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

ラジカル置換反応は、アルカンやアルケンのハロゲン化に有用です。
同様に、ラジカル置換は、ハロゲン化アルキルの水素化分解として知られるトリブチルスズ水素化物を使用して、ハロゲン化アルキルからハロゲンを除去できます。
特に、水素化スズは弱いSn-H結合を持ち、ハロゲン化アルキルと反応してハロゲンを水素で置換し、アルカンとハロゲン化スズを形成することができます。
形成された新しい結合は切断された結合よりも強いため、反応はエネルギー的に有利です。
水素化分解のメカニズムは連鎖反応をたどります。
最初に、水素化スズは光の存在下でトリブチルスズラジカルを生成し、次に有機ハロゲン化物からハロゲンを抽出してラジカル中間体を生成します。
次に、ラジカル中間体は水素化スズから水素を引き出し、最終的にアルカンとトリブチルスズラジカルを形成し、反応を伝播します。 
水素化物分解に対するハロゲン化アルキルの反応性は、ヨウ化物からフッ化物に減少しますが、フッ化アルキルは反応性がありません。
特に、塩化アルキルの還元には、反応にAIBNなどの開始剤を添加することにより、より高濃度のトリブチルスズ基が必要になります。

ラジカル置換: 水素化トリブチルスズによるハロゲン化アルキルの水素化分解

Radical Substitution: Hydrogenolysis of Alkyl Halides with Tributyltin Hydride

ラジカル置換反応を使用して、分子から官能基を除去できます。 ハロゲン化アルキルの水素化分解はそのような反応の 1 つで、水素化トリブチルスズの弱い Sn-H 結合がハロゲン化アルキルと反応してアルカンを形成します。 ここで、試薬 Bu_3SnH は副生成物としてハロゲン化トリブチルスズを生成します。

この反応で形成される結合は破壊される結合よりも強いため、エネルギー的に有利になります。 この反応はラジカルハロゲン化反応と同様のラジカル連鎖機構に従い、光の存在下で Bu_3SnH がホモリシス開裂を受けてトリブチルスズラジカルを形成します。 このラジカルはハロゲン化アルキルからハロゲンを引き抜き、アルキルラジカル中間体とハロゲン化トリブチルスズを形成します。 ラジカル中間体はさらに水素化トリブチルスズから水素を引き抜き、アルカンとトリブチルスズラジカルを生成し、反応が進行します。

C-Br および C-I 結合は C-Cl 結合よりも弱いため、臭化アルキルおよびヨウ化アルキルの水素化分解を開始するには日光だけで十分です。 対照的に、フッ化アルキルは強い C-F 結合のため反応性がありません。 Bu_3SnH を使用した塩化アルキルの水素化分解には高濃度のトリブチルスズラジカルが必要ですが、これは反応混合物に開始剤を添加することで達成されます。 AIBN は、60 °C 以上で熱均一分解を受けてニトリルで安定化されたラジカルを形成するため、最も広く使用されている開始剤です。 これらのラジカルは水素化トリブチルスズから水素を引き抜き、トリブチルスズラジカルを形成し、反応が進行します。


<img alt="Figure1" height="190" src="/files/ftp_upload/12901/PT_12901_Figure%201.svg" style="text-align: center; height: 190px;">



過酸化物は反応性が高く、有機ハロゲン化物から水素を引き抜いて望ましくない副反応を引き起こす可能性があるため、開始剤としては使用されません。

ラジカル置換反応は、分子から官能基を除去するために使用することができます。ハロゲン化アルキルの水素化分解はそのような反応の1つであり、...

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

Education

JoVE Core

Organic Chemistry

有機化学

Unit 1

ユニット1

Radical Chemistry

ラジカル化学

KEY TERMS AND CONCEPTS

主要用語と概念

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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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...

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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

organic chemistry

JoVE Core 有機化学では、立体化学、反応、メカニズムの基本概念を、簡潔でわかりやすいアニメーション ビデオ レッスンを使用して説明します。さらに、科学者の実際のビデオでは、世界中の今日の研究室で行われている古典的および独自の研究実験における関連概念の適用を示しています。

Alkyl Halide Reduction with Tributyltin Hydride

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