Acetone enolate, structure

Example The NR mass spectrum of acetone closely resembles its 70 eV El mass spectrum (Chap. 6.2.1), thereby demonstrating that the molecular ion basically retains the structure of the neutral (Fig. 2.29). [144] However, the isomeric CsHeO" ions formed by McLafferty rearrangement of 2-hexanone molecular ion are expected to have enol structure (Chap. 6.7.1), and thus the corresponding NR mass spectrum is easily distinguished from that of acetone. 

Recent general spectral studies of such nitriles include the vibration spectra of 2-pyrazinecarbonitrile and a (> 99%)15N-isotopic version 1172 the mass spectra of 2,3-pyrazinedicarbonitrile, its 5,6-diphenyl derivative, and 2,3,5,6-pyrazinetetracarboni-trile for comparison with those of analogous heterocyclic nitriles 1406 and the 15C NMR spectra of 2-pyrazinecarbonitrile and the like for correlation with their reactivities toward acetone enolate anions.251 The structure-activity relationship of pyrazinecarbonitriles as herbicides has been reported.1048... 

A particularly interesting combination of allyl ligand structure and the high reactivity toward aryl halides leads to the synthesis equivalent of nucleophilic aromatic substitution by enolate anions, but under completely different conditions (Scheme 51). In this example, the 2-methoxyallyl ligand is equivalent to an acetone enolate anion, but reacts with very different functional group compatibility (no polar... 

Isolation and identification of surface-bonded acetone enolate on Ni(l 11) surfaces show that metal enolate complexes are key intermediates in carbon-carbon bond-forming reactions in both organometaUic chemistry and heterogeneous catalysis. Based on studies on powdered samples of defined surface structure and composition, most of the results were reported for acetone condensation over transition-metal oxide catalysts, as surface intermediate in industrially important processes. With the exception of a preoxidized silver surface, all other metal single-crystal surfaces have suggested that the main adsorption occurs via oxygen lone-pair electrons or di-a bonding of both the carbonyl C and O atoms. 

Hudson, C. E. McAdoo, D. J. /. Org. Chem. 2003, 68, 2735 used DFT calculations to locate a transition structure for an antarafacial [131 hydrogen shift in acetone enolate that is lower in energy than the dissociated radical species. 

A synthetic equivalent is a reagent whose structure, when incorporated into a product, gives the appearance of having come from one type of precursor when as a reactant it actually had a different structural origin. Although it is possible to form the enolate of acetone, use of ethyl acetoacetate as a synthetic equivalent is often more convenient because its a hydrogens are so much more acidic (pA = 9-11) than those of acetone itself (p. = 19-20). If we had wanted to use the acetone enolate directly, we would have had to use a much stronger base and other special conditions (e.g., a lithium enolate. Section 18.4). 

Most Simple aldehydes and ketones exist mainly in the keto form. Acetone, for example, is 99.9997% in the keto form, with only 0.0003% of the enol present. The main reason for the greater stability of the keto form is that the C=0 plus C—H bond energy present in the keto form is greater than the C=C plus O—H bond energy of the enol form. We have already encountered some molecules, however, that have mainly the enol structure—the phenols. In this case, the resonance stabilization of the aromatic ring is greater than the usual energy difference that favors the keto over the enol form. Aromaticity would be destroyed if the molecule existed in the keto form therefore, the enol form is preferred. 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

Among numerous examples of the role of the chemical structure in tunneling rotation we select just one, connected with the effect of intramolecular hydrogen bond. In acetyl acetone in stable enol form... 

Compare electrostatic potential maps of enolates derived from 2-butanone, 4,4-dimethyl-2-pentanone, 4,4,4-trifluoro-2-butanone and l-phenyl-2-propanone with those of acetone. Which substituents cause significant changes in the electronic structure of these enolates and what are the nature of these changes Justify your answers by making drawings of any important resonance contributors. 

The excellent agreement with the experimental and calculated isotope effect (calculated for formaldehyde, 3.22, and for acetaldehyde, 3.3 experimental value 2.9) supports the computational approach. This suggests that the computed transition structure for hydride transfer in the reaction of the lithium enolate of acetone with acetaldehyde (Figure 30) is realistic. 

These two compounds have identical structures except for the position of one proton this is clearly a special case of isomerism and it is called tautomerism. A is called the keto and B the enol form of acetone. 

Comparisons of structurally related hydroxy- and methoxy-substituted cations show that hydroxy is more stabilizing by between 4 and 5 log units. This difference was recognized 20 years ago by Toullec who compared pifas for protonation of the enol of acetophenone and its methyl ether145 (-4.6 and 1.3, respectively) based on a cycle similar to that of Scheme 15, but with the enol replacing the hydrate, and a further cycle relating the enol ether to a corresponding dimethyl acetal and methoxycarbocation.146 Toullec concluded, understandably but incorrectly, that there was an error in the pA a of the ketone (over which there had been controversy at the time).147,148 In a related study, Amyes and Jencks noted a difference of 105-fold in reactivity in the nucleophilic reaction with water of protonated and O-methylated acetone and concluded that the protonated acetone lacked a full covalent bond to... 

In the former instance, the major structural condition is that one of the carbonyl components (either the aldehyde or the ketone) has no a-hydrogen and thus is unable to form an enolate ion, but is nevertheless sufficiently electrophilic at its carbonyl carbon that it reacts with the carbanion of the second carbonyl component. Examples are provided in the formation of 4-phenylbut-3-en-2-one from benzaldehyde (no a-hydrogens) and acetone in the presence of base, and the formation of benzylideneacetophenone from benzaldehyde and acetophenone (Expt 6.135). 

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