Palladium-CatalyzedOrtho-Selective CF Activation of Polyfluoroarenes with Triethylsilane: A Facile Access to Partially Fluorinated Aromatics

Owing to the unique characteristics of fluorine that often lead to profound changes in physical, chemical, and biological properties of organic molecules when it is incorporated, partially fluorinated aromatics play an important role in life and materials sciences. However, synthetic access to these fluorinated compounds is difficulties and there is only limited commercial availability of fluoroaromatic sources. One attractive approach to partially fluorinated aromatics is selective substitution of polyfluoroarenes through C F bond activation, as polyfluoroarenes are more readily available and cheaper than their mixed halo or organometallic counterparts. In the past few years, important progress has been made in this field, and the reduction of C F bonds (hydrodefluorination, HDF) in polyfluoroarenes has become a useful approach to access partially fluorinated aromatics that are difficult to obtain otherwise. However, compared to some aromatic nucleophilic substitution (SNAr) reactions of polyfluoroarenes that can regioselectively lead to substituted polyfluoroarenes with strong nucleophiles, the transitionmetal-catalyzed chemoand regioselective transformations of C F bonds remain a challenge owing to the robustness of the C F bond and difficulty in adjusting reaction selectivity of different C F bonds on an aromatic ring. Although examples of selective transition-metal-catalyzed HDF of polyfluoroarenes have been reported, most of them focus on the mechanistic understanding of the HDF cycle. In particular, for the palladium-catalyzed HDF reaction, only hydrodefluorination of pentafluoropyridine at the para position has been reported to date, in which palladium fluoro or palladium hydrido complexes were used as precatalysts. Therefore, developing a new simple catalytic system with broad substrate scope and high regioselectivity for widespread synthetic applications is appealing. Azine/diazine (e.g., pyridine, quinoline, quinoxaline) substituted fluorinated aromatics bearing a hydride ortho to the heteroaryl group are an important structural motif in light-emitting devices and photocatalysts. However, the limited synthetic methods and commercial availability of fluoroaryl patterns significantly limits the structural and functional diversity of this structural motif in further applications. Continuing our study in palladium-catalyzed polyfluoroarene chemistry, herein, we describe the first example of palladium-catalyzed, chelation-assisted ortho-selective C F activation of polyfluoroarenes with triethylsilane, in which N-containing heterocycles were employed as directing groups. This method provides a useful and facile access to a wide range of azine/diazine substituted fluoroaromatics that are difficult to synthesize otherwise. We began this study on the basis of the hypothesis that with the aid of a directing group, such as N-containing heterocycles, ortho to the fluorine to chelate and direct delivery of palladium catalyst, the oxidative addition of the C F bond to Pd would be facilitated. Subsequently, the Pd-catalyzed ortho-selective hydrodefluorination of polyfluoroarenes would be possible in the presence of a reductant, such as silane (Scheme 1). The formation of thermostable Si F bond between the newly formed Pd F complex (I or II) and silane 2 would be a driving force to promote the generation of palladium hydrido complex (III or IV) that would release ortho hydrodefluorinated aromatics V after reductive elimination.