What is Zimmerman Traxler transition state?
In 1957, Howard Zimmerman and Marjorie D. Traxler proposed that some aldol reactions have “six-membered transition states having a chair conformation.” This is now known as the Zimmerman–Traxler model. E-enolates give rise to anti products, whereas Z-enolates give rise to syn products.
What does LDA do in a reaction?
Strong organic bases such as LDA (Lithium DiisopropylAmide) can be used to drive the ketone-enolate equilibrium completely to the enolate side. LDA is a strong base that is useful for this purpose. The steric bulk of its isopropyl groups makes LDA non- nucleophilic.
What is the alkylation of Enolates?
Enolates can be alkylated in the alpha position through an SN2 reaction with alkyl halides. During this reaction an α-hydrogen is replaced with an alkyl group and a new C-C bond is formed. A very strong base, such as LDA, is often used because of its ability to form the enolate completely.
Who discovered aldol reaction?
The aldol reaction, first discovered by Wurtz in 1872,1 is one of the most powerful transformations in organic chemistry. The process unites two carbonyl partners to give β-hydroxyketones with up to two new stereocenters (Scheme 1).
Is LDA selective?
Why is LDA useful? Well, enolates are extremely useful nucleophiles, able to participate in SN2 reactions with alkyl halides as well as the aldol reaction (among many other things). The selectivity of LDA in forming the less substituted enolate makes it extremely useful.
What does LDA and THF do?
Ether solvents like tetrahydrofuran (THF) are commonly used for enolate anion formation. With the exception of sodium hydride and sodium amide, most of these bases are soluble in THF. Because of its solubility in THF, LDA is a widely used base for enolate anion formation.
Can esters form Enolates?
As we mentioned, esters can also form enolates, though in smaller concentrations than aldehydes or ketones. These enolates can then react with unreacted starting material in a condensation reaction that is exactly analogous to the aldol reaction.
Why do Enolates alkylate on carbon?
Enolates can act as a nucleophile in SN2 type reactions. Overall an α hydrogen is replaced with an alkyl group. This reaction is one of the more important for enolates because a carbon-carbon bond is formed. These alkylations are affected by the same limitations as SN2 reactions previously discussed.
What is aromatic ketone?
Solution. The compounds in which a group is attached to either two aryl groups or one aryl and one alkyl group are called aromatic ketones.
