Porphyrins contracted

At first glance, the qnestion of cation-radical basicity seems to be meaningless Cation-radicals are nsnally strong acids. Nevertheless, these are relevant examples. Corrole and isocorrole belong to the porphyrin family and are named contracted porphyrins. They are typified by their contracted... 

Boron, with a covalent radius of 0.85 A, is too small to coordinate to a porphyrin ligand through all four nitrogen atoms. There are two possible solutions to this problem, either to use a contracted porphyrin-type ligand or to coordinate more than one boron atom to a single porphyrin, and both of these have been realized. 

Since the initial disclosure of the basic corrin structure, there has been a considerable body of effort devoted toward the synthesis of macrocycles related to this chromophore that may be considered as being intermediates between porphyrin and corrin. These macrocycles, namely the dehydrocorrins (e.g., tetradehydrocor-rin 2.3) and the corroles (e.g., 2.4), represent interesting classes of contracted porphyrins that warrant specific mention here. The interest in these molecules derives in part from the fact that they could represent milestones along the biosynthetic pathway leading to vitamin B12. They are, however, also of interest from a non-biological perspective. Simply stated, this is because corrole-type macrocycles possess unique electronic and chemical characteristics, the study of which can help one to understand better the chemistry of all porphyrin analogs. 

The review presented here has a more synthetic focus than its predecessors. Detailed information about the physical properties of the corrin-related macrocycles is, therefore, not included here. Instead, the reader is referred to the earlier reviews, as well as to a number of relevant papers, for detailed descriptions of the physical properties of corrin-type systems.Still, in this chapter, a complete, up-to-date discussion of corrole and heterocorrole synthesis and metalation properties will be presented. Also, two sections will be devoted to other synthetic contracted porphyrins, including isocorroles, and several systems that contain fewer than four pyrrole-like subunits in their macrocyclic framework. 

Because of its immense scope, a detailed description of corrins (and vitamin B12) will not be presented here. The reader is instead referred to reviews of B12 chemistry and its biosynthesis that have appeared recently. Further, because they are more directly related to the corrins than are the corroles, the chemistry of the dehydrocorrins will not be discussed here. Also not included in this review are the so-called artificial porphyrins of Floriani, et al. (e.g., 2.5), since it is deemed by these authors in their review that these macrocycles are more dehydrocorrin-like than corrole-like in their nature. Other systems omitted here include the spiro porphyrins of Battersby and coworkers, the tetraphosphole macrocycles of Mathey and coworkers and the tetrapyrrolic systems of Bartczak and Smith and co-workers. Thus, the emphasis will be on those contracted porphyrins that are most removed, in structural and chemical terms, from the macrocyclic unit found in coenzyme B12 and its analogs. 

Free-base corrole (e.g., 2.6), like porphyrin, contains an 18 n-electron pathway and, as such, can sustain a diamagnetic, aromatic ring current (Scheme 2.1.1). Corroles are much stronger acids (and weaker bases) than porphyrins. Thus, unlike porphyrin, corrole forms a stable anion when treated with aqueous alkali. This anion, best represented by structure 2.7, is also an aromatic 18 Tt-electron system, which may account in part for its special stability. In any event, the neutral form of the macrocycle may be regenerated on acidification. Thus, the corrole-to-porphyrin relationship in terms of acidity allows for comparison to be made between these two systems and their smaller six n-electron counterparts benzene and cyclopentadienyl anion (Figure 2.1.2). In other words, cyclopentadienyl anion may be regarded as a contracted benzene just as corrole anion may be considered as being a contracted porphyrin . 

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