Out of the Blue! Azuliporphyrins and Related Azulene-Containing Porphyrinoids

Carbaporphyrinoids, porphyrin analogues that have one or more of the usual pyrrolic units replaced by carbocyclic rings, have been widely studied in recent years. These systems exhibit a wide range of properties and varying degrees of macrocyclic aromaticity. Azuliporphyrins have an azulene ring incorporated into the porphyrin framework and exhibit intermediary aromatic character. They were first prepared by reacting tripyrranes with 1,3-azulenedicarbaldehyde under acid-catalyzed conditions, although several alternative routes have been developed for preparing this intriguing carbaporphyrinoid system. Azuliporphyrins are easily protonated and this greatly enhances the diatropic properties for the macrocycle. Azuliporphyrins are also superior organometallic ligands and afford stable derivates with Ni(II), Pd(II), Pt(II), Ir(III) and Rh(III). Furthermore, tetraarylazuliporphyrins have been shown to undergo an oxidative metalation with copper(II) salts to generate a nonaromatic copper(II) complex. Treatment of azuliporphyrins with tert-butyl hydroperoxide under basic conditions triggers a ring contraction reaction to afford benzocarbaporphyrins. This appears to involve initial nucleophilic attack onto the seven-membered ring, followed by a Cope rearrangement and elimination of tert-butyl alcohol. These observations have led to investigations into related macrocyclic systems, including heteroazuliporphyrins with furan, thiophene or selenophene rings. In addition, an azulisapphyrin has also been reported. Dicarbaporphyrinoids with azulene-subunits have also been investigated. Azulene-containing fulvene dialdehydes have been shown to react with dipyrrylmethanes under acidic conditions to give 22-carbaporphyrins, and this system also shows intermediary aromatic character. In addition, a bis-azulenylmethane dialdehyde was shown to react with dipyrrylmethanes to give, following oxidation with DDQ, examples of diazuliporphyrins. This system was only stable in a protonated form but retained a degree of diatropic character. Reaction with palladium(II) acetate gave an unusual zwitterionic palladium(II) organometallic complex. These results demonstrate that azulene-containing porphyrinoids have unique properties and are superior precursors to organometallic derivatives.