Stability enolate

The present chapter extends our study of carbanions to the enolate ions derived from esters Ester enolates are important reagents m synthetic organic chemistry The stabilized enolates derived from p keto esters are particularly useful... 

Most of the reactions of ester enolates described so far have centered on stabilized eno lates derived from 1 3 dicarbonyl compounds such as diethyl malonate and ethyl ace toacetate Although the synthetic value of these and related stabilized enolates is clear chemists have long been interested m extending the usefulness of nonstabilized enolates derived from simple esters Consider the deprotonation of an ester as represented by the acid—base reaction... 

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... 

Tnfluoroacetonitrile oxide also reacts with stabilized enolate ions, such as that derived from 2,4-pentanedione, to give good yields of 1,3-dipolar adducts [38] (equation 38). 

In this paper Speziale and Smith 109) described experiments which led them to modify the mechanism proposed earlier 108) for the reaction of trivalent phosphorus compounds with haloamides. The first step is considered to be attack of the trivalent phosphorus compound on a chlorine atom of the halo amide (132) to produce a resonance-stabilized enolate ion (133). This is reasonable since under conditions where the trichloroamide... 

O Base abstracts an acidic alpha hydrogen from one acetaldehyde molecule, yielding a resonance-stabilized enolate ion. 

Manchmal, bei sterischer Hinderung, unter Bildung von Ketonen, die bei stabiler Enol-Form nicht weiter reduziert werden1 (Blei-Ionen wirken katalytisch2). 

Scheme 2.23 provides some examples of conjugate addition reactions. Entry 1 illustrates the tendency for reaction to proceed through the more stable enolate. Entries 2 to 5 are typical examples of addition of doubly stabilized enolates to electrophilic alkenes. Entries 6 to 8 are cases of addition of nitroalkanes. Nitroalkanes are comparable in acidity to (i-ketocslcrs (see Table 1.1) and are often excellent nucleophiles for conjugate addition. Note that in Entry 8 fluoride ion is used as the base. Entry 9 is a case of adding a zinc enolate (Reformatsky reagent) to a nitroalkene. Entry 10 shows an enamine as the carbon nucleophile. All of these reactions were done under equilibrating conditions. 

In principle, stereoselective aldol condensations can be carried out under two distinct sets of conditions. Under the influence of acid catalysis, stabilized enol derivatives of defined geometry (M = SiMea,... 

The first iridium catalysts for allylic substitution were published in 1997. Takeuchi showed that the combination of [fr(COD)Cl]2 and triphenylphosphite catalyzes the addition of malonate nucleophiles to the substituted terminus of t -allyliridium intermediates that are generated from allylic acetates. This selectivity for attack at the more substituted terminus gives rise to the branched allylic alkylation products (Fig. 4), rather than the linear products that had been formed by palladium-catalyzed allylic substitution reactions at that time [7]. The initial scope of iridium-catalyzed allylic substitution was also restricted to stabilized enolate nucleophiles, but it was quickly expanded to a wide range of other nucleophiles. 

Most allylic substitution reactions catalyzed by other metals are selective for the formation of branched products. Although this had been demonstrated for a large portion of the d-block before Takeuchi s work with iridium, most of the progress in this area was restricted to stabilized enolate nucleophiles. 

Addition to linear 1,1-disubstituted allylic acetates is slower than addition to monosubstituted allylic esters. Additions to allylic trifluoroacetates or phosphates are faster than additions to allylic carbonates or acetates, and reactions of branched allylic esters are faster than additions to linear allylic esters. Aryl-, vinyl, alkynyl, and alkyl-substituted allylic esters readily undergo allylic substitution. Amines and stabilized enolates both react with these electrophiles in the presence of the catalyst generated from an iridium precursor and triphenylphosphite. 

Iridium-Catalyzed Allylic Substitution with Stabilized Enolates... 

Recently, Smulik and Vedejs have reported that amination of ester enolates and enim-inates with 0-(p-nitrobenzoyl)hydroxylamine 21 takes place with good yields . However, reaction of enolates derived from ethyl phenylacetate and phenylacetonitrile gave lower yields compared with stabilized enolates derived from diethyl malonate, diethyl 2-phenylmalonate and 2-phenyl-2-cyanopropionate (Scheme 23). 

It is also appropriate to recognize the role of enolates stabilized by an exocyclic carbonyl function in C-glycoside synthesis. The use of LN to achieve reductive cleavage of anomeric sulfones to provide access to an ester-stabilized enolate and, ultimately, 2-deoxy- -C-glycosides has already been illustrated in Scheme 11 (Sect. 2.1.1). 

Mechanism. Removal of an a-hydrogen from the acetaldehyde by NaOH produces a resonance-stabilized enolate anion. Nucleophilic addition of the enolate to the carbonyl carbon of another acetaldehyde gives an alkoxide tetrahedral intermediate. The resulting alkoxide is protonated by the solvent, water, to give 3-hydroxybutanal and regenerate the hydroxide ion. 

Mechanism. Removal of an a-hydrogen from the ethyl acetate by NaOEt produces a resonance-stabilized enolate anion. 

---