Reactivity of Co-ordinated Enolates with Electrophiles

In the same way that the formation of the enolate may be promoted by co-ordination to a metal ion, so may the reactions of that enolate be promoted or modified. Spectacular examples of the reactions of co-ordinated enolates are observed in the electrophilic halo-genation of /3-ketoesters. These reactions are accelerated dramatically in the presence of copper(n) and other metal ions (Fig. 5-6). 

5 Stabilisation of Anions and the Reactions of Co-ordinated Ligands with Electrophiles 

It is possible to form the corresponding nitro compounds by direct nitration of the co-ordinated diketonate (Fig. 5-7), but they are also readily prepared by electrophilic nitration of the corresponding iodo compounds (which are, in turn, obtained by direct iodination) (Fig. 5-8). 

The analogy between the behaviour of these diketonate complexes and benzene derivatives is indeed remarkable, and reaction sequences very reminiscent of those observed in organic aromatic chemistry are common. In Fig. 5-9, the reduction of a nitro to an amino derivative is illustrated, a conversion reminiscent of the reduction of nitrobenzene to aniline. It is usually necessary to use kinetically inert cP or d6 metal complexes in sequences of this type. 

Once the 3-amino-2,4-diketonate has been prepared, it then shows reactivity typical of an aminobenzene derivative. For example, reaction with nitrous acid and sodium fluoride allows the formation of the 3-fluoro derivative in a Sandmeyer reaction (Fig. 5-10). This may usefully be compared to the reaction sequence from conventional aromatic chemistry 

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