Decelerating Effect of Alkenes in the Oxidative Addition of Phenyl Iodide to Palladium(0) Complexes in Heck Reactions

In DMF, the oxidative addition of PhI to Pd 0 (PPh 3 ) 4 or to the anionic Pd 0 (PPh 3 ) 3 (OAc) - is slower in the presence of an alkene (styrene, methyl acrylate). Indeed, the concentration of the reactive Pd 0 (PPh 3 ) 2 or Pd 0 (PPh 3 ) 2 (OAc) - complex decreases because of its coordination to the alkene to form the unreactive ( η 2 -CH 2 ═CHR)Pd 0 (PPh 3 ) 2 (R = Ph, CO 2 Me) or ( η 2 -CH 2 ═CHPh)Pd 0 (PPh 3 ) 2 (OAc) -, respectively. As already evidenced in palladium-catalyzed Stille reactions, this work establishes that, in palladium-catalyzed Heck reactions as well, the nucleophile plays a role in the kinetics of the oxidative addition (decelerating effect), as soon as it may coordinate Pd 0 complexes. This is an essential observation, in view of the general belief that the nucleophile enters the catalytic cycle only at the stage of the attack on the aryl−Pd II complex formed in the oxidative addition. Whenever the oxidative addition is not rate determining, the decelerating effect of the alkene on this reaction is in favor of a higher efficiency of the catalytic cycle.