Sapphyrin-like

Several years after the initial report of Woodward, et al., Sessler and coworkers found that sapphyrin 5.21 would in fact form a stable complex with uranyl cation. It was determined, however, that 1102 " complexation was accompanied by reduction of the sapphyrin macrocycle. Specifically, it was found that metal insertion occurred concurrent with addition of methanol to one of the meso-Xike carbon centers. The net result was an overall neutral complex of a modified, nonaromatic sapphyrin-like system (Scheme 5.5.2). 

Subsequent to the preparation of 8, a number of other sapphyrin-nucleobase conjugated carriers were synthesized in the authors laboratories. These include compound 9 that has two cytosine moieties appended to the sapphyrin macrocycle, as well as two guanine-bearing sapphyrins, 10 and 11. Here, as expected, the ditopic sapphyrin-guanine receptor 10 served to enhance the transport of cytosine-5 -monophosphate (but not GMP or AMP). By contrast, the two tritopic receptors, namely 9 and 11, were designed with the intention of allowing for the formation of the complexation-derived triple-helix-like C-G-C and G-C-G motifs in addition to phosphate chelation by sapphyrin. In other words, it was expected that a centrally... 

UV-vis spectroscopy was also used to probe the nature of the presumed sap-phyrin-DNA complex. Before such studies could be made, however, it proved necessary to map out the spectroscopic properties of sapphyrin itself when constrained to an aqueous medium. As the result of these latter predicative analyses, it was determined that sapphyrin can exist in three spectroscopically distinct states in aqueous media. The first spectroscopically distinct state, characterized by a Soret-like absorbance at = 450 nm, is ascribed to a monomeric form. The second, assigned to dimerized forms of sapphyrin, is readily identified by its characteristic Soret-like band at = 420 nm. The third state, identified on the basis of its signature 400-nm Soret-like absorption maximum, corresponds to the highly aggregated form of sapphyrin it is this form that is generally dominant in simple aqueous media. 

Nitrogen-bearing cyclophanes like 351 [16] and 352 [17] bind larger organic anions in water due to superposition of the hydrophobic effect and electrostatic attraction. The phenanthridinium hosts like 351 have been found to form the most stable nucleotide complexes known so far. On the other hand, free tetrapyrrolic porphyrins do not bind anions since their cavity is too small to take advantage of the convergent N-H dipoles for the complex stabilization [18]. However, expanded diprotonated porphyrins like sapphyrin 353 were shown to form stable complexes with phosphate [19a] and halide [19b] anions. 

Like porphyrins, sapphyrins are easily diprotonated in acidic media the basicity of sapphyrins... 

With the original reports of the successM synthe of the sapphyrins [26,66,152] and uranyl superphthalocyanine [112, 118, 119], interest in other expanded porphyrin systems, was kindled. The next logical step (after sapphyrin), in the expanding series of all-pyrrole systems, was the pentaphyrin macrocycle 231 which contains five pyrroles and five meso-like methine bridgra. In 1983 Gossauer et al. reported the synthesis of the first prototypical member 231 of this macrocyclic family [158, 182, 183, 185-187]. This first synthesis was achieved by a 2 + 3 MacDonald-type condensation between an oc-firee dipyrromethane 233 and a tripyrrane dialdehyde 236. More recently, the synthesis of pentaphyrin 231 has l n achieve by using a dipyrromethane 5,5 -dicarboxylic acid 235 in place of an a-firee dipyrromethane [21]. Here, as is the case in many of these kind of reactions [21,26,27,66,155], decarboxylation occurs under the reaction conditions to produce the corresponding a-free species 233 in situ. (Scheme 40) [21]. 

It is worth mentioning that an alternative 3 + 2 synthetic approach to sapphyrin has been reported. Specifically, it was found that the alkyl-substituted sapphyrin 5.19 could be prepared via the acid-catalyzed condensation of pyrrolyl-bipyrrole 5.27 with diformyl dipyrrylmethane 5.28 (Scheme 5.1.5). Following oxidation with air and chromatographic purification, this method affords a 35% yield of the desired sapphyrin. Unfortunately, this method has yet to find any kind of general applicability. This is most likely a reflection of the fact that the needed pyrrolyl-bipyrrolic precursors are hard to make. 

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... 

The methods described above for the synthesis of wc5o-arylsapphyrins are relatively new. This newness has so far limited the availability, and therefore the study, of aryl-substituted sapphyrin systems. This, however, is something that is likely to change in the near future, especially when one considers the important role that the parent me o-phenyl-substituted porphyrins have played in the development of numerous biological and materials science model systems. ... 

Because of their basic resemblance to porphyrins, it was initially expected that the sapphyrins would mimic, at least on some level, the rich coordination chemistry displayed by the porphyrins. However, the larger core size ca. 5.5 A inner N-N diameter vs. ca. 4.0 A for porphyrins), the greater number of potentially chelating heteroatom centers, and the fact that pentaazasapphyrins when fully deprotonated are potentially trianionic ligands made sapphyrin a likely candidate for large metal chelation, particularly as a potential ligand for the trivalent lanthanides and actinides. Unfortunately, in spite of extensive effort, this hope remains largely unrealized. Nonetheless, some metal complexes of sapphyrins and heterosapphyrins have been successfully prepared and characterized. Their preparation and properties are reviewed in this section. 

Through a series of experiments, Sessler and coworkers were able to determine a likely mechanism for the formation of complex 5.96. This mechanism, shown in Scheme 5.5.3, involves the initial coordination of uranyl cation to sapphyrin giving a complex such as 5.97 or 5.98. Addition of methanol (or methoxide anion) to one of... 

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