Pure and Applied Chemistry

Abstract: 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.