Palladium in Heterocyclic Chemistry 29Finally, reductive elimination forms the carbon–carbon or carbon–heteroatom bond and eliminates Pd(0), which then
returns to the oxidative addition step, completing the catalytic cycle.
Mechanisms of other reactions
The Heck reaction, and carbonylation, also proceed via an initial oxidative addition but this is followed by insertion
of the palladium–carbon bond into an alkene, or carbon monoxide, followed by elimination of HPdX or nucleophilic
attack at CO, respectively. HPdX breaks down in the presence of base to give back Pd(0).
Reactions involving electrophilic palladation
Pd(II) compounds are electrophilic species that can react with electron-rich systems, such as particular fi ve-membered
heterocycles or carboaromatics, with replacement of hydrogen. These are just standard aromatic electrophilic substitutions,
as occur, for example with Hg(II) compounds or bromine.
It is immediately obvious that the product of such a substitution is exactly the same as the product of oxidative insertion
of Pd(0) into ArX, and can therefore undergo the same transformations. However, although reactions using stoichio-
metric amounts of palladium are successful, catalytic processes using this approach cannot generally be carried out, as