Nucleophilic attack ligand-dependent pathway

The ligand-dependent pathway, represented by the 2[Y] term, is readily understood as a nucleophilic attack of Y at the metal ion. Thus, increasing steric hindrance at the metal atom markedly retards the reaction, exemplified by reaction (2), in which bulkier iV-substituents, R, on the diethylenetriamine (dien) ligand reduce 2 (9)-... 

Both intramolecular and intermolecular attack by M—OHn+ species are well established for cobalt(III) and other kinetically inert metal centres (Section 61.4.2.2.3). However, reactions of this type are not as well defined with labile metal ions. In copper(II) complexes, the pKa values for coordinated water ligands usually fall within the range pKa 6-8. If coordinated hydroxide ion is an important nucleophile in copper(II)-promoted reactions, the reactions would be expected to become independent of [OH-] at pH 8 when the bulk of the complex was converted to the active hydroxo species. Studies of the pH dependence of a number of copper(II)-promoted reactions to such pH levels have been carried out and no evidence obtained for the production of catalytically active hydroxo complexes however, some reactions do proceed by this pathway. 

In their review of phosphorus stereochemistry. Hall and Inch (1980b) have catalogued many nucleophilic substitution reactions at phosphorus whose product distribution and overall stereochemistry are dependent not only on solvent but also on other reaction conditions, in particular the presence of metal ions. They further conclude that mixed reaction pathways may be followed, with the possibility that initial attack of nucleophile may be directed apical to more than one specific ligand. This situation is easy to rationalize in five-membered rings with endocyclic N, O and S ligands, in which there is a fine balance between apical potentialities. 

The generally accepted mechanism for Pd-catalyzed allylic substitution involves association of the palladium(0) catalyst to the substrate, and oxidative addition to provide a ir-aUyl complex. The equilibrium between the neutral 7r-allyl complex and the more reactive cationic 7r-allyl complex depends on the nature/concentration of phosphine Ugand. Nucleophilic addition to the ligand involves direct attack on the ligand when stabilized enolates are employed. After dissociation of the product, the palladium is able to continue in the next catalytic cycle (Scheme 2). In general, the reaction proceeds via a Pd(0)/Pd(II) shuttle, although a Pd(II)/Pd(IV) pathway is also possible. 

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