Carbaporphyrinoids: Taking the heterocycle out of nature's [18]annulene

A "3+1" methodology has been developed for the synthesis of novel porphyrin analogues with carbon rings in place of one or more of the usual pyrrole u nits. This approach involves the acid catalyzed condensation of a tripyrrol ic intermediate (tripyrrane) with dialdehydes, followed by oxidation with a n electron-deficient quinone or aqueous FeCl3. The first examples of aromat ic porphyrinoids with benzene, cyclopentadiene, indene and cycloheptatriene have been accomplished, as well as nonaromatic macrocycles such as benzipo rphyrin and systems with borderline aromatic properties including the azule ne-containing porphyrinoid "azuliporphyrin". Azuliporphyrin undergoes a nov el oxidative rearrangement with tert-butyl hydroperoxide to give benzocarba porphyrins, systems that can be synthesized directly from diformylindene. T he new porphyrinoids show diverse spectroscopic and chemical properties and are providing new insights into the interface between porphyrin-type and a nnulene-type aromaticity. Carbaporphyrins have not as yet afforded stable m etal chelates, unlike their structural cousins the N-confused porphyrins, a lthough a highly selective oxidation occurs at the interior carbon atom wit h ferric chloride in alcohol solutions. Application of a related "4+1" rout e has allowed the synthesis of expanded carbaporphyrinoids, specifically ca rbasapphyrins, and these are also demonstrating unique chemical and spectro scopic properties. Further modification of the porphyrin core has led to th e synthesis of an aromatic dicarbaporphyrin, but it remains to be seen whet her this work can be extended to the tetracarbaporphyrin system "quatyrin".