Tautomerism enone-dienol

Cycloalkenone-2-carboxylates tautomerize to conjugated dienols in the presence of either acids or bases. Iron(III) catalysts have also been found to promote enone-dienol equilibration, and, at room temperature, dimerization64. Thus, treating 87 with 1 mol% iron(III) chloride hexahydrate in methylene chloride at room temperature affords 88 in 81% yield (equation 46). The cyclohexadiene-cyclohexanone is in a rapid equilibrium with its triendiol tautomer, 89 (equation 47). 

Scheme 8.20 Enone-dienol tautomerism and iron-catalyzed dimerization.

A dienol is also formed via hydrogen abstraction by the excited carbonyl group from a second enone molecule in (4.14). This dienol tautomerizes in C6F6 to give the (3,y-unsaturated ketone selectively, the overall reaction thus being deconjugation of the a,(3-unsaturated ketone415K... 

Rate and equilibrium constants have been measured for representative intramolecular aldol condensations of dicarbonyls. For the four substrates studied (32 n — 2, R = Me = 3, R = H/Me/Ph), results have been obtained for both the aldol addition to give ketol (33), and the elimination to the enone (34). A rate-equihbrium mismatch for the overall process is examined in the context of Baldwin s rules. The data are also compared with Richard and co-workers study of 2-(2-oxopropyl)benzaldehyde (35), for which the enone condensation product tautomerizes to the dienol (i.e. ) -naphthol). In all cases, Marcus theory can be apphed to these intramolecular aldol reactions, and it predicts essentially the same intrinsic barrier as for their intermolecular counterparts. 

After protonation of the ketone carbonyl, there is an elecftonic movement that begins on the electron pair in the dihydrofuran oxygen, produces the cleavage of one of the cyclobutane bonds, and leads to the formation of a dienol. The dienol tautomerizes to an enone. 

The enone tautomerizes to the corresponding dienol. This h ansformation is favoured by the... 

Irradiation of the a,6-unsaturated ketone 92 produced the dienol 93, which tautomerized to the 6,y-unsaturated ketone 94 (equation 12.69). The notable aspect of this reaction is that it converts a more stable, conjugated enone to a less stable, nonconjugated enone. The reactant, being more conjugated, absorbs UV-vis radiation at longer wavelength than does the product, so the reaction can be driven by radiation that the reactant absorbs but that the product does not absorb efficiently. 

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