2,4-Pentanedione enol acidity

Ketones with labile hydrogen atoms undergo enol acetylation on reaction with ketene. Strong acid catalysis is required. If acetone is used, isoptopenyl acetate [108-22-5] (10) is formed (82—85). Isopropenyl acetate is the starting material for the production of 2,4-pentanedione (acetylacetone) [123-54-6] (11). 

Comprehensive work in this field has been done by Slovak authors (98MI1, 95M1359, 96CCC269, 96CCC371, 97CCC99). They prepared 2-substituted (H, Me, Ph) 4-, 5-, 6-, and 7-nitrobenzoxazoles, which were then reduced to amines (not isolated) and subjected to subsequent nucleophilic substitution with activated enol ethers such as alkoxymethylene derivatives of malonic acid esters and nitrile, 3-oxobutanoic acid esters, pentanedione, or cyanoacetic acid esters to yield aminoethylenes 8. 

When a hydrogen atom is flanked by two carbonyl groups, its acidity is enhanced even more. Table 22.1 thus shows that compounds such as 1,3-dikotoncs (/3-diketoncs). 3-oxo esters (/3-keto esters), and 1,3-diesters are more acidic than water. This enhanced acidity of jS-dicarbonyl compounds is due to the stabilization of the resultant enolate ions by delocalization of the negative charge over both carbonyl groups. The enolate ion of 2,4-pentanedione, for instance,... 

The concept of a diastereoselective Friedel-Crafts alkylation of a-chiral benzyl alcohols was first examined by Bach and coworkers [62, 63]. The initial protocol required stoichiometric amounts of strong Brpnsted acids like HBF4 and was followed by a more valuable methodology in which catalytic amounts of AuC L were employed for the diastereoselective functionalization of chiral benzyl alcohols [64], Beside arenes, allyl silanes, 2,4-pentanediones and silyl enol ethers have been used as nucleophiles. Depending on the diastereodiscriminating group and on the catalyst (Brpnsted or Lewis acid), the authors observed either the syn or the anti diastereoisomer as the major product. 

When the more acidic 1,3-diketones are used as the nucleophiles, the active species may be either the enol form or the enolate ion. In both cases a diketonic adduct is expected to be initially formed. This is indeed found to be the case in the reaction of 2,4-pentanedione, yielding 187 as a primary adduct, which subsequently equilibrates with the enol forms 188 (Eq. 27). When 1,3-cyclopentanedione is used, only the enol forms are obtained, however, presumably because enolization is faster than the diketonic adduct formation.218... 

Two carbonyl groups greatly increase the acidity. For example, 2,4-pentanedione (acetylacetone, 2) has a pATa s 9, which is comparable to the O—H acidity of phenols (see Table 17-1). The reason is that the enolate anion 3 has the charge largely shared by the two oxygen atoms (cf. 3b and 3c). As a result, the enolate anion 3 is stabilized more with respect to the... 

Exercise 17-46 If the keto form of 2,4-pentanedione is more stable than the enol form in water solution, why does it also have to be a weaker acid than the enol form in water solution ... 

However, the acidity of the a proton gets increased if it is flanked by two carbonyl groups rather than one, for example, 1, 3-diketones ((i-di ketones) or 1,3-diesters ([i-keto esters). This is due to the fact that the negative charge of the enolate ion can be stabilised by both carbonyl groups which results in three resonance structures (Following fig.). For example, the pKa of 2, 4-pentanedione is 9. 

As Section IV will be mostly devoted to 2,4-pentanedione, acacH, it is worthwhile to dwell on this well studied species. The rate of interconversion between the keto and the enol form of acacH is rather slow at room temperature , thus they can be simultaneously detected by NMR spectroscopy it has been observed that the lower the polarity of the solvent, the higher the percentage of the enol tautomer . Electron diffraction studies indicate that in the gas phase acacH adopts the enol configuration with a keto/enol ratio of 8/92. More recently, an X-ray analysis of acacH, carried out at 110 showed that it exists as a mixture of the two enol forms 86 and 87, with the enolic hydrogen atom equally distributed over two positions close to the oxygen atoms as in 88. It should be noted that inclusion compounds containing different host molecules show different ratios of acacH in the enol form. For example, acacH exists as a dynamically averaged 1 1 mixture of 86 and 87 in an inclusion complex with l,T-binaphthyl-2,2 -dicarboxylic acid as host , while l,l-bis( >-hydroxyphenyl)cyclohexane and (4R,5R)-trawi-4,5-bis(hydroxydiphenylmethyl)-2,2-dimethyl-l,3-dioxolane include acacH in pure enolic form. ... 

Enol acetylation (I, 1174 1178). As a Grst step in the synthesis of 3-n-butyl-2,4-pentanedione, a mixture of 28.6 g. (0.25 mole) of 2-heptanone, 51.0 g. (0.50 mole) of acetic anhydride, and 1.9 g. (0.01 mole) of p-toluenesulfonic acid monohydrate contained in a stoppered 500-ml. round-bottomed flask equipped with a magnetic stirrer is stirred at room temperature for 30 min. Then 55 g. (0.43 mole) of the I 1 boron trifluoridc-acetic acid complex [Reagents, I, 69 (1967)] is added some heat is evolved... 

Kung (1974) isolated a compound responsible for a pleasant, buttery caramel aroma during GC of a coffee aroma. The MS corresponded to that of 2,3-pentanedione, but the retention time and odor did not. IR and NMR spectra of the trapped compound were more likely those of an enol form which could effectively be produced by injection at 300 °C of the dione or by action of a concentrated acid at lower temperature. The free enol form is not stable and reverts to the keto form at room temperature. This observation does not entirely exclude the occasional organoleptic contribution of enolized forms in certain states of the coffee beverage, but contrary to 2,4-pentanedione (D.55) which is enolized to a significant extent, the a-diketones are not discussed here under their enolic forms. 

Diethoxyphosphinyl)methylene ]-2,4-pentanedione [diethyl (2-acetyl-3-oxo-2-butenyl)-phosphonate] (14) A solution of the enol ether (13) (0.1 mole) in ether (100 ml) is treated with a suspension of sodium diethyl phosphite (0.1 mole) in ether (100 ml) and heated under reflux, with stirring, for 2 h. After cooling, the mixture is extracted with 20% hydrochloric acid (19 g). The ethereal phase is dried and freed from ether and unchanged enol ether in a vacuum. Two fractionations of the residue in a vacuum afford the product (61 % crude yield before distillation 80%), b.p. 127°/0.01 mm, D20 1.4622. 

Both the 1,3-diol of cyclopentene and the carboxylic acid derivative of cyclobutane have formula C5H3O2, yet there is no accessible mechanism for establishing chemical equilibrium with the 2,4-pentanedione or its enol tautomer. The chemical potentials of both molecules would be fixed at negative infinity in the system. There are additional structural isomers for which the equivalent statement can be made. 

As shown in Table 20.2, p-dicarbonyl compounds such as 2,4-pentanedione (a (3-diketone), ethyl acetoacetate (a (3-keto ester), and diethyl malonate (a 1,3-diester) have pATa s in the 9-13 range. They are stronger acids than water and alcohols so can be converted to their enolates by hydroxide and alkoxides. 

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