Enolate anions, halogenation

The methyl group of a methyl ketone is converted into an a ,a ,a -trihalomethyl group by three subsequent analogous halogenation steps, that involve formation of an intermediate enolate anion (4-6) by deprotonation in alkaline solution, and introduction of one halogen atom in each step by reaction with the halogen. A... 

Aldehydes and ketones nndergo acid- and base-catalysed halogenation in the a position. This is also dependent on enolization or the formation of enolate anions. 

To account for the role of the catalysts and the independence of the rate from the halogen concentration, the ketone necessarily must be slowly converted by the catalysts to something that can react rapidly with halogen to give the products. This something is either the enol or the enolate anion of 2-propanone ... 

The reaction of either the enol or the enolate anion (Equations 17-2 or 17-3) with Br2 resembles the first step in the electrophilic addition of halogens to carbon-carbon multiple bonds (Section 10-3A). However, the second step,... 

The reactions discussed in this chapter that depend on the formation of enolate anions (i.e., halogenation, aldol addition, and alkylation) often proceed smoothly and under milder conditions with 1,3-diketones than with monoketones. This is because the 1,3-diketones are stronger acids and therefore can form the enolate anions with weaker bases. The principal synthetic methods for preparing 1,3-dicarbonyl compounds will be discussed in Chapter 18. 

The enolization process, i,e. conversion of a carbonyl compound such as 438 into the intermediate enol 439 or enolate anion 440 is an important reaction in organic chemistry because these intermediates can further react with electrophiles to undergo either protonation, halogenation, alkylation, aldo-lization, or acylation type reactions. 

Treatment of acetone with base will yield a small amount of the enolate anion. In the presence of Cl2 as a halogen source, a chlorination can occur. Repeating this process twice would then lead to trichloroacetone. Note that after the first chlorination, the enolate will always be formed on the same side as the existing Cl since that enolate will be more stable (due to the presence of the electron-withdrawing Cl). 

Disubstitution products are obtained when dihalobenzenes (Cl, Br, I) react with aliphatic ketone enolate anions. Conversely, the reactions of o-iodohalobenzenes (X = I, Br, Cl) with the enolate anions of aromatic ketones, such as acetophenone, propiophenone and 2-naphthyl methyl ketone in DMSO yield mainly monosubstitution with the retention of one halogen (Scheme 10.7). The extent of dehalogenation is explained in terms of the energetics of the intramolecular ET from the ArCO-7t-system to the C—X bond in the monosubstituted radical anions proposed as intermediates [19]. 

Halogen substituents in a "-haloketones are easily removed even by zinc dust, because of the greater tendency of the halogen to ionisation. The reduction may be represented by a mechanism like that for acetoxy ketones, the ease of removal of halogens following their usual order of reactivity F < Cl < Br < I. In one case the intermediacy of an enolate anion was revealed by direct isolation of an enol acetate aa-chloro-Sa-cholestan 3 One was dechlorinated with zinc in acetic anhydride to give 3-acetoxycholest-2 ene [ 112]. 

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