The Alder ‘ene’ reaction

 The Diels–Alder reaction was originally called the ‘diene reaction’ so, when half of the famous team (Kurt Alder) discovered an analogous reaction that requires only one alkene, it was called the Alder ene reaction and the name has stuck. Compare here the Diels–Alder and the Alder ene reactions.

The simplest way to look at the ene reaction is to picture it as a Diels–Alder reaction in which one of the double bonds in the diene has been replaced by a C–H bond (green). The reaction does not form a new ring, the product has only one new C–C bond (shown in black on the product), and a hydrogen atom is transferred across space. Otherwise, the two reactions are remarkably similar. The ene reaction is rather different in orbital terms. For the Woodward–Hoffmann description of the reaction we must use the two electrons of the C–H bond to replace the two electrons of the double bond in the Diels–Alder reaction, but we must make sure that all the orbitals are parallel, as shown. The C–H bond is parallel with the p orbitals of the ene so that the orbitals that overlap to form the new π bond are already parallel. The two molecules approach one another in parallel planes so that the orbitals that overlap to form the new σ bonds are already pointing towards each other. Because the electrons are of two types, π and σ, we must divide the ene into two components, one π2 and one σ2. We can then have an all-suprafacial reaction with three components. All three components are of the (4q + 2)s type so all count and the total is three—an odd number—so the reaction is allowed. We have skipped the step-by-step approach we used for the Diels–Alder reaction because the two are so similar, but you should convince yourself that you can apply it here. Now for some real examples. Most ene reactions with simple alkenes are with maleic anhydride. Other dienophiles—or enophiles as we should call them in this context—do not work very well. However, with one particular alkene, the natural pine tree terpene β-pinene, a reaction does occur with enophiles such as acrylates.

The major interaction between these two molecules is between the nucleophilic end of the exocyclic alkene and the electrophilic end of the acrylate. These atoms have the largest coefficients in the HOMO and LUMO, respectively, and, in the transition state, bond formation between these two will be more advanced than anywhere else. For most ordinary alkenes and enophiles, Lewis acid catalysis to make the enophile more electrophilic, or an intramolecular reaction (or both!), is necessary for an efficient ene reaction.

The ‘ene’ component is delivered to the bottom face of the enone, as its tether is too short for it to reach the top face, and a cis ring junction is formed. The stereochemistry of the third centre is most easily seen by a Newman projection (Chapter 16) of the reaction. In the diagram in the margin we are looking straight down the new C–C bond and the colour coding should help you to see how the stereochemistry follows.

Since the twin roles of the enophile are to be attacked at one end by a C=C double bond and at the other by a proton, a carbonyl group is actually a very good enophile. These reactions are usually called ‘carbonyl ene’ reactions. The important interaction is between the HOMO of the ene system and the LUMO of the carbonyl group—and a Lewis-acid catalyst can lower the energy of the LUMO still fur ther. If there is a choice, the more electrophilic carbonyl group (the one with the lower LUMO) reacts.

It may not be obvious that an ene reaction has occurred because of the symmetry of the alkene. The double bond in the product is not, in fact, in the same place as it was in the start ing material, as the mechanism shows. One carbonyl ene reaction is of commercial importance as it is part of a process for the pro duction of menthol used to give a peppermint smell and taste to many products. This is an intramolecular ene reaction on another terpene derivative.

It is not obvious what has happened in the fi rst step, but the movement of the alkene and the closure of the ring with the formation of one (not two) new C–C bonds should give you the clue that this is a Lewis-acid-catalysed carbonyl ene reaction. The stereochemistry comes from an all-chair arrangement in the conformation of the transition state. The methyl group will adopt an equatorial position in this conformation, fixing the way the other bonds are formed. Again, colour coding should make it clearer what has happened.

اخر الاخبار

اخبار العتبة العباسية المقدسة

قسم رعاية الصحن الشريف ينهي استعداداته الخاصة بزيارة الأربعين

قسم الإعلام ينظم دورة تخصصية في تقنيات الذكاء الاصطناعي لعددٍ من منتسبيه

القسم النسوي بمجمع أبي الفضل العباس (عليه السلام) ينهي الاستعدادات لاستقبال زائرات الأربعين

قسم الشؤون الفكرية ينظّم ورشة تدريبية عن الالتزام والانضباط في الخدمة لفتية الكفيل

المزيد

الآخبار الصحية

علامات فارقة بين النوبة أو السكتة القلبية.. ماهي؟

الفلفل الأحمر.. سعرات حرارية منخفضة وفوائد صحية عديدة

8 أعراض إذا شعرت بها.. اذهب للطوارئ فورا !

تعرف على أكثر 8 أماكن اتساخا في المنزل

المزيد

الآخبار التكنلوجية

إنتاج الهيدروجين الأخضر في البحر.. هذه أحدث الدراسات

"ناسا" تخطط لبناء مفاعل نووي على سطح القمر !

ابتكار ومواهب وأموال.. مارك زوكربيرغ يعلن الحرب على أبل

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

المزيد

مواضيع ذات صلة

Non-linear Hammett plots

A complete picture of the transition state from Hammett plots

Using the Hammett ρ values to uncover mechanisms

Reactions with small Hammett ρ values

Reactions with negative Hammett ρ values

Reactions with positive Hammett ρ values

Equilibria with positive Hammett ρ values

The Hammett substituent constant σσ A

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms