Enolate anions, from

Cationic vinylidene complexes can be considered to be metal-stabilized vinyl cations in purely organic chemistry, vinyl cations can be obtained by dissociation of the super leaving group, TfO-, from the esters obtained from enolate anions and Tf20. 

Selenides eliminate readily without a base. They are generally prepared from enolate anions by reaction with diphenyldiselenide or phenylselenyl bromide to give phenylselenides. The phenylselenides are oxidized with sodium periodate, hydrogen peroxide, or peracids to the selenoxides, which eliminate even at room temperature to afford the a,p-unsamrated ketones and esters [107]. 

Each act of proton abstraction from the a carbon converts a chiral molecule to an achi ral enol or enolate ion The sp hybridized carbon that is the chirality center m the start mg ketone becomes sp hybridized m the enol or enolate Careful kinetic studies have established that the rate of loss of optical activity of sec butyl phenyl ketone is equal to Its rate of hydrogen-deuterium exchange its rate of brommation and its rate of lodma tion In each case the rate determining step is conversion of the starting ketone to the enol or enolate anion... 

Alkyl halides by reaction with the enolate anion derived from diethyl acetamidomalonate... 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymeri2ation of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkaU metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Usehil initiating systems for acrylate polymeri2ation have been prepared from complexes of ester enolates with alkak metal alkoxides (104,105). 

Reaction of Enolate Anions. In the presence of certain bases, eg, sodium alkoxide, an ester having a hydrogen on the a-carbon atom undergoes a wide variety of characteristic enolate reactions. Mechanistically, the base removes a proton from the a-carbon, giving an enolate that then can react with an electrophile. Depending on the final product, the base may be consumed stoichiometricaHy or may function as a catalyst. Eor example, the sodium alkoxide used in the Claisen condensation is a catalyst ... 

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. 

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. 

Azirines react with enolate anions. Initial nucleophilic attack on phenyl 1-azirine by the enolate anion derived from acetophenone gives intermediate (223) which undergoes 1,2-bond cleavage, cyclization and hydroxyl group elimination to give pyrrole (226). 

The reactions of ketenes or ketene equivalents with imines, discussed above, all involve the imine acting as nucleophile. Azetidin-2-ones can also be produced by nucleophilic attack of enolate anions derived from the acetic acid derivative on the electrophilic carbon of the imine followed by cyclization. The reaction of Reformatsky reagents, for example... 

The idea of kinetic versus thermodynamic control can be illustrated by discussing briefly the case of formation of enolate anions from unsymmetrical ketones. This is a very important matter for synthesis and will be discussed more fully in Chapter 1 of Part B. Most ketones, highly symmetric ones being the exception, can give rise to more than one enolate. Many studies have shown tiiat the ratio among the possible enolates that are formed depends on the reaction conditions. This can be illustrated for the case of 3-methyl-2-butanone. If the base chosen is a strong, sterically hindered one and the solvent is aptotic, the major enolate formed is 3. If a protic solvent is used or if a weaker base (one comparable in basicity to the ketone enolate) is used, the dominant enolate is 2. Enolate 3 is the kinetic enolate whereas 2 is the thermodynamically favored enolate. 

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... 

The alkylation reactions of enolate anions of both ketones and esters have been extensively utilized in synthesis. Both very stable enolates, such as those derived from (i-ketoesters, / -diketones, and malonate esters, as well as less stable enolates of monofunctional ketones, esters, nitriles, etc., are reactive. Many aspects of the relationships between reactivity, stereochemistry, and mechanism have been clarified. A starting point for the discussion of these reactions is the structure of the enolates. Because of the delocalized nature of enolates, an electrophile can attack either at oxygen or at carbon. 

These mechanistic interpretations can also be applied to the hydrogenation of cyclohexanones. In acid, the carbonium ion (19) is formed and adsorbed on the catalyst from the least hindered side. Hydride ion transfer from the catalyst gives the axial alcohol (20). " In base, the enolate anion (21) is also adsorbed from the least hindered side. Hydride ion transfer from the catalyst followed by protonation from the solution gives the equatorial alcohol (22). 

Extensions of this concept have utilized enamine hydrolysis (171, X = R N) and the quenching of the enolate anion (171, X = O ) e.g. ref. 353). a,(i-Un-saturated ketones are usually more stable than their p,y counterparts, but there are notable exceptions to this, and in such cases the deconjugated ketone may be isolated from the equilibrated system. For example, retro steroids (9, 10a) have a large proportion of A -3-ketone at equilibrium, and 17-ketones yield the more stable A -system on treatment with acid. ... 

The 17a-ethynyl compound (59) has been prepared in 88% yield from estr-4-ene-3,17-dione (58) and acetylene, at 2-3 atm pressure in tetrahydro-furan in the presence of potassium t-butoxide. Presumably the A-ring enone system is protected as the enolate anion during the course of the reaction. 

Bromination of the enolate anion from the reaction of 3j -acetoxypregna-5,16-dien-20-one (1) with methylmagnesium bromide in the presence of cuprous chloride affords (after treatment with sodium iodide to dehalo-genate any 5,6-dibromide) a mixture of 17a-bromo- and 17)5-bromo-16a-methyl compounds (11) and (12) in a ratio 9 1. The 17a-iodides can be obtained in an analogous reaction. 

The equatorial orientation of the newly introduced alkyl group may be controlled in both (3) and (6) by stereoelectronic and steric factors. The attack of the enolate anions (2) and (5) by the alkyl halide proceeds in a plane that is perpendicular to the plane of the enolate system. Products result from attack at the less hindered a- or -face, respectively... 

Acetylenedimagnesium bromide, 66, 84, 137 Acyl-alkyl diradical disproportionations, 299 Acyl-alkyl diradical recombination, 296 Alkaline hydrogen peroxide, 10, 12, 20 Alkylation of formyl ketones, 93 Alkylation via enolate anions, 86 17a-Alkynyl steroids from 17-ketones, 67 2a-Al]yl-17jS-hydroxy-5a-androstan-3 -one, 9 5 Allylic acetoxylation, 242 Allylmagnesium bromide, 64 17 -Aminoandrost-5-en-3 -ol, 145 17 a-Aminomethy 1-5 a-androstane-3, 1718-diol, 387... 

Anotheranalogy between the enolate anions derived from a,)3-unsatura ted ketones and the corresponding enamines is encountered in their alkylation reactions (57), which proceed by the kinetically controlled attack at the a-carbon atom. For instance, Stork and Birnbaum (51) found that the alkylation of the morpholine enamine of /J -octalone-2 (117) with methyl iodide gave the C-1 methylated derivative (118). 

The problem of nitrogen alkylation of enamines, which one encounters with alkyl halides, is of no consequence in alkylations with positively activated olefins, since the generation of amonium salts can be expected to be reversible in these cases. Thus such enamine alkylations are obviously attractive to the synthetic chemist. Their particular importance, however, arises from avoidance of the serious obstacles often found with parallel enolate anion reactions. 

Enolate chemistry follows from this anion acting as a carbon nucleophile. 

The enolate anion 1 may in principle be generated from any enolizable carbonyl compound 4 by treatment with base the reaction works especially well with /3-dicarbonyl compounds. The enolate 1 adds to the a ,/3-unsaturated compound 2 to give an intermediate new enolate 5, which yields the 1,5-dicarbonyl compound 3 upon hydrolytic workup ... 

Q Base removes an acidic hydrogen from the a position of the carbonyl compound, yielding an enolate anion that has two resonance structures. 

Active methylene compounds may be sulfinylated by reaction of their enolate anions with sulfinate ester7-1 This reaction has been investigated much in recent years and the compounds resulting from it have been of considerable use in asymmetric synthesis (see the chapter by Posner). Examples of the sulfinylation are given in the following paragraphs. 

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