Keto resonance

Principal resonance structures of the anion of a 3 keto ester... 

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

By analogy to phenols we would expect the isomers with —OH groups on benzene like rings to be more stable This turns out not to be true because the keto forms are also aromatic owing to amide resonance... 

IR spectroscopy has been particularly helpful in detecting the presence of keto tautomers of the hydroxy heterocycles discussed in Section 3.01.6. Some typical frequencies for such compounds are indicated in Figure 4. Here again the doublets observed for some of the carbonyl stretching frequencies have been ascribed to Fermi resonance. 

Since two quaternary atoms and four CH atoms appear in the C NMR spectrum, the latter with a benzenoid coupling constant of 7-9 Hz, this is a disubstituted benzene ring, and the C signal with 5c = 162.2 fits a phenoxy C atom. The keto carbonyl (5c = 204.9) and methyl (5c = 26.6) resonances therefore point to an acetyl group as the only meaningful second substituent. Accordingly, it must be either o- or m-hydroxyacetophenone A or B the para isomer would show only four benzenoid C signals because of the molecular symmetry. 

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... 

A hydrogen attached to the a-carbon atom of a p-keto ester is relatively acidic. Typical P-keto esters have values of about 11. Because the a-carbon atom is flanked by two electron-withdrawing carbonyl groups, a carbanion formed at this site is highly stabilized. The electron delocalization in the anion of a p-keto ester is represented by the resonance structures... 

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... 

Note the difference between tautomers and resonance forms. Tautomers are constitutional isomers—different compounds with different structures—while resonance forms arc different representations of a single structure. Tautomers have their atoms arranged differently, while resonance forms differ only in the position of their electrons. Note also that tautomers are rapidly interconvertible. Thus, keto and enol isomers are tautomers, but aJkene isomers such as 1-butene and 2-butene are not, because they don t interconvert rapidly under normal circumstances. 

Mechanism of base-catalyzed enol formation. The intermediate enolate ion, a resonance hybrid of two forms, can be protonated either on carbon to regenerate the starting keto tautomer or on oxygen to give an enol. 

Because they are resonance hybrids of two nonequivalent forms, enolate ions can be looked at either as vinylic alkoxides (C=C-0 ) or as -keto... 

According to Branchini et al. (2004), luciferase modulates the emission color by controlling the resonance-based charge delocalization of the anionic keto-form of oxyluciferin in the excited state. They proposed the structure C5 as the yellow-green light emitter, and the structure C6 as the red light emitter. 

Ions of the Type. —CO—CR—CO—. These ions, which are derived by removal of a proton from malonic esters, P-keto esters, P-diketones, and so on, are resonance hybrids ... 

Jeevarajan, A. S., M. Khaled et al. (1994a). Simultaneous electrochemical and electron paramagnetic resonance studies of keto and hydroxyl carotenoids. Chem Phys. Lett. 225 340-345. 

A resonance-stabilized lithium salt generated from a y-substituted a,P-unsaturated carboxylate derivative has been found to undergo displacement of chloride from dialkyl chlorophosphites leading (on oxidation) to the formation of P-keto-8-carboxyphosphonates (Equation 4.2 7).71... 

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]. 

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