Mechanism of palladium-catalyzed Suzuki-Miyaura reactions: multiple and antagonistic roles of anionic "bases" and their countercations.

In Suzuki-Miyaura reactions, anionic bases F(-) and OH(-) (used as is or generated from CO3(2-) in water) play multiple antagonistic roles. Two are positive: 1) formation of trans-[Pd(Ar)F(L)2] or trans-[Pd(Ar)-(L)2(OH)] (L = PPh3) that react with Ar'B(OH)2 in the rate-determining step (rds) transmetallation and 2) catalysis of the reductive elimination from intermediate trans-[Pd(Ar)(Ar')(L)2]. Two roles are negative: 1) formation of unreactive arylborates (or fluoroborates) and 2) complexation of the OH group of [Pd(Ar)(L)2(OH)] by the countercation of the base (Na(+), Cs(+), K(+)).