Visible Light-Promoted Metal-Free C-H Activation: Diarylketone-Catalyzed Selective Benzylic Mono- and Difluorination

We report herein an operationally simple method for the direct conversion of benzylic C H groups to C F. We show that visible light can activate diarylketones to abstract a benzylic hydrogen atom selectively. Adding a fluorine radical donor yields the benzylic fluoride and regenerates the catalyst. The selective formation of mono- and difluorination products can be achieved by catalyst control. 9-Fluorenone catalyzes benzylic C-H monofluorination, while xanthone catalyzes benzylic C H difluorination. The scope and efficiency of this new C H fluorination method are significantly better than those of the existing methods. This is also the first report of selective C H gem-difluorination.