Diones keto-enol tautomerism

Simultaneous ring-chain and keto-enol tautomeric equilibria were observed (90ZOR2489) in solutions of 5-hydroxy-5-phenylpentane-2,4-dione 29. Increase of the solvent polarity and proton-accepting ability in the series... 

Another approach used in the empirical characterization of liquid polarity is the study of the outcome of a chemical reaction. Earle et al. [216] report a preliminary study of the keto-enol tautomerization of pentane-2,4-dione, and create an empirical correlation between the degree of tautomerization and the dielectric constant of molecular liquids. They then predict dielectric constants for a series of ILs based on the observed keto-enol equilibrium the values range from 40 to 50, slightly higher than those of short-chain alcohols. A more detailed study by Angelini et al. [217] considers the tautomerization of a nitroketone complex in a series of five imidazolium-based ILs. The results, parameterized as a linear free energy analysis of the behavior of the equilibrium constant, indicates an overall polarity comparable to that of acetonitrile, consistent with the partitioning and spectroscopic studies referenced above. 

Refluxing 5-azidotetrazolo[l,5-a]quinazoline in 6N hydrochloric acid produced quinazo-line-2,4-dione <63ZOB2475) via nucleophilic displacement of the 2 - and 4-azido groups by water followed by keto-enol tautomerism. 

From a synthetic point of view, the CHT-NCD equilibrium has been exploited in a total synthesis of colchicine and, more recently, in syntheses of tropones (115) and tropolones (119) from 7-halobicy-clo[4,1.0]heptenones (112) or 7-halobicyclo[4.1.0]heptane-3,4 diones (116), respectively. Keto-enol tautomerism of (112) to (113) followed by ring opening to the cycloheptatrienols (114) and loss of HX would explain the formation of the tropone (115). The tropolone (119) probably results from a similar sequence. These examples represent one of few cases that exemplify 6e retro electrocyclizations presented dius far in this review. Additional examples will be presented later in Section 6.2.4.1. 

Treatment of mercuric acetate with l,l,l,2,2,3,3-heptafluoro-7,7-dimethyloctane-4,6-dione in 95% ethanol provides Hg(fod)2 quantitatively the mercury atom in this solid is bonded to only carbon, but in [ H ]acetone solution the equilibrium shown in Scheme 9 is set up, a 1 1 mixture of tautomers being obtained at — 60 °C. A dynamic n.m.r. study of this equilibrium, the first substantiated mercury keto-enol tautomerization, has revealed that the process is totally intramolecular it was not possible to decide whether or not intermolecular ligand exchange occurs this was attempted by mass spectrometric examination of an approximately equimolar mix-... 

Even more complicated processes occur in the case of ligands that isomerize during the formation and polymerization of MCMs. For example, methacryloylace-tone (5-methylhex-5-ene-2,4-dione), which is the initial monomer for the synthesis of dicarbonyl type chelates, is susceptible to keto-enol tautomerism (scheme 24). ... 

NMR spectroscopy of enols has been a key tool in determining the relative proportions of keto and enol forms in highly enolized species. For example, in pentane-2,4-dione, the keto and enol forms (Figure 17.7) can be readily identified in the NMR spectrum of the material the keto-enol tautomerism is slow compared with the timescale of the NMR spectroscopy experiment. This experiment has been extended by changing the temperature, which allows the calculation of thermodynamic parameters for the equilibrium, and the use of a range of solvents to study the effect of polarity. The process has also been studied in the gas phase, where enols predominate for almost all p-dicarbonyl compounds. In solution, the keto forms are generally some 8-9 kj moH more stable than in the gas phase. 

It is also possible to examine the effect of oxygen substituents on the stability of arenonium ions. Wirz has studied keto-enol equilibria for phenol,151 naphthol (Wirz J, Personal communication), and anthrol.152,153 The tautomeric constants may be combined with p/y,s for protonation of the keto tautomer and ionization of the phenol to provide pifas f°r protonation of the aromatic ring of phenol and the phenoxide ion. As illustrated in Scheme 18 the unstable keto tautomer of phenol 22 was produced by photolysis of the bicyclooctene dione 21. Except in the case of the anthrone a pA a for protonation of the keto tautomer has not been measured directly. However, values can be estimated from the pfor protonation of the 4,4-dimethylated analog136 with a correction for the substituent effect of the methyl groups. 

Phenylhydrazones derived from 2-oxopropionaldehyde, benzoylformaldehyde, and butane-2,3-dione led to dinuc-lear metallacycles. " The addition of NaOMe in MeOH afforded deep violet eompounds which do not contain chlorine atoms. Deprotonation takes place by means of a hydrazo-keto azo-enol tautomerization involving 92 and 93 (Scheme 17). 

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]. 

When stilboestrol (diethylstilbestrol, 62) was photolysed in aqueous methanol at 254 nm it gave the stable 4a,4b-dihydrophenanthrene dione (63). The mechanism required a photo trans-cis isomerization, photocyclization and spontaneous enol-keto tautomerism [53,54]. Previous workers had carried out a similar irradiation in dilute acetic acid and obtained the expected aromatic product (64) [55],... 

The conjugated diene dienoestrol (65) was irradiated at 254 nm in 90% aqueous methanol. Rotation and cis-trans photoisomerization gave (66) which underwent a photochemical [1, 5]sigmatropic rearrangement to give (67). Photocyclization followed by enol-keto tautomerism then gave the isolated dihydrophenanthrene dione (68) [56]. 

Propose mechanisms for the reactions shown in Problems 22-62 parts (a) and (b) and 22-63 parts (a) and (b). Pentane-2,4-dione (acetylacetone) exists as a tautomeric mixture of 8% keto and 92% enol forms. Draw the stable enol tautomer, and explain its unusual stability. 

P-Diketones (1,3-Diketones). Diketones with carbonyl groups located 1,3 with respect to each other may yield a more complicated pattern than those observed for most ketones (2,4-pentane-dione, Fig. 2.42). These yS-diketones often exhibit tautomerization, which yields an equilibrium mixture of enol and keto tautomers. Since many yS-diketones contain large amounts of the enol form, you may observe carbonyl peaks for both the enol and keto tautomers. 

In connection with this search, the chemical behavior of the carbon-centered radical was also examined [118]. The similar hydroxycyclobutenone 140 bearing Barton s ester at C-4 was photolyzed (W-lamp) to again give a 5-endo-cyclized product, 4-cyclopentene-l,3-dione 145, prior to enol-keto tautomerization (Scheme 27). 

Hydrates lose water to form an enol, which tautomerizes back to the carbonyl compound. It is known that most ketones exist in >99% as the keto form. It is also known that 2,4-pentanedione has a large percentage of the enol form in equilibrium with the keto form, depending on the solvent. Suggest a reason why this dione has such a high percentage of enol. 

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