Resonance enolate forms

A perhaps more exotic substrate for the Heck reaction is 1,2-cyclohexanedione [25], The reactivity of this molecule under Heck coupling conditions can probably be attributed to its resonance enol form. This reaction is attractive, because the literature contains relatively few examples of the preparation of 3-aryl-l,2-cyclohexane-diones. Yields varied from good to modest when classic heating and electron-rich aryl bromides were used, and reaction times typically ranged from 16 to 48 h. Similar yields were obtained under continuous microwave irradiation with a single-mode microwave reactor for 10 min at 40-50 W (Eq. 11.10) [25],... 

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... 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

Nucleophile attacks >C=C< of enol-form Acidic a-H, -CH -CHO can form resonance... 

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

Diketones usually exist as mixtures of tautomeric keto and enol forms. The enolic form does not show the normal absorption of conjugated ketones. Instead, a broad band appears in the 1640-1580 cm-1 region, many times more intense than normal carbonyl absorption. The intense and displaced absorption results from intramolecular hydrogen bonding, the bonded structure being stabilized by resonance. 

Formation of intramolecular hydrogen bonds shows up clearly in the proton nmr spectra, as we have seen previously for the enol form of 2,4-pentanedione (Section 17-ID, Figure 17-1). Figure 26-1 shows the proton resonances of the nitrobenzenol isomers, and you will see that the ortho isomer has the OH proton resonance at much lower field than either the meta or para isomer. Only for the ortho isomer are the nitro and hydroxyl groups sufficiently close together to form an intramolecular hydrogen bond ... 

Barbituric acid (83) may be regarded as 2,4,6-trihydroxypyrimidine, but in the crystalline state it exists as the triketo-form (95). In aqueous solution the compound is remarkedly acidic as the result of ionisation of the mono-enolic form (96) with the formation of a resonance stabilised anion (97). 

The enol form of phenol benefits from the resonance stabilization and aromaticity of the benzene ring, but the keto form, with the single sp3 center in the ring, cannot. 

NMR spectroscopy indicates that 3-acetyl-6,6-dimethyltetrahydrothiopyran-2,4-dione exists exclusively in a single enolic form 266 as does the derived enamine 267 <2003RJ0235> and a similar situation obtains for 4-acyltetrahydrothiopyran-3,5-diones for which the enolic proton resonates at ca. 618 <2003RJ01772>. 

The structures of the adducts were confirmed by H NMR. The regio-orientation of the pyrrole ring in compound (7) is apparent from the chemical shifts and splitting patterns of resonances due to the H-4a and H-7a protons. The doublet for H-7a is at a lower field than that for H-4a which appears as a multiplet due to spin-spin coupling with the H-5a, H-7a and H-4 (NH) protons. Proton NMR studies also reveal that there are two enolic forms of the ketone function in (7) but the major tautomer is the keto form (7a) (Scheme 1). 

There are three different resonance possibilities for the enolic double bond—viz., with the aromatic nucleus, with the cyano, and with the carbonyl group. Furthermore, the a-hydrogen of the keto form would be acidic owing to the presence of the cyano group. Accordingly, all three compounds gave positive reactions to the iron chloride enol test. Further proof for the enol form in the crystalline state is given by the infrared spectra taken in Nujol and Fluorolube. 

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. 

A carbonyl group dramatically increases the acidity of the protons on the a carbon atom because deprotonation gives a resonance-stabilized enolate ion. Most of the enolate ion s negative charge resides on the electronegative oxygen atom. The pKa for removal of an a proton from a typical ketone or aldehyde is about 20, showing that a typical ketone or aldehyde is much more acidic than an alkane or an alkene (pKa > 40), or even an alkyne (pKa = 25). Still, a ketone or aldehyde is less acidic than water (pKa = 15.7) or an alcohol (pA a = 16 to 18). When a simple ketone or aldehyde is treated with hydroxide ion or an alkoxide ion, the equilibrium mixture contains only a small fraction of the deprotonated, enolate form. 

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. 

Evidence for Resonance-Assisted Hydrogen Bonding from Crystal Structure Correlations on the Enol Form of the P-Diketone Fragment. 

This structure, however, is much less probable from energy considerations, since although 17-2 kcals have been gained on account of the redistribution of the bonds, this transformation is accompanied by a much greater decrease of the resonance energy of the benzene ring. Thus the keto is much less stable than the enol form and the latter predominates. 

In the enolic form, resonance of the two normal structures / and II with the ionic structures III to VI occurs ... 

The metal may replace the hydrogen atom of the enolic form with the formation of a salt having a structure which resonates between X and XI ... 

Resonance involves only the movement of electrons over the same atomic skeleton. Thus the keto and enol forms of acetoacctic ester are not... 

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