Enolate anions, and

A number of novel reactions involving the a carbon atom of aldehydes and ketones involve enol and enolate anion intermediates... 

Enamines and enolate anions react with benzofuroxan to give quinoxaline di-A -oxides (Scheme 38) (69AHC(10)1). Sydnones (274) with phenyl isocyanate give 1,2,4-triazoles (275) (76AHC(19)l), and from (276) the intermediate adduct (277) can be isolated (73JA8452). This is one of the few instances in which such primary cycloadducts have been isolated in the oxazole series of mesoionic compounds. 

The addition (150-157) of Grignard reagents, alkoxides, hydroxide, sulfides, cyanide, and enolate anions to pyridinium and isoquinolinium salts again provides a variety of cyclic enamines of potential synthetic use. 

A variety of enamines and enolate anions have been found to react with benzofuroxan, giving quinoxaline di-V-oxides in moderate yields [Eq, (6)]. 

Alkylation (Sections 8.8, 16.3, 18.2. 22.7) Introduction of an alkyl group onto a molecule. For example, aromatic rings can be alkylated to yield arenes, and enolate anions can be alkylated to yield a-substituted carbonyl compounds. 

Vinyl sulfones, being good Michael acceptors, have been regarded as useful reagents for carbon-carbon bond formation. Nucleophiles used often are organometallic reagents, enamines and enolate anions and the Michael addition products are usually obtained in... 

Let us now look at an example of how nature exploits the equivalent of enol and enolate anion chemistry. Enolization provides another application... 

Enamines and enolate anions react with benzofuroxan to give quinoxaline di-/V-oxides (Scheme 59) (69AHC(10)l). 

PQQ and the other quinone prosthetic groups described here all function in reactions that would be possible for pyridine nucleotide or flavin coenzymes. All of them, like the flavins, can exist in oxidized, half-reduced semiquinone and fully reduced dihydro forms. The questions to be asked are the same as we asked for flavins. How do the substrates react How is the reduced cofactor reoxidized In nonenzymatic reactions alcohols, amines, and enolate anions all add at C-5 of PQQ to give adducts such as that shown for methanol in Eq. 15-51, step a 444,449,449a Although many additional reactions are possible, this addition is a reasonable first step in the mechanism shown in Eq. 15-51. An enzymatic base could remove a proton as is indicated in step b to give PQQH2. The pathway for reoxidation (step c) might involve a cytochrome b, cytochrome c, or bound ubiquinone.445 446... 

CARBONYL COMPOUNDS II. ENOLS AND ENOLATE ANIONS. UNSATURATED AND POLYCARBONYL COMPOUNDS... 

A rare species of salts consisting of a heteroatom-stabilized carbocation and a heteroatom-stabibzed carbanion has been formed by deprotonating methyl (Z)- or (E)-3-hydroxy-2,3-dimesitylpropenoate with tetrakis(dimethylamino)methane the resonance stabilization of the cation [(CH3)2N]3C+ and enolate anion, which is of E-configuration exclusively, since the guanadinium ion is incapable of forming a chelate, prevents a spontaneous O- or C-alkylation.12... 

Figure 3-4. Generation of enol and enolate anions from the keto form of a carbonyl compound.

Given the structure of an aldehyde or ketone, write the structure of the corresponding enol and enolate anion. 

The enhanced acidity of carboxylic acids and enols relative to alcohols has long been attributed to the stabilisation of the carboxylate and enolate anions by delocalisation of their n electrons (see 1 and 2 below). Alkoxide anions, as saturated systems, are not subject to resonance stabilisation. 

From a consideration of the optimised geometries, it could be concluded that both the acids and the deprotonated anions are subject to some 7t-electron delocalisation. In accord with chemical intuition, the effect of delocalisation is more important in the carboxylate and enolate anions than in the other species. However, the geometry changes that the acids undergo under deprotonation are only partly explained by 7t delocalisation. 

First, this chapter discusses the structure, stability, and methods for generation of ends and enolate anions. The reactions of these species as nucleophiles with halo-... 

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