Synthesis of fluorinated benzophenones, xanthones, acridones, and thioxanthones by iterative nucleophilic aromatic substitution

Fluorination of fluorophores can substantially enhance their photostability and improve spectroscopic properties. To facilitate access to fluorinated fluorophores, bis(2,4,5-trifluorophenyl)methanone was synthesized by treatment of 2,4,5-trifluorobenzaldehyde with a Grignard reagent derived from 1-bromo-2,4,5-trifluorobenzene, followed by oxidation of the resulting benzyl alcohol. This hexafluorobenzophenone was subjected to sequential nucleophilic aromatic substitution reactions, first at one or both of the more reactive 4,4'-fluorines, and second by cyclization through substitution of the less reactive 2,2'-fluorines, using a variety of oxygen, nitrogen, and sulfur nucleophiles, including hydroxide, methoxide, amines, and sulfide. This method yields symmetrical and asymmetrical fluorinated benzophenones, xanthones, acridones, and thioxanthones and provides scalable access to known and novel precursors to fluorinated analogues of fluorescein, rhodamine, and other derivatives. Spectroscopic studies revealed that several of these precursors are highly fluorescent, with tunable absorption and emission spectra, depending on the substituents. This approach should allow access to a wide variety of novel fluorinated fluorophores and related compounds.