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α-Alkylation of Ketones via Enolate Ions01:10

α-Alkylation of Ketones via Enolate Ions

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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...
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Regioselective Formation of Enolates01:33

Regioselective Formation of Enolates

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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.
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Reactivity of Enolate Ions01:23

Reactivity of Enolate Ions

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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...
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Aldehydes and Ketones with Amines: Enamine Formation Mechanism01:14

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Enamine formation involves the addition of carbonyl compounds to a secondary amine through a series of reactions. The mechanism begins with the generation of carbinolamine, a nucleophilic attack followed by several proton transfer reactions. The hydroxyl group of the carbinolamine is converted into water to make a better leaving group that can push the reaction forward by eliminating a water molecule. In enamine formation, the last step involves the abstraction of a proton from the α carbon to...
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Aldehydes and ketones form enols, although only about 1% of the enol is present at the equilibrium for simple monocarbonyl compounds. The enol form is undetectable for acetaldehyde, present as only 1.5 × 10−4 % of acetone, and present as only 1.2% of cyclohexanone. Two kinds of regioisomeric enols are possible for unsymmetrical ketones, and their net composition is 1% at equilibrium. This instability is due to the lower bond energy of C=C than the C=O group. The additional...
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Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview01:16

Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview

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Primary amines react with carbonyl compounds—aldehydes and ketones—to generate imines. Imines consist of a C=N double bond and are named Schiff bases after its discoverer—the German chemist Hugo Schiff. On the other hand, secondary amines react with carbonyl compounds to give enamines. In enamines, the presence of a C=C double bond adjacent to the nitrogen atom leads to the delocalization of the lone pair.
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Solid-State and Solution Structures of Glycinimine-Derived Lithium Enolates.

Kyoung Joo Jin1, David B Collum1

  • 1Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University , Ithaca, New York 14853-1301, United States.

Journal of the American Chemical Society
|November 12, 2015
PubMed
Summary

This study reveals the complex aggregation structures of lithium enolates in solution, using techniques like X-ray crystallography and NMR spectroscopy. Findings detail monomer, dimer, tetramer, and hexamer formations influenced by solvent and chiral diamine additives.

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Area of Science:

  • Organometallic Chemistry
  • Supramolecular Chemistry
  • Organic Synthesis

Background:

  • Lithium enolates are crucial intermediates in organic synthesis.
  • Understanding their aggregation state is key to controlling reactivity.
  • Chiral diamines are often used as additives to influence stereochemistry.

Purpose of the Study:

  • To elucidate the aggregation structures of lithium enolates derived from glycinimines.
  • To investigate the influence of solvent and chiral additives on these structures.
  • To correlate solution behavior with solid-state structures.

Main Methods:

  • X-ray crystallography for solid-state structure determination.
  • (6)Li NMR spectroscopy and method of continuous variations for solution studies.
  • Density functional theory (DFT) computations for theoretical insights.

Main Results:

  • Observed crystal structures include monomers, dimers, tetramers, and hexamers.
  • NMR studies revealed the distribution of these species in solution.
  • DFT calculations provided detailed insights into bonding and energetics.

Conclusions:

  • Lithium enolate aggregation is highly dependent on solvent and additives.
  • Chiral diamines like TMCDA play a significant role in structure formation.
  • A combination of experimental and computational methods provides a comprehensive understanding.