Porphyrin, aromaticity formation from pyrrole

The synthons of porphyrin syntheses are the pyrroles, which in turn must be made from 1,4-difunctional synthons. These carbon skeletons are available by an aldol-type condensation of the enol of a 1,3-diketone with an a-nitrosylated acetoacetate (Knorr pyrrole synthesis. Scheme 1.3.4). The final reductive ring closure by Schiff base formation is again a reversible condensation reaction. After dehydration, however, a stable 7i-electron sextet is formed, which gives the resulting pyrrole aromatic stability. Hydrolysis of this enamine can now only occur in very strong acid. In water of modest acidity or basicity it is perfectly stable. 

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. 

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