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

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.