Enolization base-catalysed mechanism

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... 

Fig. Base-catalysed mechanism for keto-enol tautomerism.

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... 

Ah initio methods have been used to compare enzyme-catalysed enolization mechanisms.130 Acid- and base-catalysed stepwise mechanisms have been compared with the concerted reaction the latter is favoured by several hydrogen-bonding interactions. 

Studies of relative rates, activation parameters, kinetic isotope, and solvent isotope effects, and correlation of rates with an acidity function, have elucidated the mechanisms of cyclization of diacetyl aromatics (23-26) promoted by tetramethyl-ammonium hydroxide in DMSO.32 Rate-determining base-catalysed enolate anion formation from (24-26) is followed by relatively rigid intramolecular nucleophilic attack and dehydration whereas the cyclization step is rate determining for (23). 

The base-catalysed Aldol reaction is shown in Equation 3.11 [3, 4], and a mechanism to account for the global process in Scheme 3.1. At low concentrations of acetaldehyde, reverse of the proton-abstraction steps is fast compared with the forward bimolecular enolate capture (k [CH3CHO] <rate limiting. Under these conditions, the kinetics are second order in [CH3CHO] and show specific base catalysis, i.e. the reaction is first order in [HCY ] and, even though B is involved in the mechanism, it does not appear in the rate law [5]. According to this mechanism, therefore, the overall rate law is given by Equation 3.12 ... 

Reactions involving Enols or Enolic Derivatives.—A review of the structure and reactivity of alkali-metal enolates includes some steroidal reactions. A study of the mechanism of isomerization of androst-5-en-17/3-ol-3-one to testosterone indicated that the acid-catalysed process proceeds through rate-determining enolization whereas the base-catalysed reaction proceeds through rate-determining protonation of an enolate ion. Bromination of the 4,4,6-trimethyl-A -3-oxo-compounds (111)—(113) gave the 2a-bromo-derivatives, each of which showed anomalous o.r.d. curves. Bromination at C-2 was favoured for the... 

The base-catalysed degradation of the ring of isoxazolium salts is particularly easy, requiring only alkali metal carboxylates to achieve it. The mechanism, illustrated for the acetate-initiated degradation of 2-methyl-5-phenylisoxazolium iodide, involves initial 3-deprotonation with cleavage of the N-O bond subsequent rearrangements lead to an enol acetate which rearranges to a final keto-imide. 

The kinetics and mechanism of the acid-catalysed reactions of methylated trioses have been determined and the results were shown to be in good agreement with quantum mechanical calculations of charge distributions in substrates and intermediates. The same group has studied the kinetics and mechanism of acid-base-catalysed enolization of glycolaldehyde and methoxyacetaldehyde by polarography. Deuterium incorporation was used to establish the mechanism. ... 

The mechanism for copper(n)-catalysed dimerization of pyruvate (P ) proceeds by a CuP+ complex which undergoes a general-base catalysed enolization process,... 

The elimination is even easier in acid solution and acid-catalysed aldol reactions commonly give unsaturated products instead of aldols. In this simple example with a symmetrical cyclic ketone, the enone is formed in good yield in acid or base. We shall use the acid-catalysed reaction to illustrate the mechanism. First the ketone is enolized under acid catalysis as you saw in Chapter 21. 

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