Pyrrole and aromaticity

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. 

Arene(Cp)Fe] complexes 62 are also found to be very efficient catalysts for polymerization of pyrroles and aromatic dicyanate esters [34,35]. 

Better results for oxidation step were obtained when oxidation was carried out in solution (Table 2.61). In this procedure, cyclocondensation of pyrrole and aromatic aldehydes in the presence of acid catalyst (pTsOH) were carried mechanochemically, followed by DDQ oxidation in solution (chloroform, 2h). 

---