Aromatic tetrapyrrole macrocycles

Corroles 8 (Figure 2) are aromatic tetrapyrrole macrocycles bearing a direct... 

Corrole is a tetrapyrrolic macrocycle with a direct link between two pyrrole rings. Lacking a C-20 meso carbon bridge it has a corrin like skeleton with double bonds involving porphyrin-like conjugation. It has an 18 electron rt system and hence aromatic character. The direct link between the A and D... 

Two new expanded porphyrins, that would perhaps best be classified as vinylogous porphyrins, have also been reported in the last several months. Corriu et al. have developed a facile synthesis of new tetrapyrrolic macrocyclic derivatives [241]. A notable difference between these new vinylogous porphyrins and those described above (Section 11) is the incorporation of pyridine, in one case, as part of the macrocycle. While these new expanded porphyrins are not aromatic, they may have great potential as ligands. Indeed, the formation of a bimetallic palladium complex was described. 

Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the onter periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B. Two potential application of corroles are in tumor detection and their use in photovoltaic devices. Selective snbstitntion of corroles via nitration, hydroformylation, and chlorosulfonation for the gallinm were studied in detail and the respective mechanistic pathways and spectroscopic data were reported, (an example is shown in Fignre 27). Overall, over 139 varions corroles were synthesized and the effect of various metal complexation pertaining to their selective reactivity examined. ... 

As implied above, the first synthesis of a sapphyrin was purely accidental and came about as the result of the reaction of the linear tetrapyrrolic precursor 5.1 with formic acid and HBr. Following oxidation with iodine, instead of obtaining the desired tetrapyrrolic corrole macrocycle, a different macrocycle was obtained (Scheme 5.1.1). This unexpected by-product was best formulated as being the deca-alkyl-substituted aromatic pentapyrrolic macrocycle 5.2. The unexpected formation... 

Linear dimers of perpendicularly arranged porphyrins can also be assembled at a tetrahedral metal center such as Zn(II) or Cu(I), using ditopic, phenanthroline chelates, as shown by Crossley and coworkers. The case of Cu(l) (92), is shown in Figure 51. The phenanthroline ligands are connected to the tetrapyrrolic macrocycles via fused aromatic rings, producing ribbon-like molecules. 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

From the synthetic standpoint, porphyrins are merely cyclic tetrapyrroles in which the oxidation level has been adjusted to give a highly conjugated aromatic macrocycle. On account of the aromatic stability present in the porphyrin macro-ring, one can expect that... 

As would be expected for an aromatic macrocycle, pentaphyrins display exceptionally strong absorbance bands in the visible portion of the electronic spectrum. For instance, pentaphyrin 6.35 displays a sharp Soret-like absorbance band at 458 nm (CH2CI2, = 238 000 cm ), and two Q-like absorbances at 642 and 695 nm (e = 5990 and 3540M cm , respectively). They thus resemble spectroscopically both the sapphyrins and smaragdyrins, as well as the tetrapyrrolic porphyrins from which they are conceptually derived. Like these various aromatic cousins , the... 

The final cyclooctapyrrole reported by Vogel and coworkers is the 32 n-electron (therefore, formally non-aromatic) octaphyrin-(l.O.l.O.l.O.l.O) derivative 8.20." This macrocycle was first synthesized in 7% yield as the result of a [1 + 1] acid-catalyzed condensation between the linear tetrapyrroles 8.18 and 8.19. Subsequently, this same macrocycle was synthesized from bipyrrole 8.21 and diformyl bipyrrole 8.10 in higher yield (11%) than originally obtained via the [1 + 1] approach (Scheme 8.2.3). This finding attests to the generality of the [2 + 2] strategy certainly, it appears to be a good synthetic route to cyclic octapyrroles. 

The porphyrins and related tetrapyrrolic systems are among the most widely studied compounds for use as photosensitizers in PDT. Porphyrins are IStt-electron aromatic macrocycles that exhibit characteristic optical spectra with a strong k-k transition around 400 nm (Soret band) and usually four Q... 

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