Dual-role catalysis of iridium in photo-irradiation synthesis of 9-fluorenone through intramolecular cyclization via hydrogen evolution

Abstract

A photoredox catalyzed synthesis of 9-fluorenone from [1,1′-biphenyl]-2-carboxylic acid with 2-arylbenzo[d]oxazolyl based iridium complexes as dual-purpose catalysts has been developed. An unusual hydrogen evolution reaction accompanying the formation of 9-fluorenone has been identified firstly. The amount of the formed hydrogen was proportional to 9-fluorenone, which made the reaction atom economic. Combined with DFT calculation, a ligand dissociation process of the iridium catalyst has been put forward to explain the dehydrogenation. The catalyst [Ir(Fmpbo)₂dtbppy]PF₆ played a dual role as both the photoredox catalyst to initiate the benzoyl radical reaction and the TM catalyst to finish the aromatization and the hydrogen evolution. The photoredox/TM dual catalytic role of the Ir catalyst was proposed firstly. The reactions provide good to excellent yields of the products under mild and economic conditions.