its Pd-Metal Complex

Benziporphyrin systems are widely explored, yet alternative improved synthetic routes towards these systems are needed. Here, a fairly and efficient synthesis of the free base and its metal complex is well designed. Dimethoxybenzene dicarbinol intermediate was prepared in excellent yields by reacting 4-methoxyphenylmagnesium bromide with isophthaladehyde in diethyl ether. Reaction with equivalent pyrrole and pentafluorobenzaldehyde in the presence of trifluoroacetic acid (TFA), followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), provided good yields of bis-(meso-4-methoxyphenyl)-benziporphyrin. Metalation of the free base was performed using palladium(II) acetate salt in acetonitrile. All intermediates and the final products are fully characterized using NMR, HMRS, and UV-Vis spectroscopies and briefly discussed.

Synthesis, metalation and antiaromatic properties of 22-hydroxybenziporphyrins

Condensation of 5-alkyl-2-hydroxybenzene-1,3-dicarbaldehydes with a tripyrrane in the presence of trifluoroacetic acid (TFA), followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), gave two examples of 22-hydroxybenziporphyrins. Unlike a related meso-tetraphenyl-22-hydroxybenziporphyrin, these products were reasonably stable in solution. Dicationic species were obtained upon addition of TFA to CDCl3 solutions, and these exhibited weakly diatropic properties. However, the free base forms gave broad peaks for the macrocycle’s peripheral protons that fell into the range of 4.5–3.3 ppm, demonstrating that the system has paratropic characteristics that are consistent with the keto-tautomers. The porphyrin analogues gave broad, poorly resolved, UV-vis spectra of the type expected for nonaromatic or antiaromatic structures. Reaction of a hydroxybenziporphyrin with nickel(II), copper(II) or palladium(II) acetate gave the related coordination complexes. The nickel and palladium derivatives were structurally characterized by X-ray crystallography and exhibited buckled geometries with the phenolate units strongly pivoted away from the macrocyclic core.

Reversible Redox System of 2-Oxypyritriphyrin(1.2.1) Accompanying Interconversion between 3-Pyridone and 3-Hydroxypyridine Units

To develop a system in which a π-conjugation circuit switched by a redox reaction between 3-pyridone and 3-hydroxypyridine, a ring-contracted analog of oxypyriporphyrin, 2-oxypyritriphyrin(1.2.1) was synthesized for the first time. 2-Oxypyritriphyrin(1.2.1) contains a 14π aromatic conjugation circuit with the keto-form of the 3-oxypyridine ring. When 2-oxypyritriphyrin was treated with NaBH₄ , not only the 3-oxypyridine group, but also the meso-carbons were also reduced to give a colorless porphyrinogen analog. The reduced compound could be oxidized again to provide 2-oxypyritripyhrin in a reversible manner.

Synthesis and Structure of meso-Substituted Dibenzihomoporphyrins

Bench-stable meso-substituted di(p/m-benzi)homoporphyrins were synthesized through acid-catalyzed condensation of dipyrrole derivatives with aryl aldehydes. The insertion of a 1,1,2,2-tetraphenylethene (TPE) or but-2-ene-2,3-diyldibenzene unit in the porphyrin framework results in the formation of dibenzihomoporphyrins, merging the features of hydrocarbons and porphyrins. Single crystal X-ray analyses established the non-planar structure of these molecules, with the phenylene rings out of the mean plane, as defined by the dipyrromethene moiety and the two meso-carbon atoms. Spectroscopic and structural investigations show that the macrocycles exhibit characteristics of both TPE or but-2-ene-2,3-diyldibenzene and dipyrromethene units indicating the non-aromatic characteristics of the compounds synthesized. Additionally, the dibenzihomoporphyrins were found to generate singlet oxygen, potentially allowing their use as photosensitizers.

Kinetic versus Thermodynamic Control Over Multiple Conformations of Di-2,7-naphthihexaphyrin(1.1.1.1.1.1)

Di-2,7-naphthihexaphyrin(1.1.1.1.1.1), a non-aromatic carba-analogue of the hexaphyrin(1.1.1.1.1.1), incorporating two built-in 2,7-naphthylene moieties was synthesized as two separate, conformationally locked stereoisomers. Both conformers followed complex protonation pathways involving structurally different species, which can be targeted under kinetic and thermodynamic control. The neutralization of the ultimate dicationic product, accessible from both stereoisomers of the free base, allowed to realize the complex conformational switching cycle involving six structurally different species.

Synthesis and Characterization of N-Methylporphyrins, Heteroporphyrins, Carbaporphyrins, and Related Systems

MacDonald-type "3 + 1" condensations of an N-methyltripyrrane with a series of dialdehydes afforded a matched set of N-methylporphyrins, N-methylheteroporphyrins, N-methyloxybenziporphyrin, N-methyloxypyriporphyrin, N-methyltropiporphyrin, and a N-methylcarbaporphyrin aldehyde. meso-Unsubstituted heteroporphyrins have been little explored previously, and this strategy was also used to prepare N-unsubstituted 21-oxa-, 21-thia-, and 21-selenaporphyrins. In every case, the N-methylporphyrinoids exhibited weaker, bathochromically shifted UV-Vis absorptions compared to their core unsubstituted congeners. However, proton NMR spectroscopy demonstrated that these derivatives retained strong diamagnetic ring currents and the presence of the internal alkyl substituents had little effect on the global aromatic characteristics. Nevertheless, the UV-Vis spectra of N-methyl-oxybenzi- and N-methyl-oxypyriporphyrins were dramatically altered and gave greatly weakened absorptions. N-Methyl-oxybenzi- and N-methyltropiporphyrins reacted with palladium(II) acetate to give stable palladium(II) complexes, demonstrating that N-alkylation alters the metalation properties for these carbaporphyrinoids. The organometallic derivatives also retained strongly aromatic properties, and the proton NMR spectra showed the N-methyl resonances near -3 ppm. N-Methylcarbaporphyrin-2-carbaldehyde also gave a palladium(II) complex, but this gradually rearranged at higher temperatures to afford a C-methyl complex. The results demonstrate that core alkylation of porphyrinoids greatly alters the reactivity and spectroscopic properties for these systems.

Deprotection of a benzyl unit induces a 22π aromatic macrocycle of 3-oxypyripentaphyrin(0.1.1.1.0) with strong NIR absorption

We report aromaticity switching from a 6π pyridine ring to a 22π macrocyclic ring of 3-oxypyripentaphyrin(0.1.1.1.0). This system has potential applications in photodynamic therapy owing to macrocyclic aromaticity being selectively induced by protecting group removal and strong absorption bands produced in the NIR region especially in methanol.

Synthesis, Structure, and Optical Properties of Di-m-benzihexaphyrins (1.1.0.0.0.0) and Di-m-benziheptaphyrins (1.0.1.0.0.0.0): Blackening of m-Phenylene-Linked Dicarbaporphyrinoids by Simple π-Expansion

Acid-catalyzed condensation of a newly prepared di-m-benzipentapyrrane with appropriate mono- and diheterocyclic dialcohols selectively produced stable di-m-benzihexaphyrins and di-m-benziheptaphyrins with only two meso-carbon bridges. Single-crystal X-ray diffraction analyses reveal planar conformation with slight distortion of bridged phenylene rings. Despite the presence of m-phenylene units interrupting the global delocalization, the presence of bithiophene units in di-m-benziheptaphyrins 3a-b exhibits altered optical features covering the entire visible region (ca. 250-720 nm), exhibiting a black dye property as a "metal-free" porphyrinoid.

Antiaromatic Carbaporphyrinoids: Fluorene as a Fused Motif toward the Synthesis of meso-Fused Heterobenziporphyrins

meso-Tetraphenyl meta-benziporphyrins are nonaromatic macrocycles which can be changed to antiaromatic by fusing the core benzene ring with one of the adjacent meso-phenyl groups, as demonstrated here by adopting a premodification method. We used a fluorene moiety in place of the m-phenylene ring as a premodified fused aromatic motif to synthesize fused heterobenziporphyrins. Spectral and X-ray data indicated that the macrocycles are antiaromatic, which was supported by DFT, ACID, and NICS calculations. These macrocycles formed organopalladium complexes.

28-Hetero-2,7-Naphthiporphyrins: Horizontal Expansion of the m-Benziporphyrin Macrocycle

Replacement of the m-phenylene moiety of m-benziporphyrins with the 2,7-naphthalenyl subunit yielded 28-hetero-2,7-naphthiporphyrins-macrocycles that can be considered as expanded carbaporphyrinoids. This group retains some features of parent m-benziporphyrins, but due to larger size and different shape of the macrocyclic cavity, their coordination properties are different. Upon reduction and conformational rearrangement the 28-thia- and 28-selena-2,7-naphthiporphyrin form organophosphorus(V) complexes.

Synthesis of Tellurabenziporphyrin and Its Pd(II) Complex

An unprecedented tellurabenziporphyrin containing C, N, and Te donor atoms was synthesized by condensing benzitripyrrane and tellurophene diol under acid catalyzed conditions. The tellurabenziporphyrin readily forms a Pd(II) complex when treated with PdCl₂ in CHCl₃/CH₃CN. The crystal structures of tellurabenziporphyrin and its Pd(II) complex revealed that the benzene ring hinders the π-electron delocalization. An unusual five-membered ring formed inside the macrocycle due to the strong interaction between "Te" and "N" in the Pd(II) complex.

Late transition metal complexes of oxypyriporphyrin and the platinum(II) complex of oxybenziporphyrin

Even though oxypyriporphyrin, a pyridone-containing porphyrinoid system, has the equivalent coordination core to true porphyrins, its coordination chemistry has been little explored. In this study, the first examples of palladium(II), platinum(II) and silver(II) oxypyriporphyrins have been synthesized. Conventional conditions failed to result in the formation of a platinum(II) complex, but moderate yields were obtained when oxypyriporphyrin was reacted with platinum(II) chloride in refluxing mixtures of DMF and acetic acid containing potassium acetate. The palladium(II) and platinum(II) derivatives were characterized by X-ray crystallography. The structurally analogous carbaporphyrinoid system oxybenziporphyrin was also shown to react with platinum(II) chloride under the same conditions to give a platinum(II) hydroxybenziporphyrin complex. Unlike oxybenziporphyrin, this complex is only weakly diatropic. In the presence of base, the diatropic character was reasserted to afford an aromatic anionic species.

An alternative synthesis of benziporphyrins starting from isophthaloyl chloride

An alternative route to benziporphyrins has been developed. Reaction of an [Formula: see text]-unsubstituted pyrrole ethyl ester with isophthaloyl chloride in the presence of aluminum chloride afforded a diketone that underwent selective reduction with diborane to give a benzitripyrrane. Cleavage of the ethyl esters with sodium hydroxide in refluxing ethylene glycol, followed by acid catalyzed condensation with a pyrrole dialdehyde and oxidation with DDQ, generated the targeted benziporphyrin product. Spectrophotometric titration of the benziporphyrin with trifluoroacetic acid (TFA) demonstrated the sequential formation of mono- and dicationic species. At lower dilutions, the free base benziporphyrin showed unusually strong [Formula: see text] coupling between two adjacent methyl substituents, indicating that the connecting unit has substantial olefinic characteristics.

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.

[18]Annulene put into a new perspective

New insights into [18]annulene were gained by looking more closely at its X-ray structure, revealing a close face-to-face stacking of 3.16 Å in a herringbone-like crystal packing. Hexadehydro[18]annulene was co-crystalized in a benzene matrix, demonstrating the stabilizing role of intercalated solvent molecules in solid annulenes.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

Synthesis and spectroscopic behaviour of metal complexes of meso-alkylidenyl carbaporphyrinoids and their expanded analogues

Metal complexes of meso-diethyl malonylidene carbaporphyrinoids have been synthesized and fully characterized.

New insights into aromatic pathways of carbachlorins and carbaporphyrins based on calculations of magnetically induced current densities

The aromatic pathways of carbaporphyrins and carbachlorins that are based on magnetically induced current density DFT-GIMIC calculations are presented and discussed.

Core chemistry and skeletal rearrangements of porphyrinoids and metalloporphyrinoids

Porphyrin core alteration allows for the exploration of porphyrin-like or porphyrin-unlike coordination chemistry and provides an insight into reactions inside particularly shaped macrocyclic architecture including metal-mediated structural transformations.

Unique prototropy of meso-alkylidenyl carbaporphyrinoid possessing one meso-exocyclic double bond

The unique prototropy ofmeso-alkylidenyl-thia(m-benzi)porphyrinoid containing one exocyclic double bond at themeso-position is presented.

Benziporphyrins, a unique platform for exploring the aromatic characteristics of porphyrinoid systems

Benziporphyrins and related systems exhibit a wide range of properties and may possess nonaromatic, strongly aromatic or even antiaromatic characteristics.