Enolate anions energy

Enols are not as reactive as enolate anions, however. This lower reactivity simply reflects the presence of the additional proton in the enol, which decreases the electron density of the enol relative to the enolate. In MO terminology, the —OH and —0 donor substituents raise the energy of the tt-HOMO. 

Enolate anions (4e) that have been heated by infiared multiple photon absorption for which torsional motion about the H2C-C bond, which destabilizes the 7t orbital containing the extra electron, is the mode contributing most to vibration-to-electronic energy transfer and thus to ejection. 

However, a more favourable pathway is used, employing a more reactive nucleophile. Rather than using the enolate anion derived from acetyl-CoA, nature uses the enolate anion derived from malonyl-CoA. Malonyl-CoA is obtained from acetyl-CoA by means of an enzymic carboxylation reaction, incorporating CO2 (usually from the soluble form bicarbonate). Now CO2 is a particularly unreactive material, so this reaction requires the input of energy (from ATP) and the presence of a suitable coenzyme, biotin, as the carrier of CO2 (see Section 15.9). The... 

The effect of solvation in CH2=CHOLi was studied in detail. Earlier studies at the B3LYP/6-31+G level suggested that the lowest energy minima correspond to isolated bridged lithium enolate 2a, rather than the open-chain strnctnre 2b this is attributed to the interaction of the lithium cation with the enolate anion (Fignre 3). 

The enolate anion may be considered as an alkene with a very good powerful X -type substituent, the alkoxide oxygen. The HOMO of the n bond is strongly raised in energy... 

Because acidity depends on the difference in energy of the acid and its conjugate base, we must be sure that the stabilization of the enolate anion by electron delocalization represented by 14a and 14b is greater than the analogous stabilization of the neutral enol represented by 15a and 15b ... 

Enols are stronger acids than alcohols because of the increase in electron delocalization in enolate anions as compared to the neutral enols (see Section 15-8A). The stabilization energy of benzenol (Table 21-1) is 48 kcal mole-1, 5 kcal greater than that of benzene. We can ascribe this increase to delocalization of an unshared electron pair from oxygen ... 

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. 

These results clearly show that the potential energy surface can contain a series of minima. The fact that selectivity in re-attack by the F ions can be observed indicates that the differences between the energy barriers for the secondary reactions control the distribution of the final products. The multistep character of these processes is further illustrated by the reactions observed when enolate anions are used as reactant ions. The ambident enolate anions may react with methyl pentafluorophenyl ether at the carbon or the oxygen site. If they react with the carbon site at the fluorine-bearing carbon atoms, then the molecule in the F ion/molecule complex formed contains relatively acidic hydrogen atoms so that proton transfer to the displaced F ion may occur. An example is given in (47) where the enolate anion, generated by HF loss, is not observed. An intramolecular nucleophilic aromatic substitution occurs instead and leads to a second F ion/ molecule complex. The F" ion in this complex then re-attacks the substituted benzofuran molecule formed, either by proton transfer or SN2 substitution. 

The second mechanism. Reaction 6.32, proposes that water acts as a base to create acetone enolate anion. This enolate next adds to acetaldehyde through a TS that is 49.1 kcal mol above reactants. The final aldol product is obtained by proton transfer. While this mechanism require less energy than the first of Houk s mechanisms, it too is unlikely to be competitive, given its large barrier for formation of the acetone enolate. 

Treatment of the following enone with NaOD in D2O results in the exchange of fiw hydrogens by deuterium. Use SpartanView to obtain the energies of all enolal anions that might be produced from the enone, draw resonance structures for thedi and predict the structure of the deuterium-containing product. 

The rearrangement of cw-divinylcyclopropane usually proceeds below room tem-perature. " When an enolate anion or an enol ether is contained in the framework of the cyclopropane, the activation energy, and hence the temperature for the rearrangement, is lowered. Several examples exist of rearrangements of divinylcyclopropanes to cycloheptadienes that proceed below room temperature. " ... 

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