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
