Enolate delocalization

A simple test of the thermochemical postulate in Schemes 27 and 28 was recently reported in the literature. From a wide range of potential a-substituents with radical stabilization energies above 11 kcal/mol, ester groups were selected. Based on literature reports, it was expected that an ester carbonyl and two methyl groups would stabihze a carbon radical by >12.3 kcal/mol with respect to the unsubstituted methyl radical. The resulting radical would be stabilized by keto-enol delocalization and by the effect of the methyl substituents. Given that photochemical decarbonylation of ketodiesters had not been previously documented, it was of interest to test if their photochemistry would conform to the prediction. Three crystalline 1,3-acetonedicarboxylates, 88a to 88c, were investigated and all three reacted cleanly and smoothly in the solid state (Scheme 29). 

Enol ethers (Figure 2-58a) have two electron pairs on the oxygen atom in two different orbitals, one delocalized across the two carbon atoms, the other strictly localized on the oxygen atom (Figure 2-58b). Ionization ftom either of these two orbitals is associated with two quite different ionization potentials, a situation that cannot be handled by the present connection tables. 

The first step protonation of the double bond of the enol is analogous to the pro tonation of the double bond of an alkene It takes place more readily however because the carbocation formed m this step is stabilized by resonance involving delocalization of a lone pair of oxygen... 

Resonance forms illustrating charge delocalization in enolate of a p keto ester... 

Enolate ion (Section 18 6) The conjugate base of an enol Enolate ions are stabilized by electron delocalization... 

The alkylation reactions of enolate anions of both ketones and esters have been extensively utilized in synthesis. Both very stable enolates, such as those derived from (i-ketoesters, / -diketones, and malonate esters, as well as less stable enolates of monofunctional ketones, esters, nitriles, etc., are reactive. Many aspects of the relationships between reactivity, stereochemistry, and mechanism have been clarified. A starting point for the discussion of these reactions is the structure of the enolates. Because of the delocalized nature of enolates, an electrophile can attack either at oxygen or at carbon. 

In the presence of bases such as hydroxide, methoxide, and ethoxide, these p-diketones aie converted completely to their enolate ions. Notice that it is the methylene group flanked by the two caibonyl groups that is deprotonated. Both caibonyl groups paitici-pate in stabilizing the enolate by delocalizing its negative chaige. 

It s reasonable to ask why one would prepare a ketone by way of a keto ester (ethyl acetoacetate, for example) rather than by direct alkylation of the enolate of a ketone. One reason is that the monoalkylation of ketones via their enolates is a difficult reaction to cany out in good yield. (Remember, however, that acylation of ketone enolates as described in Section 21.4 is achieved readily.) A second reason is that the delocalized enolates of (3-keto esters, being far- less basic than ketone enolates, give a higher substitution-elimination ratio when they react with alkyl halides. This can be quite important in those syntheses in which the alkyl halide is expensive or difficult to obtain. 

Delocalized frontier orbitals provide a different kind of problem. The ester enolate shown below might react with electrophiles at two different sites. 

Is the most delocalized enolate also the most easily formed enolate Calculate relative deprotonation energies from the enolate precursors using the deprotonation energy of acetone as a standard. 

Compare the geometries of the cyclohexanone enolate and the cyclohexanone lithium enolate. Do both molecules show delocalized structures, or is the bonding in one of them more localized For comparison, examine the geometries of 1-hydroxycyclohexene md cyclohexanone. 

Examine atomic charges and display the electrostatic potential map for 2,7-octadione. Are you able to say which hydrogens (at Ci or at C3) are more likely to be abstracted by base, and conclude which is the kinetically-favored enolate Which enolate (2,7-octadione, Cl enolate or C3 enolate) is the lower in energy What do you conclude is the thermodynamically-favored enolate Is this also the enolate in which the negative charge is better delocalized Compare electrostatic potential maps to tell. 

When a hydrogen atom is flanked by two carbonyl groups, its acidity is enhanced even more. Table 22.1 thus shows that compounds such as 1,3-dikotoncs (/3-diketoncs). 3-oxo esters (/3-keto esters), and 1,3-diesters are more acidic than water. This enhanced acidity of jS-dicarbonyl compounds is due to the stabilization of the resultant enolate ions by delocalization of the negative charge over both carbonyl groups. The enolate ion of 2,4-pentanedione, for instance,... 

The radical anions of 309, 311 (Me derivative of enol), and 308 were generated electrochemically ESR spectroscopy indicated that the unpaired electron was delocalized in the triazine moiety (81MI3). 

The intensive mechanistic studies of phenoxyl self-reactions proved a great variety of mechanisms and rate constants of these reactions [2,3,6], The substituents can dramatically influence the mechanism and kinetics of self-reactions. Due to free valence delocalization the phenoxyl radical possesses an excess of the electron density in the ortho- and para-positions. Mono- and disubstituted phenoxyls recombine with the formation of labile dimers that after enolization form bisphenols [3,6],... 

Air is required for conversion of the keto-enol to the endoperoxide. The most likely reaction is autoxida-tion. The O2 makes bonds to C2 and C6, neither of which has an H atom attached for abstraction. But abstraction of H from 07 gives a radical, A, that is delocalized over 07 and C2. Addition of O2 to C2 gives a hydroperoxy radical, which abstracts H from 07 of the starting material to give a hydroperoxide and A again. The hydroperoxide thus obtained can then add to the C6 ketone in a polar fashion to give the observed hemiketal. 

Recently, Kochi et al. described a novel photochemical synthesis for a-nitration of ketones via enol silyl ethers. Despite the already well-known classical methods, this one uses the photochemical excitation of the intermolecular electron-donor-acceptor complexes between enol silyl ethers and tetranitrometh-ane. In addition to high yields of nitration products, the authors also provided new insights into the mechanism on this nitration reaction via time-resolved spectroscopy, thus providing, for instance, an explanation of the disparate behavior of a- and (3-tetralone enol silyl ethers [75], In contrast to the more reactive cross-conjugated a-isomer, the radical cation of (3-tetralone enol silyl ether is stabilized owing to extensive Tr-delocalization (Scheme 50). 

The acidity of a C-H is further enhanced if it is adjacent to two carbonyl groups, as in the 1,3-diketone acetylacetone. The enolate anion is stabilized by delocalization, and both carbonyl oxygens can participate in the process. This is reflected in the VK, 9 for the protons between the two carbonyls. 

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