Synthesis, Spectroscopy, and Reactivity of meso-Unsubstituted Azuliporphyrins and Their Heteroanalogues. Oxidative Ring Contractions to Carba-, Oxacarba-, Thiacarba-, and Selenacarbaporphyrins

Abstract

This paper reports the first detailed study on meso-unsubstituted azuliporphyrins, an important family of porphyrin-like molecules where one of the usual pyrrole rings has been replaced by an azulene subunit. Although the azulene moiety introduces an element of cross-conjugation, zwitterionic resonance contributors with tropylium and carbaporphyrin substructures give azuliporphyrins diatropic character that falls midway between true carbaporphyrins and nonaromatic benziporphyrins. Protonation affords an aromatic dication where this type of resonance interaction is favored due to the associated charge delocalization. Two different "3 + 1" syntheses of meso-unsubstituted azuliporphyrins have been developed. Acid-catalyzed reaction of readily available tripyrrane dicarboxylic acids with 1,3-azulenedicarbaldehyde, followed by oxidation with DDQ or FeCl(3), affords good yields of azuliporphyrins. Alternatively, azulene reacted with acetoxymethylpyrroles (2 equiv) in refluxing acetic acid/2-propanol to give tripyrrane analogues, and following a deprotection step, condensation with a pyrrole dialdehyde in TFA-CH(2)Cl(2) gave the azuliporphyrin system. The latter approach was also used to prepare 23-thia- and 23-selenaazuliporphyrins. However, reaction of the azulitripyrrane with 2,5-furandicarbaldehyde produced a mixture of three oxacarbaporphyrins in moderate yield. The free base forms of thia- and selenaazuliporphyrins both showed intermediary aromatic character that was considerably enhanced upon protonation. The UV-vis spectra for azuliporphyrins and their heteroanalogues showed four bands between 350 and 500 nm and broad absorptions at higher wavelengths. Addition of TFA gave dications that showed porphyrin-like spectra with Soret bands between 460 and 500 nm. In the presence of pyrrolidine, azuliporphyrins and their heteroanalogues undergo nucleophilic attack on the seven-membered ring to give carbaporphyrin adducts. These systems also undergo oxidative rearrangements under basic conditions with t-BuOOH to give benzocarbaporphyrins. The selenaazuliporphyrin afforded two benzoselenacarbaporphyrins, a previously unknown core-modified carbaporphyrin system. The proton NMR spectra for these compounds showed strong diatropic ring currents with the internal CH resonance upfield above -5 ppm, while the meso-protons resonated downfield near 10 ppm. The UV-vis spectra were also porphyrin-like and gave strong Soret bands at ca. 440 nm.

Metalation of Carbaporphyrinoid Systems

“…[25,38,39] Azuliporphyrins 6 show comparable diatropic character to the cross-conjugated NCPs 21, in both cases showing the internal CH resonances in proton NMR spectra near 3 ppm, although this property is greatly enhanced upon protonation. [25,39] Other systems that have been prepared using the '3 + 1' approach include N-confused pyriporphyrin 24, [40] oxynaphthiporphyrins 25 [41] and tropiporphyrins 8. [27,42] Further structural diversity can be introduced when modified tripyrrane structures are utilized in these syntheses (Scheme 2).…”

“…[37] Azulene dialdehydes 23 react with tripyrranes 10 to give a particularly important group of porphyrin analogues known as azuliporphyrins (6). [25,38,39] Azuliporphyrins 6 show comparable diatropic character to the cross-conjugated NCPs 21, in both cases showing the internal CH resonances in proton NMR spectra near 3 ppm, although this property is greatly enhanced upon protonation. [25,39] Other systems that have been prepared using the '3 + 1' approach include N-confused pyriporphyrin 24, [40] oxynaphthiporphyrins 25 [41] and tropiporphyrins 8.…”

“…[43,44] Azulene undergoes electrophilic substitution at the 1 and 3 positions, and this allows the formation of azulitripyrranes 30 when azulenes 31 are reacted with two equivalents of an acetoxymethylpyrrole 32 in the presence of acetic acid. [39,45,46] These tripyrrane analogues were generated with terminal tert-butyl ester protective groups. Following deprotection with TFA, the azulitripyrranes were condensed with pyrrole, furan, thiophene or selenophene dialdehydes to give azuliporphyrins and their heteroanalogues 33 in good yields.…”

“…Following deprotection with TFA, the azulitripyrranes were condensed with pyrrole, furan, thiophene or selenophene dialdehydes to give azuliporphyrins and their heteroanalogues 33 in good yields. [39,46] Resorcinol ( 34a) and 2-methylresorcinol (34b) were also shown to react with acetoxymethylpyrroles 35 in the presence of p-toluenesulfonic acid and calcium chloride to give tripyrrane analogues with terminal benzyl ester protective groups. [47] Following hydrogenolysis over 10% palladium-charcoal, the related dicarboxylic acids 36 were reacted with dialdehydes 37 (X = NH, O, S or Se) to give a series of rather insoluble hydroxyoxybenziporphyrins 38.…”

“…[38] In the presence of nucleophiles like pyrrolidine, adducts 52 may be generated that take on fully aromatic carbaporphyrin-type spectroscopic properties. [38,39] Azuliporphyrins 6 can undergo oxidative ring contractions to afford benzocarbaporphyrins 53, and this chemistry readily occurs in the presence of tert-butyl hydroperoxide and potassium hydroxide. [38,39] Although mixtures of several carbaporphyrin products may be generated, this chemistry provides an alternative route to benzocarbaporphyrins.…”