Reactivity of enols

Enols behave as nucleophiles and react with electrophiles at the a-position. 

As the oxygen atom can donate a lone pair of electrons to the double bond, enols are more nucleophilic than alkenes 

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One of the general features of the reactivity of enolate anions is the sensitivity of both the reaction rate and the ratio of C- versus O-alkylation to the degree of aggregation of the enolate. For example, addition of HMPA fiequently increases the rate of enolate alkylation... 

Reactivity of Enols The Mechanism of Alpha-Substitution Reactions... 

The rate of alkylation of enolate ions is strongly dependent on the solvent in which the reaction is carried out.41 The relative rates of reaction of the sodium enolate of diethyl n-butylmalonate with n-butyl bromide are shown in Table 1.3. Dimethyl sulfoxide (DMSO) and iV,Ai-dimethylformamide (DMF) are particularly effective in enhancing the reactivity of enolate ions. Both of these are polar aprotic solvents. Other... 

The reactivity of enolates is also affected by the metal counterion. For the most commonly used ions the order of reactivity is Mg2+ < Li+ < Na+ < K+. The factors that are responsible for this order are closely related to those described for solvents. The smaller, harder Mg2+ and Li+ cations are more tightly associated with the enolate than are the Na+ and K+ ions. The tighter coordination decreases the reactivity of the enolate and gives rise to more highly associated species. 

The versatile reactivity of enol silyl ethers has also dominated the chemistry of C—C bond formation such as... 

The one-electron oxidation of enol silyl ether donor (as described above) generates a paramagnetic cation radical of greatly enhanced homolytic and electrophilic reactivity. It is the unique dual reactivity of enol silyl ether cation radicals that provides the rich chemistry exploitable for organic synthesis. For example, Snider and coworkers42 showed the facile homolytic capture of the cation radical moiety by a tethered olefinic group in a citronellal derivative to a novel multicyclic derivative from an acyclic precursor (Scheme 8). 

DMSO and /V, A- dime th y I fo nn a in i d c (DMF) are particularly effective in enhancing the reactivity of enolate ions, as Table 1.2 shows. Both of these compounds belong to the polar aprotic class of solvents. Other members of this class that are used as solvents in reactions between carbanions and alkyl halides include N-mcthyI pyrro I i donc (NMP) and hexamethylphosphoric triamide (HMPA). Polar aprotic solvents, as their name implies, are materials which have high dielectric constants but which lack hydroxyl groups or other... 

Reactivity of enols and enolates at carbon. Selectivity in reactions by ... 

The reactivity of enols was shown by Kresge61 to be very low with little carbanion character. Therefore, upon breaking the carbon-carbon bond in the decarboxylation of these acids, the adjacent leaving group possesses minimal carbanion character and will not be subject to significant carboxylation by carbon dioxide. Alternatively, if decarboxylation leads to the enolate, which has carbanion character, the internal return of C02 would become a competing factor. 

Mendez F, Gazquez JL. Chemical reactivity of enolate ions the local hard and soft acids and bases principle viewpoint. J Am Chem Soc 1994 116 9298-9301. 

Specific enol equivalents are intermediates that still have the reactivity of enols or enolates but are stable enough to be prepared in good yield from the carbonyl compound. That was all we needed to know in Chapter 26. Now we know that... 

As emphasized in Section IV of this chapter, the lithiotropy is of much consequence in the reactivity of enolates, the O and C sites competing toward electrophiles. This problem has been examined recently by Meneses and coworkers202, who described a local hardness parameter that can be used as a selectivity index, in particular for a set of ketone lithium enolates. 

Hi Reactivity of enolates derived from a-selenocarbonyl compounds... 

The Hell-Volhard-Zelinskii Reaction 909 Acidity of Alpha Hydrogen Atoms Enolate Ion Formation Reactivity of Enolate Ions 915... 

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