Colorful Friedel−Crafts Chemistry of <i>m</i><i>eso</i>-Tetraarylporphyrins. An Unexpected Route to Porphyrinic Spiro Dimers

Under Friedel−Crafts conditions, <i>meso</i>-tetraarylporphyrins give porphyrin spiro dimers in good yield. This reaction involves acylation, acid-catalyzed cyclization, and dimerization. A stable dimer possessing three additional six-membered rings could be isolated and its structure determined. By modifying the substrate and reagents, intermediates could be isolated and characterized. The reactivity of the substrates, the side-reactions, and the concentration requirements to form the dimers, all explain why this reaction remained apparently unexplored in the widely used <i>meso</i>-tetraarylporphyrin series.