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

Neben Pinacol- und McMurry-Reaktionen ist die Acyloin-Kondensation eine weitere reduktive Kupplungsreaktion, an der Ester beteiligt sind.
Bei dieser Reaktion reagieren Ester in Gegenwart einer metallischen Elektronenquelle in einem aprotischen Lösungsmittel zu einem α-Hydroxyketon, auch Acyloin genannt.
Die Reaktion verläuft durch die Bildung von Ketylen, die radikal dimerisiert werden, um ein instabiles tetraedrisches Zwischenprodukt zu bilden, und dann weiter kollabieren, wodurch ein 1,2-Diketon entsteht.
Da ein 1,2-Diketon gegenüber Elektrophilen und Reduktionsmitteln vergleichsweise reaktiver ist als ein Keton, reduziert die Übertragung von zwei Elektronen das 1,2-Diketon leicht zu einem Enediolat.
Schließlich ergibt die Ansäuerung von Enediolat ein α-Hydroxyketon mit einer guten Ausbeute. Die Reaktivität von nukleophilem Enedilat führt jedoch häufig zur Bildung von Nebenprodukten.
Um unerwünschte Nebenreaktionen zu verhindern, wird Trimethylsilylchlorid zugesetzt, um das Enediolat zu siliylieren, wodurch ein Bis-Silylether entsteht, der weiter mit wässriger Säure hydrolysiert wird, um ein α-Hydroxyketon als Endprodukt zu erhalten.

α-hydroxyketone durch reduktive kopplung von estern: Übersicht über die acyloin-kondensation

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

Bei den Pinakol- und McMurry-Reaktionen handelt es sich um die reduktive Kopplung von Ketonen oder Aldehyden. In ähnlicher Weise ergibt die bimolekulare reduktive Kopplung zweier Estermoleküle in Gegenwart von Natriummetall in einem aprotischen Lösungsmittel ein α-Hydroxyketonprodukt. Das α-Hydroxyketon wird auch Acyloin genannt, daher wird die Reaktion als „Acyloinkondensation“ bezeichnet.

<img alt="Figure1" src="/files/ftp_upload/12908/12908_PT_Figure_1.svg" style="text-align: center; height: 65px;">

Bei der Acyloin-Kondensationsreaktion werden Ester in Ketyle umgewandelt, die einer radikalischen Dimerisierung unterliegen und ein instabiles tetraedrisches Zwischenprodukt bilden. Darüber hinaus kollabiert dieses Zwischenprodukt und ergibt ein 1,2-Diketon-Zwischenprodukt. Die 1,2-Diketone sind aufgrund der geringeren π*-Energie vergleichsweise reaktiver als Ketone gegenüber Elektrophilen und Reduktionsmitteln. Folglich werden zwei Elektronen nacheinander auf das 1,2-Diketon übertragen, um ein Endiolat zu bilden. Abschließend wird das Endiolat-Zwischenprodukt mit Säure gequencht, um in guter Ausbeute ein α-Hydroxyketon zu ergeben.

Aufgrund der Reaktivität des nukleophilen Endiolats fallen häufig neben dem Hauptprodukt auch einige Nebenprodukte an. Um Nebenreaktionen zu vermeiden und die Ausbeute eines Acyloins zu verbessern, wird Trimethylsilylchlorid zur Silylierung des Endiolats verwendet. Dies führt zu einem Bissilylether, der mit wässriger Säure weiter hydrolysiert wird, um α-Hydroxyketone zu ergeben.

α-Hydroxyketone durch reduktive Kopplung von Estern: Übersicht über die Acyloin-Kondensation

Die Pianacol- und McMurry-Reaktionen beinhalten die reduktive Kopplung von Ketonen oder Aldehyden. In ähnlicher Weise ist die bimolekulare reduktive Kopplung zweier Ester ...

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

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Organic Chemistry

Organische Chemie

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Radical Chemistry

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KEY TERMS AND CONCEPTS

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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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&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 Organic Chemistry explains basic concepts of stereochemistry, reactions, and mechanisms using concise and easy-to-understand animated video lessons. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Acyloin Condensation: Ester Coupling Pathway

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