Porphyrin on a half-shell! Synthesis and characterization of corannulenoporphyrins

An efficient synthetic method for preparing corannulenoporphyrins is described. Nitrocorannulene was reacted with ethyl isocyanoacetate in the presence of a phosphazene base to generate the pivotal corannulenopyrrole intermediate. Following cleavage of the ethyl ester moiety with KOH in ethylene glycol at 180-190 degrees C, the heptacyclic system was reacted with acetoxymethylpyrroles under mildly acidic conditions to afford tripyrranes. The proton NMR spectra for these tripyrrolic intermediates suggest that they can take on helical geometries, but the conformations are dependent on the nature of the terminal ester groupings and may be altered by solvent interactions. Treatment of a di-tert-butyl ester tripyrrane with TFA cleaved the protective groups, and subsequent condensation with diformylpyrroles in TFA-CH(2)Cl(2), followed by oxidation with DDQ or ferric chloride, gave excellent yields of corannulenoporphyrins. Nickel(II), copper(II), and zinc complexes of this system were also prepared, and the nickel derivative was also further characterized by X-ray crystallography. The same synthetic strategy was also used to prepare a porphyrin with fused acenaphthylene and corannulene subunits. The fused bowl-shaped corannulene provides these porphyrins with a unique structural component that increases solubility by reducing pi-pi stacking interactions.