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 obtained from which a tribenzoyl derivative with benzoyl groups at the 2, 4, and γ-meso positions could be isolated in low yield. Except for the known Vilsmeier formylation, this constitutes the first example of a Friedel-Crafts acylation taking place at the meso position of a porphyrin chelate. The expected lack of regioselectivity of this reaction, even at low conversion rates, and the difficulty to separate the products limit, of course, its synthetic value for the preparation of the particular benzoyl derivatives. This limitation cannot be overcome by starting with a completely β-substituted porphyrin or with a symmetrically substituted porphyrin, since attempts to benzoylate Zn(II) mesoporphyrin-IX dimethyl ester lead exclusively to demetallation. However, these findings add to the knowledge of the fundamental properties of porphyrin reactivity. A rationalization based on calculations at a semiempirical level, which are in acceptable agreement with the experimental results, is presented.