KetoEnol Tautomerism

Although this synthesis is extremely simple, its mechanism is not yet clearly understood. It has been proposed that such reactions can be rationalized348 in terms of 1,4-additions, where the initial attack by a proton occurs at the carbonyl oxygen, giving rise to a very stable carbocation that is not only allylic but also stabilized by the mesomeric donating oxygen. This step is followed by the nucleophilic addition of SnCl3( f and by a ketoenol tautomerism (Scheme 27). 

NMR spectroscopy has been utilized in the quantitative analysis of the racemic mixtures. The important condition is that the proton signals of the components are well separated. Ketoenol tautomerism has been studied in this manner. This is illustrated by acetyl acetone. 

In further support of the notion of electronic tautomerism, Fry cited the 1905 paper by E. C. C. Baly and C. H. Desch in which they described ketoenol tautomerism as the result of the motions of a labile hydrogen atom that functions as a potential ion, "inasmuch as the bond of attraction or Faraday tube of force must be considered to be lengthened sufficiently to allow the interchange of the atom from the one position to the other within the molecule." Thus, the difference between "electrolytes" (electrical molecules) and nonelectrolytes (chemical molecules) has to do with conditions that determine the varying lengths of the Faraday tubes. 125 Baly and Desch s hypothesis was a result of their rumination on Wislicenus s earlier hypothesis (i.e., that the influence of "electrochemical polarization" explains the acidic properties of acetoacetic ester) in combination with thinking about J. J. Thomson s ideas. 126... 

The hydroxyphenylbenzotriazoles are the most important. They absorb the damaging UV light and rapidly convert it into harmless heat (ketoenol tautomerism) [5.11]. 

Application of the UMP2 and G3 methods has revealed the complex mechanisms for reaction of O3 with 1-butene (CH3CH2CH=CH2), for both adiabatic and non-adiabatic reaction channels, and shown that the minimum energy crossing points play a key role. The site selectivity of the addition of O3 to either carbon atom of the double bond of 1-butene is unremarkable. The mechanisms of formation of butenols are also discussed and shown to occur not only by ketoenol tautomerization but also by a rearrangement and decomposition reaction involving the intermediate epoxide compound. ... 

Stable metal complexes can be favorably formed when a bidentate metal-binding site is available, such as a- and -diketone moieties which are the tautomeric forms of a- and /3-ketoenols. Some /S-diketonate complexes of paramagnetic lanthanides such as Pr(III), Eu(III) and Yb(III) have been extensively utilized as paramagnetic shift reagents for structural assignment of molecules with complicated stereochemistry prior to 2D techniques in NMR spectroscopy. Their syntheses and application are discussed in separate chapters in this volume. The examples below provide some dynamic and structural basis for better understanding of metal enolates in biomolecules and biochemical processes. 

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