Keto-enol tautomerism base-catalyzed

Figure 25.8 Fructose, a ketose, is a reducing sugar because it undergoes two base-catalyzed keto-enol tautomerizations that result in conversion to an aldose.

Show how enols and enolate ions act as nucleophiles. Give mechanisms for acid-catalyzed and base-catalyzed keto-enol tautomerisms. 

D presentation of the chromatogram of 5-(+)-naproxen dissolved and stored in the acidic medium (development in the second direction). In this figure, we can see the single 3D peak of 5-(+)-naproxen only. In that way, we provided sufficient experimental evidence in favor of the base-catalyzed keto-enol tautomerism as a driving force for the configuration change of the chirally pure 5-(+)-naproxen. 

Problem 17.3 Compare the mechanisms for (a) base-catalyzed and (b) acid-catalyzed keto-enol tautomerism. 

Keto-enol tautomerism of carbon) ] compounds is catalyzed by both acids and bases. Acid catalysis occurs by protonation of the carbonyl oxygen atom to give an intermediate cation that Joses H+ from its a carbon to yield a neutral enol (Figure 22.1). This proton loss from the cation intermediate is similar to what occurs during an El reaction when a carbocation loses H+ to form an alkene (Section 11.10). 

Pyridine is an important cofactor in the reaction system that leads to cleaner reactions and better yields of products than in its absence.15 It can act either as a u-donor for Pb(rv) or as a base catalyzing the keto-enol tautomerism. The u-donor effect was evidenced spectroscopically by the formation of adducts of pyridine with lead tetraacetate.45,4511 Moreover, pyridine catalyzed the ligand redistribution of twcfc-methoxyphenyllead triacetate to bis( r/ -mcthoxy-phenyl)lead diacetate. Other u-donor catalysts can be used and their nature is highly important for the success of the reaction. NaOMe and HOBT showed a modest effect, but a thousand-fold increase in rate over the uncatalyzed reaction was observed when 1,10-phenanthroline was employed and near quantitative yields of arylation products were obtained (Equation (16)).44... 

Compare the base-catalyzed and acid-catalyzed mechanisms shown for keto-enol tautomerism. In base, the proton is removed from the a carbon, then replaced on oxygen. In acid, oxygen is protonated first, then the a carbon is deprotonated. Most proton-transfer mechanisms work this way. In base, the proton is removed from the old location, then replaced at the new location. In acid, protonation occurs at the new location, followed by deprotonation at the old location. 

Deuterium can be exchanged for a-hydrogens catalyzed by acid or base through formation of the enol during keto-enol tautomerization. 

Tautomerizations involve the shift of a hydrogen atom across a tt system. The most typical tautomerization is a 1,3-shift, and the focus of this section is the interconversion of a ketone (or aldehyde) and an enol, often termed keto-enol tautomerization. The reaction can be catalyzed by acid or base, and it is technically an isomerization, a class of reactions we will cover later in this chapter. However, knowledge of the mechanism of keto-enol tautomerizations is crucial to understanding enol and enolate chemistry, and therefore we cover it here. 

Keto-enol tautomerization is an equilibrium process that is catalyzed by even trace amounts of acid (or base). Glassware that is scrupulously cleaned will still have trace amounts of acid or base adsorbed to its surface. As a result, it is extremely difficult to prevent a keto-enol tautomerization from reaching equilibrium. 

The interconversion between an enol and a ketone is called keto-enol tautomerization and is catalyzed by trace amounts of acid or base. 

Ketones and aldehydes bearing a hydrogens are in equilibrium with their enol forms, although for simple ketones and aldehydes the carbonyl forms are greatly favored. This equilibrium is the keto—enol tautomerization. Equilibration with the enol form can be either acid- or base-catalyzed. The enol form can be favored in special cases. Esters and other acid derivatives also have acidic a hydrogens. LDA is a strong base that can be used to drive ketones, aldehydes, or esters completely to their corresponding enolates. 

As usual, this base-catalyzed reaction has its add-catalyzed counterpart. For the add-catalyzed reaction the catalyst is not hydroxide, but H30, and the active ingredient is not the enolate anion, but the enol itself The first step in the reaction is acid-catalyzed enol formation, the keto-enol tautomerization we have come to know (Fig. 19.67). 

Know how to recognize or draw the keto and enol forms of a molecule, and know the mechanism of a keto-enol tautomerization under acid- and base-catalyzed conditions. 

Based on the energy diagram in CTQ 11, which is rate-limiting in acid-catalyzed alpha-halogenation, keto-enol tautomerization or halogenation (circle one)... 

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