BENZOYLATION AT THE MESO POSITION OF A ZINC(II) DEUTEROPORPHYRIN

By short treatment of zinc(II) deuteroporphyrin-IX dimethyl ester with benzoic anhydride/SnCl4, a mixture of benzoylated derivatives was obt ained from which a tribenzoyl derivative with benzoyl groups at the 2, 4, and gamma-meso positions could be isolated in low yield. Except fo r the known Vilsmeier formylation, this constitutes the first example of a Friedel-Crafts acylation taking place at the meso position of a p orphyrin chelate. The expected lack of regioselectivity of this reacti on, even at low conversion rates, and the difficulty to separate the p roducts limit, of course, its synthetic value for the preparation of t he particular benzoyl derivatives. This limitation cannot be overcome by starting with a completely beta-substituted porphyrin or with a sym metrically substituted porphyrin, since attempts to benzoylate Zn(II) mesoporphyrin-IX dimethyl ester lead exclusively to demetallation. How ever, these findings add to the knowledge of the fundamental propertie s of porphyrin reactivity. A rationalization based on calculations at a semiempirical level, which are in acceptable agreement with the expe rimental results, is presented.