Porphyrinoids

Corrin is the porphyrinoid chromophore of the vitamin parent compound cobyrinic acid. Corrin itself has not yet been synthesized, but routes to cobyrinic acid and several other synthetic corrins have been described by A. Eschenmoser (1970, 1974) and R.B. Woodward (1967). 

Given stringent requirements for effective sensitizers and the desire to use wavelengths further to the red for therapeutic appHcations, definition of newer sensitizers has been a principal area of research since about 1987. Expanded theoretical and experimental understanding of photophysics has been a key element in identifying new classes of potential sensitizers (93—98). Research has focused on cationic derivatives of Nile Blue (93), metaHo-phthalocyanines (94), naphthalocyanines (95), chlorin-type compounds (96), expanded ring porphyrinoids (97), as well as porphyrins other than hematoporphyrin and its derivatives (98). This work has also been reviewed (10,91). Instmmentation for photodynamic therapy has been reviewed (99). 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

The seiectivities of metal complexes of cofacial porphyrinoids (porphyrins, corroles, and phthalocyanines) reported in the literature by mid-2007 are summarized in Fig. 18.15. The data are organized by the type of catalyst as well as in order of decreasing Mav Whereas ORR catalysis by certain cofacial porphyrins, such as (FTF4)Co2 and (DPY)Co2 (Y = a, B) has been smdied extensively by a number of groups, and the values of av are known with high degree of confidence, those for most other catalysts... 

Borovkov VV, Inoue Y (2006) Supramolecular Chirogenesis in Host-Guest Systems Containing Porphyrinoids. 265 89-146... 

Nitrite reductase and sulfite reductase are enzymes found in choroplasts and in prokaryotes that reduce nitrite to ammonia and sulfite to sulfide (Scott et al., 1978). Sulfite reductase also catalyzes reduction of nitrite at a lower rate. Both enzymes contain a siroheme prosthetic group linked to an iron-sulfur cluster. In siroheme, the porphyrinoid moiety is present in the more reduced chlorin form. Because NO lies between nitrite and ammonia in oxidation state, it is a potential intermediate. 

Porphyrin analogs, homologs, and porphyrinoids - The class of tetrapyrrole chromophores can be enlargened by alteration of the numbers of carbon atoms linking the pyrrole rings [101]. A beautiful porphyrin isomer, porphycene, has been prepared by Vogel and coworkers [102], This porphyrin isomer, H2(Pyc),... 

This section will not be concerned with the detailed description of the synthetic methods leading to the appropriate precursors we will limit our attention to the crucial step of the synthesis of corrinoids, i.e. the formation of the tetrapyrrolic ring. The corrinoid macrocycle has been synthesized following two different procedures the first one involves the cyclization of a proper linear precursor, while the second involves ring contraction of a porphyrinoid structure. 

Heteroporphyrins Core modification Tetrathiaannulenes Porphycenes Porphyrinoids Expanded porphyrinoids. 

Following the standard system of nomenclature, the names of porphyrinoids consist of three parts (i) a number in the square bracket corresponds to the number of jt-electrons in the shortest conjugation pathway (ii) a core name representing the number of pyrroles or other heterocycles in the overall system and (iii) numbers in round brackets specify the number of bridging C-atoms between each pyrrole subunit, starting with the largest. 

We have compiled synthetic chemistry and properties of porphyrin analogues containing only furan, imidazole, or thiophene subunits in place of pyrrole subunit(s). Expanded porphyrinoids have been excluded. This write-up is not intended to give a comprehensive treatment, but is aimed to complement earlier reviews (06CCR468, 08CSR215). In the following two sections, the chemistry of core-modified porphycenes and porphyrins is presented. 

Bis(heterocyclyl)methanes constitute highly valuable building blocks of natural and unnatural porphyrinoids (00MI1), which are of immense importance in biological, industrial, and material science applications... 

Leunig, M. et al. (1993) Tumour localisation kinetics of photofrin and three synthetic porphyrinoids in an amelanotic melanoma of the hamster, Br. J. Cancer 68, 225-234. 

Abstract Porphyrins and their analogues constitute one of the most important families of aromatic macrocycles. The present review discusses aromaticity of porphyrinoids, focusing mainly on non-expanded systems. The effect of structural modifications on the aromaticity-dependent properties of porphyrin-like macrocycles is described. It is shown that delocalization modes observed in porphyrinoids can be classified using a simple valence-bond approach. Aromaticity of porphyrinoids is further discussed as a function of tautomerism, coordination chemistry, and the oxidation state of the macrocycle. 

Keywords Aromaticity, Electronic structure, NMR spectroscopy, Porphyrinoids,... 

Variations on the basic structural theme of 1 have led to a plethora of unusual macrocyclic systems, collectively known as porphyrin analogs or porphyrinoids. These molecules, often nontrivial to synthesize, exhibit remarkable physical and chemical properties and their chemistry has been extensively reviewed [2-18], With the impressive range of structural modifications introduced so far, the term porphyrinoid has ultimately expanded to encompass a wide range of often exotic macrocycles, some of which contain no pyrrole rings at all, or have a structural outline barely resembling that of porphyrin. Some of the generic modification types are shown in Fig. 1. Combination of these design concepts provides a virtually inexhaustible source of structural diversity. 

A great number of porphyrin analogs possess circular conjugation pathways, and often exhibit aromaticity comparable with that of the parent system 1. Moreover, the JT-electron delocalization of many porphyrin-like molecules is strongly affected by structural detail, redox chemistry, and prototropic tautomerism. The aim of the present review is to provide a description of porphyrinoid aromaticity and its connection with tautomeric equilibria. Our main focus will be on the physical manifestations of aromaticity, with a special emphasis on NMR spectroscopy. The reactivity of porphyrin analogs, including their coordination chemistry, will be discussed only to the extent it has a bearing on their aromaticity. 

Most of the porphyrinoid molecules synthesized so far possess a number of peripheral substituents. In fact, many of the important macrocyclic systems were obtained with several distinct substitution patterns. Usually they correspond to one of the generic substitution types of regular porphyrins meso-aryl or p-alkyl, as exemplified by 5,10,15,20-tetraphenylporphyrin and 2,3,7,8,12,13,17,18-octaethylporphy-rin, respectively (Fig. 2). For the sake of brevity we will assume the following conventions in the entire manuscript ... 

Many of the above deficiencies were removed in further refinements of the Hiickel method, which was anyway made obsolete by the development of ab initio and DFT techniques. Still, organic chemists adhere to the original Hiickel description, which is often sufficient to make qualitative predictions about the nature of n-conjugated systems. In particular, the Hiickel model finds widespread use in porphyrinoid chemistry. The so-called annulene model, which will be used throughout this review, is outlined below for the parent porphyrin macrocycle. 

The porphyrin ring contains 22 electrons in its n orbitals. As explained above, the Hiickel rule cannot be applied to this electron count, because the molecule is not monocyclic. However, the porphyrin ring can be formally derived from neutral [18]annulene, by introduction of appropriate heteroatoms and bridges (Fig. 3). The macrocycle is thus shown to be aromatic in the Hiickel sense, and is denoted [18]porphyrin. As we will show in subsequent sections, this approach is readily generalized to other porphyrinoids, whose aromatic character can be predicted by defining a neutral annulenoid pathway in the macrocycle. These pathways will be... 

Alternatively, the porphyrin ring can be constructed starting from [16]annulene. In the first step, two electrons are added to form the corresponding [16]annulene dianion, which is transformed into porphyrin Cl") by adding bridges and heteroatoms. Unlike the structure derived from [18]annulene, the dianion-based model has a fourfold symmetry, and was considered suitable for the description of metal complexes [23] (see Sect. 2.3.2). In yet another approach [24], based on the so-called perimeter model, the porphyrin macrocycle is derived from the [20]annulene dication (I "). Both the [16]- and [20]annulene models were employed to describe electronic absorption spectra and magnetic circular dichroism of porphyrinoids [24, 25],... 

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