Mechanistic Origin of Antagonist Effects of Usual Anionic Bases (OH-, CO32-) as Modulated by their Countercations (Na+, Cs+, K+) in Palladium-Catalyzed Suzuki-Miyaura Reactions

The mechanism of the reaction of trans-ArPdBrL2 (Ar=p-Z-C6H4, Z=CN, H; L=PPh3) with Ar'B(OH)2 (Ar'=p-Z'-C6H4, Z'=H, CN, MeO), which is a key step in the SuzukiMiyaura process, has been established in N,N-dimethylformamide (DMF) with two bases, acetate (nBu4NOAc) or carbonate (Cs2CO3) and compared with that of hydroxide (nBu4NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M+ (e.g., M+OH-), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M+ (Na+, Cs+, K+) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L2, partly inhibiting its transmetalation with Ar'B(OH)2. A decreasing reactivity order is observed when M+ is associated with OH-: nBu4N+> K+> Cs+> Na+. Acetates lead to the formation of trans-ArPd(OAc)L2, which does not undergo transmetalation with Ar'B(OH)2. This explains why acetates are not used as bases in SuzukiMiyaura reactions that involve Ar'B(OH)2. Carbonates (Cs2CO3) give rise to slower reactions than those performed from nBu4NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH-. The mechanism of the reaction with carbonates is then similar to that established for nBu4NOH, involving ArPd(OH)L2 in the transmetalation with Ar'B(OH)2. Due to the low concentration of OH- generated from CO32- in water, both transmetalation and reductive elimination result slower than those performed from nBu4NOH at equal concentrations as Cs2CO3. Therefore, the overall reactivity is finely tuned by the concentration of the common base OH- and the ratio [OH-]/[Ar'B(OH)2]. Hence, the anionic base (pure OH- or OH- generated from CO32-) associated with its countercation (Na+, Cs+, K+) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L2, acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar'B(OH)3- and by complexation of ArPd(OH)L2 by M+.