Conjugated macrocycles related to the porphyrins. Part 18: Synthesis and spectroscopic characterization of electron-rich benzi- and oxybenziporphyrins: influence of steric and electronic factors on porphyrinoid aromaticity

Abstract

Although 4,6-dihydroxyisophthalaldehydes failed to condense with tripyrranes to produce porphyrinoid products, the related dimethoxydialdehydes reacted with tripyrrane 9 to give, following oxidation with aqueous ferric chloride solution, nonaromatic dimethoxybenziporphyrins 20 in excellent yields. In the presence of TFA, dications were generated that showed weakly diatropic properties by proton NMR spectroscopy. This observation was attributed to the ability of the methoxy units to facilitate charge delocalization. Treatment of 20 with 50 equiv. of boron tribromide cleaved one of the methyl ethers to produce the related methoxyoxybenziporphyrins 24. These porphyrinoids showed strong diamagnetic ring currents due to the presence of 18 π-electron delocalization pathways within these structures. On treatment with TFA, the resulting dications show significantly reduced ring currents although their diatropic character is retained to a far greater extent than when the methoxy substituents are absent. For both 20 and 24, the methoxy groups are more effective at charge delocalization, which results in increased diatropicity, when there is no adjacent alkyl substituent. Treatment of 20a with HBr in refluxing acetic acid cleaved both methyl ethers to give the highly insoluble hydroxyoxybenziporphyrin 17. Although the spectroscopic properties of the free base could not be assessed, this compound was soluble in TFA–chloroform and the resulting dication proved to be highly aromatic as judged by NMR and UV–vis spectroscopy. Treatment of the 20b with HBr–acetic acid again cleaved both methyl ethers but also gave an unexpected oxidation to give a diketone bearing a 3° alcohol unit. This novel porphyrinoid retains an 18 π-delocalization pathway, shows porphyrin-like UV–vis spectra and is highly diatropic as the free base and in protonated form as judged by proton NMR spectroscopy.