Resonance keto-enol

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

Although Eibner elucidated the structure between 1904 and 1906, it was only through IR and nuclear magnetic resonance spectroscopy (NMR) that the chro-maticity of these molecules could be attributed to keto-enol tautomerism and simultaneous hydrogen bond formation (structures 137a = 137b) [2]. 

The difference in conjugation between neutral molecules and their ion-radicals can also be traced for keto-enol tautomerism. As a rule, enols are usually less stable than ketones. Under the equilibrium conditions, enols exist only at a very low concentration. However, the situation becomes different in the corresponding cation-radicals, where gas-phase experiments have shown that enol cation-radicals are usually more stable than their keto tautomers. This is because enol cation-radicals profit from allylic resonance stabilization that is not available to ketones (Bednarek et al. 2001, references therein). 

Keto-enol-tautomerization is not resonance. The ketone and enol forms are different compounds that are in equilibrium. 

Aldehydes or ketones with an a-hydrogen exist as an equilibrium mixture of keto (H-Ca-C=0) and enol (Ca=C-OH) tautomers. The keto form usually predominates. An a-hydrogen is weakly acidic and can be removed by a base to produce a resonance-stabilized enolate anion. Deuterium exchange of a-hydrogens provides experimental evidence for ends as reaction intermediates. 

High-coordination-number complexes of 0-keto-enolates continue to be obtained with the metals such as zirconium(IV),8 hafnium(IV),8 cerium(IV),9 and the lanthanons(III),10 the last being tetrakis anionic species. At least one example of a volatile tetrakis 0-keto-enolate salt has been reported,11 Cs[Y(CF3-COCHCOCF3)4]. The ionic charge on the 0-keto-enolate complex has been shown to produce12 a high field nuclear magnetic resonance for anions and low field shifts for cations, relative to the positions observed for the neutral species. 

In the presence of strong bases, ketones and aldehydes act as weak proton acids. A proton on the a carbon atom is abstracted to form a resonance-stabilized enolate ion with the negative charge spread over a carbon atom and an oxygen atom. Reprotonation can occur either on the a carbon (returning to the keto form) or on the oxygen atom, giving a vinyl alcohol, the enol form. 

Hydroxy derivatives. 2-Hydroxy derivatives usually exist as the oxo tautomers, unless the hydroxy tautomer is appreciably stabilized by electron-withdrawing or chelating substituents. The tendency for enolic hydroxy compounds to revert to the oxo form can be understood by reference to simple aliphatic ketones where the keto-enol equilibrium constants are of the order of 108. In the five-membered heterocycles under consideration, this tendency will be in opposition to the loss of aromatic resonance energy that increases in the order furan << thiophene < pyrrole. For the 2-hydroxy compounds 217 some extra stabilization of the oxo tautomers 218 and 219 is derived from the resonance energy of the X-C=0 group, which by analogy with open-chain compounds should increase in the sequence thiolester, ester << amide. 

Bulky H should not diffuse or show marked oscillational movement as indicated by magnetic resonance studies. The hydridic model actually provides a reasonable explanation for the mean amplitude of H vibrations (ca. 0.2 A.) and is noncommittal about diffusion. Conceivably, the barrier to diffusion comprising an Is2- configuration about the proton is in effect lowered by the distance of the barrier from the mean position of the nucleus. If the movement of hydrogen is quasitautomeric—for example, in keto-enol tautomerism—one may consider that it moves from one potential well to another as a proton. 

Table 1 summarizes these parameters characterizing the keto-enol equilibria, where A refers to the difference between the enol and keto forms. The enol forms are significantly more stable, consistent with the inclusion of an intramolecular hydrogen bond in the structures and concurrent resonance stabilization. The low frequency torsional vibration of the keto forms can account for their significantly greater relative entropy. 

Draw keto-enol tautomers of carbonyl compounds, identify acidic hydrogens, and draw the resonance forms of enolates. 

Because the p-keto ester formed in Step [3] has especially acidic protons between its two carbonyl groups, a proton is removed under the basic reaction conditions to form an enolate (Step [4]). The formation of this resonance-stabilized enolate drives the equilibrium in the Claisen reaction. 

These carbanions can be formed (Figure 5.8) by proton abstraction from ketones resulting in aldol condensations, by proton abstraction from acetyl CoA, leading to Claisen ester condensation, and by decarboxylation of p-keto acids leading to a resonance-stabilised enolate, which can likewise add to an electrophilic centre. It should be noted that the reverse of decarboxylation also leads to formation of a carbon—carbon bond (this is again a group transfer reaction involving biotin as the carrier of the activated CO2 to be transferred). 

Phenol can be considered as the enol of cyclohexadienone. While the tautomeric keto-enol equilibrium lies far to the ketone side in the case of aliphatic ketones, for phenol it is shifted almost completely to the enol side. The reason of such stabilization is the formation of the aromatic system. The resonance stabilization is very high due to the contribution of the ortho- and / ara-quinonoid resonance structures. In the formation of the phenolate anion, the contribution of quinonoid resonance structures can stabilize the negative charge. 

The coordination of transition metals is known to influence the keto-enol tautomerism in the condensed phase" . The effect of coordination of bare Fe+ ions on the keto-enol equilibrium of phenol was investigated by means of generation of various cyclic [Fe,Cg, He, 0]+-isomers. These isomers were characterized by collisional activation (CA) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry" . It was shown that the energy difference between the phenol-iron complex 65 and the keto isomer 66 is not perturbed by the presence of the iron cation in comparison with the uncom-plexed isomers 3 and 4 (equation 25). Thus, the energy difference for both the neutral and the Fe+-coordinated systems amounts to ca 30 kJ moC in favor of the phenolic tautomer. 

Resonance energies, in keto/enol forms of phenol 39... 

The combined ND (20 K) and XRD (8.4 K) study on benzoylacetone [91] provides an experimental verification of the resonance-assisted HB model, according to which the structure is stabilized via ir-delocalization [92] of the 0=C-C=C-0-H keto-enol group. This is an especially challenging problem, because the diffraction image of a statistically disordered keto-enol system is practically indistinguishable from that of an ordered but delocalized system. The analysis of the ND data excludes the disorder and the experimental BCP indices show fine details of the bonding situation supporting the resonance model there are clear indications for ir-delocalization over the 0=C-C=C-... 

Alpha hydrogens are hydrogens on carbons directly attached to a carbonyl group. They are weakly acidic and can be abstracted by base to form a carbanion. The carbanion is called an enolate ion and is resonance stabilized. Neutralization of the enolate ion results in an enol, a compound in which an alcohol group is directly bonded to a carbon involved in a carbon-carbon double bond. The enol is in equilibrium with the original aldehyde or ketone in an equilibrium referred to as keto-enol tautomerism. The equilibrium usually favors the keto form. 

The process by which enols are converted to aldehydes or ketones is called keto-enol isomerism (or keto-enol tautomerism) and proceeds by the sequence of proton transfers shown in Figure 9.6. Proton transfer to the donble bond of an enol occurs readily because the carbocation that is produced is a very stable one. The positive charge on carbon is stabilized by electron release from oxygen and may be represented in resonance terms as shown on the following page. 

---