Dual role of nucleophiles in palladium-catalyzed Heck, Stille, and Sonogashira reactions

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 triffate to a Pd-0 complex. As soon as the nucleophiles possess a C = C or a C drop C bond capable of coordinating the Pd-0 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 Pd-0 complex. Indeed, its concentration decreases due to the formation of either unreactive complexes (eta(2)-RCH=CH2)(PdL2)-L-0 (R = Ph, CO2Et, L = PPh3; R = Bu3Sn, L = AsPh3), (eta(2)-RCH = CH2)(PdL2)-L-0(OAc)(-) (R = Ph, L = PPh3), (eta(2)-R-C drop CH)(PdL2)-L-0 (R = Ph, L = PPh3 or more slowly reactive complexes (eta(2)-R-C drop CH)(PdL2)-L-0 (R = CO2Et, L = PPh3), (eta(2)-RCH = CH2)(PdL2)-L-0 (R = CO2Me, L-2 = 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.