External attack of nucleophiles on alkene coordinated to electrophilic metal complexes

In the first step in the main catalytic cycle, a rt type coordination of ethylene to Pd + catalyst center takes place to give a rt-bonded ethylene complex of palladium(II). Attack of an OH anion on the coordinated ethylene molecule 

The /1-hydroxyethylpalladium species is subsequently transformed into a-hydroxyethylpalladium species by skeletal isomerization involving /I-hydrogen abstraction and transfer of the hydride hgand to the methylene carbon of vinyl alcohol formed. The a-hydroxyethylpalladium liberates acetaldehyde as shown in the scheme with generation of Pd(0). The key of success in realizing the catalytic process was to re-oxidize the Pd(0) species produced to Pd(II) with Cu(II), which is reduced in the auxiliary redox process to Cu(I) that is in turn re-oxidized by oxygen to Cu(II). 

When a-olefins such as propylene are employed, 2-ketones are catalytically produced [113]. The process involves the nucleophilic attack at the substituted carbon in the terminal double bond. Skeletal isomerization through -hydrogen elimination and re-addition of the hydrido ligand on the terminal methylene provides a branched metal alkyl with CH3 and OH substituents. Ketone can be liberated from the Pd(II) complex with release of Pd(0) (Eq. 1.18). 

External attack of an acetate anion on the ethylene molecule coordinated to Pd(II) is operative in industrial production of vinyl acetate, first reported by Moiseev et al. [114] and later commercialized by Kuraray in Japan after modification of the process. In certain cases the process of attack by an external nucleophile is reversible and the nucleophile bonded may dissociate to convert the a-bonded complex back into the jt complex. 

In certain cases decrease in the optical activity in the catalytic allylation products is observed with increase of the catalyst. The cause can be ascribed to the intervention of a bimolecular process involving attack of the coordinated r -allyl ligand by a Pd(0) species from the opposite side of the allyl plane [118]. 

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