Nucleophilic substitution ligand attacks

Since enolates also add via a ligand attack process, the regioselectivity that they exhibit is quite comparable to soft caibon nucleophiles. Alkyl or aiyl substituents at the allyl termini direct attack to the less substituted terminus (equations 228-232) functional groups such as COaMe and halogen at one allyl terminus direct attack to the remote terminus (equations 233 and 234). Remote functionalities such as —OR also direct addition to the allyl terminus more removed from the substituent (equations 23S and 236). 

The addition of Grignards and organolithium reagents proceeds by attack at the metal center in ir-allylpalladium complexes. The regiochemical selectivity exhibited by these hard carbon nucleophiles with ir-allyl complexes substituted at the termini with alkyl or aryl groups is comparable to the soft carbon nucleophiles (ligand attack) in most cases, with addition occurring predominantly at the less substituted terminus (equations 248 and 249).1591387... 

Within the past 20 years, ferrates, i.e. anions possessing iron as the center atom, have found increasing application as nucleophilic complexes in substitution chemistry. In these reactions, the ferrate replaces the leaving group X in a first nucleophilic substitution event. A transfer of one ligand from the metal atom (i.e. a reductive elimination, path A, Scheme 7.2) or substitution of the metal atom via external attack of the nucleophile (path B) concludes this mechanistic scenario. However, the exact mechanism in ferrate-catalyzed nucleophilic substitutions is still under debate. Apart from the ionic mechanism, radical processes are also discussed in the literature. 

The kinetics of the reaction between Irons- Ml N2Me)Br(dppe)21 and methyl iodide in tetrahydrofuran exhibit a first-order dependence in the concentration of complex and first-order in the concentration of methyl iodide. When M = W, the reaction with methyl iodide is 38 times faster than the reaction with ethyl iodide, which is typical ofSN2 reactions. Therefore, it is concluded that the secondary alkylation is a bimolecular nucleophilic substitution (Scheme 10) in which nucleophilic attack of the diazenido ligand on the carbon atom of the alkyl halide is the rate-limiting step (93). 

Other neutral cyclopentadienyl alkyne complexes have been prepared by chloride substitution reactions with sulfur nucleophiles pentafluoro-phenylthiolate products have been reported [Eq. (35)] (95). These reactions are sensitive to the identity of the sulfur nucleophile (96), and attack at an alkyne carbon to yield 772-vinyl ligands is common (97). 

Intermolecular Nucleophilic Substitution with Heteroatom Nucleophiles. A patent issued in 1965 claims substitution for fluoride on fluorobenzene-Cr(CO)3 in dimethyl sulfoxide (DMSO) by a long list of nucleophiles including alkoxides (from simple alcohols, cholesterol, ethylene glycol, pinacol, and dihydroxyacetone), carboxylates, amines, and carbanions (from triphenyhnethane, indene, cyclohexanone, acetone, cyclopentadiene, phenylacetylene, acetic acid, and propiolic acid). In the reaction of methoxide with halobenzene-Cr(CO)3, the fluorobenzene complex is ca. 2000 times more reactive than the chlorobenzene complex. The difference is taken as evidence for a rate-limiting attack on the arene ligand followed by fast loss of halide the concentration of the cyclohexadienyl anion complex does not build up. In the reaction of fluorobenzene-Cr(CO)3 with amine nucleophiles, the coordinated aniline product appears rapidly at 25 °C, and a carefiil mechanistic study suggests that the loss of halide is now rate limiting. 

The difficulties of direct oxidative insertion with metals other than Mg or Li mean that o-complexes are often made from organo-lithium or Grignard reagents by metal exchange. This reaction amounts to a nucleophilic substitution at the metal without a change of oxidation state so the metal is used in whatever oxidation state is finally needed. Attack of methyl lithium on a Cu(I) halide gives methyl copper 50, a o-complex of Me- and Cu(I). If an excess of MeLi is present an ate complex is formed, lithium dimethylcuprate 51. This is formally a compound of a copper anion 51a, just as BF4 is a borate. The term ate complex refers to such formally anionic complex in which the metal has one extra anionic ligand. Its true structure is dimeric 51b and it exists as an equilibrium with 52 in solution.20... 

The number of studies of inorganic reaction mechanisms by theoretical methods has increased drastically in the last decade. The studies cover ligand substitution reactions, insertion reactions oxidative addition, nucleophilic and electrophilic attack as well as metallacycle formation and surface chemistry, in addition to homogeneous and heterogeneous catalysis as well as metalloenzymes. We can expect the modeling to increase further both in volume and in sophistication [173],... 

Another consideration is that with increasing size of the ligands, an unsymmetrical 7t-allyl complex is formed with palladium shifted to the less substituted side. Attack of the nucleophile occurs at the more substituted end to minimize loss of palladium-carbon bond energy. Preference for the trans-prod net is due to the favorable formation of a chair conformation for the axial attack, in contrast to a boat conformation for an equatorial attack34. 

As stated by Chatt and co-workers, their early speculation on the role of trans-n bonding groups in ligand substitution of platinum(ii) complexes was based on the assumption that the reactions proceed by an 8 2 mechanism. However, at the time (1955) most of the observations reported on such reactions were qualitative and little had been done to use detailed kinetic studies in attempts to elucidate the mechanism of ligand substitution. Since the valence bond theory in use then assigned dsp hybridisation to the square-planar plati-num(ii) complexes, coordination chemists believed an entering nucleophile would readily attack the low energy vacant p orbital on the metal and substitution would take place by an 8 2 mechanism. Furthermore, a coordination... 

More nucleophilic entering ligands react more readily. Solvent attack may also provide a route for ligand substitution, depending on the nature of Y and the solvent. 

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. 

Since the relative apicophilicity of a hydroxy ligand is greater than that of a sulphydryl, TBPs with apical oxygen will be favoured over those with apical sulphur. Thus with c Jo-adjacent nucleophilic attack, nucleophilic substitution on the S p isomer of cAMP(S) will be favoured over attack on the Rp isomer. This provides one explanation for the selectivity of beef-heart cAMP phosphodiesterase for the Sp isomer of cAMP(S). Analysis based... 

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