Figure 1. (a) Proposed photocatalytic cycle for HDF. (b) Reduction potentials and density functional theory (DFT) computed electrostatic surface potential maps (inset) of Py and FA.
Electron transfer (ET) plays a critical role in many organic transformations. In photoredox catalysis, the ET usually follows light-induced non-adiabatic outer-sphere ET mode, which requires suitable overpotential to achieve efficient ET kinetics. Inner-sphere ET, on the other hand, can be achieved adiabatically between closely bind species, resulting in significantly faster ET than that predicted by the outer-sphere mode due to strong electronic coupling. It can be envisioned that fast inner-sphere ET can be achieved when photocatalyst closely binds with substrate, which, however, has not been developed yet.
Polyfluoroarene–arene (also known as “π-hole–π”) interaction is a directional and non-covalent intermolecular force, attributed to the weak electrostatic interaction between polyfluoroarenes (positive surface potential due to the flipped quadruple moment) and arenes (negative surface potential). Dr. Zhang and co-workers discovered that light-induced efficient inner-sphere ET between FA and photocatalyst can be achieved when photocatalyst is an electron-rich arene (for example, Py), owing to the formation of “π-hole–π” complex between FA and photocatalyst (Figure 1a). More importantly, this process proceeds smoothly against a large underpotential (Figure 1a) and subsequently promotes a HDF reaction to yield partially fluorinated arenes (Figure 1b).
The formation of “π-hole–π” complexes were proved by 1H NMR titration and were further confirmed by DFT-based structure optimization. Furthermore, the stronger electrostatic interaction in “π-hole–π” complexes than the typical π–π stacking force was revealed by the average interplane distances in single crystal analysis. The team applied the optimized reaction condition to a series of FA and successfully access to the HDF products with good to excellent yield up to 93%. They also demonstrated the potential utility of Py in the metal-free C-F reductive alkylation. This work points to the further development of the design paradigm for photoredox catalysis where the size and shape of photocatalyst can be fine-tuned to enhance the overall catalytic activity. This work also constitutes a new example of the utility of weak, non-covalent interaction in small molecule catalysis.This work was just accepted by JACS.
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