Macrocycle hexapyrrolic

Another antiaromatic hexapyrrolic macrocycle, [24]hexaphyrin(l. 0.1.0.1.0), rosarin 167 has been prepared by acid-catalyzed condensation of bipyrrole 166 and an aromatic aldehyde (Scheme 69) (1992JA8306). 

Prior to developing the above synthesis, Sessler and coworkers discovered a different route to meso-axy sapphyrins. This synthetic strategy, like so many in the area of sapphyrin-related research, is based upon a reaction that was not initially intended to produce sapphyrins. Specifically, it involves the reaction between bis(pyrrolyl)bipyrrole 5,38 and diformyl dipyrrylmethane 5.39, and was found to produce, quite unexpectedly, sapphyrin 5,41 in ca. 5% yield, with none of the intended hexapyrrolic macrocycle 5,40 being detected (Scheme 5.2.3). It was thus considered that such a 4 + 2 = 5 approach might prove useful for the synthesis of we o-phenyl sapphyrins. This indeed proved to be the case. In fact, this strategy has... 

Pentaphyrins have also been isolated as side-products of 3 + 3 reactions used to prepare a class of hexapyrrolic macrocycles called hexaphyrins (discussed in... 

A reductive McMurry-type approach similar to that used for the synthesis of porphycenes (Chapter 3) and ozaphyrins (Chapter 6) has also been applied to the synthesis of hexapyrrolic macrocycles. This approach has been explored independently by Johnson and Ibers Cava and Merz and Neidlein " and has led to a range of hexaphyrin-(2.0.0.2.0.0)-type expanded porphyrins (e.g. 7.55) that are gen-erically known as homoporphycenes or bronzaphyrins. As is often the case, the trivial name bronzaphyrin, originally coined by Johnson and Ibers ° reflects the color of the macrocycle observed in organic media. Indeed, in solution, these macro-cycles typically exhibit a distinctive bronze color. For the purpose of continuity in this chapter, the name bronzaphyrin will be universally employed for all macrocycles of the hexaphyrin-(2.0.0.2.0.0) type. 

Core-Expanded Benzoporphyrins Core-expanded porphyrins had a variety of beautiful colors, and this was due to their characteristic electronic spectra showing a strong Soret-band absorption around 400-500 nm and several weak Q-band absorptions over 500 nm [94]. Hexapyrrolic macrocycles of red-colored rubyrin (hexaphyrin(l. 1.0.1.1.0)) [95] and amethyrin (hexaphyrin( 1.0.0.1.0.0)) [96] were named after their mby-Iike and amethyst-Uke colors of precious stones. Among the expanded porphyrins, sapphyrin (penta-phyrin(l.l.l.l.O)) was the first example, which was seren-dipitously discovered during the synthesis of vitamin B12 by Woodward and coworkers in 1966 [97]. These electronic spectra could be easily modified by 71-expansion by the rDA method. 

As the number of pyrrole rings in a macrocycle increases, the number of possible combinations for a given set of pyrroles and meso-carbons also increase. As a consequence, at least six hexapyrrolic systems have been reported (03ACI5134, 02P1SC311). However, only the latest information on the hexapyrrolic systems is summarized. 

A third class of a hexapyrrolic expanded porphyrins (e.g., 7.36) was reported in 1992 by Sessler, et al. These macrocycles contain three fewer mesoAike bridging carbons than do the hexaphyrins, and may thus be formally considered as being triply contracted hexaphyrins. However, they are more commonly referred to as being rosarins . This is because the protonated derivatives appear pink to red in organic solution. 

In recent years, the desire to generate new expanded porphyrins has led to the preparation of ever larger macrocycles. Most of these have consisted of tetra-, penta-, and hexapyrrole-type macrocycles. However, a few higher order systems, expanded porphyrins containing more than six pyrrole-type subunits, are now known. It is a review of these systems, still limited to macrocycles containing eight and ten pyrrole-type subunits, that is the subject of the present chapter. 

Good fluoride anion binding affinities were observed in the case of the two hexapyrrolic calix[4]pyrrole receptors 9 reported in 2014 by Anzenbacher and coworkers [24]. The incorporation of two additional carbonyl ester pyrrole units on either side of the macrocycle produced the cis- and traws-configurational isomers, 9a and 9b, respectively (cf. Fig. 12.7). In acetonitrile solution, a value of 1.0 X 10 for fluoride anion binding was estimated for both 9a and 9b on the... 

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