Structures of Enolates

The Partition Principle as Applied to the Structure of Enolic Sodium... 

The conditions for the preparation and the proposed structures of enolates such as 19 or 20 are presented in Table 2. 

It is known that the chemistry of enolates depends on the nature of the metal. Moreover, the metals are an integral part of the structures of enolates. Lithium enolates are most frequently employed, and in the solid state the lithium cations definitely are associated with the heteroatoms rather than with the carbanionic C atoms. Presumably the same is true in solution. The bonding between the heteroatom and the lithium may be regarded as ionic or polar covalent. However, the heteroatom is not the only bonding partner of the lithium cation irrespective of the nature of the bond between lithium and the heteroatom ... 

Assuming ionic Li 0 or Li NR interactions, it may be appropriate to draw a parallel between the structures of enolates and ionic crystals of the Li 0 or Li NR types. In the latter structures, every lithium atom is coordinated by six neighboring anions. 

The structures of enolates have been examined through magnetic resonance studies (NMR) and with X-ray crystallographyIt has been observed that solvated enolates exist as dimers, tetramers or hexa-mers, depending on the enolate structure, the nature of the cation and the solvent. 

Fig. 6.P26. HF/6-31G structures of enolates 1-4. Reproduced from Helv. Chim. Acta, 85, 4216 (2002), by permission of Wiley-VCH.

Figure 2. Chemical structure of enol form of fructoseglycine and L-scorbamic acid.

The enolate O atom, having most of the negative charge of the resonance hybrid, is more tightly associated with a counterion such as Na" " or Li+. The counterion has a shielding effect that inhibits reactions with electrophiles, an effect that is amplified by the aggregate structure of enolate anions in solution. 

Of course, phenols also react with FeCla (p. 188). However, in methanolic solution only those phenols react which carry in the ortho position of the phenolic hydroxyl a group capable of the formation of a chelate bond, as, for example, in catechol or salicylic acid. Trans-enols do not produce a color in methanolic solutions, but do in aqueous solution. For more detailed information of the relationship between the reactivity and the color and the structure of enols see (3). 

The bridged lithium enolate 13a is more stable than the unbridged structure 13b. The structures of the enolate moiety in 13b change only slightly as lithium is replaced by sodium, potassium, rubidium, and cesium i.e., these structures are those of salts of the enolate ion, but the free enolate ion structure does change significantly on coordination with the alkali metal, an effect attributed to the important polarization effect of the metal cation. The polarization structure 12c is now known to be an important contributor to the electronic structure of enolate ions, and its effect would be reduced by association with a cation as in 13b. 

Transmetallation of the lithium or potassium enolates is also a reliable method for the preparation of palladium and nickel enolates, as illustrated in Scheme 2.50. Clear evidence for the C-bound structure of enolates 172 and 173 thus prepared was provided by NMR spectroscopy and - for nickel enolate 172 (M = Ni, L = Cp ) - by a crystal structure analysis. The reaction of C-bound nickel and palladium enolates 172 and 173 with aldehydes is much more sluggish and much less uniform than the analogs of that of the polar main-group metals. In addition to P-hydroxyketones or esters, products resulting from a Tishchenko reaction were also observed [164b]. 

Keto-enol Tautomerism and Structures of Enol Molecules... 

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