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α-Substituted ketones or aldehydes can be synthesized from enamines by the Stork enamine reaction, named after its pioneer Gilbert Stork. Enamines are useful synthetic intermediates where the lone pair on nitrogen is in conjugation with the C=C bond. They resemble enolate ions, as the resonance forms of both species have a nucleophilic α carbon.
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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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In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...
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By replacing an α-hydrogen with a halogen, acid-catalyzed α-halogenation of aldehydes or ketones yields a monohalogenated product
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Another method of radical formation is the elimination process. It is the opposite of the addition route and is driven by the instability of the radical. For example, as depicted in Figure 1, dibenzoyl peroxide yields a pair of unstable radicals upon homolysis. Given its instability, this radical spontaneously undergoes elimination via a C–C bond cleavage to form a relatively more stable phenyl radical. The mechanism involves cleavage of the bond between the α and β positions...
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Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...
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A Radical Route to α-Substituted Enones.

Bartosz Bieszczad1, Samir Z Zard1

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A new method enables α-substitution of enones using unactivated alkenes via α-xanthyl-β-hydroxy ketones. This approach offers a versatile route to complex molecules, mimicking alkenyl radicals.

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Area of Science:

  • Organic Chemistry
  • Synthetic Chemistry

Background:

  • Enone functionalization is crucial for synthesizing complex organic molecules.
  • Direct α-substitution of enones with unactivated alkenes remains challenging.
  • Alkenyl radicals are highly reactive and difficult to control synthetically.

Purpose of the Study:

  • To develop a versatile strategy for the α-substitution of enones.
  • To utilize stable synthetic equivalents of alkenyl radicals.
  • To enable the formal fusion of enones and unactivated alkenes.

Main Methods:

  • Formation and in situ use of α-xanthyl-β-hydroxy ketones.
  • Employing these intermediates as surrogates for alkenyl radicals.
  • One-pot reaction conditions for efficient synthesis.

Main Results:

  • Successful α-substitution of enones with unactivated alkenes.
  • Demonstrated versatility of the α-xanthyl-β-hydroxy ketone strategy.
  • Extended the methodology to an α,β-unsaturated ester in one instance.

Conclusions:

  • The developed method provides a robust and versatile approach for enone α-functionalization.
  • The use of α-xanthyl-β-hydroxy ketones simplifies the handling of reactive alkenyl radical equivalents.
  • This strategy opens new avenues for constructing substituted carbonyl compounds.