Cyclo-addition reactions involve the conversion of two π-bonds into σ-bonds to form a cyclic product. [2+2] cyclo-addition reactions specifically require photochemical conditions rather than thermal conditions. Under photochemical conditions, symmetry imbalance is resolved by combining the ground state of one alkene with the excited state of another. This allows reactions that are forbidden thermally. Cyclo-addition reactions are useful in organic synthesis as they allow for regio- and stereoselective formation of polycyclic structures.

1. Cyclo-addition reaction
by photochemical mode.
By Sameen Fatima Roll No. 908529
Bs Chemistry 2019-2023 Semester 7th (M)
Submitted to Mrs.
Rukhsana Kausar

2. 2
CYCLO-ADDITION REACTION BY PHOTOCHEMICAL REACTION
 Cyclo addition reaction
That pericyclic in which two 𝜋 bonds of reactants are converted new single cyclic
product by forming σ bonds.
For Example:
[1]
 Types of cyclo addition reaction
 [4+2] Cyclo-addition reaction: A thermal mode reaction.
 [2+2] Cyclo-addition reaction: A photochemical mode reaction.
 Sigma tropic rearrangement: A sigma bond migration reaction.
 Group transfer reaction: A group transfer reaction.
[2]
 [2+2] Cyclo-addition Reactions (photochemical reactions)
Cyclo-addition reactions that are [2+2] are done thermodynamically (these are also
called Diels Alders reaction) but that which reactions do not are done photo
chemically e;g
[3]
This is understood that [2+2] cyclo-addition reactions are only done by photochemical mode.

3. 3
Definition: “A photochemical [2+2] cyclo-addition reaction when occurs in two alkenes
between their π electrons, instead of them sigma bonds are formed and a cyclic product is
obtained.”
[4]
Explanation: Only 6 π electrons system is studied under the thermal control. 4 π electrons
system that is a suprafacial is studied under the photochemical conditions. . Under photo-
chemical conditions, the rules switch such that all the cyclo-additions that are not allowed
thermally are allowed photo-chemically. The vast majority of [2+2] photo-cyclo-addition
reactions involve enone– alkene cyclo-addition which is conveniently achieved through direct
excitation or sensitization by UV irradiation. This only needs because of the incompatible
symmetry between the two different alkenes.
[5]
Explanation with molecular orbitals: The following molecular orbitals are of
explanation of 1st
electron excitation state.
The question of symmetry imbalance arises here. To resolve this issue combines the ground
state of one alkene with the excited state of another alkene. If we mix two π orbitals and make
a molecular orbital in which two electrons go down and one go up. And when we mix two π*
orbitals then the molecular orbital has one electron in down state and none in the upper state.

4. 4
In this way two alkenes are dimerized photochemically. As its’ mechanism is difficult to
draw bonding between ground and excited state. It is more difficult if conjugated system is
present in one alkene in this way UV light absorb by one alkene so it goes to excited state and
one retains in ground state. The bonding diagram is as under.
[6]
As its’ mechanism is difficult but some chemists perform the diradical system. That is as
under:
Mechanism of diradical system:
[7]
Regioselectivity in [2+2] Cycloaddition reaction: If we combine the HOMO of one
alkene and LUMO of the enone so it gives the cycloaddition by thermal reactivity but if we
are talking about the photochemical reactivity then regioselectivity found in it. Ionic reaction
does not give regioselectivity. It is that the Following reaction only give the combination of
LUMO to LUMO reaction if occur photochemically.

5. 5
[8]
The forbidden state of this reaction when HOMO and LUMO combine then the reaction is as:
[9]
The orbital diagram of the LUMO to LUMO reaction is:
[10]
Stereo specificity in Cyclo-addition reaction: In any way some stereospecificity is
also found in photochemical cyclo addition reaction. For this the following example is best to
expalin. In which the two old rings keep out of each other’s way during the reaction and the
conformation of the product looks reasonably unhindered.

6. 6
[11]
Use of Photochemical cyclo-addition reaction in organic synthesis: Cyclo-
addition Reactions in Organic Syntheses help synthetic organic chemists to advance their
research and develop new functional materials and applications in chemical research,
pharmaceuticals, and materials science. It is a wonderful tool for chemists to form
carbocyclic and heterocyclic structures. They can do many achievements pharmaceuticals,
agrochemicals, dyes, and optics.
[12]
Importance: Cyclo-addition reactions are among the most useful methods in organic
synthesis, having their ability to provide multiple bond formations with regio and
stereo=chemical control leading to polycyclic, carbo-cycles and hetero-cycles through
efficient process.
[13]

7. 7
REFERENCE
[1]
https://www.sciencedirect.com/topics/chemistry/cycloaddition#:~:text=A%20cycloaddition%
20reaction%20indicates%20the,Diels%E2%80%93Alder%20reaction%20(Fig.
[2]
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_
Organic_Chemistry_(Roberts_and_Caserio)/13%3A_Polyfunctional_Compounds_Alkadiene
s_and_Approaches_to_Organic_Synthesis/13.03%3A_Cycloaddition_Reactions
[3]
https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book%3A_Basic_Principles_of_
Organic_Chemistry_(Roberts_and_Caserio)/13%3A_Polyfunctional_Compounds_Alkadiene
s_and_Approaches_to_Organic_Synthesis/13.03%3A_Cycloaddition_Reactions
[4] https://aklectures.com/lecture/cycloaddition-reactions/photochemical-2-2-cycloaddition-
reaction
[5] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[6] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[7] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[8] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[9] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[10] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[11] Organic chem._2_edition_by_Jonathan Clayden, Nick Greeves and Stuart Warren.
[12]
https://www.wiley.com/enus/Methods+and+Applications+of+Cycloaddition+Reactions+in+
Organic+Syntheses-p-9781118299883
[13] https://www.sciencedirect.com/topics/chemistry/cycloaddition