Synthesis of Phenaliporphyrin, a PAH-Porphyrin Hybrid, from an Acenaphthene-Fused Cyclopropane Dialdehyde

Phenaliporphyrin, a new aromatic benziporphyrin derivative, has been synthesized from an annulated cyclopropane dialdehyde intermediate. Corey-Chaykovsky cyclopropanation of dimethyl acenaphthylene-1,2-dicarboxylate gave a cyclopropane diester, and this was converted to the corresponding bis-Weinreb amide. Reduction with lithium aluminum hydride afforded an unstable dialdehyde and subsequent acid-catalyzed condensation with a tripyrrane dicarboxylic acid, followed by oxidation with aqueous ferric chloride, gave the targeted porphyrinoid system. This involves ring expansion of the acenaphthylene precursor to generate the phenalene unit. Phenaliporphyrin exhibits a porphyrin-like UV-vis spectrum with a Soret band at 443 nm and a relatively intense Q-band at 601 nm. The proton NMR spectrum demonstrated that the system has a strong diamagnetic ring current, and the external <i>meso</i>-protons showed up downfield at 9.64 and 9.34 ppm while the internal C-H appeared upfield at -6.48 ppm. Although DFT calculations show that the phenalene ring is twisted to relieve steric interactions, single crystal X-ray diffraction analysis indicated that the macrocycle is nearly planar. The discrepancy was attributed to crystal packing forces and a distorted conformation is likely to be favored in solution. The aromatic properties for phenaliporphyrin were confirmed by nucleus independent chemical shift (NICS) calculations and anisotropy of induced current density plots.