Carbenes react with alkenes to give cyclopropanes

This reaction is the most important way of making cyclopropanes, and is probably the most important reaction of carbenes. The mechanism of this type of reaction (an example is shown in the margin) depends on whether the carbene is a singlet or a triplet, and the out come of the reaction can provide our first chemical test of the conclusions we came to in the previous section. Singlet carbenes, like this one here (remember that substituents with lone pairs stabilize the singlet spin state) can add to alkenes in an entirely concerted manner: the curly arrows for the process can be written as shown in the margin. Because the process is concerted, we expect that the geometry of the alkene should be pre served in the product—the reaction ought to be stereospecific. The two examples below show that this is indeed the case. It is more impressive that the Z alkene gives the cis-cyclopropane as this is less stable than the trans-cyclopropane and would change to E if it could.

The alkene insertion reaction is stereospecific only for singlet carbenes. For triplet carbenes, the reaction is non-stereospecific. In the example below, a triplet carbene gives a mixture of cyclopropane diastereosiomers from a pure Z alkene.

The mechanism of the reaction must be different with a triplet carbene. In fact, a concerted reaction is impossible for triplet carbenes because of the spins of the electrons involved. The spins of a triplet carbene aren’t paired, so once the carbene has added to the alkene in a radical reaction, the diradical (triplet) intermediate must wait until one of the spins inverts (‘flips’) before the second C–C bond can be formed with paired electrons.

Spin-flipping, which can occur only through collision with another molecule (of solvent usually), is relatively slow on the time-scale of molecular rotations and, by the time the electrons are in a fit state to pair up, the stereochemistry of the starting material has been scram bled by free rotation in the intermediate.

The same constraints arising from the need for conservation of electron spin apply to the formation as well as to the reaction of carbenes. When a carbene forms by α elimination, say, from a molecule with all electrons paired, it must be formed as the singlet, whether or not the triplet state is lower in energy. Only later may the carbene undergo spin-flipping to the triplet state. Since most carbene reactions are very rapid, this means that carbenes that are known to have triplet ground states may, in fact, react in their fi rst-formed singlet state because they don’t have time to spin-fl ip to the triplet. This is true for: CH2 produced from CH₂N₂, which adds stereospecifically to double bonds because it is formed as a singlet and because the singlet state is more reactive than the triplet.

The addition of a triplet carbene to an alkene can be considered to be rather like a radical addition to a double bond. The concerted addition of a singlet carbene, on the other hand, is a peri cyclic reaction, you should be able to classify it as a [1 + 2] cycloaddition.

As a cycloaddition, singlet carbene addition to an alkene must obey the rules of orbital symmetry discussed in Chapters 34 and 35. We might consider the empty p orbital of the carbene (LUMO) interacting with the π bond (HOMO) of the alkene or the lone pair of the carbene in its filled sp2 orbital (HOMO) interacting with the π* antibonding orbital of the alkene (LUMO).

You can immediately see that there is a problem when we try to interact these orbitals constructively to build two new bonds—direct approach of the carbene to the alkene is impossible because there is always an antibonding interaction. Two new bonds can be formed, however, if the carbene approaches the alkene in a ‘sideways-on’ manner.

The cyclopropane product must, of course, have a more or less tetrahedral arrangement about the carbon atom that was the carbene so that, even if the carbene approaches in a sideways-on manner, it must then swing round through 90° as the bonds form.

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

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

Alkyl radicals from boranes and oxygen