Enolate anions geometry

Do changes in geometries, charges and size and shape of the HOMO between the enolate anion and its lithium salt suggest differences in reactivities If so, what differences are to be expected ... 

The stereochemical outcome of the addition of lithium enolates of aldehydes and ketones to nitroalkenes is dependent upon the geometry of the nitroalkene and the enolate anion. The synjanti selectivity in the reaction of the lithium enolates of propanal, eyelopentanone and cyclohexanone with ( )- and (Z)-l-nitropropene has been reported1. 

Axial protonation is not strongly favored. They concluded that in practice this type of experiment is complicated by the fact that protonation of an enolate anion can occur either at the carbon (to give 468 or 469) or at the oxygen atom (to yield the enol). Further reaction of the enol with aqueous acid also yields the two possible ketones 468 and 469. Furthermore, since the protonation steps of this strongly basic anion (either at C or 0) are diffusion-controlled (144), it is possible that the transition state geometries for both reactions resemble the geometry of the enolate anion, so the energy difference between the direction of attack on the enolate is small. 

From a consideration of the optimised geometries, it could be concluded that both the acids and the deprotonated anions are subject to some 7t-electron delocalisation. In accord with chemical intuition, the effect of delocalisation is more important in the carboxylate and enolate anions than in the other species. However, the geometry changes that the acids undergo under deprotonation are only partly explained by 7t delocalisation. 

When a ketone reacts with a suitable base (secs. 9.1, 9.2) an enolate anion is formed by removal of the a-proton. In the case of an unsymmetrical ketone such as 30, a mixture of (Z)-enolate (31) and ( )-enolate (32) usually results (secs. 9.2.E, 9.5.A). This mixture influences the diastereoselectivity and enantioselectivity of enolate condensation reactions (sec. 9.5). Such a mixture of geometrical isomers generates both syn- and antiproducts upon reaction with aldehydes so it is important to control or at least identify the geometry of the enolate. Several solutions to this problem have been developed, including formation of stable and separable enolate isomers and controlling reaction conditions to maximize production of one isomer. 

C Enolate Anions (ElZ) Geometry in Enolate Formation Table 9.3. The Influence of Ketone Structure and Base on Enolate Geometry. 

Lithium enolates exist as large aggregates and their approach to the electrophile is restricted by steric and electronic considerations, as well as by the relative geometry of the molecule. Despite the structural complexity remarkably good predictions for reactivity and diastereoselectivity can be made based on the steric requirements that would be present in a monomeric system. For example, in most reactions of enolate anions the electrophile will be delivered to the less hindered face of the enolate to give the major product. In all models used to describe reactivity (secs. 9.5.A.iii-9.5.A.v), a monomeric enolate will be shown but the facial and orientational bias of the enolate is clearly influenced by the state of aggregation in solution. 

Whether or not the highly electropositive alkali metals or magnesium form an ionic instead of a covalent bond to the oxygen of the enolate is less important. Even if there is a contact ion pair of the metal cation and the oxygen anion, the geometry of the six-membered chair transition state, as outlined above, will be maintained. 

Decarboxylation leading to a delocalized anion (which could be an anion, an enol, or an enamine) could be treated by a model with three dimensions bond change, geometry change at carboxylate, and geometry change at the enolate carbon. Some neutral carboxylic acids might react by... 

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