Efficient Synthesis of Anthraquinones from Diaryl Carboxylic Acids via Palladium(II)-Catalyzed and Visible Light-Mediated Transformations

Abstract

Irradiation of 9-ester-substituted anthracenes with visible light results in the formation of endoperoxides in the absence of a photocatalyst, which further undergo base-assisted fragmentation to afford anthraquinones. The excited state species of anthracene generated by energy transfer, interacts with O-3(2) to afford O-1(2) by energy transfer and undergoes cycloaddition with O-1(2). By employing palladium(II)-catalyzed and visible light-mediated transformations, we have developed an efficient synthetic protocol for accessing diverse anthraquinones from readily available diaryl carboxylic acids. The optimal result was obtained with palladium(II) acetate, Ac-Ile-OH, benzoquinone and potassium carbonate in tert-amyl alcohol under O-2 at 90 degrees C with irradiation from a 30 W fluorescent light bulb.