Direct Excitation of Aldehyde to Activate the C(sp2)−H Bond by Cobaloxime Catalysis toward Fluorenones Synthesis with Hydrogen Evolution

Abstract

AbstractA new way to form fluorenones via the direct excitation of substrates instead of photocatalyst to activate the C(sp2)−H bond under redox‐neutral condition is reported. Our design relies on the photoexcited aromatic aldehyde intermediates that can be intercepted by cobaloxime catalyst through single electron transfer for following β‐H elimination. The generation of acyl radical and successful interception by a metal catalyst cobaloxime avoid the use of a photocatalyst and stoichiometric external oxidants, affording a series of highly substituted fluorenones, including six‐membered ketones, such as xanthone and thioxanthone derivatives in good to excellent yields, and with hydrogen as the only byproduct. This catalytic system features a readily available metal catalyst, mild reaction conditions and broad substrate scope, in which sunlight reaction and scale‐up experiments by continuous‐flow approach make the new methodology sustainable and amenable for potentially operational procedures.