Naphthiporphyrins

Abstract

Benziporphyrins, cross-conjugated porphyrin analogues with a benzene ring in place of one of the usual pyrrole units, have varying degrees of macrocyclic aromaticity because the 6π electron arene needs to give up its aromatic characteristics to facilitate conjugation over the entire system. As naphthalene would lose less resonance stabilization energy in giving up one of its benzene units, it was proposed that naphthiporphyrins would exhibit enhanced diatropicity compared to the related benziporphyrins. A naphthiporphyrin was prepared using the "3 + 1" variant of the MacDonald condensation by reacting 1,3-naphthalenedicarbaldehyde with a tripyrrane in the presence of TFA, followed by oxidation with DDQ. Although the free base form of naphthiporphyrin showed no overall diatropicity, the corresponding dication in TFA-CDCl(3) demonstrated a significant diatropic ring current where the internal CH shifted upfield to between 4.0 and 4.6 ppm. Naphthiporphyrin was converted to the corresponding palladium(II) complexes by reaction with Pd(OAc)(2) in acetonitrile, and the complex was further characterized by X-ray crystallography. Oxynaphthiporphyrins were similarly prepared by the "3 + 1" methodology from 4-methoxy-1,3-naphthalene-dicarbaldehyde, and these showed slightly enhanced diatropic character compared to oxybenziporphyrins. Reaction of oxybenziporphyrins or oxynaphthiporphyrins with silver(I) acetate afforded the corresponding silver(III) organometallic derivatives. A meso-tetraphenyl naphthiporphyrin was also synthesized in 4% yield by reacting a 1,4-naphthalene dicarbinol with 2 equiv of benzaldehyde and 3 equiv of pyrrole in the presence of BF(3).Et(2)O, followed by oxidation with DDQ. However, this 1,4-naphthiporphyrin showed reduced diatropic character compared to the corresponding p-benziporphyrin system. The NMR spectra for the 1,4-naphthiporphyrin show that the naphthalene unit pivots over the macrocycle and this presumably leads to further steric interactions that reduce the planarity of the macrocycle. These results demonstrate that while naphthiporphyrins can show enhanced aromatic properties as predicted, other factors may overwhelm this effect.