Name: allylic bromination with nbs
Uploaded: 2025-05-22T09:05:43.000Z
Duration: 1 min 27 s
Channel: JoVE

アルケンのアリルハロゲン化には、低濃度のハロゲンと高温が必要です。
室温でアリル酸臭素化を行うには、試薬としてN-ブロモスクシンイミド(NBS)を使用します。臭素濃度を低く保ち、臭素ラジカルの供給源として機能します。
例えば、プロペンは光または過酸化物の存在下でNBSと反応し、3-ブロモプロペンを形成します。
アリル臭素化のメカニズムには、開始、増殖、および終了のステップが含まれます。
開始ステップでは、NBSは光によるN-Br結合のホモリシス性切断を受け、臭素ラジカルを形成します。
最初の増殖ステップでは、臭素ラジカルがプロペンと反応してアリル水素を分離し、アリルラジカルとHBrを形成します。
生成されたHBrはNBSと反応して、反応のさらなる伝播に必要な臭素分子を生成します。
第2の増殖ステップでは、臭素分子がアリルラジカルと反応して臭化アリルと臭素原子が得られます。
最後に、終了ステップでは、異なるラジカルが互いに結合して非ラジカル生成物を生成し、それ以上の反応を停止します。

ラジカル置換: アリル臭素化

Radical Substitution: Allylic Bromination

有機合成では、反応条件を変更することで生成物の形成を変えることができます。 たとえば、プロペンを室温で臭素で処理すると、ジブロモ付加生成物が形成されます。 対照的に、プロペンは非極性溶媒中で高温でアリル置換を受け、3-ブロモプロペンが得られます。 付加反応を避けるためには、反応全体を通じて臭素濃度をできるだけ低く保つ必要があります。 これは、臭素分子の代わりに試薬として N-ブロモスクシンイミド (NBS) を使用することで実現できます。

プロペンは光または過酸化物の存在下でラジカル置換を介して NBS と反応し、臭化アリルまたは 3-ブロモプロペンを形成します。 ラジカル反応と同様に、アリル臭素化メカニズムには、開始、伝播、および停止の 3 つのステップが含まれます。 開始ステップでは、NBS は光または過酸化物の存在下で弱い N-Br 結合のホモリシス開裂を受け、臭素ラジカルを形成します。 最初の伝播ステップ中に、生成された臭素ラジカルがアリル水素を引き抜いて、共鳴安定化されたアリルラジカルと HBr を生成します。 形成された HBr はイオン反応ですぐに NBS と反応して Br_2 を生成し、これが第 2 の伝播ステップに関与します。 最終的に、停止ステップで異なるラジカルが結合し、その結果非ラジカル生成物が形成され、反応が停止します。 反応全体を通じて、HBr および Br_2 濃度は最小限に保たれます。 これらの条件下、つまり臭素濃度が非常に低い非極性溶媒中では、Br_2 のイオン付加はラジカル臭素化とうまく競合しません。

アリル置換アルケンのラジカル臭素化により、生成物の混合物が形成されます。 これは、形成されたアリルラジカル中間体の共鳴安定化により、いずれかの部位からハロゲンを引き抜くことができるためです (図 1)。

<img alt="Figure1" height="150" src="/files/ftp_upload/12900/PT_12900_Figure%201.svg" style="text-align: center; height: 150px;">

有機合成では、反応条件を変えることで生成物の形成を変化させることができます。例えば、ジブロモ添加製品は...

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

allylic bromination with nbs

Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form.

The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene instead undergoes allylic substitution to form 3-bromopropene. When propene reacts with NBS in the presence of light or peroxide, it forms allyl bromide, which is also called 3-bromopropene.

The mechanism follows three familiar radical steps: initiation, propagation, and termination. During initiation, NBS undergoes homolytic cleavage of its weak N–Br bond when exposed to light or peroxide. This forms a bromine radical. In the first propagation step, that radical removes an allylic hydrogen and leaves behind a resonance-stabilized allylic radical plus HBr.

HBr does not build up. It reacts right away with NBS in an ionic step that produces Br2. The Br2 then takes part in the second propagation step. In the termination step, radicals combine to form nonradical products, which stops the chain reaction.

Keeping both HBr and Br2 at very low levels is important throughout the reaction. Under these conditions, the ionic addition of Br2 does not compete well with radical bromination. For allylic substituted alkenes, the reaction can give a mixture of products because the resonance-stabilized allylic radical can abstract halogen from either site.

Allylic Bromination with NBS

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Allylic bromination with NBS changes propene into allyl bromide under radical conditions. In this reaction, the goal is to favor substitution at the allylic position instead of bromine addition across the double bond. Using N-bromosuccinimide, or NBS, helps keep the bromine concentration low so the addition product is less likely to form. The reaction conditions matter. Propene gives a dibromo addition product with bromine at room temperature. In non-polar solvents at high temperature, propene...

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key terms and concepts

See how NBS helps propene undergo allylic bromination instead of addition. Learn the radical steps, low bromine conditions, and product mix.

Allylic Bromination with NBS | 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。

Allylic Bromination with NBS

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