Photochemical [2 + 2] cycloadditions

We shall now leave six-electron cyclodadditions such as the Diels–Alder and ene reactions and move on to some four-electron cycloadditions. Clearly, four is not a (4n + 2) number, but when we described the Woodward–Hoffman rules we used the term ‘thermally’. All suprafacial cycloadditions with 4n electrons are allowed if the reaction is not thermal (that is, driven by heat energy) but photochemical (that is, driven by light energy). Under photo chemical conditions, the rules switch such that all the cycloadditions that are not allowed thermally are allowed photochemically. This works because the problem of the incompatible symmetry in trying to add two alkenes together is avoided by converting one of them into the excited state photochemically. First, one electron is excited by the light energy from the π to the π* orbital.

Now, combining the excited state of one alkene with the ground state of another solves the symmetry problem. Mixing the two π orbitals leads to two molecular orbitals, and two electrons go down in energy while only one goes up. Mixing the two π* orbitals is as good—one electron goes down in energy and none goes up. The result is that three electrons go down in energy and only one goes up. Bonding can occur.

Alkenes can be dimerized photochemically in this way, but reaction between two different alkenes is more interesting. If one alkene is bonded to a conjugating group, it alone will absorb UV light and be excited while the other will remain in the ground state. It is difficult to draw a mechanism for these reactions as we have no simple way to represent the excited alkene. Some people draw it as a diradical (since each electron is in a different orbital); others prefer to write a concerted reaction on an excited alkene marked with an asterisk.

The reaction is stereospecific within each component but there is no endo rule—there is a conjugating group but no ‘back of the diene’. The least hindered transition state usually results. The dotted lines on the central diagram simply show the bonds being formed. The two old rings keep out of each other’s way during the reaction and the conformation of the prod-uct looks reasonably unhindered.

You may be wondering why the reaction works at all, given the strain in a four-membered ring: why doesn’t the product just go back to the two starting materials? This reverse reaction is governed by the Woodward–Hoffmann rules, just like the forward one, and to go back again the four-membered ring products would have to absorb light. But since they have now lost   their π bonds they have no low-lying empty orbitals into which light can promote electrons. The reverse photochemical reaction is simply not possible because there is no mechanism for the compounds to absorb light.

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