Keto-enols 1704 theory

For the acid-catalyzed ketone -> enol reaction of typical unhydrated ketones, AS appears to be close to the normal or collision theory value for a second-order reaction. For example, AS for the acid-catalyzed bromination of acetone is —12 e.u. (Rice and Kilpatrick, 1923). Since for normal keto-enol equilibria, AS0 is close to zero, the conclusion is that AS for the ketonization reaction would also be close to —12 e.u. 

An important, but not widely recognized effect in electron transfer chemistry is the phenomenon that the thermochemical stability order of several tautomeric systems can be inverted upon one-electron oxidation. Hitherto, most of the data stem from gas-phase measurements (photoelectron and mass spectrometry data) or from calculations at several levels of theory (Table 2). While the explicit numbers still differ for each method they all agree on the inversion of the stability order for keto/enol, alkine/allene, imine/enamine, nitrile/isonitrile and aldimine/aminocarbene pairs. One-electron reduction, on the other hand, does not necessarily lead to a thermochemical stability inversion, as demonstrated in the case of acetaldehyde 7cthenol [41]. 

KETO-ENOL ISOMERISM ON TRANSITION METAL SURFACES, A DENSITY FUNCTIONAL THEORY STUDY... 

Figure 22 Optimised structures for the two hydrogen-bonded cis-fi-keto-enol isomers at the B3L YPj6-3llG(d,p) level of theory...

Both J(C-3,C-6) and j(C-5,C-6) were used successfully as probes to examine the keto-enol tautomerism in derivatives of 2-OH-pyridine 52 in solution, which is strongly dependent on the solvent s dielectric constant and type/position of the substitution [96], Experimental J values of the pyridine-pyridone tautomers were measured in natural abundance with the INADEQUATE technique, and theoretical values were computed at DFT level of theory considering the solvent by the SCRF-PCM model [96],... 

As noted above (Section 1.4.2.2) reduction of carbonyl compounds under these conditions proceeds with hydrogen transfer to afford an equimolar mixture of alkoxide and enolate, plus varying quantities of dimeric reduction products. As a consequence, at least in theory, this procedure should afford an equimolar mixture of recovered ketone and reduction product. This appears to be the case if less than one equivalent of metal is used however, with excess metal, camphor, " some 12-keto steroids2 and several 1 -decalones2 afforded 70-99% yields of secondary alcohols. The explanation which has been offered is that the product enolate is protonated by NH3 to regenerate the starting ketone, which is recycled through the reduction process. ... 

This range of possible adsorption modes for acetone prompted us to study the alternative adsorption possibilities for formaldehyde, as an example where the eno/ form is not possible, and acetone on Pt and Pd surfaces using periodic density functional theory. A previous theoretical study in this area has been published by Dumesic e al. however they have concentrated on the reaction pathway for hydrogenation of acetone over Pt(lll) assuming the r (0)-acetone is the only adsorbed state present. In this contribution we compare adsorption of acetone in the enol and keto isomers on both Pt(l 11) and Pd(l 11). 

Two mechanistic hypotheses are considered when furans are formed via the alkyne hydration reaction (Scheme 20.1). Density functional theory (DFT) calculations for gold(l)-catalyzed furan formation were performed, and these calculations indicate that the keto pathway is preferred over the enol pathway. 

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. ... 

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