Enolate ions intermediates

Reactions of Aldehydes and Ketones That Involve Enol or Enolate Ion Intermediates... 

The preferential exchange of the 6 -hydrogen under strongly acidic conditions is attributed to the formation of a A -dienol intermediate (29) which then undergoes deuterium attack from the -side at C-6. In contrast, in neutral or alkaline media the exchange proceeds via an enolate ion intermediate in which deuterium addition occurs in the following order C-4 >... 

As noted previously, conjugate addition of a nucleophile to the j3 carbon of an cr,/3-unsaturated aldehyde or ketone leads to an enolate ion intermediate, which is protonated on the a carbon to give the saturated product (Figure 19.16). The net effect is addition of the nucleophile to the C=C bond, with the carbonyl group itself unchanged. In fact, of course, the carbonyl group is crucial to the success of the reaction. The C=C bond would not be activated for addition, and no reaction would occur, without the carbonyl group. 

Figure 22.7 The biosynthesis of indolmycin from indolylpyruvate occurs through a pathway that includes an alkylation reaction of a short-lived enolate ion intermediate.

The a-substitution reaction of a carbonyl compound through either an enoi or enolate ion intermediate is one of the four fundamental reaction types in carbonyl-gron p chem istrv. 

Two of the four general carbonyl-group reactions—carbonyl condensations and basic conditions and involve enolate-ion intermediates. Because the experimental conditions for the two reactions... 

Step 2 of Figure 29.3 Conjugate Addition of Water The a,(3-unsaturated acyl CoA produced in step 1 reacts with water by a conjugate addition pathway (Section 19.13) to yield a jG-hydroxyacyl CoA in a process catalyzed by enoyl CoA hydratase. Water as nucleophile adds to the 3 carbon of the double bond, yielding an enolate ion intermediate that is protonated on the a position. 

Steps 7-8 of Figure 29.12 Hydration and Oxidation The final two steps in the citric acid cycle are the conjugate nucleophilic addition of water to fumarate to yield (S)-malate (L-malate) and the oxidation of (S)-malate by NAD+ to give oxaloacetate. The addition is cataiyzed by fumarase and is mechanistically similar to the addition of water to ris-aconitate in step 2. The reaction occurs through an enolate-ion intermediate, which is protonated on the side opposite the OH, leading to a net anti addition. 

Scheme 23). An enol or an enolate ion intermediate is involved in the process, the intermediate subsequently ketonizing (equation 23). 

The enolate ion intermediate is now much more basic than the anion of the nitro compound so it removes a proton from the nitro compound and provides another molecule of anion for the seeond round of the reaction,... 

Reaction of (R)-2-methylcyclohexanone with aqueous base is shown below. Reaction with aqueous acid proceeds by a related mechanism through an enol, rather than an enolate ion, intermediate. 

Halogenation of aldehydes and koUmes fiocura under boch baaic and conditions. As you naight cxpc ct, the base-promoted reaction occurs t1 an enolate ion intermediate. Even relatively weak bases such as hvdi ion are effective for halo nntion because it s not necessary to convert ketone completely into its enolate too. As soon as a. small amount of eoo late is generated, it reacts immediately with the halogen. 

Alpha-substitution reactions occur at the position next to the carbonyl group—the position—and involve the substitution of an a hydrogen atom by an electrophile, E, through either an enol or enolate ion intermediate. Let s begin by learning more about these two species. 

Two of the four general carbonyl-group reactions—carbonyl condensations and a substitutions—take place under basic conditions and involve enolate ion intermediates. Since the experimental conditions for the two reactions are so similar, how can we predict which will occur in a given case When we generate an enolate ion with the intention of carrying out an < alkylation, how can we be sure that a carbonyl condensation reaction won t occur instead ... 

An explanation of the observed isomer distributions is that sp -7i overlap is possible in conformations A and B, but not in C. Conformation A is favored because the cw-decalin-type conformation B experiences additional 1,3-destabilizing interactions at the concave face. Thus, the enolate ion intermediates formed during dissolving... 

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