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-CH2
CHR)Pd0(PPh3)2
(R = Ph, CO2Me) or (η2-CH2
CHPh)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.
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