Name: alkyl halide reduction with tributyltin hydride
Uploaded: 2023-04-30T11:22:27.000Z
Duration: 1 min 26 s
Channel: JoVE

Radikalsubstitutionsreaktionen sind nützlich für die Halogenierung von Alkanen und Alkenen.
In ähnlicher Weise können radikalische Substitutionen Halogen aus Alkylhalogeniden unter Verwendung von Tributylzinnhydrid entfernen, das als Hydrogenolyse von Alkylhalogeniden bekannt ist.
Insbesondere Zinnhydrid hat eine schwache Sn-H-Bindung und kann mit Alkylhalogeniden reagieren, um das Halogen durch Wasserstoff zu ersetzen, wobei Alkan und Zinnhalogenid entstehen.
Die Reaktion wird energetisch begünstigt, da die neu gebildeten Bindungen stärker sind als die gebrochenen Bindungen.
Der Mechanismus der Hydrogenolyse folgt einer Kettenreaktion.
Zunächst erzeugt Zinnhydrid in Gegenwart von Licht ein Tributylzinnradikal, das dann Halogen von einem organischen Halogenid abstrahiert und ein radikalisches Zwischenprodukt erzeugt.
Als nächstes abstrahiert das radikalische Zwischenprodukt Wasserstoff aus Zinnhydrid, um schließlich ein Alkan und einen Tributylzinnrest zu bilden, der die Reaktion fortsetzt. 
Die Reaktivität von Alkylhalogeniden gegenüber der Hydrogenolyse nimmt von Jodid zu Fluorid ab, wo die Alkylfluoride nicht reaktiv sind.
Insbesondere erfordert die Reduktion von Alkylchloriden eine höhere Konzentration an Tributylzinnradikalen, die durch Zugabe eines Initiators wie AIBN zur Reaktion erreicht wird.

radikalische substitution: hydrogenolyse von alkylhalogeniden mit tributylzinnhydrid

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

Radikalische Substitutionsreaktionen können zur Entfernung funktioneller Gruppen aus Molekülen eingesetzt werden. Die Hydrogenolyse von Alkylhalogeniden ist eine solche Reaktion, bei der die schwache Sn-H-Bindung in Tributylzinnhydrid mit Alkylhalogeniden unter Bildung von Alkanen reagiert. Das Reagenz Bu_3SnH liefert dabei Tributylzinnhalogenid als Nebenprodukt.

Die bei dieser Reaktion gebildeten Bindungen sind stärker als die gebrochenen Bindungen, so dass sie energetisch günstig ist. Die Reaktion folgt einem Radikalkettenmechanismus, ähnlich wie bei radikalischen Halogenierungsreaktionen, bei denen Bu_3SnH in Gegenwart von Licht homolytisch gespalten wird und ein Tributylzinnradikal bildet. Dieses Radikal abstrahiert das Halogen von einem Alkylhalogenid und bildet ein Alkylradikal-Zwischenprodukt und Tributylzinnhalogenid. Das radikalische Zwischenprodukt abstrahiert Wasserstoff aus Tributylzinnhydrid und erzeugt ein Alkan und ein Tributylzinnradikal, das die Reaktion fortführt.

Da C-Br- und C-I-Bindungen schwächer sind als C-Cl-Bindungen, reicht Tageslicht aus, um die Hydrogenolyse von Alkylbromiden und -jodiden einzuleiten. Im Gegensatz dazu sind Alkylfluoride aufgrund der starken C-F-Bindungen unreaktiv. Die Hydrogenolyse von Alkylchloriden mit Bu_3SnH erfordert eine hohe Konzentration von Tributylzinnradikalen, die durch Zugabe eines Initiators zum Reaktionsgemisch erreicht wird. AIBN ist der am häufigsten verwendete Initiator, da er oberhalb von 60 °C eine thermische Homolyse durchläuft und nitrilstabilisierte Radikale bildet. Diese Radikale entziehen dem Tributylzinnhydrid Wasserstoff und bilden das Tributylzinnradikal, das die Reaktion in Gang setzt.

<img alt="Figure1" src="/files/ftp_upload/12901/PT_12901_Figure%201.svg" style="text-align: center; height: 190px;">

Peroxide werden nicht als Initiatoren verwendet, da Peroxidradikale hochreaktiv sind und Wasserstoff von organischen Halogeniden abziehen können, was zu unerwünschten Nebenreaktionen führt.

Radikalische Substitution: Hydrogenolyse von Alkylhalogeniden mit Tributylzinnhydrid

Radikalsubstitutionsreaktionen können verwendet werden, um funktionelle Gruppen aus Molekülen zu entfernen. Die Hydrogenolyse von Alkylhalogeniden ist eine solche Reaktion, bei der die ...

radical-substitution-hydrogenolysis-alkyl-halides-with-tributyltin

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Radical Chemistry

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alkyl halide reduction with tributyltin hydride

Alkyl halide reduction with tributyltin hydride removes a halogen and replaces it with hydrogen. This reaction turns alkyl halides into alkanes. The reagent Bu3SnH has a weak Sn–H bond, and the reaction forms tributyltin halide as a byproduct. Because the new bonds are stronger than the bonds that are broken, the reaction is energetically favorable.

The reaction proceeds by a radical chain mechanism, which is similar to radical halogenation. Light causes Bu3SnH to split homolytically, meaning the bond breaks evenly to form a tributyltin radical. That radical removes the halogen from the alkyl halide. This step creates an alkyl radical intermediate and tributyltin halide.

Next, the alkyl radical takes hydrogen from tributyltin hydride. This forms the alkane product and a new tributyltin radical. The new radical then continues the chain reaction.

Bond strength affects how easily the reaction starts. C–Br and C–I bonds are weaker than C–Cl bonds, so daylight can start the hydrogenolysis of alkyl bromides and iodides. Alkyl fluorides do not react because C–F bonds are very strong.

Alkyl chlorides need more tributyltin radicals, so an initiator is added to the reaction mixture. AIBN is the most common initiator. Above 60 °C, it undergoes thermal homolysis and forms nitrile-stabilized radicals. These radicals remove hydrogen from tributyltin hydride and create the tributyltin radical that keeps the reaction going.

Peroxides are not used as initiators in this reaction. Their radicals are very reactive and can remove hydrogen from organic halides. That can lead to unwanted side reactions.

Alkyl Halide Reduction with Tributyltin Hydride

alkyl-halide-reduction-with-tributyltin-hydride

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key terms and concepts

Learn how tributyltin hydride removes halogens from alkyl halides through a radical chain reaction, and why light, AIBN, and bond strength matter.

Alkyl Halide Reduction with Tributyltin Hydride | JoVE K12

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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.

Alkyl Halide Reduction with Tributyltin Hydride

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