Pentapyrrolic expanded porphyrin

Figure 4. (a) Tetra- and pentapyrrolic expanded porphyrins, (b) Hexapyrrolic expanded porphyrins. 

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... 

The idea that expanded porphyrins, as "bigger porphyrins", could serve as ligands for cations that are too large to be accommodated as 1 1 in-plane adducts by the tetrapyrrolic porphyrins is, of course, not new. Indeed, it has its antecedents in the early work of Marks and Day and in even earlier studies carried out by Woodward and coworkers. In the case of the latter group, for instance, it became apparent sometime between 1966, the lime of the initial disclosure of sapphyrin 3 (a disclosure, incidentally, that represented the first documented report of an expanded porphyrin), and 1983, the time of the follow-up full paper, that the pentapyrrolic sapphyrins, with an inner core of ca. 5.5 A diameter, should be well-suited for the complexation of uranyl ion, U02 (in-plane ionic radius 0.81-0.86 A depending on coordination number ). 

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