Macrocycles porphyrinoid

According to a recently proposed definition, expanded porphyrins possess more than 16 atoms in the inner or, more precisely, smallest circuit of the macrocycle (porphyrin 1 contains exactly 16 atoms in the smallest macrocyclic circuit) [11], In addition to porphyrinoid macrocycles traditionally recognized as expanded, i.e., possessing more than four cyclic subunits or more than four meso bridges, this new definition encompasses some other systems, such as p-benziporphyrin 150. Regardless of the exact definition, expanded porphyrins constitute an unusually rich and diverse family of structures, and have been the subject of several excellent reviews [2, 5, 11, 146], The relationship between aromaticity and tautomerism in... 

In other physical chemical analyses, the perimeter model [73] has been employed to analyze the spectral intensities and MCD signals for a series of porphyrinoid macrocycles derived from the C20H20 perimeter, including the parent, benz-free analogue of texaphyrin 158 [74], These calculations were then compared with the MCD spectra of a number of substituted cadmium texaphyrins (e.g. 116,143, and 148-152, c.f. Scheme 16) [75]. The results confirmed that the perimeter model accounts in a simple way for the signs of the MCD B terms associated with the low-lying electronic transitions of these metallotexaphyrins. 

Novel porphyrinoid macrocycles and their metal complexes , Vogel, E., J. Heterocycl Chem., 1996, 33, 1461. 

The early collaboration between Gray and Bendix, like that between Ballhausen and Gray, involved the development of electronic and molecular descriptions of novel transition metal complexes. During his fellowship at Caltech, Bendix examined the manganese [22, 23] and gallium [24] complexes of a class of porphyrinoid macrocycles known as corroles [25-28]. Corroles were once difficult to prepare in the laboratory, but they have become more accessible due to the development of new syntheses [29-37]. According to a Scifinder [38] search performed on June 5, 2011,... 

The presence of a carbon in the core of the ring indicates that these macrocycles will form organometallic interactions upon metal binding. In biological systems, naturally occurring organometallic complexes are rare. The cobalamin cofactor is one of the more important examples, which possesses an axial metal-carbon bond in a cobalt porphyrinoid macrocycle (77). Another common example is seen in carbon-monoxide deactivated ferrous heme (22) ... 

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. 

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. 

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. 

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. 

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],... 

The X-confused systems 100 and 101 exhibit borderline macrocyclic aromaticity, which results from the onium-type contributions 100 and 101, characterized by the presence of 18-electron aromatic circuits. For instance, the NH protons in 100 resonate at ca. 5.8 ppm, while those in the corresponding dication are observed at 5.3—4.4 ppm. These chemical shifts are intermediate between values expected of a nonaromatic porphyrinoid (S > 10 ppm) and those typical of aromatic systems (S < 0 ppm). The macrocyclic aromaticity is fully restored in the 3-substituted derivatives containing an sp3 carbon (104-105) or a carbonyl group (102-103), which is an additional reason for the enhanced reactivity of 100 and 101. In comparison, the pyrrolyl-substituted derivative 106 shows only moderate diatropicity, which is noticeably enhanced upon protonation [247], This effect is explained by cross-conjugation of the pyrrolyl substituent in the dication 107, which enables an 18-electron aromatic circuit in the macrocycle. 

Chapter 3 by Marcin Slcpien and Lechoslaw Latos-Grazynski deals with jt-elec-tron delocalization in relation to tautomerism and chemical properties in porphy-rines and porphyrinoids. An important and very interesting problem discussed here is that for the title systems aromaticity may be concerned either locally (for one or a few rings) or for a whole macrocycle. The body of the chapter is based on analyses of II NMR and I IWVis spectroscopies and takes into account the relations between n-clectron delocalization and tautomeric equilibria. 

The X-ray crystal structures of pyrazine V.JV -dioxide (134) <02AX(E)1253>, the P-polymorph of phenazine (135) <02AX(C)181>, cobalt(III) complexes of pyrazine-2,6- and pyridine-2,6-dicarboxylic acids <02JIC458>, and bis-urea-substituted phenazines <02ZN(B)937> were reported. Fluorescent pyrido[l,2-a]quinoxalines 136 prepared as pH indicators were examined by X-ray crystallography <02JCS(P2)181>, as were macrocyclic quinoxaline-bridged porphyrinoids obtained from the condensation of dipyrrolylquinoxalines 137 and 1,8-diaminoanthracene... 

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