Concentration effects

You know that R• is perfectly capable of abstracting H from Bu₃SnH because that is what happened in the dehalogenation reaction, but here a different reaction happens: addition to the alkene. In fact, the rate constant for reaction of R• with Bu₃SnH is about the same as that for reaction with acrylonitrile (CH₂=CHCN), so the only way in which good yields can be obtained is by ensuring that the concentration of acrylonitrile is always at least 10 times that of the tin hydride. The difference in rates will then be sufficient to give 10 times as much addition to the alkene as reduction by the tin hydride. Too much acrylonitrile in the reaction mixture causes problems with side reactions, so a good way of achieving this is to add the tin hydride very slowly during the reaction—often a device known as a syringe pump is used for this. Of course, for complete reaction, a whole equivalent of hydride is necessary, but this can be added over a period of hours. An elegant alternative is to use a technique conceptually similar to the use of NBS to provide a low concentration of Br₂ for radical allylic substitution. Instead of adding one equivalent of Bu3SnH, a catalytic amount (usually 0.1–0.2 equivalents) of Bu₃SnCl is added at the beginning of the reaction, with 1 equivalent of NaBH₄. NaBH₄ will reduce Bu3SnHal to Bu₃SnH, so about 0.1 equivalent of Bu₃SnH is formed immediately. With each cycle of the chain reaction, a molecule of this Bu₃SnH is converted to Bu₃SnBr, which NaBH₄ can reduce back to Bu3SnH. Only as much Bu3SnH is produced as is needed because the rate of production is limited by the rate of reaction.

This method was used in the following example, in which an enantiomerically pure lac tone, a useful synthetic building block, was made from naturally occurring glyceraldehyde.

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قسم الشؤون الخدمية يواصل تنفيذ أعمال الوقاية ومكافحة الحشرات في مواقع العتبة العباسية المقدسة

المجمَع العلمي للقرآن الكريم يواصل تنفيذ مشروع بيوت النور القرآني

قسم الشؤون الفكرية يبحث التعاون في مجال البحث مع كلية التقنيات المتعددة في نيجيريا

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الآخبار الصحية

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9 مسببات شائعة للالتهاب المزمن قد تهدد صحتك

باحثون يطورون علاجا لهشاشة العظام.. النتائج مذهلة

تحمي من أمراض عديدة.. 6 فوائد لتناول البطاطا الحلوة

المزيد

الآخبار التكنلوجية

عودة التعاون الفضائي بين روسيا وأميركا.. وبحث ملفات "هامة"

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بشرى لمرضى الشلل.. شريحة في الدماغ قد تتيح التحكم بالأدوات

مدينة غارقة.. هل يخفي قاع البحر الكاريبي سرّ حضارة مفقودة؟

المزيد

مواضيع ذات صلة

Attack of carbenes on lone pairs

Nitrenes are the nitrogen analogues of carbenes

Rearrangement reactions

Insertion into C–H bondsInsertion into C–H bonds

Making cyclopropanes

Carbenes react with alkenes to give cyclopropanes

The structure of carbenes depends on how they are made

Carbenes can be divided into two types

اكتشاف العناصر

العناصر

Diazomethane makes methyl esters from carboxylic acids

Intramolecular radical reactions are more efficient than intermolecular ones