The Woodward–Hoffmann description of the Diels–Alder reaction

Kenichi Fukui and Roald Hoffmann won the Nobel prize in 1981 (Woodward died in 1979 and so couldn’t share this prize: he had already won a Nobel prize in 1965 for his work on synthe sis) for the application of orbital symmetry to pericyclic reactions. Theirs is an alternative description to the frontier orbital method we have used and you need to know a little about it. They started by considering a more fundamental correlation between the symmetry of all the orbitals in the starting materials and all the orbitals in the products. This is rather too complex for us to cover here, and we shall concentrate only on a summary of the conclusions—the Woodward–Hoffmann rules. The most important of these states:

● Woodward–Hoffmann rules in a thermal pericyclic reaction the total number of (4q + 2) s and (4r) a component must be odd.

This needs some explanation. A component is a bond or orbital taking part in a pericyclic reaction as a single unit. A double bond is a π2 component. The number 2 is the most important part of this designation and simply refers to the number of electrons. The prefix π tells us the type of electrons. A component may have any number of electrons (a diene is a π4 component) but may not have mixtures of π and σ electrons. Now look back at the rule. Those designations (4q + 2) and (4r) simply refer to the number of electrons in the component where q and r are integers. An alkene is a π2 component and so it is of the (4q + 2) kind while a diene is a π4 component and so is of the (4r) kind. You have already seen the importance of 4n + 2 numbers in aromaticity; here the significance is closely related. Now what about the suffixes ‘s’ and ‘a’? The suffix ‘s’ stands for suprafacial and ‘a’ for antara facial. A suprafacial component forms new bonds on the same face at both ends while an antarafacial component forms new bonds on opposite faces at both ends. If you find it easier to understand, you can think of the Woodward–Hoffmann rules like this:

● Woodward–Hoffmann rules: alternative version In an allowed thermal pericyclic reaction this sum: number of suprafacial components with 2, 6, or 10 electrons + number of antarafacial components with 0, 4, or 8 electrons = an odd number It’s the number of relevant components that must be odd, not (obviously) the number of electrons, and you must ignore any components which aren’t mentioned in the sum (for example you can have as many suprafacial components with four electrons as you like—they just don’t count).

See how this works for the Diels–Alder reaction. Here is the routine.

1. Draw the mechanism for the reaction (we shall choose a general one).

 2. Choose the components. All the bonds taking part in the mechanism must be included and no others.

3. Make a three-dimensional drawing of the way the components come together for the reaction, putting in orbitals at the ends of the components (only!). The orbitals are just unshaded p orbitals, and do not make up HOMOs or LUMOs nor any particular molecular orbital. Don’t attempt to mix frontier orbital and Woodward–Hoffmann descriptions of pericyclic reactions.

4. Join up the components where new bonds are to be formed. Coloured dotted lines are often used.

5. Label each component ‘s’ or ‘a’ depending on whether new bonds are formed on the same or on opposite sides. In all of the cycloadditions you have seen so far (and indeed the vast majority of those you will ever see), both components react suprafacially.

6. Count the number of (4q + 2)s and (4r)a components. If the total count is odd, the reaction is allowed. In this case, there is one (4q + 2)s component (the alkene) and no (4r)a components. Total = 1 so it is an allowed reaction. Components of the other symmetry, that is (4q + 2)a and (4r)s components, do not count. You can have as many of these as you want.

You may well feel that there is very little to be gained from the Woodward–Hoffmann treat ment of the Diels–Alder reaction. It does not explain the endo selectivity nor the regioselectivity. However, the Woodward–Hoffmann treatment of other pericyclic reactions (particularly electrocyclic reactions, in the next chapter) is very helpful.

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قسم رعاية الصحن الشريف ينهي استعداداته الخاصة بزيارة الأربعين

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

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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