Synthesis, Structural Characterization, Aromatic Characteristics, and Metalation of Neo-Confused Porphyrins, a Newly Discovered Class of Porphyrin Isomers

Neo-confused
porphyrins represent a unique family of porphyrin isomers that retain overall aromatic
characteristics by virtue of a 17-atom 18π electron delocalization
pathway. These porphyrin analogues have a pyrrolic subunit linked
in a 1,3-fashion so that a nitrogen atom is directly connected to
a meso-bridging carbon. Pyrrole-3-carbaldehydes were
shown to react with sodium hydride and 5-acetoxymethylpyrrole-2-carbaldehydes
in DMF to give the crucial neo-confused dipyrrolic dialdehyde intermediates.
MacDonald “2 + 2” condensation of the dialdehydes with
a dipyrrylmethane afforded a dihydroporphyrinoid, and subsequent oxidation
with 0.2% aqueous ferric chloride generated a series of fully conjugated
neo-confused porphyrins. Unusual dihydroporphyrin byproducts were
also identified. Reaction of neo-confused porphyrins with nickel­(II)
or palladium­(II) acetate in refluxing acetonitrile gave excellent
yields of the corresponding organometallic derivatives. Proton NMR
spectroscopy demonstrates that the diatropic character of this system
is diminished compared to regular porphyrins, although neo-confused
porphyrins retain porphyrin-like UV–vis spectra. Protonation
led to the sequential formation of mono- and dicationic species. Proton
NMR spectra for the dications showed the presence of enhanced diamagnetic
ring currents.