Simple Methodology for Syntheses of Porphyrins Possessing Multiple Peripheral Substituents with an Element of Symmetry

Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions─Synthesis and Transport

Molecular junctions offer significant potential for enhancing thermoelectric power generation. Quantum interference effects and associated sharp features in electron transmission are expected to enable the tuning and enhancement of thermoelectric properties in molecular junctions. To systematically explore the effect of quantum interferences, we designed and synthesized two new classes of porphyrins, P1 and P2, with two methylthio anchoring groups in the 2,13- and 2,12-positions, respectively, and their Zn complexes, Zn-P1 and Zn-P2. Past theory suggests that P1 and Zn-P1 feature destructive quantum interference in single-molecule junctions with gold electrodes and may thus show high thermopower, while P2 and Zn-P2 do not. Our detailed experimental single-molecule break-junction studies of conductance and thermopower, the latter being the first ever performed on porphyrin molecular junctions, revealed that the electrical conductance of the P1 and Zn-P1 junctions is relatively close, and the same holds for P2 and Zn-P2, while there is a 6 times reduction in the electrical conductance between P1 and P2 type junctions. Further, we observed that the thermopower of P1 junctions is slightly larger than for P2 junctions, while Zn-P1 junctions show the largest thermopower and Zn-P2 junctions show the lowest. We relate the experimental results to quantum transport theory using first-principles approaches. While the conductance of P1 and Zn-P1 junctions is robustly predicted to be larger than those of P2 and Zn-P2, computed thermopowers depend sensitively on the level of theory and the single-molecule junction geometry. However, the predicted large difference in conductance and thermopower values between Zn-P1 and Zn-P2 derivatives, suggested in previous model calculations, is not supported by our experimental and theoretical findings.

Classic highlights in porphyrin and porphyrinoid total synthesis and biosynthesis

Porphyrins feature prominently in nature, be it as enzymatic cofactors, electron and exciton shuffles, as photoactive dyes, or as signaling substances. Their involvement in the generation, storage and use of oxygen is pivotal to life, while their photochemical properties are central to the biochemical functioning of plants. When complexed to metals, porphyrins can engage in a multitude of contemporary applications ranging from solar energy generation to serving as catalysts for important chemical reactions. They are also able to function as useful theranostic agents, and as novel materials for a wide range of applications. As such, they are widely considered to be highly valuable molecules, and it almost goes without saying that synthetic organic chemistry has dramatically underpinned all the key advances made, by providing reliable access to them. In fact, strategies for the synthesis of functionalized porphyrins have now reached a state of refinement where pretty well any desired porphyrin can successfully be synthesized with the approaches that are available, including a cornucopia of related macrocycle-modified porphyrinoids. In this review, we are going to illustrate the development of this exciting field by discussing a number of classic syntheses of porphyrins. Our coverage will encompass the natural protoporphyrins and chlorophylls, while also covering general strategies for the synthesis of unsymmetrical porphyrins and chlorins. Various industrial syntheses of porphyrins will also be discussed, as will other routes of great practical importance, and avenues to key porphyrinoids with modified macrocycles. A range of selected examples of contemporary functionalization reactions will be highlighted. The various key syntheses will be described and analyzed from a traditional mechanistic organic chemistry perspective to help student readers, and those who are new to this area. The aim will be to allow readers to mechanistically appreciate and understand how many of these fascinating ring-systems are built and further functionalized.

Synthesis and reactivity of iron-dinitrogen complexes bearing anionic methyl- and phenyl-substituted pyrrole-based PNP-type pincer ligands toward catalytic nitrogen fixation

Iron-dinitrogen complexes bearing methyl- and phenyl-substituted pyrrole-based anionic PNP-type pincer ligands are prepared and characterized by X-ray analysis. The former complex is found to work as a more effective catalyst than that bearing a non-substituted PNP-type pincer ligand toward the transformation of nitrogen gas into ammonia and hydrazine under mild reaction conditions.

A Cost-Efficient Method for Unsymmetrical Meso-Aryl Porphyrin Synthesis Using NaY Zeolite as an Inorganic Acid Catalyst

Herein we report the synthesis of unsymmetrical meso-aryl substituted porphyrins, using NaY zeolite as an inorganic acid catalyst. A comparative study between this method and the several synthetic strategies available in the literature was carried out. Our method presented a better, more cost-efficient rationale and displayed a significantly lower environmental impact. Furthermore, it was possible to verify the scalability of the process as well as the reutilization of the inorganic catalyst NaY (up to 6 times) without significant yield decrease. In addition, this method was applied to the synthesis of several other unsymmetrical porphyrins, from a low melting point porphyrin to mono-carboxylated halogenated unsymmetrical porphyrins, in yields higher than those found in the literature. Additionally, for the first time, two acetamide functionalized halogenated porphyrins were prepared in high yields. This methodology opens the way to the preparation of high yielding functionalized porphyrins, which can be easily immobilized for a variety of applications, either in catalysis or in biomedicine.

Myelin-targeted, texaphyrin-based multimodal imaging agent for magnetic resonance and optical imaging

Reliable methods of imaging myelin are essential to investigate the causes of demyelination and to study drugs that promote remyelination. Myelin-specific compounds can be developed into imaging probes to detect myelin with various imaging techniques. The development of multimodal myelin-specific imaging probes enables the use of orthogonal imaging techniques to accurately visualize myelin content and validate experimental results. Here, we describe the synthesis and application of multimodal myelin-specific imaging agents for light microscopy and magnetic resonance imaging. The imaging agents were synthesized by incorporating the structural features of luxol fast blue MBS, a myelin-specific histological stain, into texaphyrins coordinated to Gd^III . These new complexes demonstrated absorption of visible light, emission of near-IR light, and relaxivity values greater than clinically approved contrast agents for magnetic resonance imaging. These properties enable the use of optical imaging and magnetic resonance imaging for visualization of myelin. We performed section- and en block-staining of ex vivo mouse brains to investigate the specificity for myelin of the new compounds. Images obtained from light microscopy and magnetic resonance imaging demonstrate that our complexes are retained in white matter structures and enable detection of myelin. Copyright © 2016 John Wiley & Sons, Ltd.

Cost-efficient method for unsymmetrical meso-aryl porphyrins and iron oxide-porphyrin hybrids prepared thereof

Synthesis, characterisation and photophysical properties of magnetic iron oxide-porphyrin hybrids starting from unsymmetrical porphyrins, which are prepared by a cost-efficient methodology.

Access to pyrrole-based heterocyclic compounds via addition of pyrrole to C=C and C=N bonds

A series of metal triflate-catalyzed addition reactions of pyrrole to C=C and C=N bonds have been investigated to access pyrrole-based heterocyclic compounds. The addition of pyrrole to different α,β-unsaturated compounds or N-tosyl imines afforded suitable structures for the construction of [5-5] bicyclic systems or porphyrins, respectively. Intramolecular cyclization reactions were applied for the synthesis of pyrrolizine derivatives. In the other reaction mode, intermolecular cyclization reactions gave A4- and trans-A2B2-meso-substituted porphyrins under mild reaction conditions with low scrambling.

Syntheses and chemistry of porphyrins

Common methods available for the preparation of useful porphyrins from naturally derived heme and by total synthesis from monopyrroles are presented. Such porphyrins can be used for numerous reactivity and structural modification studies of the type which will be presented in minisymposia (including that entitled ‘Syntheses and Chemistry of Porphyrins’) at the First International Conference on Porphyrins and Phthalocyanines (ICPP-1).

Modification of the porphyrin chromophore by ring fusion: identifying trends due to annelation of the porphyrin nucleus

The effects exerted by fused aromatic rings on the UV-vis spectra of porphyrins are surveyed. Modified porphyrin chromophores with fused benzene, 1,2-naphthalene, 9,10-phenanthrene or phenanthroline rings are surprisingly little affected even when a maximum number of ring fusions are incorporated. Linearly annealed naphtho- or anthraporphyrins show large red shifts to the Q bands but the Soret absorptions are weakened and undergo only minor bathochromic shifts. Fluoranthoporphyrins give multiple bands in the Soret region, but the Q band region is virtually unaffected by this tetracyclic ring system. On the other hand, metal chelates of fluoranthoporphyrins show surprisingly strong bands near 600 nm. Benzothiadiazole rings split and weaken the Soret band, but the Q bands region is unexceptional. However, metal coordination again produces relatively intense bands near 600 nm. The most significant results were obtained for porphyrins with fused acenaphthylene rings. Monoacenaphthoporphyrins (41) have three Soret bands at 387, 431 and 454 nm, and the longest wavelength Q band is shifted to 658 nm. opp-Diacenaphthoporphyrin (43) further shifts these bands with two Soret absorbances at 443 and 470 nm, and an additional strong peak is observed at 692 nm. The metal complexes of these systems also show strong bands between 602 and 656 nm. Still larger effects are produced by tetraacenaphthoporphyrin (47), the dication for which in trifluoroacetic acid (TFA)–chloroform has a Soret absorption at 528 nm. Tetraaryltetraacenaphthoporphyrins (48) are even more red shifted, showing Soret bands between 556 and 570 nm for the free bases and 565 to 588 nm for the related dications. The lead(II) chelate for tetraphenylporphyrin (48a) shows an additional 'hyper' spectral shift that brings the Soret band to 604 nm, and this effect can also be achieved by introducing four meso-phenylethynyl substituents onto the tetraacenaphthoporphyrin nucleus (49). In addition, by combining these two factors for the lead(II) chelate of 49, a record-breaking value for the Soret band of 642 nm can be achieved. Spectral shifts due to ring annelation in porphyrin analogues are also discussed, including those for oxybenziporphyrins, oxypyriporphyrins, carbaporphyrins and sapphyrins.

Porphyrins with Exocyclic Rings: Part 9 [1] Synthesis of Porphyrins by the ‘3 + 1’ approach

Tripyrrane dibenzyl esters are readily available from the reaction of two equivalents of 5-acetoxymethylpyrrole-2-carboxylates with 2,5-diunsubstituted pyrroles in refluxing acetic acid-ethanol. Hydrogenolysis of the benzyl ester moieties affords the corresponding dicarboxylic acids, and subsequent condensation with 2,5-diformylpyrroles in 5% trifluoroacetic acid-dichloromethane, followed by neutralization with triethylamine and oxidation with DDQ, gives porphyrin products in good to excellent yields. The isomeric purity of these products has been rigorously demonstrated by proton and carbon-13 NMR spectroscopy. This chemistry allows the direct synthesis of porphyrin structures that would otherwise only be accessible via multistep procedures. In addition, this approach provides a convenient synthetic entry to the geochemically important tetrahydrobenzoporphyrins and cycloalkanoporphyrins. The first examples of tripyrranes with tetrahydroindole subunits are reported and their utility in the synthesis of novel cycloalkanoporphyrins is disclosed.

Porphyrins with β,β'-fused five-membered rings

Recent studies involving porphyrin systems bearing fused five-membered rings are reviewed, these including pyrrolo[3,4-b]porphyrins and fused metallocenoporphyrins. Diels-Alder cycloadditions between pyrroloporphyrins, to give benzoporphyrins, are discussed. The syntheses of mono-, and the two regioisomers (adj- and opp-) of bis-sulfolenoporphyrins are then discussed and their reactions with dienophiles such as dimethyl acetylenedicarboxylate are outlined.

Establishing a library of porphyrin building blocks for superstructured assemblies: porphyrin dienes and dienophiles for cycloaddition reactions

The synthesis and utility of a series of porphyrins with (masked) diene and dienophile functionality are described. The key porphyrin diene is synthesised from a sulfolenopyrrole by a 3+1 strategy. A range of Diels-Alder cycloadducts is readily accessed from the diene by mild thermal extrusion of sulfur dioxide from the sulfolenoporphyrin, which produces the reactive porphodimethylidene. Each of these cycloadducts is fused to the porphyrin nucleus through a cyclohexene ring thus retaining some conformational flexibility in the resultant structures. The structures can be rigidified by mild oxidation to the corresponding benzo-derivatives. Diels-Alder reaction of the porphyrin 1,3-diene resulting from the sulfolenoporphyrin with norbornadiene produces the norbornene derivative, which can serve as a dienophile or dipolarophile in subsequent cycloaddition reactions. Nevertheless, a preferred route to this structure is through a corresponding 1+3 route, where the norbornene component is part of the tripyrrane. Extension of the synthetic protocols allows ready access to a “mixed function” porphyrin, containing both diene and dienophile components. Likewise, the synthesis of a bis-norbornene porphyrin is described. A collection of each of these reactive components is the basis for a library of building blocks which allows easy and simple entry to a wide variety of complex porphyrin-containing superstructures.

Synthesis, characterization and spectral properties of substituted tetraphenylporphyrin iron chloride complexes

A series of substituted tetraphenylporphyrin iron chloride complexes [RTPPFe(III)Cl, R=o/p-NO₂, o/p-Cl, H, o/p-CH₃, o/p-OCH₃] were synthesized by a novel universal mixed-solvent method and the spectral properties of free base porphyrins and iron porphyrin compounds were compared with each other. The experimental results showed that the one-pot mixed solvent method was superior to the two-step method in the yields, reaction time and workup of reaction mixtures for the synthesis of iron porphyrin compounds. The highest yields (28.7%-40.4%) of RTPPFe(III)Cl were obtained in the mixed solvents propionic acid, glacial acetic acid and m-nitrotoluene under reflux for 2 h. A detailed analysis of ultraviolet-visible (UV-vis), infrared (IR) and far-infrared (FIR) spectra suggested the transformation from free base porphyrins to iron porphyrins. The red shift of the Soret band in ultraviolet-visible spectra due to the presence of p-nitrophenyl substituents and the blue shift of Fe-Cl bond of TPPFeCl in far-infrared spectra were further explained by the electron transfer and molecular planarity in the porphyrin ring.

Rational synthesis of tripyrranes

The rational, chromatography-free synthesis of two regioisomeric 5,10-diaryltripyrranes from pyrrole and aromatic acids has been developed. The strategy is general and should be applicable to a broad range of acids. This methodology was successfully applied to the synthesis of monoprotected dipyrrane. The oxidation of N,N'-bis-mesyltripyrrane under basic conditions led to the formation of both known tripyrrin-1-one and unknown 1-methoxytripyrrin-a fluorescent dye strongly absorbing green light.

Synthesis, cellular uptake and animal toxicity of a tetra(carboranylphenyl)-tetrabenzoporphyrin

A water-soluble nido-carboranyl-tetrabenzoporphyrin has been synthesized in 43% overall yield, by condensation of butanopyrrole with a carboranylbenzaldehyde, followed by metal insertion, oxidation, demetallation and deboronation reactions. This compound accumulated within human glioblastoma T98G cells to a significant higher extent than a structurally related nido-carboranylporphyrin, and localized preferentially in the cell lysosomes. Animal toxicity studies using male and female BALB/c mice revealed that both compounds are non-toxic even at a dose of 160 mg/kg, administered intraperitoneally as a single injection at a concentration of 4 mg/mL. It is concluded that the tetra(carboranylphenyl)-tetrabenzoporphyrin is a promising new sensitizer for the treatment of malignant tumors.

Benzoporphyrins via an olefin ring-closure metathesis methodology

A new route to benzoporphyrins is reported in which readily available vicinal dibromoporphyrins are bis-allylated using the Suzuki reaction, cyclized by way of olefin metathesis and finally oxidized to give mono-, di-, or tri-benzoporphyrins.

Synthesis and characterization of a carboranyl-tetrabenzoporphyrin

An expeditious synthetic route to a carboranyl-substituted tetrabenzoporphyrin is reported. The absorption and emission spectra of water-soluble tetrabenzoporphyrin 4 are distinct from those of a known carboranylporphyrin (5). Both tetrabenzoporphyrin 4 and porphyrin 5 were found to be non-toxic toward V79 hamster lung fibroblast cells at 300 microM, using an MTT assay. The X-ray structure of a Cu(II)-carboranyl-tetrabenzoporphyrin is presented and discussed.

Novel Versatile Synthesis of Substituted Tetrabenzoporphyrins

A novel general synthetic route to tetraaryltetrabenzoporphyrins (Ar(4)TBP) with various peripheral functional groups is developed. The procedure includes (i) Barton-Zard condensation of 1-nitro- or 1-phenylsulfonylcyclohexenes with isocyanoacetic acid esters, (ii) condensation of the resulting 4,5,6,7-tetrahydroisoindoles with aromatic aldehydes to give fused tetraaryltetracylohexenoporphyrins (Ar(4)TCHP), and (iii) aromatization of the metal complexes of Ar(4)TCHP's into the corresponding Ar(4)TBP's. Cu and Zn complexes of Ar(4)TBP's are further demetalated to give the corresponding Ar(4)TBP free bases. The overall yields for the sequence range from 15% to 40%, making the method suitable for the preparation of gram quantities of Ar(4)TBP's in a single run. The scope of the method, the selection of the peripheral substituents, the choice of the metal ions, and their influence on the yields of aromatization are discussed. The basic spectroscopic properties of newly synthesized Ar(4)TBP's and Ar(4)TCHP's are reported together with the first X-ray crystallographic structure of the NiAr(4)TBP complex.

Conjugated macrocycles related to the porphyrins. 21. Synthesis, spectroscopy, electrochemistry, and structural characterization of carbaporphyrins

The "3 + 1" variant of the MacDonald condensation has been shown to be an excellent methodology for synthesizing carbaporphyrins. In particular, 1,3-indenedicarbaldehyde condenses with tripyrranes in the presence of TFA to give, following oxidation with DDQ, a series of benzocarbaporphyrins in excellent yields. Triformylcyclopentadienes also afford carbaporphyrin products, albeit in low yields ranging from 5 to 8%. These hybrid bridged annulene structures have porphyrin-like electronic absorption spectra with strong Soret bands near 420 nm and a series of Q-bands through the visible region. The proton NMR spectrum confirms the presence of a strong diamagnetic ring current, and the meso-protons show up at 10 ppm, while the internal CH is shielded to approximately -7 ppm. Carbaporphyrins undergo reversible protonation with TFA. Initial addition of acid affords a monocation, although mixtures of protonated species are observed in the presence of moderate concentrations of TFA. However, in the presence of 50% TFA a C-protonated dication is generated. The dications relocate the pi-delocalization pathway through the benzo moiety of benzocarbaporphyrins, and these therefore represent bridged benzo[18]annulenes, although they nevertheless retain powerful macrocyclic ring currents. Carbaporphyrins with fused acenaphthylene and phenanthrene rings have been prepared, and the former demonstrated significantly larger bathochromic shifts in UV-vis spectroscopy that parallel previous observations for acenaphthoporphyrins. A diphenyl-substituted benzocarbaporphyrin 19b was also characterized by X-ray crystallography, and these data show that the macrocycle is reasonably planar although the indene subunit is tilted out of the mean macrocyclic plane by 15.5 degrees. The structural data indicates that the preferred tautomer in the solid state has the two NH's flanking the pyrrolene unit in agreement with previous spectroscopic and theoretical studies. Cyclic voltammetry for carbaporphyrin 19a was more complex than for true porphyrins, showing five anodic waves and two quasi-reversible reductive couples.

Dihydroporphyrin Synthesis: New Methodology

Selective formation of trans-nitrochlorins 16-19, cyclopropylchlorins 14, 15, and 20-23, or functionalized trans-chlorins 5-13 by reaction of 2-nitro-5,10,15,20-tetraphenylporphyrin 1 with "active" methylene compounds such as malonates or malononitrile in the presence of base has been achieved. Reaction control is accomplished via sequential Michael additions, followed by intramolecular nucleophilic displacement of a secondary nitro group. Steric as well as thermodynamic effects have been found to govern the selectivity of product formation. Ambient temperature or bulky carbanion substituents lead to nitrochlorins and/or cyclopropylchlorins. Increased reaction temperatures, combined with sterically less encumbered carbanion substituents, favor the formation of disubstituted trans-chlorins. Nucleophilic ring-opening reactions of cyclopropyl-derivative 14afford disubstituted trans-chlorin products 5 and 25 and provide additional mechanistic evidence for the intermediacy of the cyclopropylchlorin. Use of porphyrins with modified meso-phenyl positions illustrates the generality of this methodology and allows a novel method for the preparation of a wide range of reduced porphyrins, which may find application in fields such as the photodynamic therapy (PDT) of cancer.