Dicarbonyl compounds tautomers

Although nitrosation of (l-dicarbonyl compounds becomes increasingly more facile upon successive replacement of the a alkyl groups with perfluoroalkyl groups because of the increased ionization of the perfluorinated enolate (equation 7), the stability of the nitrosodiketone tautomers decreases Thus, 1,1,1-trifluoro pentane-2,4-dione and 1,1,1,5,5,5-hexafluoropentane 2,4 dione mtrosate much faster than penta-2,4 dione but yield ketoximes, which decompose upon workup... 

A mechanistic study of acetophenone keto-enol tautomerism has been reported, and intramolecular and external factors determining the enol-enol equilibria in the cw-enol forms of 1,3-dicarbonyl compounds have been analysed. The effects of substituents, solvents, concentration, and temperature on the tautomerization of ethyl 3-oxobutyrate and its 2-alkyl derivatives have been studied, and the keto-enol tautomerism of mono-substituted phenylpyruvic acids has been investigated. Equilibrium constants have been measured for the keto-enol tautomers of 2-, 3- and 4-phenylacetylpyridines in aqueous solution. A procedure has been developed for the acylation of phosphoryl- and thiophosphoryl-acetonitriles under phase-transfer catalysis conditions, and the keto-enol tautomerism of the resulting phosphoryl(thiophosphoryl)-substituted acylacetonitriles has been studied. The equilibrium (388) (389) has been catalysed by acid, base and by iron(III). Whereas... 

Hydroxycoumarin can be considered as an enol tautomer of a 1,3-dicarbonyl compound conjugation with the aromatic ring favours the enol tautomer. This now exposes its potential as a nucleophile. Whilst we may begin to consider enolate anion chemistry, no strong base is required and we may formulate a mechanism in which the enol acts as the nucleophile, in a simple aldol reaction with formaldehyde. Dehydration follows and produces an unsaturated ketone, which then becomes the electrophile in a Michael reaction (see Section 10.10). The nucleophile is a second molecule of 4-hydroxycoumarin. 

Interestingly, 2,3-dicarbonyl derivatives formed by the condensation of a-dicarbonyl compounds and 2-aminophenols (80JHC1625) exist in the 2Z/-form (81 Scheme 36). However, when there is an additional carbon atom in the side-chain of the 3-substituent so that hydrogen bonding with the NH group may now occur, as in the benzoxazinone (82), the equilibrium is directed strongly in favour of the 4Z/-tautomer (Scheme 37) (76JHC681). 

When drawing disconnections it doesn t matter which tautomer you use. If we disconnect both C-N bonds 63 we get a keto-acid 64 and removal of the alkene gives a simple 1,5-dicarbonyl compound 65. Reaction with ammonia followed by oxidation gives the pyrimidone. 

Furthermore, some fused tropolones give typical reactions of o-dihydroxy or o-diketo (389) tautomers they form o-dimethoxy compounds (e.g., 418a) or derivatives of dicarbonyl compounds (dioximes and quinoxalines like 513), respectively (65MI3). Another diketone exhibits remarkable stability against enolization to form indolotropolone 324 (Scheme 83), due to steric interaction (78BCJ3579). 

Table 20.1 shows that the amount of enol tautomer that is present at equilibrium in the case of simple aldehydes and ketones is very small. Simple carboxylic acid derivatives, such as esters, have an even smaller enol content. However, 1,3-dicarbonyl compounds... 

BTIB-oxidations of aromatic ethers can also be directed to C,C-bond formation. Intermodular examples include oxidative heterocouplings of 1,4-dimethoxybenzene with /1-dicarbonyl compounds, presumably via their enol tautomers, to the arylated /1-dicarbonyl derivatives 102... 

The abbreviations which refer to /3-dicarbonyl compounds are meant to refer to all those species which comprise the /3-dicarbonyl system, i.e. the keto and the various enol tautomers. 

For over 20 years the structures of /8-dicarbonyl compounds have been determined by X-ray, electron and neutron diffraction and microwave spectroscopy. Agreement as to how the data should be interpreted has led to centred and non-centred hydrogen bonds being proposed for the same substance. In almost all cases the cis enol is the stable tautomer, but in 1971 Karle et al.71 reported the structure of AA, in both this and the diketo form, by electron diffraction. 

Enols are much more easily oxidized than the corresponding ketone tautomers, however, their tendency to ketonize rapidly has precluded extensive electrochemical and other one-electron oxidation studies so far. Hence, oxidation potentials were only determined of few relatively stable enols of fi-dicarbonyl compounds or a-cyano ketones, where the enol content may be as... 

AMI semiempirical calculations have shown that, as far as tautomerism is concerned, there is a structural relationship between jS-dicarbonyl compounds and NH-pyrazoles, and in a wide variety of NH-pyrazoles studied " the most stable tautomer was found to be that having the largest single-bond character between the C(3)—C(4) bond. The problem of proton transfer in NH-pyrazole crystals has been subjected to a detailed theoretical study, while a study of the tautomerism of 2-aryl and 2-heteroaryl derivatives of benzimidazole has indicated that tautomerism takes place by the intermolecular relay of protons between stacked molecules. The first report of the stable co-existence of two different histidine tautomers in one peptide crystal structure has appeared. Ab initio calculations have been used to study the tautomerism of both histamine and pyrazolo[3,4-i/]pyridazine in the gas phase and in aqueous solution, and a theoretical study of the NH tautomerism in free-base porphyrin has been undertaken. ... 

The only reference to this ring system describes the heterocycle as a furazono[3,4-h]pyrazine/ The compounds are prepared by condensation of 3,4-diaminofurazan (25) with a-dicarbonyl compounds. Benzil gave the 5,6-diphenyl derivative 26 (R = Ph), and oxalic acid gave the dihydroxy compound 26 (R = OH). The presence of a band at 1695 cm in the solution infrared spectrum of 26 (R = OH) is taken to indicate the presence of the 0x0 tautomer (27). 

Since the preparation of acetylacetone and similar /J-dicarbonyl compounds in the latter half of the nineteenth century, organic chemists have had considerable interest in their properties and reactions. The best known phenomenon of -dicarbonyls is their participation in tautomeric equilibria where an apparently pure compound is actually a mixture of two substances having keto and enol structures. Early studies with / -dicarbonyls revealed that these compounds exhibited chemical properties indicative of both the dicarbonyl and unsaturated hydroxy ketone structures. In an unsuccessful attempt to explain this anomalous behavior Laar (7) in 1885 coined the word tautomerism" which has survived to the present. By the turn of the century it was generally held that the dichotomous properties of -dicarbonyls resulted from the pure compound existing as a mixture of both keto and enol tautomers. Since... 

Pyrazoles have a high degree of aromaticity with a Bird Ia of 90. They also are involved in a tautomeric equilibrium, with the NH tautomer being preferred (Scheme 9.31). Generally, they are synthesized from a preformed compound with a N-N bond, usually a hydrazine. Reaction with a beta-dicarbonyl compound gives the pyrazole structure. 

This reaction was first reported by Knorr in 1883. It is the preparation of pyrazole derivatives from 1,3-dicarbonyl compounds and hydrazines. Similarly, pyrazole derivatives can also be prepared from hydrazines and equivalent molecules of 1,3-dicarbonyl compounds, such as a-ethoxymethylene a-phenylacetone. In a special case, -carbonyl aldehyde reacts with a simple hydrazine to afford tautomers of 3- and 5-substituted pyra-zoles, of which the proton oscillates rapidly back and forth between two nitrogen atoms. These two tautomers are distinguishable only at sufficiently low temperatures." It should be pointed out that the reaction of hydrazine with j8-carbonyl ester leads to the formation of pyrazolone. ... 

The classic de Mayo reaction involves the [2 + 2] photocycloaddition of an alkene to the hydrogen-bonded enol tautomer of a P-dicarbonyl compound as exemplified by the formation of 1,5-diketone 9 from pentane-2,4-dione and cyclohexene (vide supra). In addition to alkenes, allenes are also used as the olefinic component. For example, irradiation of a mixture of dimedone and allene results in the formation of 3,3-dimethyl-7-methylenecycloocta-l,5-dione 12 via the cyclobutane intermediate 11, together with the corresponding head-to-tail product 13, which spontaneously dimerizes to the hetero Diels-Alder adduct 14. Diketone 12 is a versatile building block for the preparation of substituted cyclooctadienones and 8-valerolactones. 

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