Tetraaryldimethoxybenziporphyrins. At the edge of carbaporphyrinoid aromaticity

A series of eight dimethoxybenziporphyrins were prepared in three steps from 1,3-dimethoxybenzene or 2,6-dimethoxytoluene. Dibromination, followed by lithium-halogen exchange and reaction with benzaldehyde gave dicarbinol intermediates. These reacted with pyrrole and aryl aldehydes in the presence of BF3.Et2O in chloroform, followed by oxidation with DDQ, to give the benziporphyrins in 15-25% yield. These compounds readily gave nickel(II) and palladium(II) organometallic derivatives and could be selectively reduced with sodium borohydride to give unstable benziphlorins. Regioselective oxidation with silver acetate afforded the related 22-acetoxybenziporphyrins in 52-64% yield. The dimethoxybenziporphyrins showed chemical shifts by proton NMR spectroscopy that were consistent with weakly diatropic macrocycles. However, addition of TFA gave dications that showed far more significant shifts that are attributed to the presence of a more substantial diatropic ring current. The internal CH for 11H2(2+) was observed at 3.5 ppm, but this effect was diminished for the 3-methylbenziporphyrins 12H2(2+) where this resonance appears at 4.7 ppm. Even in the absence of the methoxy substituents, the dication derived from tetraphenylbenziporphyrin 8H2(2+) shows an upfield shift for this resonance to 5.5 ppm. The dications of the 22-acetoxybenziporphyrins also show similar effects despite the presence of an internal ester moiety. These results demonstrate that a spectrum of diatropic character can manifest even in highly crowded benziporphyrin derivatives.