Metal ions electrophilic assistance

Very often the configuration of the organometallic reagent used is unknown. But even if it is known, predictions are difficult to make, since the bonding character or the ion-pair structure of organometallic compounds, and therefore the selectivity, is not only influenced by the character of the metal but also by important external factors such as solvent, complexation and temperature24. In addition, the nature of the electrophile itself influences the mode of reaction. Complexation of the electrophile at the metal atom (electrophilic assistance) in general is believed to favor metalloretentive attack. 

Metal cations can lend electrophilic assistance to weaken the Pd—X bonds in the intermediate R-Pd —X. Either full fission of this bond, leading to the realization of a polar mechanism, or partial polarization, might take place. Soft Lewis acids (the cations of Cu, Ag, Tl) are used most often (see Chapter 9.8 for a discussion of how metal ions act as Lewis-acid catalysts). 

The above mechanism assigns an electrophilic function to the metal ion. During decarboxylation, an electron pair initially associated with the carboxylate ion group is transferred to the rest of the molecule. A metal ion, because of its positive charge, should assist this transfer (46). 

Analysis ofthe kinetic data shows that the barium salt of 7, as well as the analogous salts of its higher homologues, perform much less efficiently than 4-Ba. The Ba complex of 7 turns over with a very low efficiency, caused by the extreme slowness of the deacylation step. Only a minor fraction ofthe liberated pNPOH in the steady-state phase is due to the expected double displacement mechanism. A larger fraction is most likely ascribable to the metal ion not sequestered by 7, and thereby available in solution for electrophilic assistance to direct methanolysis of the ester reactant. 

Figure 21. Metal ion effect on the acid hydrolysis of phosphosulfate (a) electrophilic catalysis and (b) metal-ion-assisted, proton-transfer catalysis...

In Chapter 17 you saw epoxides acting as electrophiles in Sn2 reactions. They can be used to alkylate enolates providing epoxide opening is assisted by coordination to a Lewis acidic metal ion in this case the lanthanide yttrium(III). The new C-C bond in the product is coloured black. Note that the ketone starting material is unsymmetrical, but has protons only to one side of the carbonyl group, so there is no question over which enolate will form. The base is one of the LDA variants we showed you on p. 668—LHMDS. 

It is common practice to consider the traditional Werner octahedral complex ions [MlLNle]" [M = Co(III), Rh(III), Ir(III), Cr(III), Ru(III), Pt(IV) LN = donor atom of unidentate or polydentate ammine or amine] as well as square-planar [M(LN)4p [M = Pt(II), Pd(II)] as kinetically inert compounds. Bound ammonia is generally less labile than bound water, and it has been suggested that this observation can be related to the presence of an extra and exposed electron pair in water. This may make it more sensitive to electrophilic groups in the solvation sheath, which could assist its dissociation from the metal ion (274). If we take the stance of assigning lability as a property of the ligand in such complexes, then ammonia and amines in general can be... 

The possible assistance given to leaving groups by electrophiles in the form of solvent or metal ions represents one part of a ligand replacement reaction normally referred to as SE2(cyclic). Structure 19 shows the interaction at the transition state. It can be seen as simultaneous attack of the electrophile E at the leaving group X and of the... 

The activation of C-H bonds by an electrophilic pathway is shown schematically in eq. (12) and has been observed with a number of late transition metal ions [9], A driving force for the reaction shown in eq. (12) is the stabilization of the leaving group, H", by solvation in polar solvents. The related four-center electrophilic activation by transition, lanthanide, and actinide metal centers has also been reported, (eqs. (13a) and (13b)) [9b,c,g, 27]. In these instances, a ligand on the metal assists the reaction by acting as the base. 

Metal ions, which have a positive charge, contribute to the catalytic process by acting as electrophiles (electron-attracting groups). They assist in binding of the substrate, or they stabilize developing anions in the reaction. They can also accept and donate electrons in oxidation-reduction reactions. 

Although PI is a plausible intermediate on the reaction path in such reactions, it has not been detected directly by experiment and, hence, its proposed existence remains largely speculative. However, indirect proof for the presence of PI on the reaction path comes from the observed isomerization between 3, 5 -Unkage and 2, 5 -linkage only under acidic pH where PI can exist in the monoanionic form (PIH). Thus, an alternative mechanism (Scheme 2.26) for the metal-ion-assisted hydroxide-ion- and general base ( B)-catalyzed hydrolysis of RNA and RNA-model compounds cannot be completely ruled out if pentacoordinated intermediate (PIH) does exist on the reaction path. Nearly 10 -fold enhanced catalytic effects of Co " complexes are attributed to the occurrence of intramolecular electrophilic catalysis as shown in the transition state TS,g. 

The C-H bonds at the P position relative to R3M- (M = Si, Ge, Sn, Pb) substituents are activated towards attack by electrophilic reagents. Two types of electrophilic attack at a P C-H bond are considered in Scheme 3 Path 1 involves hydride abstraction by the electrophile, resulting in the formation of a carbenium ion intermediate, a process that is assisted by the metal P-effect. Such a pathway might be expected to be followed by strongly Lewis acidic reagents, such as carbenium ion reagents. 

There is an increase in the importance of electrophilic catalysis by zinc cation relative to acetic acid for deprotonation of the a-carbonyl carbons of hydroxyace-tone, a substrate which provides a second stabilizing chelate interaction between the hydroxy group at the substrate and the metal dication that is expressed at transition state for proton transfer [19]. For example, the third-order rate constants kx for the Zn +-assisted acetate-ion-promoted deprotonation of the a-CHs and a-CH20H groups of hydroxyacetone are 32-fold and 770-fold larger, respectively, than the corresponding second-order rate constants kAco for proton transfer to acetate anion assisted by solvent water that is present at 55 M (Scheme 1.12). This shows that Zn + stabilizes the transition state for proton transfer from the a-CHs... 

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