adj-Dicarbaporphyrinoid Systems: Synthesis, Spectroscopic Characterization, and Reactivity of 23-Carbabenziporphyrins.

A new family of adj-dicarbaporphyrinoids has been prepared using the "2 + 2" MacDonald methodology. Dibutylboron triflate catalyzed condensation of 3-iodo-4-methoxybenzaldehyde with an indene enamine afforded an iodofulvene aldehyde, and a related dimethoxyfulvene was similarly prepared in two steps from 2,4-dimethoxybenzaldehyde. Following protection as the corresponding dimethyl acetals, the iodofulvenes were metalated with Bu3MgLi at -100 °C and reacted with dimethylformamide to give the required fulvene dialdehyde intermediates. Acid-catalyzed condensation with three different dipyrrylmethanes afforded a series of benzo-23-carbabenziporphyrins in 52-70% yields. The proton nuclear magnetic resonance spectra for these adj-dicarbaporphyrinoids indicate that these macrocycles are slightly diatropic. Monoprotonation afforded cationic species with slightly larger aromatic ring currents, and under strongly acidic conditions, C-protonated dications were generated with substantial diatropic properties. The aromatic character of these structures was supported by nucleus-independent chemical shifts and anisotropy of induced current density calculations. The computational results indicate that the dications favor 23-atom 22π electron delocalization pathways. The benzo-23-carbabenziporphyrins were selectively oxidized with silver(I) acetate in dichloromethane-methanol to give stable nonaromatic structures with two additional methoxy substituents connected to sp3 hybridized bridging carbons. The intriguing reactivity and unique spectroscopic properties of benzo-23-carbabenziporphyrins make these novel structures promising candidates for further investigations.