Nucleophile-electrophile interaction

Weak nucleophile-electrophile interactions (and the donor-acceptor complexes) are considered precursors in aromatic electrophilic substitutions133 and in additions of electrophiles to C=C double bond of olefins the first step (the addition of the electrophile to an electron-rich substrate) is probably the same for both reactions. 

Some striking demonstrations of metal-metal bond lability are provided by cluster rearrangements due to protonation. This is the case for some anionic osmium clusters (cf. Section VI). It involves ligand activation for some tetrairon clusters (51-53). Thus, the clusters 9 and 11 open up upon protonation, and compensation for the lost iron - iron bonds in the products 10 and 12 comes from the bonding between one iron atom and a carbonyl oxygen. The relation of these unusual nucleophile-electrophile interactions to cluster-induced CO transformations is obvious. 

Deprotonation reactions if the 2-benzopyrylium cation has an a-alkyl substituent or a group with a mobile hydrogen atom in any other fragment of the molecule. This conversion takes place over a wide range of conditions, and it often accompanies nucleophilic additions. Moreover, the problem of the acid-base and nucleophilic-electrophilic interactions for 2-benzopyrylium salts concerns not only the primary step of their reactions with nucleophiles, but also the ring-open forms, as shown in Section III,C,4. 

NUCLEOPHILE-ELECTROPHILE INTERACTIONS AND REDOX REACTIONS INVOLVING ORGANIC SUBSTANCES... 

At first sight, it may appear that a discussion of nucleophile-electrophile interactions does not fit into a chapter on redox processes, but the transfer of electron pairs from a nucleophile to an electrophile, such as in hydrolysis or in chlorination of a phenol, may be looked at as a reaction between an oxidant and reductant, although we are aware that mechanistically, in many real redox processes involving organic substances, the electron transfers often occur in one-electron steps. 

Pericyclic reactions, such as the addition of a carbene to an alkene and the Diels-Alder cycloaddition (Review Table 1, reactions Ih and li), involve neither radicals nor nucleophile-electrophile interactions. Rather, these processes take place in a single step by a reorganization of bonding electrons through a cyclic transition state. We ll look at these reactions mate closely in Chapter 30. 

The most favorable reactive channels along the nucleophile/electrophile interaction. 

For other electrophiles, such as esters, much less covalent bonding in the transition state may occur and ionic bonding may be large. The accepting orbitals may be concentrated in space, similar to that of the proton. Clearly, 1 will not be a good nucleophile, compared to F (for an interesting analysis of nucleophile-electrophile interactions that does not use orbital theory, see reference 26). 

The intermolecular donor-acceptor hardness (>/da, as defined by Equation (12.6)), provides a more rigorous description of these nucleophile/electrophile interactions. The /da values for the whole series of N4 macrocycles are collected in Table 12.4 and plotted in Figure 12.6. 

The HS AB principle provides a good description of many nucleophile-electrophile interactions soft-soft and hard-hard interactions are both thermodynamically and kinetically favored. 

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