Equilibria keto/enol tautomeric

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerisni. The individual keto and enol forms are said to be tautomers, a word used to describe constitutional isomers that interconvert rapidly. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely... 

Keto-enol tautomerism equilibrium of ethyl acetoacetate at ca. 20°C. 

Scheme 2. Keto-enol tautomeric equilibrium with indication of the most important canonical structures.

It is also possible to induce aldol condensation reactions under acidic conditions. When 137 was treated with tosic acid in hot benzene (40-50°C), the normal keto-enol tautomerism equilibrium was shifted to favor enol 138. The enol attacked the carbonyl on the pyridone in an intramolecular aldol cyclization, producing aldol 139. Elimination of water under the reaction conditions gave an 84% yield of 140 in Comins synthesis... 

Figure 1 Keto-enol tautomerism equilibrium and equalization of the keto-enol bond lengths upon ligandation for acetylacetone derivatives...

An analysis of the influence of substituents on the keto-enol tautomeric equilibrium in substituted 2-OH pyridines have been studied by De Kowalew-ski et al.f Jcc and Jcc couplings have been measured and calculated by the use of the DFT method for the keto and enol forms of the compounds studied. 

K Akao, Y Yoshimura. Keto-enol tautomeric equilibrium of acetylacetone in tri-fluoromethane near the critical temperature. J Chem Phys 94 5243, 1991. 

The keto-enol tautomeric equilibrium of acetylacetone is an intramolecular hydrogen exchange process. High-pressure NMR was used to study changes in this equilibrium over a pressure range to 2.5 kbar and temperatures to 145 C (51). With an increase in temperature at constant pressure, the equilibrium distribution shifted to the keto tautomer. An increase in pressure did not change the keto-enol distribution at any temperature. From the high-pressure experiments as a function of temperature the reaction enthalpy, A/J, and entropy, AS, were determined to be 2.80 0.02 kcal/mol and 7.2 0.3 cal/K mol, respectively. 

The keto-enol tautomeric equilibrium of 3-hydroxythiophenes 50(OH) [thiophene-3(2H)-ones 50(=O)] was systematically studied [91]. H and NMR spectra were analyzed completely (e.g., 50(OH) 5(C-2) =98.0ppm, i5(C-3) = 155.1 ppm 50(=O) 5(C-2) = 36.6ppm, 5(C-3) = 203.4ppm) and the equilibria determined. The amount of keto tautomer is greatest in nonpolar solvents [91]. Using the same methodology, the tautomeric equihbrium of the thione-thiol forms of l,3-thiazolidine-2-thione was examined [92]. 

---