Mild and Efficient Ni-Catalyzed Biaryl Synthesis with Polyfluoroaryl Magnesium Species: Verification of the Arrest State, Uncovering the Hidden Competitive Second Transmetalation and Ligand-Accelerated Highly Selective Monoarylation

Employing a nickel
catalyst and electron-deficient polyfluoroaryl
magnesium species, a highly selective monoarylation of polyfluoroarenes
containing multiple identical coupling sites has been achieved for
the first time, which represents a long-standing problem due to the
competitive reactivity between the desired products and the starting
polyfluoroarenes. Because of the negative fluorine effect, a surprisingly
stable cis [Ni­(ArF4)2(DPEPhos)]
species 4 (ArF4 = 2,3,5,6-tetrafluorophenyl)
confirmed by X-ray crystallography is isolated, which acts as catalyst
arrest state as proven by a thermal decomposition test. Further retro-transmetalation
experiments uncover a hidden secondary transmetalation between ArF4-Ni-Ph and excess ArF4-MgCl that competes with
the desired but reluctant reductive elimination at the high-valent
nickel center. Accordingly, through the cooperation of newly developed
DMM-DPEPhos, and a dioxane-mediated Schlenk equilibrium with Grignard
reagent, the formation of the corresponding arrest state is remarkably
inhibited. An excellent coupling efficiency and an excellent monoarylation
selectivity are therefore generally accomplished with a widespread
electrophile scope and good functional group tolerance under mild
conditions. Importantly, our novel method shows great power in the
gram-scale synthesis of thienyl-2,3,5,6-tetrafluorophenyl units that
represent key components in materials science.