Acylation of nickel meso-tetraarylporphyrins:: Porphyrin to corrole ring contraction and formation of seco-porphyrins

Friedel-Crafts acylation of nickel meso-tetraarylporphyrins with aryl anhydrides, followed by air oxidation in the presence of pyridine, DMAP, and excess anhydride, produced corroles in 8-21% yields. Other products include a porphyrinic ketone, an additional corrole whose bridge has been retained as an acyl group attached toa pyrrole, and a lactone resulting from the insertion of an oxygen atom into an alpha,beta-group pyrrole bond. The ring contraction is best explained by a pinacolic rearrangement reminiscent of the one taking place in the formation of corrinoids, a benzoyloxy group replacing the acetic side chain found in the natural products.