Of keto-enol tautomerism

For an extensive study, see Mills, S.G. Beak, P. J. Org. Chem., 1985,50,1216. For keto-enol tautomerism in aqueous alcohol solutions, see Blokzijl, W. Engberts, J.B.F.N. Blandamer, M.J. J. Chem. Soc., Perkin Trans. 2,1994,455 For theoretical calculations of keto-enol tautomerism in aqueous solutions, see Karelson, M. Maran, U. Katritzky, A.R. Tetrahedron, 1996, 52, 11325. 

The possibility of keto-enol tautomerism exists. In basic medium, the enolic form is favored. Furthermore, the presence of the CONH group in the trans form could be proved from the ultraviolet absorption spectrum of niclosamide in methanol. 

The more complex structures are inappropriate for consideration here, but the two compounds orsellinic acid and phloracetophenone exemplify nicely the enolate anion mechanisms we have been considering, as well as the concept of keto-enol tautomerism. 

Glucose 6-phosphate is then isomerized to fructose 6-phosphate. This conversion of an aldose sugar to a ketose sugar is easy to rationalize in terms of keto-enol tautomerism (see Box 10.1). 

An investigation of keto-enol tautomerism for perfluorinated keto-enol systems was undertaken. N-methylpyrrolidone (NMP) catalyzes equilibration of the keto and enol forms, but if used in more than trace amounts, it drives the equilibrium strongly toward enol because of hydrogen bonding to the amide. The enol is much more thermodynamically stable than its ketone, and it was found that in mildly Lewis basic solvents, such as ether, THE, acetonitrile, and NMP, the enohzation equilibrium lies too far right to allow detection of ketone (Correa et al., 1994). 

Internal alkynes undergo acid-catalysed addition of water in the same way as alkenes, except that the product is an enol. Enols are unstable, and tautomer-ize readily to the more stable keto form. Thus, enols are always in equilibrium with their keto forms. This is an example of keto-enol tautomerism. 

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. 

KINETIC STUDIES OF KETO-ENOL TAUTOMERIC EQUILIBRIA... 

Scheme 2 Acid-, base-, and uncatalyzed reaction paths of keto-enol tautomerism.

Koch A, Thomas S, Kleinpeter E (1997) Ab initio study, semiempirical calculation and NMR spectroscopy of keto-enol tautomerism of triazolopyrimidines. J Mol Struct (Theochem) 401 1—14... 

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 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 the last decades there has been a growing interest in coordination componnds with diketonate ligands. These systems have attracted the interest of both the organic and inorganic chemical communities . Here we very briefly describe the chemistry of fi-diketone ligands which are capable of keto-enol tautomerism (Scheme 1). 

Similar reactions occur with a-chloromercury ketones. Reactions of type (a) are less representative than those of type (b) acyl chlorides, aryl sulfonyl chlorides, and dialkylchlorophosphites react according to type (b) and display reaction site transfer. Is this a result of keto-enol tautomerism ( mercurotropy ) Can metallotropic tautomerism occur Or, alternatively, can the ambivalent behavior be explained by reaction site transfer, which in this case involves the a, 77-conjugated system ... 

Silyl-Enolethers and -Ethers — New Results of Keto-Enol Tautomerism... 

The involvement of keto-enol tautomerism in the process is supported by the observation that the methoxy compound (123) is inert under the present conditions. 

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