Weakly nucleophilic potassium aryltrifluoroborates in palladium-catalyzed Suzuki–Miyaura reactions: relative reactivity of K[4-RC(6)F(4)BF(3)] 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-RC(6)F(4)BF(3)] (R = H, Bu, MeO, EtO, PrO, iPrO, BuO, t-BuO, CH(2)=CHCH(2)O, PhCH(2)O, PhCH(2)CH(2)O, PhO, F, pyrazol-1-yl, pyrrol-1-yl, and indol-1-yl) with ArX (4-BrC(6)H(4)CH(3), 4-IC(6)H(4)F and 3-IC(6)H(4)F). An assumed role of silver(I) compounds Ag(m)Y (Y = O, NO(3), SO(4), BF(4), F) consists in polarization of the Pd–X bond in neutral complex ArPdL(n)X with the generation of the related transition state or formation of [ArPdL(n)][XAg(m)Y] with a highly electrophilic cation and subsequent transmetallation with the weakly nucleophilic borate. Efficiency of Ag(m)Y as a polarizing agent decreases in order Ag(2)O > AgNO(3) ≈ Ag(2)SO(4) > Ag[BF(4)] > AgF. No clear correlation between the reactivity of K[4-RC(6)F(4)BF(3)] and substituent electron parameters, σ(I) and σ(R)°, of the aryl group 4-RC(6)F(4) was found.