Enolate anions described

Since most of the synthetically useful enolate anions described in the previous section are prepared by the reactions of enolizable substrates with alkali metal amide bases, it is appropriate to note a few structures of these amide bases. The common bases in synthetic organic chemistry include LDA and LHMDS. The structures of both of these bases are known as the THF solvates.Both of these compounds form bis-solvated dimers corresponding to structure (201). The diethyl ether solvate of LHMDS also forms a bis-solvated dimer (202).Sodium hexamethyldisilazide crystallizes as an unaggregated monomer from benzene solution.Two different cryst line forms of KHMDS are known as the polymeric dioxane solvate (203), ° and the unsolvated dimer (204). ... 

Of the two resonance forms of the enolate anion, that with the charge on the electronegative oxygen will be preferred over that with charge on the carbon. Note the distinct difference between resonance as shown here, a redistribution of electrons, and tautomerism, as described above. Tautomers are isomers in equilibrium and have the atoms arranged differently. 

Any process that produces an enol, including the formation of enolate anions. Enols (i.e., entities containing the moiety HO—C(Ri)=C(R2Rs)) appear as intermediates in a wide variety of enzyme-catalyzed reactions, and Rose has presented the following diagram to describe... 

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... 

The reactions of the nitrites described have also been observed under FA conditions at much higher pressures. The increased pressure might explain the occurrence of an additional pathway observed in the reaction between enolate anions and nitrites and exemplified in (25b) (King et al., 1981). Path (b) has also been reported to occur in the reaction between the enolate anion of diisopropyl ketone and methyl nitrite in an ICR instrument operating at the relatively high pressure of 10 3 Pa (Klass and Bowie, 1980). 

In the localised VB the orbitals are only allowed to extend over part of the molecule. For instance, for an enolate anion, the basic wavefunction would have a doubly occupied 7i-orbital, localised only on the oxygen atom and a doubly occupied 7t-orbital extending over both carbon atoms. Thus, while describing structure 2a in Fig. 14, a single determinant is still employed. Since the orbitals are completely optimised, the a-system can partly counteract the charge separation. 

A similar process is described in the Focus On box titled The Reverse Aldol Reaction in Metabolism on page 880. The Claisen ester condensation also has an equilibrium step in which an enolate anion leaves in the reverse of the step (see Figure 20.4). 

Because of the importance of carbon nucleophiles in synthesis, organic chemists have spent considerable effort developing others in addition to the enolate anions that have already been described. Several of these other carbon nucleophiles are presented in this and the following section. This section describes the use of enamines. 

The powerful electron-withdrawing nature of the nitro group means that deprotonation is possible even with very mild bases (the pkTa of MeNO is 10). The anions react with carbon electrophiles and a wide variety of nitro-containing products can be produced. The anions are not, of course, enolates, but replacing the nitrogen with a carbon should help you to recognize the close similarity of these alkylations with the enolate alkylations described later. 

Silyl enolates are a class of electron-rich, non aromatic compounds which can be described as masked enols or enolates since hydrolysis following their reaction yields ketones they can be purified by distillation or chromatography, and then converted back to the enolate anion. The electron-rich character of these species can be used for oxidation reactions and examples have been described in the preceding sections. In this section, additional examples of chemical, PET and electrochemical redox reactions involving silyl enolates will be discussed, for a better appreciation of these interesting species in organic synthesis. 

The classical example of a stabilized carbanion is the enolate anion, formed from carbonyl compounds upon treatment with a base. Such an ion can be represented as a resonance hybrid of two canonical structures, namely 29, with the charge on carbon, and 30, with the charge on oxygen (Scheme 2.15). The structure is perhaps more properly described by the hybrid structure 31, where the dashed line and the charge sign indicate that the electron pair is spread over... 

The preparation of 4,4-dimethyl steroids (6) by alkylation of the enolate anion 2) of a A 3-ketone was described by... 

This method was first described by Weitz and Scheffer. The oxidation begins with a reversible attack by the nucleophilic hydroperoxy anion on the conjugated system, and an oxirane conjugated with an electron-attracting group is formed by ring-closure following the loss of OH from the intermediate enolate anion (Eq. 15). 

There is a report describing intramolecular, stereoselective cyclopropanations by utilizing the ylide reaction Preparation of tricyclic compounds, such as 24, has been accomplished by intramolecular reaction of phosphorous ylide generated in situ by addition of enolate anion to vinylphosphonium salt (equation 82) . Optically active... 

The guidelines we use for obtaining enolate anion crystals are to find a suitable solvent, concentration and temperature combination such that the crystals grow in a matter of 24 h or so. Typically this involved concentrations of 0.5 to 1.0 M in a solvent combination (hydrocarbon plus donor) that idlows for complete dissolution of the anion and slow crystallization. It is preferable to crystallize the enolate anions in the range of -20 to -SO °C since the crystals obtained at this temperature can often be transferred directly to the diffractometer with a minimum effort. Exact conditions for the crystallizations of many of the compounds described in this chapter are described in the original literature. 

It is noteworthy that Etter has described a recent technique of cociystallizing stable organic compounds with triphenylphosphine oxide.It is possible that additional enolate anions can be crystallized by addition of this ad nd to assist with the solid phase formation however, many of the carbanions already include donors such as TMEDA, THE, etc. summary the crystallization of enolate anions differs little from the crystallization of neutral organic molecules, except that it is often carried out at somewhat lower temperatures. The patience, skill and experience of the chemist often determine whether the crystallization procedure is successful. 

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