In palladium-catalyzed Heck, Stille, or Sonogashira reactions, the nucleophiles-respectively, alkenes, vinylstannanes, or alkynes-are involved in carbopalladation or transmetallation steps that follow the oxidative addition of an aryl halide or triflate to a Pd0 complex. As soon as the nucleophiles possess a C=C or a C 'triple-bond' C bond capable of coordinating the Pd0 complex active in the oxidative addition, they play a dual role since they interfere in the oxidative addition by a decelerating effect due to a partial coordination of the active Pd0 complex. Indeed, its concentration decreases due to the formation of either unreactive complexes (η2-RCH=CH2)Pd0L2 (R = Ph, CO2Et, L = PPh3 ; R = Bu3Sn, L = AsPh3), (η2-RCH=CH2)Pd0L2(OAc)- (R = Ph, L = PPh3), (η2-R-C 'triple-bond' CH)Pd0L2 (R = Ph, L = PPh3) or more slowly reactive complexes (η2-R-C 'triple-bond' CH)Pd0L2 (R = CO2Et, L = PPh3), (η2-RCH=CH2)Pd0L2 (R= CO2Me, L2 = dppf). Whenever the oxidative addition is faster than the ensuing carbopalladation or transmetallation, the decelerating effect of the nucleophiles in the oxidative addition is in favor of a better efficiency for the catalytic cycle by bringing the rate of the fast oxidative addition closer to that of the slow carbopalladation or transmetallation steps.
