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α-Alkylation of Ketones via Enolate Ions

α-Alkylation of Ketones via Enolate Ions

Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the...

Reactivity of Enolate Ions

Reactivity of Enolate Ions

Enolate ions are formed by the acid–base reaction of a carbonyl compound with a base. This leads to deprotonation of the α hydrogen atom, leading to a resonance-stabilized enolate ion where one of the contributing structures is an oxyanion, which imparts additional stability. Therefore, the proton on the α carbon is more acidic in nature than that of other sp3-hybridized C–H bonds but less acidic than those in O–H bonds where the negative charge in the conjugate...

Regioselective Formation of Enolates

Regioselective Formation of Enolates

As depicted in the figure below, the unsymmetrical ketones can form two possible enolates: less substituted or more substituted enolates. Usually, the thermodynamic enolates are formed from the more substituted α-carbon atom, while the kinetic enolates are formed faster by deprotonation from the less substituted position. The thermodynamic enolates have lower energy, so they are more stable. But the energy required to form kinetic enolates is less.

Stereochemical Effects of Enolization

Stereochemical Effects of Enolization

The chiral α-carbon of the carbonyl compound is the stereocenter of the molecule. As shown in the figure below, when such a carbonyl compound undergoes racemization under an acidic or basic condition, an achiral enol is formed.

Enolate Mechanism Conventions

Enolate Mechanism Conventions

When a carbonyl compound is treated with a strong base, the α position gets deprotonated to give a resonance-stabilized intermediate called an enolate. Enolates are ambident nucleophiles because they possess two nucleophilic sites that can attack an electrophile owing to the delocalization of the negative charge between the α carbon and oxygen atoms. When the oxygen atom attacks an electrophile, it is called O-attack, whereas electrophilic attack via the α carbon is known as...

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.

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Related Experiment Video

Updated: Mar 13, 2026

1,3,5-Triphenylbenzene and Corannulene as Electron Receptors for Lithium Solvated Electron Solutions

1,3,5-Triphenylbenzene and Corannulene as Electron Receptors for Lithium Solvated Electron Solutions

Published on: October 10, 2016

Conformational Polymorphism of Lithium Pinacolone Enolate.

Jie Guang¹, Qiyong Liu¹, Russell Hopson¹

¹Department of Chemistry, Brown University , Providence, Rhode Island 02912, United States.

Journal of the American Chemical Society

|October 21, 2016

Summary

Researchers discovered a stable hexameric crystal structure for lithium pinacolone enolate (LiOPin). This structure changes in different solvents and can form mixed aggregates with lithium aldolate (LiOA).

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Area of Science:

Organometallic Chemistry
Crystallography
Supramolecular Chemistry

Background:

Lithium enolates are key intermediates in organic synthesis.
Understanding their aggregation state is crucial for controlling reactivity.
Polymorphism in organolithium compounds presents synthetic challenges.

Purpose of the Study:

To report a novel metastable, polymorphic hexameric crystal structure of lithium pinacolone enolate (LiOPin).
To investigate the influence of solvent polarity on LiOPin aggregation.
To explore the formation of mixed aggregates with lithium aldolate (LiOA).

Main Methods:

Synthesis of LiOPin via three distinct preparation methods.
Nuclear Magnetic Resonance (NMR) spectroscopy for structural characterization.
Solvent-dependent aggregation studies in aromatic and nonaromatic hydrocarbons.

Main Results:

Characterization of a metastable, polymorphic hexameric crystal structure of LiOPin.
Demonstration of LiOPin hexamer deaggregation to a tetramer in toluene.
Observation of LiOPin retaining its hexameric structure in cyclohexane.
Formation of a 3:1 mixed aggregate (LiOPin₃·LiOA) in the presence of LiOA.

Conclusions:

The aggregation state of LiOPin is sensitive to solvent environment.
Lithium aldolate (LiOA) can significantly alter LiOPin aggregation behavior.
The findings provide insights into the structural diversity and reactivity of lithium enolates.