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 units. This approach involves the acid catalyzed condensation of a tripyrrolic intermediate (tripyrrane) with dialdehydes, followed by oxidation with an electron-deficient quinone or aqueous FeCl₃. The first examples of aromatic porphyrinoids with benzene, cyclopentadiene, indene and cycloheptatriene have been accomplished, as well as nonaromatic macrocycles such as benziporphyrin and systems with borderline aromatic properties including the azulene-containing porphyrinoid "azuliporphyrin". Azuliporphyrin undergoes a novel oxidative rearrangement with tert-butyl hydroperoxide to give benzocarbaporphyrins, systems that can be synthesized directly from diformylindene. The new porphyrinoids show diverse spectroscopic and chemical properties and are providing new insights into the interface between porphyrin-type and annulene-type aromaticity. Carbaporphyrins have not as yet afforded stable metal chelates, unlike their structural cousins the N-confused porphyrins, although a highly selective oxidation occurs at the interior carbon atom with ferric chloride in alcohol solutions. Application of a related "4 + 1" route has allowed the synthesis of expanded carbaporphyrinoids, specifically carbasapphyrins, and these are also demonstrating unique chemical and spectroscopic properties. Further modification of the porphyrin core has led to the synthesis of an aromatic dicarbaporphyrin, but it remains to be seen whether this work can be extended to the tetracarbaporphyrin system "quatyrin".

porphyrin analogues - pyrroles - tripyrranes - sapphyrins - aromaticity - "3 + 1" condensation