Stability enolate anions

These last reagents, where the anion is stabilized both by the adjacent carbonyl group (as an enolate) and by the adjacent P=0 group, are just one of many examples of enolate anions stabilized by... 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

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... 

Enolate ion formation (Section 18.6) An a hydrogen of an aldehyde or a ketone is more acidic than most other protons bound to carbon. Aldehydes and ketones are weak acids, with pK s in the 16 to 20 range. Their enhanced acidity is due to the electron-withdrawing effect of the carbonyl group and the resonance stabilization of the enolate anion. 

The reaction can be performed with base catalysis as well as acid catalysis. The former is more common here the enolizable carbonyl compound 1 is depro-tonated at the a-carbon by base (e.g. alkali hydroxide) to give the enolate anion 5, which is stabilized by resonance ... 

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 ... 

These reactions accomplish the same overall synthetic transformation as the acylation of ester enolates, but use desulfurization rather than decarboxylation to remove the anion-stabilizing group. Dimethyl sulfone can be subjected to similar reaction sequences.232... 

Among Michael acceptors that have been shown to react with ketone and ester enolates under kinetic conditions are methyl a-trimethylsilylvinyl ketone,295 methyl a-methylthioacrylate,296 methyl methylthiovinyl sulfoxide,297 and ethyl a-cyanoacrylate.298 Each of these acceptors benefits from a second anion-stabilizing substituent. The latter class of acceptors has been found to be capable of generating contiguous quaternary carbon centers. 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

STABILIZATION OF ENOLATE ANIONS THE ENOLASE SUPER FAMILY... 

Stabilization of Enolate Anions The Enolase Super Family.173... 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

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],... 

Reactions involving ketones are generally controlled by the thermodynamic stability of the enolate anion. However, 2-phenylcyclohexanone reacts with bulky Michael acceptors to form the 2,6-regioisomer preferentially [17], indicating that the reaction is mainly kinetically controlled with the approach of the Michael acceptor to the substituted 2-position being sterically hindered. 

The third mechanism starts with addition of the AT-allylamine 103 to the cumulated acceptor system of an allene carbonester 108 (Acc=CHC02Me) to form an intermediate iV-allyl ammonium amide enolate 109 (allene carbonester Claisen rearrangement). The anion stabilizing group is exclusively placed... 

The CPop intermediate is the j5-cuprio ketone intermediate widely debated in mechanistic discussions of conjugate addition (cf. Scheme 10.3). On the basis of recent theoretical analysis, two limiting structures for CPop may now be considered these are shown in the bottom box in Scheme 10.5. The reason for the exceptional stability of CPop as a trialkylcopper(III) species can be readily understood in terms of the j5-cuprio(III) enolate structure, with the internal enolate anion acting as a strong stabilizing ligand for the Cu state [82]. 

The acidity of a C-H is further enhanced if it is adjacent to two carbonyl groups, as in the 1,3-diketone acetylacetone. The enolate anion is stabilized by delocalization, and both carbonyl oxygens can participate in the process. This is reflected in the VK, 9 for the protons between the two carbonyls. 

An enolate anion behaves as a carbanion nucleophile, the carbonyl group stabilizing the anion by... 

---