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.