Acetone enolate anion from

The vertical electron affinity (EA) of acetone is given as —1.51 eV by Jordan and Burrow386. Lifshitz, Wu and Tiernan387 determine—among other compounds—the excitation function and rate constants of the slow proton transfer reactions between acclone-Ih, acetone-Dg and other ketones. The acetone enolate anion has been produced in a CO2 laser induced alkane elimination from alkoxide anions by Brauman and collaborators388-390. These show, e.g. that the methane elimination from t-butoxide anion is a stepwise process ... 

What is the strength of the base required to produce an enolate anion from a carbonyl compound The strongest base that can exist in aqueous solution is the hydroxide ion. Because the pA of water is 15.7 and the pA of acetone is 19, the amount of the enolate anion of acetone that can form in a 1 M NaOH solution is very small. Although the concentration of enolate is low, it may be sufficient for some reactions. As the enolate reacts, more enolate forms, maintaining the equilibrium. 

Other possible ambident nucleophiles include cyanii anion (CN ), methyl sulfinate anion (CH3SO2 ), ar acetone enolate (CH3COCH2 ). Identify the most electro rich atom(s) in each anion (based on charges alone), ar indicate the major product that should result from an S, reaction with methyl bromide at this atom(s). 

The Mannich reaction is best discussed via an example. A mixture of dimethylamine, formaldehyde and acetone under mild acidic conditions gives N,N-dimethyl-4-aminobutan-2-one. This is a two-stage process, beginning with the formation of an iminium cation from the amine and the more reactive of the two carbonyl compounds, in this case the aldehyde. This iminium cation then acts as the electrophile for addition of the nucleophile acetone. Now it would be nice if we could use the enolate anion as the nucleophile, as in the other reactions we have looked at, but under the mild acidic conditions we cannot have an anion, and the nucleophile must be portrayed as the enol tautomer of acetone. The addition is then unspectacular, and, after loss of a proton from the carbonyl, we are left with the product. 

C NMR studies of various heteroaromatic nitriles, including 1/7-1,2,3- and 2/7-1,2,3-triazolyl nitriles, reveal a correlation between a value derived from the chemical shifts, cn and c-cn and the reactivity of nitriles in forming the corresponding enaminoketones via reactions with the enolate anion of acetone. Based on the correlation, a new reactivity index y is proposed <82H(l9)22l>. 

Another base-catalyzed reaction is the addition of enolate anions derived from ketones to the 4 position of the pyridine nucleotides (Eq. 15-19). The adducts undergo ring closure and in the presence of oxygen are converted slowly to fluorescent materials. While forming the basis for a useful analytical method for determination of NAD+ (using 2-butanone), these reactions also have created a troublesome enzyme inhibitor from traces of acetone present in commercial NADH.132... 

Enolization of cationic ketones is accelerated by electrostatic stabilization of the enolate anion. Rate constants for water-, acetate-, and hydroxide ion-catalysed enolization of 2-acetyl- 1-methylpyridinium ion (94) have been measured13811 and compared with a 2-acetylthiazolium ion (95), a simple analogue of 2-acetylthiamine pyrophosphate.13811 For (94), qh = 1.9 x 102 M-1 s 1, about 1.1 x 106 times that for a typical methyl ketone such as acetone. Thermodynamically, it is >108 times more acidic (pAa values of 11.1 vs 19.3). These increases in kinetic and thermodynamic acidity are derived from through-bond and through-space effects, and the implications for enzymatic catalytic sites with proximal, protonatable nitrogen are discussed. The results for (94) suggest a pAa value of 8.8 for (95), a value that cannot be measured directly due to competing hydrolysis. 

Now suppose that we want to use the enolate anion derived from acetone (pKa = 20) as a nucleophile in a substitution reaction. This anion requires the use of a very strong base to generate it, and its high reactivity often causes low yields of the desired product. Instead, we may choose to use its synthetic equivalent, the enolate anion derived from ethyl acetoacetate (pA a =11) ... 

Note, again, that the final product, 2-hexanone, is the same product that would result from the reaction of the enolate anion of acetone with 1-bromopropane. 

In this example the enolate anion can be derived only from acetone. The electrophile is the more reactive carbonyl carbon, that of benzaldehyde. 

Quinoline and isoquinoline salts were reported to undergo nucleophilic additions with high efficiency under sonication. The trichloromethyl anion generated from sodium trichloro-acetate in acetonitrile adds to the salts, with substantial differences in regioselectivity under reflux or sonication. An example is shown in Fig. 16. The acetone enolate ion adds similarly, with a predominant selectivity in favor of the 1,2-process. ... 

The conjugate bases formed from 16-19 are carbanions, and the resonance contributor of enolate anion 9A derived from acetone is also a carbanion. In terms of its reactivity, it is reasonable to classify 9 as a carbanion because the resonance-stabilized enolate anion has a larger concentration of electron... 

As just discussed, the condensation of an enolate derived from one aldehyde or ketone with a different aldehyde or ketone is a desirable reaction. In the reaction of acetone and benzaldehyde to give 26, the reaction gave 26 because benzaldehyde has no a-proton. Therefore, the only enolate possible is the one from acetone (see 27). Self-condensation of acetone may be a problem, but it is not possible from benzaldehyde because there is no enolate anion. The reaction of acetone and 3-pentanone is not this simple because an enolate anion may be formed from both ketones. Before this reaction is discussed (in Section 22.4.3), it is useful to describe a method that allows selective formation of the enolate anion of acetone or the enolate anion of 3-pentanone. 

Draw the enolate anions of acetone and 3-pentanone. Draw the self-condensation aldol products expected from acetone and from 3-pentanone. 

When acetone reacts with NaOEt in ethanol to form enolate anion 27, it is a reversible acid-base reaction. Therefore, unreacted ketone or aldehyde always remains in the reaction, and this fact allows self-condensation to occur. Is it possible to choose a base that will generate the enolate anion, but the equilibrium is pushed far to the right (toward the enolate anion product) If such a base is available, self-condensation is much less of a problem, which is particularly important for mixed aldol condensation reactions. As chemists experimented to find such a base, it was discovered that amide bases (RaNr), derived from secondary amines (R2NH) accomplished this goal. 

Tautomerism differs from resonance in that both electrons and hydrogen atoms are moved in order to make the new tautomer, but only electrons may be moved to create a new resonance form. The best way to generate a tautomer of a particular structure is using a thought experiment. First, remove a proton that gives a plausible anion. Write an alternative resonance form of the anion that you produced. Finally, replace the proton on the new resonance form. This is shown in Figure 8.33 for acetone. We make the enolate anion by removing a proton from one of the methyl... 

Interconversion of the hydroxy and carbonyl forms of these heterocycles proceeds through an anion (as 247) or a cation (as 248) just as the enol (249) and keto forms (252) of acetone are interconverted through the ions (250) or (251). Reactions of the various species derived from the heterocyclic... 

Photostimulated, S r k 1 reactions of carbanion nucleophiles in DMSO have been used to advantage in C—C bond formation (Scheme 1).25-27 Thus, good yields of substitution products have been obtained from neopentyl iodide on reaction with enolates of acetophenone and anthrone, but not with the conjugate base of acetone or nitromethane (unless used in conjunction, whereby the former acts as an entrainment agent).25 1,3-Diiodoadamantane forms an intermediate 1-iodo mono substitution product on reaction with potassium enolates of acetophenone and pinacolone and with the anion of nitromethane subsequent fragmentation of the intermediate gives derivatives of 7-methylidenebicyclo[3.3.1]nonene. Reactions of 1,3-dibromo- and 1-bromo-3-chloro-adamantane are less effective.26... 

Products with mass equal to the sum of the reagent masses also form, to different extents, in the reactions of 02 with ketones, namely acetone, CF3COCH3 and (CF3)2C0264. These adducts were tentatively assigned the structure of the bound tetrahedral adduct of nucleophilic carbonyl addition. While this reaction is the only one observed with acetone, it competes with H+ abstraction in the case of CF3COCH3 to form the stabilized enolate [CH2=C (CT)CF3] and with ET in the case of (CF3)2CO (electron affinity is ca 33.7 kcal moF1). In this latter case it was concluded that reaction of (CF3)2CO with Of occurs exclusively via ET and that the adduct forms in a secondary process via reaction of the primary product, the radical anion of (CF3)2CO with 302 present in the flow from the preparation of 02 (see Scheme 39). 

The anion of nitromethane (31) failed to react with Phi under irradiation in DMSO, as was the case with 1-AdI. However, when the reaction was carried out in the presence of acetone (27a) or pinacolone (27b) enolate ions, l-phenyl-2-nitroethane (161) was formed as major product, together with small amounts of benzene and toluene (equation 104). No products from the coupling with the ketone enolate ions were found169... 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

---