Efficient Fluoren-9-ones Construction through CO/I Exchange of Diaryliodonium Salts

Fluoren-9-ones derivatives have attracted much attention due to their extensively applications in pharmaceuticals, natural products and photoelectric materials. In recent decades, C-H bond functionalization is the most powerful method to access fluorenone skeleton. Although these interesting studies exploited highly efficient routes to the fluoren-9-one, in many examples, it is easy to produce two isomers in the meta-substituted substrates because of the existence of two different C-H bonds in the ortho-position. It is still indispensable to develop efficient methods for fluoren-9-ones construction. Diaryliodonium salt as a stable and easily prepared reagent reported by Hartmann and Meyer since 1894, which has been one of the most efficient arylation reagents in organic synthesis. Generally, diaryliodonium salt was employed as a single arylation reagent. In the past few years, the both aryl employments of diaryliodonium salt were explored due to the improvement of atom economy. Recently, we developed the atom exchange reactions of intramolecular and intermolecular diaryliodonium salts for sulfide, selenide, sulfone, acridine and carbazole constructions, which could employ both aryl groups of diaryliodonium salt. Continuous with our research of using such atom exchange method for significant molecular construction, herein, a CO/I exchange method of diaryliodonium salts with carbon monoxide was developed for construction of functional fluoren-9-ones. Both aryl groups in diaryliodonium salt were fully exerted in this transformation, which proceeded smoothly in a CO balloon atmosphere to afford the desired products in moderate to excellent yields with good functional-group compatibility. Note that this protocol avoided the emerging of isomers, which were easy to be formed in C-H bond functionalization method. A representative procedure for this reaction is as following:Under a CO atmosphere, Pd(OAc)₂ (0.01 mmol), dppf (0.012 mmol), K₃PO₄ (0.2 mmol), diaryliodonium salts 1 and toluene (1 mL) were added to a flame-dried Schlenk tube. The resulting mixture was stirred at 80℃ for 24 h. Water (5 mL) was added and the solution was extracted with ethyl acetate, organic layers were combined, dried over sodium sulfate. After evaporation of solvent, the residue was purified by column chromatography to give the corresponding products.