Ligands nucleophilicity

It is surprising that the second-order rate coefficients decrease as the basicity of the entering ligand increases it appears that ligand nucleophilicity is compounded of basicity and micropolarizability in no simple manner. 

The effect of the nature of ion pairs as nucleophiles in a metal-catalysed substitution reaction has been investigated by determining product ratios for the Pd-catalysed allylic alkylations of substrates (9)-(ll) under various conditions, particularly with respect to catalyst ligands, nucleophiles, and counterions. Each dienyl acetate ionizes to form initially the vinyl (7r-allyl)-Pd intermediate corresponding most closely to the leaving group, i.e. (12) from (9), (13) from (11), but (12) and (13) from (10). The initial intermediate can then either be trapped by the nucleophile or it can begin to equilibrate to some mixture of vinyl 7r-allyl intermediates. If nucleophilic addition occurs before full equilibration, the product ratio is different for each substrate if... 

Asymmetric allylic substitutions are widely applied in organic synthesis, using various metal complexes, chiral ligands, nucleophiles and allyl systems [39]. Although Pd is often the metal of choice, this is not the case for monosubstituted allylic substrates, where most Pd catalysts predominantly produce the achiral linear product. In contrast. Mo, W and Ir catalysts preferentially give rise to the desired branched products and, in recent years, a number of very effective Ir catalysts for various substrates have been developed [40]. Since, to the best of our... 

This mechanism does not explain what is happening in the rhodium(m) complex, [Rh(bpy)3]3+, where exchange occurs first at the 6-position. Increasing the charge on the metal ion will increase the tendency towards attack by a nucleophile and also the acidity of the pyridine ligand. Nucleophilic attack will always occur at the 4- and 6-positions of... 

The electron-rich ruthenium center can render the bound X ligand nucleophilic as has been demonstrated by the reactions of (rj5-C5H5)-(PPh3)2Ru—C=N (30) with a variety of electrophiles. Baird and Davies have shown, for example, that addition of alkyl halides to 30 gives the corresponding isocyanides in moderate yield [Eq. (32)] (29). Other... 

A stereochemical possibility unique to the associative mechanism is adjacent displacement in which the bond angle between entering and leaving groups is 90°. This possibility results from the ability of the pen-tacoordinate phosphorane intermediate to undergo pseudorotation (9,10) which, with certain restrictions (11) interchanges axial and equatorial ligands. Nucleophilic displacements on stable phosphoranes appear to proceed via an associative mechanism with hexacoordinated phosphorus intermediates (12, 13). The biochemical relevance of such displacements has not been established. 

The enhancement of ligand nucleophilicity has its origins in the large pKa perturbations (3—4 pKa units) which result from coordination to the metal ion. These large pKa perturbations arise from the resultant Coulombic force field between the dipositively-charged zinc ion and the weak-acid dipole. This effect can be quantitatively described by Coulomb s law when the effective dielectric constant of the microenvironment is considered [Kirkwood and Westheimer (29)]. The pKa perturbation obviously can bring about a large increase in the effective concentration of the nucleophilic species provided the pKa perturbation does not decrease nucleophilicity in a manner which is proportional to the decrease in basicity. 

Strongly backbonding metal -alkyl, hydrohen R groups -X2-type ligands -Nucleophilic at carbon... 

---