Synthesis of para- or ortho-Substituted Triarylbilindiones and Tetraarylbiladienones by Coupled Oxidation of Tetraarylporphyrins

Coupled oxidation of [tetraarylporphyrinato]iron (III) chloride carrying substituents in the ortho or para positions was performed by allowing the iron porphyrin to react with dioxygen, ascorbic acid, and pyridine to give biladienone as the major product and bilindione as a minor one. Efforts to find reaction conditions and workup procedures to obtain bilindione improved the yields of triarylbilindiones ranging between 2% and 19%. Electron-withdrawing substituents in the parts position on the aryl groups increased the selectivity of bilindione relative to biladienone: the isolated yields of bilindione and biladienone were 2% and 85% (OMe), 6% and 44% (COOMe), and 7% and 28% (CN), respectively. Electronic effects of substituents affected both isolation procedures and the spectroscopic properties of bilindiones. Tri(4-methoxyphenyl)bilindione showed a red-shifted electronic absorption compared to unsubstituted and 4-methoxycarbonyl substituted analogues. This was ascribed to the destabilization of the HOMO-1 level by the methoxy groups.