Enolizable hydrogen atom

The title compound has been deuterium labelled1 with 2H at C(3) (1), and in the vinyl group (2) by deuterium exchange of the enolizable hydrogen atoms in 3 followed by... 

If an asymmetric carbon atom has an enolizable hydrogen atom, a trace of acid or base allows that carbon to invert its configuration, with the enol serving as the intermediate. A racemic mixture (or an equilibrium mixture of diastereomers) is the result. 

If you want a more controlled reaction with addition of formaldehyde to an aldehyde or ketone without the reduction step, you can sometimes succeed with a weaker base such as potassium carbonate. Typically in these reactions all the enolizable hydrogen atoms (green) are replaced by molecules of formaldehyde (black). 

Coupling of ketones with two different enolizable hydrogen atoms gives a mixture of 1,4-diketones in which the less crowded product predominates ... 

Only a-hydrogen atoms are exchanged because the reaction occurs through an enolate ion intermediate. All possible enolizable hydrogen atoms are eventually replaced. The hydrogen atoms are lost in the solvent, which contains many more deuterium atoms than the number of hydrogen atoms transferred from the carbonyl compound. 

Although some authors propose that an enolizable /3-dicarbonyl system is essential for inflammatory activity, two analogues in which this hydrogen atom at carbon 4 has been substituted, suxibuzone (717) and pipebuzone (718), are used as antiinflammatory agents, and the latter also possesses antipyretic and analgesic properties. However, these compounds are probably not active per se and their activity is due to metabolism to phenylbutazone. 

If the products contain an enolizable hydrogen, overoxidation is a concern. When cyclopentanones are involved, the cyclic products are often more rapidly oxidized than the starting materials. However, further oxidation of the cyclic products is not necessarily undesirable. For example, Snider has developed a simple synthesis of phenols in which a manganese(III)-mediated 6-endo radical cyclization is followed by oxidative aromatization.154 An example of this process is provided in Scheme 34. Snider has also shown that the overoxidation can be blocked with a chlorine atom, as shown by the second example in Scheme 34.153... 

In addition to its mechanistic importance, keto-enol tautomerism affects the stereochemistry of ketones and aldehydes. A hydrogen atom on an a carbon may be lost and regained through keto-enol tautomerism such a hydrogen is said to be enolizable. 

Sometimes a Lewis acid such as zinc iodide is added in catalytic amounts. Side reactions may sometimes occur if acyl cyanides carry hydrogen atoms in the a-position and are, therefore, enolizable. In those examples trimethylsilyl chloride, which is formed during the reaction, may react to give trimethyl-silylated enol ethers (8). This methodology can be applied successfully to the synthesis of such com-... 

Surfaces with basic sites form enolates from both the aldehydes and ketones, leading to multiple aldol condensations and Michael additions. " Candidate molecules must be enolizable, i.e., contain an a-hydrogen atom. Aldol condensation / Michael addition products cover the range from a,p-unsaturated aldehydes, saturated aldehydes, hydrogenated products (alcohols), and the heavier aromatics resulting from multiple condensations. The presence of coordina-... 

The reaction of compounds having an active hydrogen atom with non-enolizable aldehydes and ammonia or primary or secondary amines to give rise to the formation of aminomethylated product exclusively is commonly known as the Mannich Reaction and the product is invariably termed as the Mannich Base, as depicted below ... 

Silyl derivatives, which are widely used for gas chromatographic applications, are formed by the replacement of active hydrogens atoms from acids, alcohols, thiols, amines, amides, enolizable ketones, and aldehydes with trimethylsilyl groups. A wide variety of reagents are available. These... 

Friedlander condensation is considered as an atom-economic reaction proceeding through double condensation between 2-aminoaryl carbonyl compoimds with other carbonyl components that show enolizable hydrogens. 

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