Synthesis, metalation and antiaromatic properties of 22-hydroxybenziporphyrins

Abstract

Condensation of 5-alkyl-2-hydroxybenzene-1,3-dicarbaldehydes with a tripyrrane in the presence of trifluoroacetic acid (TFA), followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), gave two examples of 22-hydroxybenziporphyrins. Unlike a related meso-tetraphenyl-22-hydroxybenziporphyrin, these products were reasonably stable in solution. Dicationic species were obtained upon addition of TFA to CDCl3 solutions, and these exhibited weakly diatropic properties. However, the free base forms gave broad peaks for the macrocycle’s peripheral protons that fell into the range of 4.5–3.3 ppm, demonstrating that the system has paratropic characteristics that are consistent with the keto-tautomers. The porphyrin analogues gave broad, poorly resolved, UV-vis spectra of the type expected for nonaromatic or antiaromatic structures. Reaction of a hydroxybenziporphyrin with nickel(II), copper(II) or palladium(II) acetate gave the related coordination complexes. The nickel and palladium derivatives were structurally characterized by X-ray crystallography and exhibited buckled geometries with the phenolate units strongly pivoted away from the macrocyclic core.