Benzoate Cyclometalation Enables Oxidative Addition of Haloarenes at a Ru(II) Center

The first Ru­(II)-catalyzed arylation of substrates without a directing group was recently developed. Remarkably, this process only worked in the presence of a benzoate additive, found to be crucial for the oxidative addition step at Ru­(II). However, the exact mode of action of the benzoate was unknown. Herein, we disclose a mechanistic study that elucidates the key role of the benzoate salt in the C–H arylation of fluoroarenes with aryl halides. Through a combination of rationally designed stoichiometric experiments and DFT studies, we demonstrate that the aryl–Ru­(II) species arising from initial C–H activation of the fluoroarene undergoes cyclometalation with the benzoate to generate an anionic Ru­(II) intermediate. The enhanced lability of this intermediate, coupled with the electron-rich anionic Ru­(II) metal center renders the oxidative addition of the aryl halide accessible. The role of an additional (NMe4)­OC­(CF3)3 additive in facilitating the overall arylation process is also shown to be linked to a shift in the C–H pre-equilibrium associated with benzoate cyclometalation.