Alkyl radicals from boranes and oxygen

Although the tin hydride + alkyl halide method is very efficient, tin compounds are falling out of favour because of their toxicity. The reaction between boranes and oxygen provides a simple alternative, and many of the reactions carried out formerly using the tin hydride radical chemistry can now be done using the method we are about to describe. Mixing trialkylboranes with α, β-unsaturated carbonyl compounds in the presence of water gives conjugate addition of one of the alkyl groups from the boron. The carbonyl compound can be an aldehyde (R² = H) or a ketone (R² = alkyl)

There was a dispute at fi rst as to whether this was a radical reaction or an ionic reaction. The ionic reaction might have been a kind of pericyclic reaction with the intermediate boron enolate being hydrolysed by the water.

However, H. C. Brown discovered that the reaction was completely inhibited by just 5% of the stable radical galvinoxyl , known to be an efficient scavenger for radicals. But where were the radicals coming from? Further experiments showed that small amounts of oxygen were needed to make the reaction work. oxygen is a triplet diradical and displaces alkyl radicals from the trialkyl borane. This reaction looks at first like an SN2 and is called an SH2 (second order homolytic displacement), but in reality the oxygen adds to the empty p orbital of planar trigonal boron to release an alkyl radical and start the chain reaction.

The alkyl radical now adds to the enone to give a delocalized intermediate that can be rep resented as a carbon- or oxygen-centred radical.

The chain is completed by displacement of another alkyl radical from a trialkylborane at oxygen by the delocalized radical and the formation of the same boron enolate proposed in the ionic reaction. This alkyl radical adds in its turn to the enone, and the boron enolate which forms is hydrolysed to the ketone product. Only small amounts of oxygen are needed to initiate the chain and it is not surprising that the air around the reaction mixture is enough to start a typical reaction. The water (which is inert to radicals, so can be present in the reaction mixture) again hydrolyses the boron enolate.

By combining a hydroboration step, which forms the borane, with the radical addition it is possible to carry out transformations such as the one below.

اخر الاخبار

اخبار العتبة العباسية المقدسة

استعدادًا لزيارة الأربعين العتبة العباسية المقدسة تعقد اجتماعًا لتنظيم عمل الهيئات التطوّعية في الصحن الشريف

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

المزيد

الآخبار الصحية

تسارع الشيخوخة.. متى يبدأ وأي الأعضاء أكثر تأثرا؟

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"

المعلمون في خطر.. خاصية جديدة في "تشات جي بي تي" تشرح الدروس

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