Kinetic Data for the Transmetalation/Reductive Elimination in Palladium‐Catalyzed Suzuki–Miyaura Reactions: Unexpected Triple Role of Hydroxide Ions Used as Base

Abstract

The mechanism of the reaction of trans-[ArPdX(PPh(3))(2)] (Ar=p-Z-C(6)H(4); Z=CN, F, H; X=I, Br, Cl) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4); Z'=CN, H, OMe) has been established in DMF in the presence of the base OH(-) in the context of real palladium-catalyzed Suzuki-Miyaura reactions. The formation of the cross-coupling product ArAr' and [Pd(0)(PPh(3))(3)] has been followed through the application of electrochemical techniques. Kinetic data have been obtained for the first time, with determination of the observed rate constant, k(obs), of the overall reaction. trans-[ArPdX(PPh(3))(2)] is not reactive in the absence of the base. The base OH(-) plays three roles. It favors the reaction: 1) by formation of trans-[ArPd(OH)(PPh(3))(2)], a key complex which, in contrast to trans-[ArPdX(PPh(3))(2)], reacts with Ar'B(OH)(2) (rate-determining transmetalation), and 2) by unexpected promotion of the reductive elimination from the intermediate trans-[ArPdAr'(PPh(3))(2)], which generates ArAr' and a Pd(0) species. Conversely, the base OH(-) disfavors the reaction by formation of the unreactive anionic Ar'B(OH)(3)(-). As a consequence of these antagonistic effects of OH(-), the overall reactivity is controlled by the concentration of OH(-) and passes through a maximum as the concentration of OH(-) is increased. Therefore, the base favors the rate-determining transmetalation and unexpectedly also the reductive elimination.

Synthesis, spectral characteristics and electrochemistry of symmetrically substituted hybrids derived from 2,5-bis(4-bromophenyl)-1,3,4-oxadiazole under Suzuki cross-coupling reaction

“…The role of base in Suzuki cross-coupling reactions is still under investigation. 40 However, there is no doubt that it plays at least a dual action in the whole transformation. The base participates both in exchanging halogen ions in the molecule of the starting halide 3 at the surface of catalyst and in activating boronic acid 4 to facilitate transmetallation step, which requires increased amounts of this reagent.…”

Halide-Enhanced Catalytic Activity of Palladium Nanoparticles Comes at the Expense of Catalyst Recovery

“…Filtered seawater would constitute a readily available solvent if suitable. The reported rate-decreasing effect of added halide on palladium transmetallation reactions [3], however, would suggest that the salinity of seawater of approximately 600 mM would negatively affect reactivity. Our aim was therefore to study the effect of added chloride and bromide on the oxidative homocoupling reaction or arylboronic acids catalyzed by our polystyrene-polypyrrole-palladium nanocomposite.…”

“…Added halides often exert a rate-retarding effect on the SMXR and the OHR. This rate-retarding effect has been attributed to the formation of unreactive halo-palladium complexes [3]. The effects of added chloride can be more complicated [4,5] including as a result of leveling the energy profile around the catalytic cycle [6].…”

Palladium catalysed sequential imine arylation/Suzuki–Miyaura coupling: synthesis of α-(biarylyl)benzylamines

“…18,19 On the other hand, recent kinetic studies and extensive computational data suggest that the boronic acid should be activated by the base (forming a boronate) in order for the transmetalation to occur. 20,21 In our case, we experimentally used a variety of conditions, which included boronic acid reagents (together with a base), or NaPh 4 B (see Tables 1, 3 and 4).…”