Ester enolate anion

Enmatin B [917-13-5] Enmatin C [19893-23-3] Enociamna Enol Enolase Enolate anions Enolboration Enol esters... 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymeri2ation of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkaU metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Usehil initiating systems for acrylate polymeri2ation have been prepared from complexes of ester enolates with alkak metal alkoxides (104,105). 

Reaction of Enolate Anions. In the presence of certain bases, eg, sodium alkoxide, an ester having a hydrogen on the a-carbon atom undergoes a wide variety of characteristic enolate reactions. Mechanistically, the base removes a proton from the a-carbon, giving an enolate that then can react with an electrophile. Depending on the final product, the base may be consumed stoichiometricaHy or may function as a catalyst. Eor example, the sodium alkoxide used in the Claisen condensation is a catalyst ... 

The alkylation reactions of enolate anions of both ketones and esters have been extensively utilized in synthesis. Both very stable enolates, such as those derived from (i-ketoesters, / -diketones, and malonate esters, as well as less stable enolates of monofunctional ketones, esters, nitriles, etc., are reactive. Many aspects of the relationships between reactivity, stereochemistry, and mechanism have been clarified. A starting point for the discussion of these reactions is the structure of the enolates. Because of the delocalized nature of enolates, an electrophile can attack either at oxygen or at carbon. 

Enolate anions of p-keto esters react with some fluoroolefms, initially by replacement of a vinylic fluorine atom, to give ultimately heterocyclic products [2S, 29] (equation 25). 

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

The reactive species is the corresponding enolate-anion 4 of malonic ester 1. The anion can be obtained by deprotonation with a base it is stabilized by resonance. The alkylation step with an alkyl halide 2 proceeds by a Sn2 reaction ... 

In the presence of a strong base, the ot carbon of a carboxylic ester can condense with the carbonyl carbon of an aldehyde or ketone to give a P-hydroxy ester, which may or may not be dehydrated to the a,P-unsaturated ester. This reaction is sometimes called the Claisen reaction,an unfortunate usage since that name is more firmly connected to 10-118. In a modem example of how the reaction is used, addition of tert-butyl acetate to LDA in hexane at -78°C gives the lithium salt of ferf-butyl acetate, " (12-21) an enolate anion. Subsequent reaction a ketone provides a simple rapid alternative to the Reformatsky reaction (16-31) as a means of preparing P-hydroxy erf-butyl esters. It is also possible for the a carbon of an aldehyde or ketone to add to the carbonyl carbon of a carboxylic ester, but this is a different reaction (10-119) involving nucleophilic substitution and not addition to a C=0 bond. It can, however, be a side reaction if the aldehyde or ketone has an a hydrogen. 

Besides ordinary esters (containing an a hydrogen), the reaction can also be carried out with lactones and, as in 16-38, with the y position of a,p-unsaturated esters (vinylogy). There are also cases, where the enolate anion of an amide was condensed with an aldehyde. ... 

Active methylene compounds may be sulfinylated by reaction of their enolate anions with sulfinate ester . This reaction has been investigated much in recent years and the compounds resulting from it have been of considerable use in asymmetric synthesis (see the chapter by Posner). Examples of the sulfinylation are given in the following paragraphs. 

SRNl substitution include ketone enolates,183 ester enolates,184 amide enolates,185 2,4-pentanedione dianion,186 pentadienyl and indenyl carbanions,187 phenolates,188 diethyl phosphite anion,189 phosphides,190 and thiolates.191 The reactions are frequently initiated by light, which promotes the initiating electron transfer. As for other radical chain processes, the reaction is sensitive to substances that can intercept the propagation intermediates. 

Parts A and B of the procedure correspond to preparation of lithium tetramethylpiperidide, and its use in the in situ preparation and addition of dibromomethyllithium to the ester 1 producing tetrahedral intermediate 2. In Part C a mixture of lithium hexamethyldisilazide and lithium ethoxide is prepared for addition in Part D to the solution of 2. The silazide base serves to deprotonate the mono and dibromo ketones that are formed on initial warming of the reaction to -20°C, thus protecting them as the enolate anions 4 and 3. Addition of the sec-butyllithium in Part... 

The preferential -configuration of the enol esters, derived from p-dicarbonyl compounds under phase-transfer conditions, contrasts with the formation of the Z-enol esters when the reaction is carried out by classical procedures using alkali metal alkoxides. In the latter case, the U form of the intermediate enolate anion is stabilized by chelation with the alkali metal cation, thereby promoting the exclusive formation of the Z-enol ester (9) (Scheme 3.5), whereas the formation of the ion-pair with the quaternary ammonium cation allows the carbanion to adopt the thermodynamically more stable sickle or W forms, (7) and (8), which lead to the E-enol esters (10) [54],... 

---