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

Abstract

In DMF, the oxidative addition of PhI to Pd0(PPh3)4 or to the anionic Pd0(PPh3)3(OAc)- is slower in the presence of an alkene (styrene, methyl acrylate). Indeed, the concentration of the reactive Pd0(PPh3)2 or Pd0(PPh3)2(OAc)- complex decreases because of its coordination to the alkene to form the unreactive (η2-CH2CHR)Pd0(PPh3)2 (R = Ph, CO2Me) or (η2-CH2CHPh)Pd0(PPh3)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 Pd0 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−PdII 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.