Keto-enol tautomerism acid-catalyzed

Under acidic conditions, the proton first adds at its new position on the adjacent carbon atom, and then is removed fran its dd position in the hydroxyl group. 

For example, the mercuric-catalyzed hydration of 1-butyne gives l-buten-2-ol as an intermediate. In the acidic solution, the intermediate quickly equilibrates to its more stable keto tautomer, 2-butanone. 

When 2-pentyne reacts with mercuric sulfate in dilute sulfuric acid, the product is a mixture of two ketones. Give the structures of these products, and use mechanisms to show how they are formed. 

Hydroboration-Oxidation In Section 8-7 we saw that hydroboration-oxidation adds water across the double bonds of alkenes with anti-Markovnikov orientation. A similar reaction takes place with alkynes, except that a hindered dialkylborane must be used to prevent addition of two molecules of borane across the triple bond. Di(sec-ondary isoamyl)borane, called disiamylborane, adds to the triple bond only once to give a vinylborane. (Amyl is an older common name for pentyl.) In a terminal alkyne, the boron atom bonds to the terminal carbon atom. 

Oxidation of the vinylborane (using basic hydrogen peroxide) gives a vinyl alcohol (enol), resulting from anti-Markovnikov addition of water across the triple bond. This enol quickly tautomerizes to its more stable carbonyl (keto) form. In the case of a terminal alkyne, the keto product is an aldehyde. This sequence is an excellent method for converting terminal alkynes to aldehydes. 

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

Aldol condensations also take place under acidic conditions. The enol serves as a weak nucleophile to attack an activated (protonated) carbonyl group. As an example, consider the acid-catalyzed aldol condensation of acetaldehyde. The first step is formation of the enol by the acid-catalyzed keto-enol tautomerism, as discussed earlier. The enol attacks the protonated carbonyl of another acetaldehyde molecule. Loss of the enol proton gives the aldol product. 

The mechanism of the acid-catalyzed aldol reaction involves an initial acid-catalyzed keto-enol tautomerization to provide the enol form protonation of a second molecule on the carbonyl oxygen creates an electrophilic oxonium ion that is then attacked by the nucleophilic enol, followed by loss of a proton to give the j8-hydroxy aldehyde or ketone product. 

DRAWING THE MECHANISM OF ACID-CATALYZED KETO-ENOL TAUTOMERIZATION... 

Drawing the Mechanism of Acid-Catalyzed Keto-Enol Tautomerization... 

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

The study of prototropic tautomerization is intimately related to the study of proton transfer reactions. The study of the dynamics of proton transfer is as old as the study of reaction kinetics itself Indeed, the first reactions studied, that is, the inversion of sugar by Wilhehny in 1850 [65], involves a proton transfer as the elementary step in the reaction. In the first studies on the dynamics of tautomerization, primarily keto-enol tautomerization in acetone-like compounds were studied, which is a slow process involving a number of reaction steps of which the acid catalyzed keto-enol conversion was taken as the rate determining one [66]. In the past century, since 1910, nearly 2000 papers have been published on the kinetics of tautomerization, and in the first 60 years most of those were devoted to the ground-state reactions of the keto-enol type involving a C atom. Until the mid-1950s, only a handful of papers can be found this was obviously due to experimental hmitations. Two things are needed a method to start the reaction, and a method to follow it. In Dawson s experiments [66], the rate could be influenced by the amount of acid present, and the reaction could be followed because the enol produced... 

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. 

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). 

Another approach for the synthesis of enantiopure amino acids or amino alcohols is the enantioselective enzyme-catalyzed hydrolysis of hydantoins. As discussed above, hydantoins are very easily racemized in weak alkaline solutions via keto enol tautomerism. Sugai et al. have reported the DKR of the hydantoin prepared from DL-phenylalanine. DKR took place smoothly by the use of D-hydantoinase at a pH of 9 employing a borate buffer (Figure 4.17) [42]. 

The initial product has a hydroxy group attached to a carbon-carbon double bond. Compounds such as this are called enols (ene + ol) and are very labile—they cannot usually be isolated. Enols such as this spontaneously rearrange to the more stable ketone isomer. The ketone and the enol are termed tautomers. This reaction, which simply involves the movement of a proton and a double bond, is called a keto—enol tautomerization and is usually very fast. In most cases the ketone is much more stable, and the amount of enol present at equilibrium is not detectable by most methods. The mechanism for this tautomerization in acid is shown in Figure 11.6. The mercury-catalyzed hydration of alkynes is a good method for the preparation of ketones, as shown in the following example ... 

Keto-enol tautomerism is also catalyzed by acid. In acid, a proton is moved from the a carbon to oxygen by first protonating oxygen and then removing a proton from carbon. 

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. 

Problem 27.5 Acrolein, CH2 CHCHO, is prepared by heating glycerol with sodium hydrogen sulfate, NaHS04. (a) Outline the likely steps in this synthesis, which involves acid-catalyzed dehydration and keto-enol tautomerization. (Hinf Which OH is easier to eliminate, a primary or a secondary ) (b) How could acrolein be converted into acrylic acid ... 

The mushroom tyrosinase-catalyzed oxidative decarboxylation of 3,4-dihydroxyphenyl mandelic acid (111, R = H) and a-(3,4-dihydroxyphenyl) lactic acid (111, R = Me) proceeds via the quinone methide intermediate 112. The coupled dienone-phenol rearrangement and keto-enol tautomerism transforms the quinone methide 112 into 1-acyl-3,4-dihydroxyphenyl compounds 113 (equation 48) . ... 

STEP 1 Keto-enol tautomerism (Section 12.8A). A small amount of enol is formed under acid-catalyzed conditions ... 

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