Structurally modified porphyrins: some highlights

A variety of structurally modified porphyrins including porphyrin isomers, heteroatom-substituted porphyrins, and expanded and contracted porphyrins have been synthesized in recent years; highlighting a few papers:

“Porphyrinoids (summary expression for the porphyrins and their structural variants) are of fundamental importance in organic chemistry. Commanding interdisciplinary interest, porphyrinoids have been subjected over the years to virtually all pertinent methods of physical and chemical investigation available to the organic chemist. Porphycene, a tetrapyrrole macrocycle with effective D2h symmetry, is the most simple structural variant of porphyrin.”

(B. Frydman, J. Org. Chem. 1998 (63) 9385-9391)
“Porphin, the parent compound of the porphyrins, contains a central 16-membered ring that corresponds to the dianion of [16]-85-annulene in configuration and conformation (planar) [...]. Porphin may be regarded alternatively as a diaza[18]annulene (outer pathway of conjugation) or as a tetraaza[16]annulene dianion (inner pathway of conjugation). The [16]annulene dianion model appears to be especially suited for the understanding of the spectroscopic properties of metalloporphyrins.”

(E. Vogel, Angew. Chem. Int. Ed. 1986 (25) 257-259)
“The past decade has seen increasing attention being paid to the synthesis of novel porphyrinoid aromatic compounds. A large number of these macrocyclic structures, including ‘expanded porphyrins’, has been reported recently. The interest in cyclic polypyrrole systems and their heteroanalogues is based on their potential application as photosensitizers in the treatment of cancer by photodynamic therapy, and as highly selective catalysts and organic electrical conductors.”

(R. Pandey, Angew. Chem. Int. Ed. 1999 (38) 126-128)
b-Substituted porphyrins are usually synthesized by the ‘3+1’ technique known as MacDonald condensation. Moreover, this procedure affords new aromatic porphyrinoid systems, including benzene- and pyridine-containing macrocycles and carba porphyrins, by utilizing other aromatic aldehydes.”

(S. Taniguchi, Synlett 1999 (1) 73-74)
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