Pentapyrrolic macrocycle

It is noteworthy2211- that the condensation between a dipyrrylmethanedicarbaldchydc and a tripyrrane or a tripyrranedicarboxylic acid in the presence of hydrobromic acid yields a porphyrin, instead of the expected pentapyrrole macrocycle, as a product. 

Pentapyrrolic macrocycles, 2,888 2,1,2-Pen tathiadiazol e-4,7-dicarbonitrile in hydrogen production from water, 6, 508 Pentatungstobis(organophosphonates), 3, 1053 4-Penten-l-al reaction with ethylene catalysts, rhodium complexes, 6, 300... 

PENTAPYRROLIC MACROCYCLES AND RELATED COMPOUNDS 21.1.4.1 General Properties... 

These pentapyrrolic macrocycles contain a 22-jr-electron ring system, and are expected to be the largest members of the [An + 2] n aromatics. In fact, their spectroscopic characteristics, i.e. the intense absorption band in visible region, the large diamagnetic shift of peripheral H and inner NH NMR signals, and relatively strong parent (M+) and dicationic (M2+) mass peaks, indicate the existence of a delocalized cyclic aromatic n system (Table 11). 

Table II H NMR Chemical Shifts and Absorption Maxima (Soret Band) of Pentapyrrolic Macrocycles...

In porphyrin H2P (71, also known as porphine) the tautomerism is degenerate i.e., tautomers 71a and 71b are identical. Also, in the anion HP-72, the four tautomers are all degenerate the kinetics of proton transfer have been studied. In chlorins, bacteriochlorins, and isobacteriochlorins, all the tautomers are different, but all the evidence indicates that the most stable tautomers are those with the inner protons in a trans disposition, represented by 73, 74, and 75, all having an 18-rr-delocalized system. Gossauer s penta-phyrin, a 22-77-electron pentapyrrole macrocycle with two inner hydrogen atoms, should also be prone to annular tautomerism [83JCS(CC)275]. 

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... 

In the full paper describing Woodward s sapphyrin-directed studies, it was noted that small amounts of sapphyrins could also be prepared via the HBr-catalyzed reaction between a bipyrrole dicarboxaldehyde and a dipyrrylmethane dicarboxylic acid. Additionally, the HBr-catalyzed reaction of 3,4-dimethylpyrrole 5.5 with 2,5-diformyl-3,4-dimethylpyrrole 5.3 was reported to afford small yields of decamethyl-sapphyrin 5.6 (Scheme 5.1.3). The fact that any sapphyrin could be isolated from these reactions, or even from the procedures of Schemes 5.1.1 and 5.1.2, attests to the stability of this pentapyrrolic macrocycle. 

In 1995, Latos-Grazyhski and coworkers reported an efficient, albeit somewhat low-yielding, synthesis of tetraphenylsapphyrin 5.50. In this work, it was found that the acid-catalyzed, oxidative reaction of benzaldehyde (5.30) with pyrrole (5.29) in a 1 3 molar ratio gave, as expected, tetraphenylporphyrin and an isomeric N-inverted porphyrin as the main macrocyclic products (cf. discussion in Chapter 3). However, careful examination of the minor fractions revealed that one side-product of this reaction was in fact the pentapyrrolic tetraphenylsapphyrin 5.50a (Scheme 5.2.6). Unfortunately, this species could only be isolated in 1.1% yield. Nonetheless, this synthesis represented the first synthetic entry into the area of tetrakis-mc o-substituted sapphyrins. 

The interest and excitement aroused by the successful synthesis of sapphyrin led Grigg and Johnson to suggest that a pentapyrrolic macrocycle containing two directly linked pyrroles (e.g., 5.142, shown in its unsubstituted, free-base form Figure 5.7.1) could exist in a planar, strain-free conformation." They referred to this class of molecules as norsapphyrins since they are formally derived from sapphyrin by the removal of one me5o-carbon from its macrocyclic nucleus. Thus, the norsapphyrins (later called smaragdyrins by Woodward see below) would be expected to bear the same relation to sapphyrin as corrole does to porphyrin. 

In the preceding chapter the principal player in the area of pentapyrrolic expanded porphyrins, namely sapphyrin, was introduced. In this chapter, a number of other pentapyrrole-derived macrocycles are discussed that are less well studied than this prototypic system. However, most of these other, less well-studied pentapyrrolic macrocycles were only discovered recently and, as a result, have not been examined in detail. In fact, nearly half of the new pentapyrrolic expanded porphyrins that have been reported in the literature have appeared in the past 2 years. This dramatic increase in number reflects, to a certain extent, the fact that the field of expanded porphyrins is one that is still emerging. 

The smallest pentapyrrolic expanded porphyrin to be reported to date is the so-called orangarin macrocycle (alternatively referred to as [20]pentaphyrin-(2.1.0.0.1) ). It was prepared in the form of the decaalkyl-substituted system 6.9 via an HCl-catalyzed condensation between the a-free terpyrrole 6.7 and the diformyl... 

An interesting non-cofacial salt-bridged system with an association constant of 2600 500 M" involving pentapyrrolic macrocycles sapphyrin was reported [640]. The singlet energy transfer was well explained by the Forster mechanism (24). 

In 1990, Sessler and co-workers developed an efficient synthesis of sapphyrin 56, a pentapyrrolic macrocycle initially discovered by Woodward et al. [108]. X-ray structure and NMR analyses of its bis HPFe salt revealed the presence of a fluoride anion bound within the diprotonated pentapyrrcUc macrocycle (Fig. 5.44) [109]. A PFs counter anion was also found in the lattice but not proximate to the nearly planar sapphyrin skeleton. The fluoride anion itself was found to be within hydrogen bonding distance of all five proton-bearing nitrogen atoms. 

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