Editing Heck reaction (section)

== Reaction mechanism ==
The mechanism of this [[vinylation]] involves [[organopalladium]] intermediates. The required palladium(0) compound is often generated [[in situ]] from a palladium(II) precursor.<ref>{{cite journal |author1=Ozawa, F. |author2=Kubo, A. |author3=Hayashi, T. | journal = [[Chemistry Letters]] |volume=21 | year = 1992 | pages = 2177–2180 | doi = 10.1246/cl.1992.2177 | title = Generation of Tertiary Phosphine-Coordinated Pd(0) Species from Pd(OAc)<sub>2</sub> in the Catalytic Heck Reaction | issue = 11}}</ref><ref>{{cite journal|last1=Bradshaw|first1=Michael|last2=Zou|first2=Jianli|last3=Byrne|first3=Lindsay|last4=Swaminathan Iyer|first4=K.|last5=Stewart|first5=Scott G.|last6=Raston|first6=Colin L.|author-link6=Colin L. Raston|title=Pd(II) conjugated chitosan nanofibre mats for application in Heck cross-coupling reactions|journal=[[Chem. Commun.]]|year=2011|volume=47|issue=45|pages=12292–12294|doi=10.1039/C1CC14717J|pmid=22011792}}</ref>

For instance, [[palladium(II) acetate]] is reduced by [[triphenylphosphine]] to bis(triphenylphosphine)palladium(0) ('''1''') concomitant with oxidation of triphenylphosphine to [[triphenylphosphine oxide]]. Step '''A''' is an [[oxidative addition]] in which palladium inserts itself in the aryl-bromide bond. The resulting palladium(II) complex then binds alkene ('''3'''). In step '''B''' the alkene inserts into the Pd-C bond in a [[syn addition]] step. Step '''C''' involves a [[beta-hydride elimination]] (here the arrows are showing the opposite) with the formation of a new palladium - alkene π complex ('''5'''). This complex is destroyed in the next step. The Pd(0) complex is regenerated by [[reductive elimination]] of the palladium(II) compound by [[potassium carbonate]] in the final step, '''D'''. In the course of the reaction the carbonate is stoichiometrically consumed and palladium is truly a catalyst and used in catalytic amounts. A similar palladium cycle but with different scenes and actors is observed in the [[Wacker process]].

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[[File:Heck Reaction Mechanism.svg|center|600px|Heck Reaction Mechanism]]
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! Heck Reaction Mechanism
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This cycle is not limited to vinyl compounds, in the [[Sonogashira coupling]] one of the reactants is an [[alkyne]] and in the [[Suzuki coupling]] the alkene is replaced by an aryl [[boronic acid]] and in the [[Stille reaction]] by an aryl [[stannane]]. The cycle also extends to the other [[group 10 element]] [[nickel]] for example in the [[Negishi coupling]] between aryl halides and organozinc compounds. Platinum forms strong bonds with carbon and does not have a catalytic activity in this type of reaction.