Enolization acid catalysed

In practice, the Konstanecki-Robinson synthesis of chromones commences with O-benzoylation not C-benzoylation, to afford ester 9.24. Base-catalysed rearrangement produces the required 1,3-diketone 9.21, via intramolecular benzoylation of the intermediate enolate. Acid-catalysed dehydration then affords flavone 9.19. 

The original asymmetric carbon( ) in the keto form loses its asymmetry and is changed to the enol form. But owing to its symmetry the enol form can produce another keto form in which the configuration about the asymmetric carbon would be the mirror image of the original and hence would lead to racemisation. This type of enolization is a case of keto-end tautamerism and it has also been found that in such cases the change is catalysed acids or bases. The acid-catalysed enolization is as follows ... 

In acidic solution the enolization of simple ketones is general acid catalysed (Bell, 1973 Toullec, 1982), but measurements of inverse solvent... 

Bell and Timimi, 1973. The reference intramolecular reaction is the enolization of the substrate conjugate acid catalysed by ethanolamine (pA, 9.50), and EM is corrected for the different pA, using p = 0.8 and for the effect of the protonated nitrogen... 

When a solution of the ester (155 R = H) in dioxan is warmed at 80 °C, the phosphonate (156) is produced together with about 5% yields of each of dimethyl hydrogen phosphonate and diazoacetoacetic ester (Scheme 8) the enol phosphate (157) could not be detected. The explanation for this sequence of reactions also relies on proton mobility, and the reaction is known to be acid-catalysed.124... 

Trimethylsilylation of enolizable carbonyl compounds and alcohols has also been accomplished by the fluoride ion promoted reaction with hexamethyldisilane and ethyl trimethylsilylacetate [48, 49], with high stereospecificity giving Z-enol ethers from ketones [50]. l-Trimethylsilyl-(l-trimethylsilyloxy)alkanes, produced from the reaction of aldehydes with hexamethyldisilane, undergo acid-catalysed hydrolysis during work up to yield the trimethylsilylcarbinols [51]. In the case of aryl aldehydes, the initially formed trimethylsiloxy carbanion produces the pinacol (Scheme 3.1). 

Lewis acid-catalysed deprotection of enol ethers to yield carbonyl compounds is aided by the addition of tetra-n-butylammonium fluoride. Optimum yields were obtained with equimolar amounts of the enol ether in dichloromethane with the fluoride and boron trifluoride etherate [20, 21]. 

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. 

The aza-Payne rearrangement and its use as a synthetically valuable equilibration process has been reviewed. Unusual diazadioxabicyclo[2.2.2]octanes (352) have been obtained by the acid-catalysed rearrangement of A -quinazolinonyl- and A -phthalimido-aziridines derived from 3-phenylcyclohex-2-enol. ° A probable mechanism is outlined in Scheme 108. A-Acyl-2,2-dimethylaziridines have been isomerized by sodium iodide into three isomers whose yields appear to depend... 

The intermediacy of enols or enolate anions may be demonstrated by hydrogen exchange reactions (see Section 4.11.2). Both acid-catalysed and base-catalysed tautomerism mechanisms involve removal... 

Two mechanisms are shown above. The base-catalysed mechanism proceeds through the enolate anion. The acid-catalysed process would be formulated as involving an enol intermediate. Note that the terminal hydrogens in pentan-3-one are not exchanged, since they do not participate in the... 

A related mechanism can be drawn for acid-catalysed halogenation. Again, the halogen concentration does not figure in the rate equation, and the rate of enolization controls the rate of reaction. 

We thus have standard imine formation in this case, the secondary amine leads to an iminium cation. This is attacked by the ketone nncleophile. This cannot be an enolate anion because of the mild acidic conditions under which the reaction proceeds, so we formnlate it as involving the enol tantomer. Accordingly, we need to include an acid-catalysed enolization step. 

The electrochemical cyclization of enol ethers in methanol uses an undivided cell and 2,6-lutidine is added as a proton scavenger. Acid catalysed hydrolysis of the enol function is thus avoided. An advantage is gained by diluting the methanol witli a non-nucleophilic co-solvent. This lowers the extent of dimethoxylation of... 

A series of disteroidal enol ethers [e.g. (357)], which have, in some cases, long-lasting uterotrophic activity, has been prepared by acid-catalysed exchange... 

Internal alkynes undergo acid-catalysed addition of water in the same way as alkenes, except that the product is an enol. Enols are unstable, and tautomer-ize readily to the more stable keto form. Thus, enols are always in equilibrium with their keto forms. This is an example of keto-enol tautomerism. 

For acid-catalysed aldol condensations (which are less frequent), the homogeneous mechanism [371] can again be accepted. The enol form of the hydrogen donor interacts with the protonated form of the hydrogen acceptor, viz. 

Selective oxidation of methyl ethyl ketone to diacetyl has been studied by passing a mixture of the ketone in artificial air over vanadium phosphorus oxide catalysts in the temperature range 200-350 C. Products observed included diacetyl, methyl vinyl ketone, acetaldehyde, acetic acid and carbon dioxide. C4 products were favoured at low temperatures and at low or zero oxygen partial pressures. These results are rationalised in terms of two pathways for C2 products, namely oxidation of the double bond in the enol form of methyl ethyl ketone to yield acetic acid and acetaldehyde, and acid catalysed hydration of the keto form to yield acetaldehyde only. The C4 products are envisaged to go through a common intermediate, namely, CH3COCHOHCH3, formed by interaction between methyl ethyl ketone and lattice oxygen. 

Rates of acid-catalysed enolization of isobutyrophenone and its ot-d analogue have been measured in H2O and D2O, by bromine scavenging.1403 Results include a solvent isotope effect, ku /kDi, of 0.56, and a substrate isotope effect, h/ d, of 6.2 (both for the enolization reaction). Combination of the data with that for ketonization in D2O140b gives the first isotope effect for the keto-enol equilibrium of a simple ketone e(H20)/ e(D20) = 0.92. The results are discussed in terms of the isotopic fiuctionation factors and the medium effect. 

A study of acid-catalysed enolization and carbon-acid ionization of isobutyrophenone has combined the solvent isotope effect k /kv = 0.56 and substrate isotope effect kH/kD = 6.2 determined for the enolization in H2O and D2O with literature information in order to estimate the solvent isotope effect on the enolization equilibrium, A e(H20)/A e(D20) = 0.92, and on the CH ionization of butyrophenone, kf (R20)/kK(D20) = 5.4.130 This is the first report of an isotope effect on AY forketo-enol equilibrium of a simple aldehyde or ketone. 

The Clemmensen reduction was accompanied by D/H exchange via acid-catalysed enolization of the carbonyl group, resulting in the production of deuteriated compounds 57 and 58 with various numbers of deuterium atoms. The mixture of the compound 59 obtained by the Wolf-Kishner reduction was predominantly labelled at position 2. This has been proved by the 13C-NMR spectrum. Isotope shift and loss intensivity on substituted C(2) carbon signals have been observed54,55. 

It was reported that the structure of the product obtained (68% yield) by acid-catalysed (8% HC1, reflux, 18 hr) hydrolysis and decarboxylation of the (3-keto ester 1 was not the expected diketone, but the stable enol 2 (mp 128-130°C), and that the equilibrium could not be reversed from enol to ketone, even on treatment of 2 with dilute alkali for 10 days. The evidence cited in support of 2 was entirely spectroscopic, as follows "C14H16O3 (232) M+ 232, Xmax (ethanol) 266 nm (log e, 4.09). The IR spectrum of 7 (= 2) showed a broad band between 3000-3500 cm 1 (enolic OH) and a sharp peak at 1705 cm-1 (ring -C=0) in the NMR spectrum (CDCI3) of 7 (s 2) a broad peak was present at 85.95 (1H, olefmic) and a downfield signal (exchangeable with D2O) at 811.1 (1H) indicating the presence of an enolic OH group."... 

Assess this evidence in terms of structure 2, show that the claim to have prepared 2 as a stable enol is specious, and, on the basis of mechanism, devise a structure for the product from the acid-catalysed hydroysis of 1 (the molecular formula C14H16O3 for the product is correct). 

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