Name: acyloin condensation: ester coupling pathway
Uploaded: 2023-04-30T11:23:12.000Z
Duration: 1 min 19 s
Channel: JoVE

Outre les réactions du pinacol et de McMurry, la condensation de l’acyloïne est une autre réaction de couplage réductrice qui implique des esters.
Dans cette réaction, les esters réagissent en présence d’une source métallique d’électrons dans un solvant aprotique pour former une cétone α-hydroxy, également appelée acyloïne.
La réaction se déroule par la formation de cétyles, qui subissent une dimérisation radicalaire pour former un intermédiaire tétraédrique instable, puis s’effondrent davantage, donnant une 1,2-dicétone.
Étant donné qu’une 1,2-dicétone est comparativement plus réactive qu’une cétone vis-à-vis des électrophiles et des agents réducteurs, le transfert de deux électrons réduit facilement la 1,2-dicétone à un énédilate.
Enfin, l’acidification de l’énédiolate donne une α-hydroxycétone avec un bon rendement. Cependant, la réactivité de l’énedilate nucléophile conduit souvent à la formation de sous-produits.
Pour éviter les réactions secondaires indésirables, du chlorure de triméthylsilyle est ajouté pour silyler l’ènediolate, ce qui donne un éther bis-silylique, qui est ensuite hydrolysé avec de l’acide aqueux pour donner une cétone α-hydroxy comme produit final.

α-hydroxycétones via couplage réducteur d'esters : aperçu de la condensation de l'acyloïne

&alpha;-Hydroxy Ketones via Reductive Coupling of Esters: Acyloin Condensation Overview

Les réactions pinacol et McMurry impliquent le couplage réducteur de cétones ou d'aldéhydes. De même, le couplage réducteur bimoléculaire de deux molécules d'ester en présence de sodium métallique dans un solvant aprotique donne un produit α-hydroxy cétone. L’α-hydroxycétone est également appelée acyloïne, c’est pourquoi la réaction est appelée « condensation de l’acyloïne ».

<img alt="Figure1" src="/files/ftp_upload/12908/12908_PT_Figure_1.svg" style="text-align: center; height: 65px;">

Dans la réaction de condensation de l'acyloïne, les esters sont convertis en cétyles, qui subissent une dimérisation radicalaire pour former un intermédiaire tétraédrique instable. De plus, cet intermédiaire s'effondre pour donner un intermédiaire 1,2-dicétone. Les 1,2-dicétones sont comparativement plus réactives que les cétones envers les électrophiles et les agents réducteurs en raison de leur énergie π* plus faible. Par conséquent, deux électrons sont transférés séquentiellement vers la 1,2-dicétone pour former un enédiolate. Enfin, l’intermédiaire énédiolate est trempé avec un acide pour donner une α-hydroxy cétone avec un bon rendement.

Souvent, certains sous-produits sont obtenus à côté du produit principal en raison de la réactivité de l'énédiolate nucléophile. Pour surmonter les réactions secondaires et améliorer le rendement en acyloïne, le chlorure de triméthylsilyle est utilisé pour silyler l'énédiolate. Il en résulte un éther bis-silylique, ensuite hydrolysé avec un acide aqueux pour donner des α-hydroxycétones.

Les réactions du pinacol et de McMurry impliquent le couplage réducteur de cétones ou d’aldéhydes. De même, le couplage réducteur bimoléculaire de deux esters ...

hydroxy-ketones-via-reductive-coupling-esters-acyloin-condensation

Education

Enseignement

JoVE Core

Organic Chemistry

Chimie organique

Unit 1

Unité 1

Radical Chemistry

Chimie radicalaire

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

acyloin condensation: ester coupling pathway

Acyloin condensation is a reaction that joins two ester molecules to form an alpha-hydroxy ketone. The product is also called an acyloin. In this reaction, sodium metal is used in an aprotic solvent, which supports the reductive coupling step.

The reaction is related to the pinacol and McMurry reactions, which also involve reductive coupling of ketones or aldehydes. Here, the ester first becomes a ketyl. These ketyl species undergo radical dimerization and form an unstable tetrahedral intermediate. That intermediate then collapses to give a 1,2-diketone intermediate.

The 1,2-diketone is more reactive than a ketone toward electrophiles and reducing agents because it has lower pi-star energy. Two electrons then transfer in sequence to form an enediolate. The enediolate is finally quenched with acid to produce the alpha-hydroxy ketone. This reaction can give a good yield, but side products may also form because the enediolate is nucleophilic.

Trimethylsilyl chloride can be used to reduce those side reactions and improve the acyloin yield. It silylates the enediolate to form a bis-silyl ether. Aqueous acid then hydrolyzes this intermediate to give the alpha-hydroxy ketone.

Acyloin Condensation: Ester Coupling Pathway

acyloin-condensation-ester-coupling-pathway

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EPR Hyperfine Splitting in Organic Radicals

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Peroxide Effect in HBr Addition to Alkenes

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Hydrogen Bromide Addition to Alkenes by Radicals

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HBr Selectivity in Alkene Addition Thermodynamics

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&alpha;-hydroxy ketones via reductive coupling of esters: acyloin condensation overview

Acyloin condensation is a reaction that joins two ester molecules to form an alpha-hydroxy ketone. The product is also called an acyloin. In this reaction, sodium metal is used in an aprotic solvent, which supports the reductive coupling step. The reaction is related to the pinacol and McMurry reactions, which also involve reductive coupling of ketones or aldehydes. Here, the ester first becomes a ketyl. These ketyl species undergo radical dimerization and form an unstable tetrahedral...

key terms and concepts

Learn how acyloin condensation couples two esters with sodium metal to form alpha-hydroxy ketones, including ketyls, enediolates, and yield improvement.

Acyloin Condensation: Ester Coupling Pathway | JoVE K12

organic chemistry

JoVE Core Chimie Organique explique les concepts de base de la stéréochimie, des réactions et des mécanismes à l'aide de leçons vidéo animées concises et faciles à comprendre. De plus, les vidéos de scientifiques à l'action présentent l'application de ces concepts grâce à des expériences de recherche classiques et originales réalisées dans des laboratoires modernes.

Acyloin Condensation: Ester Coupling Pathway

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