Keto-enol equilibrium acetylacetone

Enolizable compounds can be used for Meerwein reactions provided that the keto-enol equilibrium is not too far on the side of the ketone for example, P-dicar-bonyl compounds such as acetylacetone are suitable (Citterio and Ferrario, 1983). The arylation of enol esters or ethers (10.12) affords a convenient route for arylating aldehydes and ketones at the a-carbon atom (Scheme 10-48). Silyl enol ethers [10.12, R = Si(CH3)3] can be used instead of enol ethers (Sakakura et al., 1985). The reaction is carried out in pyridine. 

Free ligands have been studied in order to obtain an insight into their structure, both in solution and in the solid state, and for comparison with their metal complexes. H NMR spectroscopy has been used to investigate the keto-enol equilibrium and the nature of the hydrogen bonds. In the case of optically active Schiff bases UV and CD spectra provided information about structure in solution. The Schiff bases that have been most widely examined are derivatives of acetylacetone, salicyl-aldehyde and hydroxymethylenecamphor, whose prototypes with en are shown in Figure 13. 

The keto-enol equilibrium of the 1,3-diketones has been the subject of intensive studies using various physical techniques and theoretical calculations [78-80], Recently, X-ray crystal analysis of acetylacetone (83) was carried out at 110 K, and it was found that it exists as an equilibrium mixture of the two enol forms 83b and 83c [81]. Room-temperature studies show an acetylacetone molecule with the enolic H-atom centrally positioned, which can be attributed to the dynamically averaged structure 83d. Application of a crystal engineering technique showed that a 1 1 inclusion complex of83 can be formed with l,l/-binaphthyl-2,2/-dicarboxylic acid in which the enol form is stabilized by a notably short intramolecular hydrogen bond [82],... 

Fig. 4-3. Effect of solvent and concentration on the keto/enol equilibrium of acetylacetone in four solvents of different polarity at 37 + 2 °C ecu (O), CHCU (A), CHzCU ( ), and HC0N(CH3)2 (A) [50].

The addition of comparatively less polar alcohols to solutions of acetylacetone in water shifts its keto/enol equilibrium in favour of the less polar m-enolic form (4b), which has been quantitatively rationalized in terms of so-called pairwise solute/solvent interactions [245], The keto/enol equilibrium of ethyl acetoacetate and acetylacetone has also been studied in polar supercritical fluids such as CHF3 (//= 1.65 D) and CCIF3 [fi = 0.50 D) [246], In polar trifluoromethane, the dipolar keto form was found to be favoured, although the change in the equilibrium constant with increasing sc-fluid density [i.e. increasing pressure) was quite minor. For ab initio calculations of the relative stabilities of various enols of acetylacetone in the gas phase, and theoretical calculations of keto/enol equilibria in aqueous solutions, see references [247] and [248], respectively. 

Spectra of two molecules, one keto and one enol form. The rate of interconversion is too slow to average out the signals of individual forms. Intensities of peaks associated with each form can be used as a very precise measure of the keto-enol equilibrium (Grimley, 1963) and the method has great advantages over the destructive chemical methods. Reeves (1957) investigated the effects of solvents on the equilibrium in acetylacetone. The spectrum of acetylacetone is a superposition of the spectra of the two molecules below with 18-6 0-6% of keto form at 298°K. 

Alcohols do not normally behave as acids in water, but the presence of an double bond adjacent to the OH group can substantially decrease the pATa by the mechanism of keto-enol tautomerism. Ascorbic acid is an example of this effect. The diketone 2,4-pentanedione (acetylacetone) is also a weak acid because of the keto-enol equilibrium. In... 

To indicate the importance of enolization, equilibrium constants for a number of substrates are shown in Table 10.1. These equilibrium constants are only approximate, and they do depend very much on the solvents employed. Nevertheless, we can see that the equilibrium constant K = [enol]/[keto] is very small for substrates like acetaldehyde, acetone, and cyclohexanone, with only a few molecules in every million existing in the enol form. However, in ethyl acetoac-etate, enol concentrations are measured in percentages, and in acetylacetone the equilibrium constant indicates the enol form can be distinctly favoured over the normal keto form. In hexane solution, only 8% of acetylacetone molecules remain in the keto form. 

The potential of carbon-13 NMR in the analysis of keto-enol tautomerism has been demonstrated for 2,4-pentanedione (acetylacetone) and dimedone [293]. Quantitative evaluation of equilibrium concentrations is possible by application of the inverse gated decoupling technique illustrated in Fig. 2.23. 

The keto-enol tautomerization of acetylacetone (CH3-CO-CH2-CO-CH3), a prototype /3-diketone, has been extensively studied experimentally, and attention has been paid to its solvent effect. Although the enol form is more stable than the keto in the gas phase owing to the intramolecular hydrogen bonding, the equilibrium is known to shift toward the keto in solution as the solvent polarity increases. The tautomerization in various types of solution, which includes H2O, dimethyl sulfoxide (DMSO), and carbon tetrachloride (CCI4), was examined by means of RISM-SCF method. [18]... 

The results from the calculation of the equilibrium constants of keto-enol tautomerism for some aliphatic ketones in CCl are given in Table 7. The data for acetylacetone and 2-naphthylmethylketone are not presented in the table because in the former case the rate of reaching the equilibrium is commensurable with the rate of ozone consumption and in the latter case the ozone reacts with the double bonds in the naphthyl ring. The equilibrium constants do not differ from those found within the temperature range of 21°C to 3°C and agrees with data from the literature [59-62],... 

Figure 1 Keto-enol tautomerism equilibrium and equalization of the keto-enol bond lengths upon ligandation for acetylacetone derivatives...

K Akao, Y Yoshimura. Keto-enol tautomeric equilibrium of acetylacetone in tri-fluoromethane near the critical temperature. J Chem Phys 94 5243, 1991. 

The keto-enol tautomeric equilibrium of acetylacetone is an intramolecular hydrogen exchange process. High-pressure NMR was used to study changes in this equilibrium over a pressure range to 2.5 kbar and temperatures to 145 C (51). With an increase in temperature at constant pressure, the equilibrium distribution shifted to the keto tautomer. An increase in pressure did not change the keto-enol distribution at any temperature. From the high-pressure experiments as a function of temperature the reaction enthalpy, A/J, and entropy, AS, were determined to be 2.80 0.02 kcal/mol and 7.2 0.3 cal/K mol, respectively. 

The tautomerism of slow keto-enol and fast enol-enol tautomeric equilibria of a number of l-(2-hydroxyphenyl)-3-naphthylpropane-l,3-diones can be easily monitored also by NMR spectroscopy, making this nucleus as suitable as and for this kind of studies [diketo form 5( 0) 469 and 548ppm, respectively enol forms 5( 0) 332 to 313 ppm (higher double bond character) and i5( 0) 156 to 135 ppm (lower double bond character)] [34]. Also computed chemical shifts were included in the analysis of the extremely fast tautomeric equilibrium of the two enol forms in asymmetric 1,3-diketones [35], for example, acetylacetone [5(C=0) 473.8 ppm 5(0-H) 191.2 ppm]. The equilibrium constants fCp thus obtained were compared with earlier experimental results based on 5( 0) in model or blocked tautomeric structures. The theoretical methodology could complement some inadequacies in experimental NMR techniques in evaluating equilibrium constants of compounds with rapid dynamic exchange [35]. 

Figure 3.6 Top Keto-enol tautomerization in acetone. The equilibrium for this molecule strongly favors the keto-tautomer. Bottom The tautomeric equilibrium for 2,4-pentanedione (acetylacetone) favors the enol form because the double bond is conjugated with the remaining carbonyl group and the hydrogen of the enol can form a hydrogen bond with the carbonyl oxygen that creates a very stable six-membered ring.

Enols The familiar tautomeric equilibrium of keto and enol forms of acetylacetone is described in Section 3.8.3.1 (see Figure 3.45). The enol form predominates over the keto form under the conditions described. 

Ordinarily we do not write the enol form of acetone or the keto form of phenol, although minuscule amounts do exist at equilibrium. But both forms of acetylacetone are seen in the NMR spectrum because equilibration is slow enough on the NMR scale and the enol form is stabilized by intramolecular hydrogen bonding. The enol form of acetone and the keto form of phenol are not thus stabilized furthermore, the aro-... 

The proton NMR spectrum of acetylacetone is shown in Figure 8.7. The O—H proton of the enol form (not shown) appears very far downfield, at 8 = 15.5 ppm. The vinyl C—H proton is at 5 = 5.5 ppm. Note also the strong CH, peak from the enol form (2.0 ppm) and compare it with the much weaker CH, peak from the keto form (2.2 ppm). Also note that the CH2 peak at 3.6 ppm is weak. Clearly, the enol form predominates in this equilibrium. The fact that we can see the spectra of both tautomeric forms, superimposed on each other, suggests that the rate of couversion of keto form... 

The )S-dicarbonyl compound (/S-dikH) generally exists as an equilibrium mixture of the tautomeric keto and enol forms. The rate of spontaneous interconversion between these forms is rather slow at room temperature, and their simultaneous NMR spectroscopic observation is possible. For instance, the NMR spectrum of neat acetylacetone (acacH) is composed of OH, CH, CHj, and CH3 signals in accordance with the following equilibrium in ppm from internal Me4Si) ... 

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