Homoenolate ion

CH2 groups are not acidic enough for this base (2) recovered 18 was racemized 20 is symmetrical and can be attacked equally well from either side (3) when the experiment was performed in deuterated solvent, the rate of deuterium uptake was equal to the rate of racemization and (4) recovered 18 contained up to three atoms of deuterium per molecule, though if 19 were the only ion, no more than two could be taken up. Ions of this type, in which a negatively charged carbon is stabilized by a carbonyl group two carbons away, are called homoenolate ions. 

Cyclopropanol may be regarded as the simplest homoenol, the cyclopropylalkoxide ion structure is one of the pair of hybrid structures which constitute the parent homoenolate ion (equation 14). 

The complexes generated by oxidative addition of haloarenes and haloalkenes to palladium(O) are electrophilic at the metal-substituted center, and can therefore react with carbon nucleophiles other than alkenes, especially with enolate and homoenolate ions to form new C—C bonds [176, 177]. 

Enolisation 1 involves the removal of the a-proton from a carbonyl compound to form an enolate ion 2. Homoenolisation involves the removal of a (i-proton 3 to form the homoenolate ion 4 or 5. Both the enolate and the homoenolate can be represented as carbanions, but whereas the enolate version 2b is merely a different way of representing a single delocalised structure, the homoenolate 5 is a different compound from the cyclopropane 4. No literal examples of homoenolates 5 are known so they have the status of synthons which may be represented in real life by reagents derived from cyclopropanols 4 among many other possibilities.1... 

Homoenolate ions. Cyclopropyloxyzinc iodides are generated from a-sulfonyloxy carbonyl compounds on reaction with CH2(ZnI)2. 

An a,p-unsaturated aldehyde adds to a nitrone to give y-hydroxylaminoalkanoic ester when the substrates are exposed to an azolecarhene, and the reaction mixture is quenched by an alcohol. Homoenolate ion generated from the aldehyde and the carbene is the nucleophile. The use of carbene 109 engenders chiral products. ... 

The synthetic use of acylsilanes depends to a large extent on the ease of migration of Si to an alternative site in the molecule after addition of a suitable nucleophile. For instance, enolate anions add to the carbonyl carbon atom of a-chloroacyltrimethylsilanes, the McsSi unit migrating to the a-C atom with displacement of Cl. In this case, the product can be desilylated to give a /8-diketone. Alternatively, the Si can migrate to the carbonyl O atom, as in Scheme 14, where either a homoenolate ion is set up for further alkylation or, in the case of 2-furyl-lithium addition, the furyl ring opens to give a cumulated dienolate system. 

From the point of view of the synthesis of dissonant systems the most important finding reported by Reissig [19c] is the opening of cyclopropanes by fluoride ion-induced desilylation to give carboxylic ester stabilised "homoenolate" anions, from which a series of 4-oxoalkanoic esters (21a ). with a 1,4-D relationship, were prepared (Table 5.6) ... 

Marino and his coworkers [20a], on the other hand, studied the fluoride ion-induced desilylation of ethyl 2-silyloxycyclopanecarboxylates 24 and the resulting "homoenolate" anion 25 was allowed to react with different electrophiles, such as Michael acceptors, to give dissonant cyclopentene rings (26) via a [3 -i- 2] annulation strategy (Scheme 5.16). 

The formation of activated iminium intermediates derived from nitrogen heterocycles has been reported by Comins and co-workers.163,163a The activation of pyridine derivative with phenyl chloroformate provides pyridinium salt, which smoothly reacts with the zinc homoenolate (Equation (94)).163 163a 164 The reaction of unsaturated amide with Ph jC 1 BI 4 produces iV-acyliminium ions, which react with PhaZn in CH2CI2 producing the desired a--substituted amine (Equation (95)).165... 

Reformatsky reagent which then adds to carbonyl derivatives according to Scheme 20. These authors also observed conjugate addition of the homoenolate anion to a,)5-unsaturated ketones in the presence of copper ion as shown ... 

The acyl cation 2a or acylium ion 2b is a familiar intermediate in the Friedel-Crafts reaction. It is easy to make (acid chloride + Lewis acid 1) and it can be observed by NMR as it expresses the natural reactivity pattern of the acyl group. The acyl anion by contrast has umpolung or reverse polarity.1 One might imagine making it from an aldehyde by deprotonation 3 and that it would be trigonal 4a or possibly an oxy-carbene 4b. Such species are (probably) unknown and their rarity as well as their potential in synthesis has led to many synthetic equivalents for this elusive synthon. The acyl anion, the d1 synthon, is the parent of all synthons with umpolung2 and should perhaps have been treated before the homoenolates dealt with in the previous chapter. 

The requisite homoenolates were prepared through treatment of cirmamaldehyde derivatives, e.g., 147, with (V-heterocyclic carbenes derived from imidazolium ion 148 and DBU. 

It is proposed that primarily a nucleophilic carbene 24 (cf. p. 221) is generated by deprotonation of 22, which adds to the a,f -unsaturated aldehyde to give a homoenolate equivalent 25 as umpoled R-CH=CH-CH=0 synthon. Protonation of 25 in P-position leads to the 2-substituted imidazohum ion 26 on reaction with 26, the sahcylalde-hyde is O-acylated 27), the imidazohum ion 22 is regenerated, and (base-induced) intramolecular Claisen condensation of 27 provides the coumarin (23). 

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