Unusual phlorins from the oxidative coupling of pentapyrromethanes: their facile conversion to meso-substituted porphyrins

Direct, oxidant-mediated intramolecular coupling of pentapyrromethanes afforded 5-aryl-5-(2′-pyrryl)-10,15,20-triarylporphyrin (phlorins) as single products. The outcome of the reaction is comparable with the oxidative coupling of tetrapyrromethanes. Treatment of obtained phlorins with trifluoroacetic acid resulted in exclusive elimination of the peripheral pyrrole affording meso-tetraarylporphyrins quantitatively. The rate of pyrrole elimination depends on the acid concentration. The selective binding with fluoride anion was also observed with a large association constant.