Photocatalytic synthesis of 10-phenanthrenols via intramolecular cycloaromatization under oxidant-free conditions

A single cobaloxime-catalyzed acceptorless dehydrogenative cyclization of ortho biaryl-appended 1,3-dicarbonyl compounds was developed. In contrast to the well-established preparation of these compounds via dehydrogenative benzannulation pathways, our method does not require any bases or oxidants, affording a novel cyclization strategy with an extremely high atom economy for the synthesis of 10-phenanthrenols. Preliminary mechanistic studies revealed that photocycloaddition occurred initially by photoirradiation and the subsequent dehydrogenative aromatization proceeded efficiently, promoted by both light and cobaloxime. Hydrogen gas is released as the only by-product during the reaction.