Weakly nucleophilic potassium aryltrifluoroborates in palladium-catalyzed Suzuki-Miyaura reactions? Relative reactivity of K[4-RC₆F₄BF₃] and the role of silver-assistance in acceleration of transmetallation

Small differences in the reactivity of weakly nucleophilic potassium aryltrifluoroborates are revealed in the silver-assisted Pd-catalyzed cross-coupling of K[4-RC₆F₄BF₃] (R = H, Bu, MeO, EtO, PrO, iPrO, BuO, t-BuO, CH₂=CHCH₂O, PhCH₂O, PhCH₂CH₂O, PhO, F, pyrazol-1-yl, pyrrol-1-yl, and indol-1-yl) with ArX (4-BrC₆H₄CH₃, 4-IC₆H₄F and 3-IC₆H₄F). An assumed role of silver(I) compounds Ag_mY (Y = O, NO₃, SO₄, BF₄, F) consists in polarization of the Pd-X bond in neutral complex ArPdL_nX with the generation of the related transition state or formation of [ArPdL_n][XAg_mY] with a highly electrophilic cation and subsequent transmetallation with the weakly nucleophilic borate. Efficiency of Ag_mY as a polarizing agent decreases in order Ag₂O > AgNO₃ ≈ Ag₂SO₄ > Ag[BF₄] > AgF. No clear correlation between the reactivity of K[4-RC₆F₄BF₃] and substituent electron parameters, σ_I and σ_R °, of the aryl group 4-RC₆F₄ was found.