“π-Hole−π” Interaction Promoted Photocatalytic Hydrodefluorination via Inner-Sphere Electron Transfer

Abstract

We describe a metal-free, photocatalytic hydrodefluorination (HDF) of polyfluoroarenes (FA) using pyrene-based photocatalysts (Py). The weak "π-hole-π" interaction between Py and FA promotes the electron transfer against unfavorable energetics (ΔG up to 0.63 eV) and initiates the subsequent HDF. The steric hindrance of Py and FA largely dictates the HDF reaction rate, pointing to an inner-sphere electron transfer pathway. This work highlights the importance of the size and shape of the photocatalyst and the substrate in controlling the electron transfer mechanism and rates as well as the overall photocatalytic processes.

Hydrogen Bond Directed Photocatalytic Hydrodefluorination: Overcoming Electronic Control

“…Second, given that the reaction is triggered by an electron transfer, it is likely that there are numerous other catalysts which could facilitate the photo-HDF reaction and relieve the iridium issue altogether. 15 Until that is demonstrated, we wanted to probe the robustness of the photocatalytic system for this reaction (eqn 12). In the standard reaction conditions, we used relatively low catalyst loadings (0.25 mol%), although presumably much lower Ir-loading would be needed in order for the process to be amenable to scaling.…”

Transition‐Metal‐Free Catalytic Hydrodefluorination of Polyfluoroarenes by Concerted Nucleophilic Aromatic Substitution with a Hydrosilicate

“…[1,2] Fluoroarenes have been synthesized by two complementary approaches: the introduction of af luorine atom into an aromatic ring, or the substitution of af luorine atom of polyfluoroarenes by other atoms/groups.Aryl fluorides have been obtained by the former approach through various fluorination methods, including transition-metal-catalyzed C À Fb ond formation through the substitution of heteroatoms and directinggroup-assisted C À Hb ond activation, [3] both of which are alternatives to more traditional methods,s uch as the Balz-Schiemann reaction [4] and the halex process. The second approach, that is,the substitution of afluorine atom of polyfluoroarenes by other atoms/groups,i ncluding nucleophilic substitution and the activation of CÀFb onds in polyfluoroarenes, [6][7][8][9][10][11] circumvents these obstacles.H ydrodefluorination (HDF), that is,t he substitution of af luorine by ah ydrogen atom, should be ap romising approach for the synthesis of polyfluorinated building blocks from readily available fluorinated bulk chemicals. The second approach, that is,the substitution of afluorine atom of polyfluoroarenes by other atoms/groups,i ncluding nucleophilic substitution and the activation of CÀFb onds in polyfluoroarenes, [6][7][8][9][10][11] circumvents these obstacles.H ydrodefluorination (HDF), that is,t he substitution of af luorine by ah ydrogen atom, should be ap romising approach for the synthesis of polyfluorinated building blocks from readily available fluorinated bulk chemicals.…”

“…[5] These reactions,h owever, require the preinstallation of heteroatoms or directing groups,which renders polyfluorination difficult. [7][8][9][10][11][12] Substantial effort has been devoted to the development of transition-metal-catalyzed HDF reactions involving the activation of inert C À Fb onds ( Figure 1a). [7][8][9][10][11][12] Substantial effort has been devoted to the development of transition-metal-catalyzed HDF reactions involving the activation of inert C À Fb onds ( Figure 1a).…”

“…[7][8][9][10][11][12] Substantial effort has been devoted to the development of transition-metal-catalyzed HDF reactions involving the activation of inert C À Fb onds ( Figure 1a). For catalytic HDF reactions,e lectron-rich late transition metals have been combined with fluorophilic reductants,s uch as hydrosilanes.Recently,photocatalytic HDF reactions with an Ir complex [9] or ap yrene derivative [10] with diisopropylethylamine as the hydride source have been reported, for which high catalytic turnover numbers (TONs) were achieved owing to the low propensity to form deactivating M À Fb onds.A reaction with triethylphosphine has also been reported, although high temperatures and long reaction times were required. For catalytic HDF reactions,e lectron-rich late transition metals have been combined with fluorophilic reductants,s uch as hydrosilanes.Recently,photocatalytic HDF reactions with an Ir complex [9] or ap yrene derivative [10] with diisopropylethylamine as the hydride source have been reported, for which high catalytic turnover numbers (TONs) were achieved owing to the low propensity to form deactivating M À Fb onds.A reaction with triethylphosphine has also been reported, although high temperatures and long reaction times were required.…”

Photocatalytic Hydrodehalogenation for the Removal of Halogenated Aromatic Contaminants

“…Reduction of chlorinated and fluorinated arenes is more challenging due to the smaller thermodynamic driving force, as well as the sluggish kinetics of bond cleavage after electron transfer . Noncovalent secondary interactions, such as an intermolecular halogen bond, π–π interaction, or intermolecular hydrogen bond can promote electron transfer and dehalogenation.…”