Keto-enol tautomerism Subject

Space does not permit any further detailed discussion except for a brief account of two interesting subjects. The first is concerned with keto-enol tautomerism. The classical example is ethyl acetoacetate, which can exist in the keto form (I) and the enol form (II) ... 

Dibenzoylmethane (8b) has been the subject of much interest as regards the possibility that its polymorphism is associated with keto-enol tautomerism. Chemical and spectroscopic studies showed that this is not so (33a). This compound had previously been reported to be trimorphic (33b), but one form appears, in fact, to be a eutectic mixture of the other two. The molecules in these two polymorphs are both in the same state of tautomerism they differ in the torsional angle about the (CH)-(CO) bond and in the type of hydrogen bonding in which they participate. It is noteworthy that solutions prepared from these forms at low temperature have differences in chemical and spectroscopic properties that are maintained for some time. For example, such solutions prepared and held at —35° react at different rates with FeCl3. 

All the isomers 27-31 of benzoxazinotropone are subject to keto-enol tautomerism (Figure 4), which was proven by their O-acetylation. However, the keto forms were predicted to be favored energetically by calculations using the... 

Hydroxypyridines react very much like phenols they are not subject to keto-enol tautomerism to a significant degree. In aqueous alkali the oxyanion is the reactive species with bromine entering positions ortho and para to that substituent. It has proved possible to achieve a high proportion of monobromination by the use of one molar equivalent of bromine. As the proportion of bromine to substrate was increased more... 

Also, all the -situated hydrogen atoms of ketones are readily replaceable by deuterium in an aqueous alkaline medium, since these atoms take part one after the other in the reversible keto-enol tautomerism. This applies, for instance, to the four -positions of cyclohexanone and to the one of 2,2,6-tri-methylcyclohexanone, whereas no exchange occurs in camphor or camphor-quinone which cannot enolize owing to the particular stereochemistry of the molecules.90 Open-chain ketones and also steroidal ketones91 that are not subject to this limitation are often used for exchange reactions. The following description of the preparation of [D6]acetone illustrates the point 29... 

Keto acids containing an a-hydrogen atom on a carbon adjacent to the keto group are subject to keto-enol tautomerism. This applies to 2,3,4 and other 0X0 acids and results in the formation of two derivatives on esterification, the enol-ether and the free oxo compound. These separate on gas chromatography, making qualitative and quantitative work difficult. In addition, esterification of the free oxo acid with diazomethane and analogous... 

Oxazolones and their sulfur counterparts exist predominately as the keto rather than enol forms for the nitrogen analogues, the aminooxazole structure prevails. The subject of substituent tautomerism was covered in depth in <84CHEC-I(6)177>. There have been no studies since 1984 on substituent tautomerism reported in the literature. 

The dispute was in fact resolved in 1896 by Claisen [37], who isolated acetyldiben-zoylmethane as two separate solid forms, each with different melting points and chemical properties (interaction with metallic salts). Claisen correctly diagnosed them as the enol and keto forms having the structures 36b and 36a, respectively. More important still was the observation that, if either the keto or the enol form was heated in a solvent such as alcohol, or fused in the absence of solvents, a mixture was obtained from which both the keto and enol forms could be isolated. As result of this discovery, the pseudomerie/ortisomerie theory about the real existence of the isomers was proven to be correct. Ironically, the term tautomerism came into use to describe the process. In some natural way, according to the early reviews [38-46], tautomerism was considered and it is still considered in most of the cases as an equilibrium between forms coexisting in solution, and was defined as one of the most difficult subjects of experimental science. ... 

---