Metal complexes, the nucleophilicity

Metal complexes, the nucleophilicity of towards organic molecules, 23,1 Methyl transfer reactions, 16,87... 

Certain metal cations are capable of electrophilic attack on alkenes. Addition is completed when a nucleophile adds to the alkene-cation complex. The nucleophile may be the solvent or a ligand from the metal ion s coordination sphere. 

The susceptibility of a metal complex to nucleophilic attack is enhanced by a positive charge on the complex. This fact, and the fact that most metal isocyanide complexes are cationic, probably explains why no nucleophilic reactions of uncharged isocyanide complexes have yet been reported. It is... 

The reactivity of peroxo metal complexes as nucleophilic oxidants is a known process ". To visualize this type of reactivity one has to refer to peroxo metal complexes as a 1,3-dipolar reagent M+-0-0 interacting in a bimolecular fashion with electrophilic dipo-larophiles such as electron-poor olefins " (equation 15), to form peroxymetallacycle intermediates. 

Allylic substitution using hard nucleophiles proceeds through a different mechanism. Instead of attacking the allyl group of the 71 allyl-metal complex, hard nucleophiles attack the metal first and the product is subsequently formed by reductive elimination. Nickel(O) complexes have often been used for this purpose. Reports of good enantioselectivities in this type of reaction are limited. 

The chelate ligand in dionato complex 3 is planar and it is particularly stabilized by 71-delocalization. In addition to this thermodynamic stability, the iron center has 17 valence electrons in an octahedron, hence its coordination sphere is kinetically labile. By ligand exchange, the acceptor 41a is coordinated at a vacant site to form species 44 (Scheme 8.18). The function of the center metal is not only to hold the acceptor in proximity to the donor. Additionally, the acceptor is activated by Lewis acidity of the center metal. Subsequently, the nucleophilic carbon atom of the dionato ligand is... 

One of the simplest and widely used methods of forming C-S bonds involves nucleophilic attack of a thiolate on a suitable C-centred electrophile such as an alkyl halide (Fig. 5-74). Co-ordinated thiolate ligands behave as nucleophiles in exactly the same manner, and the method has been extensively used for the preparation of thioethers and their metal complexes. The method has been particularly commonly utilised in the formation of macrocyclic ligands in templated syntheses (see Chapter 6). 

The 1,2- and 1,7-isomers are susceptible to degradation with strong base, which removes one boron atom from each of the cages. In the 1,2-isomers either the 3- or 6-boron atom is removed while for the 1,7-isomers either the 2- or 3-boron atom is removed. The boron atoms capable of extraction in each isomer are adjacent to both heteroatoms. These boron atoms are the most electron-deficient and the most susceptible to nucleophilic base attack. The 11-atom fragments are not normally isolated as the ollide ions themselves but in a protonated form (i.e., 1,2-B9C2H12-). The extra protons are easily removed with base prior to metal complexation. The ollide ions discussed in this section may well exist as open-face icosahedral fragments (Fig. 2). This point has not been definitely determined yet and an alternative... 

Here, either X = (I T2C(0)-R j and ox = Jones reagent or X — CN and ox = CeIv. It has been emphasized that the addition of nucleophiles to (arene)Mn(CO)3+ complexes does not occur through an initial ET from the nucleophile to the metal center [2]. This represents an additional advantage since such redox reactions frequently lead to the decomposition of the metal complex, a typical example being the reductive deligation of bis(arene)Fe2+ complexes [48]. On the other hand, intramolecular charge transfer from the arene to the metal not only induces an electron deficiency in the arene ring (which is critical for effective attack of the nucleophile), but it also results in an attenuation of the electrophilicity of the metal center so as to avoid undesired ET reactions of the metal with the nucleophile. 

Bis(crown ethers) connected by a flexible spacer are a source of intramolecular sandwich-type complexes with alkali metal ions <1992MI1>. A conformational analysis (based on a combination of semiempirical and ab initio methods) performed on 12-crown-3 38 and 12-crown-4 39 predicted that, in the case of sandwich-type complexation, the nucleophilic cavity of 12-crown-3 rather than that of 12-crown-4 is more prone to complexation with the Na+ ion. Accordingly, ion-selective electrodes based on bis(12-crown-3) derivatives with dialkylmalonate spacers displayed the highest selectivity for Na+ ions among the alkali and alkaline earths investigated, and superior to the Na+ selectivity reached with the bis( 12-crown-4) analogue <2003ANA(480)291>. 

A number of r-allylmetal complexes react with nucleophiles resulting in transfer of the allyl group from the metal to the nucleophile. As previously shown the most developed are the r-allylpalladium complexes. However, tt-allyltricarbonylcoblat(I) complexes also react with carbanions to produce allylic alkylation products in good yield. Acylated r-allylcobalt complexes when reacted with a nucleophile result in an overall acylation/alkylation of 1,3-dienes, (equation 71). 

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