Porphyrinogen aromatization

Porphyrin, 5,10,15,20-tetraphenyl-, 4, 386 Porphyrin, vinyl-synthesis, 4, 278, 279 Porphyrin coenzymes in biochemical pathways, 1, 258-260 Porphyrinogen, mcso-tetraaryl-synthesis, 4, 230 Porphyrinogens, 4, 378, 394 pyrazoles, 5, 228 synthesis, 4, 231 Porphyrins, 4, 377-442 acetylation, 4, 395 aromatic ring current, 4, 385 basicity, 4, 400 biosynthesis, reviews, 1, 99... 

Systematic investigations have deary demonstrated that in the presence of metal ions the conjugated hexahydroporphyrin forms are thermodynamically favored by complexation whereas in the absence of metal ions the porphyrinogen form with isolated aromatic pyrrole rings is the thermodynamically stable tautomer. 

An acid catalyzed double Schiff-base condensation of 5 with o-phenylenediamine, 6, then gives the cyclic non-aromatic sp3 texaphyrin ring 7, a macrocyclic product that can be viewed as being an expanded porphyrinogen [24], The sp3 form can be oxidized with ferrocenium ion to give the corresponding aromatic derivative, namely the metal-free sp2 form 8 (Scheme 2) [25],... 

The most simple method of obtaining a porphyrin on the gram scale is to reflux a dilute benzaldehyde-pyrrole mixture in propionic acid (141°C) for 30 minutes, cool, and filter. A 20% yield of crystalline porphyrin is easy to achieve (Scheme 6.3.1) (Lindsey et al., 1987, 1994 Prathapan et al., 1993). Even pentamers have been obtained by such a reaction in one step from a porphyrin benzaldehyde building block and pyrrole. One just has to carry out the primary formation of the porphyrinogen at a relatively low concentration (10 M) and under nonoxidative conditions, which allow the rearrangement of undesired polymers. Up to 50% of the colorless porphinogens are obtained by acid-catalyzed condensation of pyrrole and aromatic aldehydes under nitrogen. 

Synthesis of Tetraarylporphyrins. Triflyl chloride was used under aerobic oxidation conditions to produce tetraarylporphyrins from pyrrole and various aromatic aldehydes. The reaction proceeds through condensation of pyrrole to the aromatic aldehyde to form an intermediate aryl triflate which then undergoes polymerization and cyclization of the tetrapyrrolic oligomer. Air oxidation of the porphyrinogen intermediate completes the sequence (eq 15). 

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