Dicarbonyl compounds, tautomerism

Ring-chain tautomerism of hydrazones of 1,3-dicarbonyl compounds 99MI21. 

Ultraviolet and infrared spectroscopy indicate that quinoxaline-2,3-dione type structures are preferred to tlie tautomeric 3-hydroxy-quinoxalin-2 One or 2,3-dihydroxyquinoxaline forms. The light absorption properties (UV) of quinoxaline-2,3-dione have been compared with those of its NN -, ON-, and OO -dimethyl derivatives (79, 80, and 81), and also its N- and 0-monomethyl derivatives (43 and 82). The parent dicarbonyl compound and its mono- and di-A -methyl derivatives show very strong carbonyl absorption near to 1690 cm split into two peaks. 

In azo couplings with carbonyl compounds, three tautomeric products are possible, compared with only two for phenols and aromatic amines (discussed in Section 12.1). The ketohydrazone 12.75 is most often dominant, but for easily enolizable 1,3-dicarbonyl compounds (X=CO-R and similar structures) the azoenol 12.76 is the major product. The azoketone 12.77 is often postulated as primary product, but has rarely been identified in an unambiguous fashion using modern methods. The CH2 group should be easily detectable in the lH NMR spectrum. 

Hydroxymethylenecyclopropanols (340) have been shown" to be intermediates in the photochemical rearrangement of a, -unsaturated carbonyl compounds (339) to 1,4-dicarbonyl compounds (341). The products are eventually obtained by double tautomerization of the enol and cyclopropanol portions of (340). 

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... 

A -Acylhydrazones of 1,3-dicarbonyl compounds can exist in three tautomeric forms enhydrazine 75A, hydrazone 75A, and 5-hydroxypyrazoline... 

The asymmetric allylic C-H activation of cyclic and acyclic silyl enol ethers furnishes 1,5-dicarbonyl compounds and represents a surrogate of the Michael reaction [136]. When sufficient size discrimination is possible the C-H insertion is highly diastereoselective, as in the case of acyclic silyl enol ether 193 (Eq. 22). Reaction of aryldia-zoacetate 192 with 193 catalyzed by Rh2(S-DOSP)4 gives the C-H insertion product 194 (>90% de) in 84% enantiomeric excess. A second example is the reaction of the silyl enol ether 195 with 192 to form 196, a product that could not be formed from the usual Michael addition because the necessary enone would be in its tautomeric naphthol form (Eq. 23). 

The first two reactions of the sequence are similar to reactions that occur in acidic medium. The 1,2- and 2,3-enediols, and the unsaturated elimination-products derived from them, are present both in acidic and basic solutions. In general, however, reactions in basic solution are much faster than in acidic solution, because of the greater catalytic effect of the hydroxyl ion on the transformation reactions Mechanistic differences between the media become operative in steps c and d. In acid, further dehydration, if it is possible, occurs rapidly, before equilibrium of the deoxy-enediol with the dicarbonyl compound has been established,17 and the products are furans. In alkaline solution, the rapid formation of the tautomeric dicarbonyl compound permits the benzilic acid rearrangement42 to proceed. 

The treatment of 1,2-diols with Dess-Martin periodinane may lead either to a 1,2-dicarbonyl compound,14 or to an oxidative breakage of a C-C bond14,72 depending on stereoelectronic factors. When a 1,2-dicarbonyl compound is obtained, very often, one of the carbonyl groups tautomerizes to the enol form. Under controlled conditions, very often, it is possible to selectively oxidize one of the alcohols in a 1,2-diol, particularly when this alcohol is an allylic one.73... 

Alkyl methyl ketones undergo nitrosation at the reactive methylene group when treated with nitrous acid or an alkyl nitrite [Method (fi)]. The presence of hydrogen on the a-carbon permits tautomeric rearrangement to the oxime of a 1,2-dicarbonyl compound. Acidic hydrolysis of the oxime, which is best carried out in the presence of a hydroxylamine acceptor such as laevulinic acid,143 affords a further useful route to the 1,2-dicarbonyl system. 

Reviews have appeared that in part cover recent advances in the ring-chain tautomerism of the nitrogen-containing derivatives of 1,3-dicarbonyl compounds (95ZOB705) of the reaction products of alkenals, alkenones, and alkane-1,3-diones with hydrazines and hydroxylamines (950PP519) and of other compounds [95H(41)1805 95H(41 )2057]. 

Ring-chain tautomerism of nitrogen-containing derivatives of /3-dicarbonyl compounds 90MI8. 

The influence of solvents on chemical equilibria was discovered in 1896, simultaneously with the discovery of keto-enol tautomerism in 1,3-dicarbonyl compounds (Claisen [14] acetyldibenzoylmethane and tribenzoylmethane Wislicenus [15] methyl and ethyl formylphenylacetate Knorr [16] ethyl dibenzoylsuccinate and ethyl diacetylsuccinate) and the nitro-isonitro tautomerism of primary and secondary nitro compounds (Hantzsch [17] phenylnitromethane). Thus, Claisen wrote Es gibt... 

In general, 1,3-dicarbonyl compounds, which include y9-dialdehydes, y9-ketoaldehydes, y -diketones, and y -ketocarboxylic esters, can exist in solution or as the pure compound in three tautomeric forms the diketo form (4a), the cw-enoUc (4b), and the trans-enolic form (4c). 

The applicability of Eq. (4-24) was demonstrated by O. Dimroth using 3-benzoyl camphor (7a,b) as an example [37]. This was particularly suitable, since both the diketo and the enol form are separately isolable, and the solubility of each is readily determined due to the slowness of their tautomerization. Although the ratio of the S terms varies by a factor of 8, the value of G is constant within an error of 5%. This is understandable, since for each 1,3-dicarbonyl compound, the same characteristic difference between diketo and enol forms should, in principle, be observed. 

The first attempt to introduce an empirical relationship between an equilibrium constant and solvent polarity was made in 1914 by K. H. Meyer [24]. Studying the solvent-dependent keto-enol tautomerism of 1,3-dicarbonyl compounds, he found a proportionality between the equilibrium constants of various tautomeric compounds in the same set of different solvents cf. Table 4-2 in Section 4.3.1). He therefore split the tautomeric equilibrium constant Ky into two independent factors according to Eq. (7-9). 

Pentanedione (acac) and related jS-dicarbonyl compounds are an extremely important class of hgands that have been studied widely for many years. In general, jS-dicarbonyl compounds exist as mixtures of tautomeric keto and enol forms (equation 15). These compounds are usually easily deprotonated to form monoanions, which form the basis for a large class of coordination compounds, encompassing vir-tuaUy every element. In addition, coordination compounds of the dianions and trianions of jS-diketones have been observed, as well as complexes of the neutral molecules. ... 

Other reversible transformations involve tautomeric structures. This is, for instance, the case for 1,3-dicarbonyl compounds particularly dibenzoylmethane (DBM), the parent compound of the UV A filter butylmethoxydibenzoylmeth-ane (BM-DBM). The photochemistry of DBM (Fig. 6) has been studied by Veierov (17), Markov (18,19), Yankov (20), Moriyasu (21) and others. 

Moriyasu M, Kato A, Hashimoto YJ. Kinetic studies of fast equilibrium by means of high-performance liquid chromatography. Part 11. keto-enol tautomerism of some P-dicarbonyl compounds. Chem Soc Perkin Trans 1986 11 515-520. 

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. ... 

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