Tetraarylporphyrin synthesis by electrochemical oxidation of porphyrinogens

The electrochemical synthesis of tetraarylporphyrins has been reinvestigated. Whereas the direct electrolysis of a mixture of benzaldehyde and pyrrole gave only a 1% yield of tetraphenylporphyrin (A. Stanienda, Z. Naturforschg. 22b (1967) 1107), the electrochemical oxidation for 2 h of preformed porphyrinogens, obtained under the Lindsey conditions, can be successfully performed using catalytic amount of quinone as mediator. Furthermore, when condensation involves 3,4-difluoropyrrole, the electrolysis. at 1.2-1.45 V versus ECS, can be realized without any mediator, owing to the good electrochemical stability of the compound. The porphyrins yields (21-41%) depend on the nature of the aldehyde (benzaldehyde, pentafluorobenzaldehyde) and of the pyrrole (pyrrole, 3-fluoropyrrole. 3,4-difluoropyrrole). They are slightly smaller in the case of a redox catalysis and a little larger for the direct electrolysis. An improvement of the purification step was obtained by decreasing the quantity of quinone and performing the chromatography of the dication porphyrins. (C) 2001 Elsevier Science Ltd. All rights reserved.