Keto-enol tautomerism cyclic

Cyanuric acid exists in two tautomeric forms corresponding to keto-enol tautomerism in carbonyl compounds. The keto form predominates, and most of the reactions of cyanuric acid have their counterparts in the chemistry of the cyclic imides. Many of the reactions involve the replacement of all three imido hydrogens (Scheme 31). Usually, the reaction cannot be controlled to produce the mono- or di-substituted isocyanurates specificially, but there are exceptions, e.g. the reaction between cyanuric acid and aziridine (Scheme 31) (B-79MI22001, 63JOC85, 63AHC(2)245). 

Acid-catalysed hydrogen-deuterium exchange in norcamphor has also been investigated by Werstiuk and Banerjee (1977) (DOAc—D20—DC1 medium). It was observed that exo-deuteron addition to the enol is also preferred, but with a slightly smaller selectivity (x 190). This would mean that, if torsional factors cause preferential base-catalysed exo-exchange, they also occur for acid-catalysed keto-enol tautomerism. However, the absence of important torsional strain effects on the rate constants of acid-catalysed enolisation of cyclic and bicyclic ketones contradicts this assumption. 

A theoretical/NMR study of keto-enol tautomerism in 2-(2-methoxycarbony-lacetyl)pyrazine (277/278) and other similarly substituted azines has been undertaken the foregoing pyrazine exists in its enolic form (278) to the extent of 35% in deuterochloroform.411 l,4-Diacetyl-l,4-dihydropyrazine (279) gave the persistent radical cation (279) + on one-electron oxidation (cyclic voltammetry in MeCN— Bu4NC104).167... 

The coordination of transition metals is known to influence the keto-enol tautomerism in the condensed phase" . The effect of coordination of bare Fe+ ions on the keto-enol equilibrium of phenol was investigated by means of generation of various cyclic [Fe,Cg, He, 0]+-isomers. These isomers were characterized by collisional activation (CA) and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry" . It was shown that the energy difference between the phenol-iron complex 65 and the keto isomer 66 is not perturbed by the presence of the iron cation in comparison with the uncom-plexed isomers 3 and 4 (equation 25). Thus, the energy difference for both the neutral and the Fe+-coordinated systems amounts to ca 30 kJ moC in favor of the phenolic tautomer. 

Kung (1974) mentioned that numerous reports appear in the literature on the study of keto -enol tautomerism of carbonyl compounds, triacylmethanes and cyclodiketones depending on the state as pure liquid, diluted in an organic solvent or in a gas phase. Except for compound D.45, which will be discussed, the aliphatic structures are represented as a-diketones and the cyclic structures under the keto-enolic form. 

Perez and Perez [414] have discussed the special case of carbon acids where proton dissociation occurs from the central carbon in 1,3-dicarbonyl compounds. In these cases complications arise not only from keto-enol tautomerisms, but also because of geometric flexibility or lack thereof. Notable differences occur between rigid cyclic P-diketones and more flexible acyclic forms, as in the cases of Meldrum s acid (cyclic, pK = 7.32 in DMSO)... 

The mechanism of a Claisen rearrangement involves a concerted redistribution of six electrons in a cyclic transition state as described above. The product of this rearrangement is a substituted cyclohexadienone, which undergoes keto-enol tautomerism to reform the aromatic ring. A new carbon-carbon bond is formed in the process. 

An interesting type of keto-enol tautomerism, called cyclic anhydride-end tautomerism (Scheme 5.14), was published [45]. NMR spectroscopy was employed to estimate the equilibrium the ratio of the anhydride tautomer 20a and the enol form 20b was 7 1 [45]. 

Ab initio and DFT studies of keto-enol equilibria of deltic acid (2,3-dihydroxycyclo-prop-2-en-l-one) in gas phase and aqueous solution have deflned a bimolecular proton transfer mechanism. Effects of ring size on the tautomerism and ionization reaction of cyclic 2-nitroalkanones in cyclohexane have been studied by H NMR and DFT calculations. " ... 

P-Diacids are unstable to heat. They decarboxylate (lose CO2), resulting in cleavage of a carbon-carbon bond and formation of a carboxylic acid. Decarboxylation is not a general reaction of all carboxylic acids. It occurs with P-diacids, however, because CO2 can be eliminated through a cyclic, six-atom transition state. This forms an enol of a carboxylic acid, which in turn tautomerizes to the more stable keto form. 

The mechanism involves acid-catalysed conversion of the keto form of the cyclic P-diketone into the enol form, which is able to attack the protonated enone. The mechanistic detail is precisely analogous to the attack of an enolate the only difference is that both reactants are protonated. The product is the enol form of the triketone, which rapidly tautomerizes to the more stable keto form. 

Decarboxylation, or loss of CO2, is not a typical reaction of carboxylic acids under ordinary conditions. However, j8-ketoacids are unusually prone to decarboxylation for two reasons. First, the Lewis basic oxygen of the 3-keto function is ideally positioned to bond with the carboxy hydrogen by means of a cyclic six-atom transition state. Second, this transition state has aromatic character (Section 15-3), because three electron pairs shift around the cyclic six-atom array. The species formed in decarboxylation are CO2 and an enol, which tautomerizes rapidly to the final ketone product. 

The decarboxylation of acetoacetic acid, a P-keto acid, occurs by way of a cyclic transition state in which a proton is transferred from the carboxylate atom to the carbonyl oxygen to give an enol that rapidly tautomerizes to give acetone. 

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