Palladium! 11 , addition with nucleophiles electronic effects

Palladium (II) compounds coordinate to alkenes to form 71-complexes. Roughly speaking, the decrease of alkene electron density caused by coordination to an electrophilic Pd(II) compound enables an attack by nucleophiles on the coordinated alkenes. The attack of a nucleophile with concomitant formation of a carbon-palladium e-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. Unlike the products of mercuration which are stable and isolable, palladation product 1 is usually unstable and undergoes rapid decomposition. Palladation is followed by two reactions. The elimination of H-Pd-X from 1 to form the vinyl compounds 2 is one path, resulting in nucleophilic substitution of the alkene. Displacement of the Pd in 1 by an other nucleophile effects nucleophilic addition of the alkene to give 3. Depending on the reactants and conditions, either nucleophilic substitution of the alkene or the nucleophilic addition to the alkene takes place [4,5]. 

Palladium(O) exhibits a degree of nucleophilic character, thus electron-with-drawing substituents increase the reactivity of aryl halides in oxidative additions. This is exemplified in the heterocyclic context the inductive effect of C=N units allows 2-chloropyrimidine (it is slightly less reactive than bromobenzene), and even 3-chloropyridine to react (even the moderate inductive effect at the P-position gives rise to a significantly higher rate of reaction relative to chlorobenzene) although a more reactive catalyst is required for the latter case (cf. section 2.7.2.2). 

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