This is for Vth Semester students of UG courses

1. Cycloaddition reactions
Dr. Satyajit Dey
Department of Chemistry
Tamralipta Mahavidyalaya
Tamluk
Midnapore (East)
Under Graduate
Organic Chemistry Course (Vth Semester)
Session: 2020-21

2. Cycloaddition reactions may be defined as the pericyclic processes involving
formation of a ring through the formation of two new -bonds at the termini
of the -systems by concerted combination of two or more independent -
systems. The reverse process is called cycloreversion or is referred to as a
retro-reaction.
New -bond at the termini
of the two -Systems
Interaction of the termini
of the two -Systems
Interaction of the termini
of the two -Systems
New -bond at the termini
of the two -Systems

3. Classification of cycloaddition reactions

4. The FMO Theory Can Answer Reactivity
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0 %

5. FMO Theory Explains Stereospecificity of Cycloadditions

6. FMO Theory Explains Difference in Rates of Cycloadditions

7. FMO Theory Explains Regiochemistry of Cycloadditions

8. Terminology: Suprafacial & Antarafacial mode of addition
Suprafacial mode: The suprafacial mode of interaction involves the face-
to-face addition i.e., the addition occurs at same face of the pi-system
Antarafacial mode: The antarafacial mode of interaction involves the
opposite face of the pi-system.

9. Suprafacial-Suprafacial Interaction:

10. Suprafacial-Antarafacial Interaction:

11. The frontier molecular orbitals of both reactants must be considered: the
HOMO of one component and the LUMO of the other.
Analysing Cycloaddition Reactions: FMO Theory

12. FMO analysis of [2+ 2] Cycloaddition Reactions:
[2+ 2] Cycloaddition Reactions are thermally forbidden but are
photochemically allowed.

13. FMO analysis of [4+ 2] Cycloaddition Reactions:

15. Few important Characteristic features of Diels-Alder reaction
2. With substituted diene and substituted dienophile the reaction is highly stereo
specific, i.e. the configurational relationship of the diene and the dienophile are
retained in the Diels-Alder adduct.
1. The reaction is reversible and the backward reaction is called retro Diels-Alder
reaction.

16. Explanation for stereospecificity in Diels-Alder Reaction

17. Explanation for stereospecificity contd........

18. Few important Characteristic features of Diels-Alder reaction,
contd....
3. For the Diels-Alder reaction to occur the diene component must be in the s-cis
(cisoid) conformation because both ends of the diene must be close to the pi bond of
the dienophile for effective overlap in a concerted fashion.

19. Few important Characteristic features of Diels-Alder reaction,
contd....
Explanation:
4. The Diels-Alder reaction is stereoselective.

20. Few important Characteristic features of Diels-Alder reaction,
contd....

21. Another example of endo - exo selectivity
O
O O
HOMO - Diene
O
O O
HOMO - Diene
LUMO - dienophile

22. Few important Characteristic features of Diels-Alder reaction,
contd....
5. Moderate temperature favors the formation of products in a Diels-Alder
reaction but very high temperature ( > 200 oC) favors retro-Diels-Alder reaction.
Q. The entropy change (DS)for a Diels-Alder reaction is usually large and
negative. Explain.

23. Diels-Alder of Normal Electron Demand
OMe
CHO
OMe
CHO
OMe
CHO
Heat
Electron releasing
group Electron withdrawing
group

24. Diels-Alder of inverse Electron Demand
CO2Me
OMe
OMe
CO2Me
OMe
CO2Me
Heat
Electron releasing
group
Electron withdrawing
group

25. The out-endo-cis rule
Out-endo-cis rule states that the out group in diene and endo group in
dienophile (electron withdrawing group) will have cis-stereo chemical
relationship to each other in the Diels-Alder adduct

26. 1,3-dipolar Cyclo-addition reactions

27. [2 + 2] Cyclo-addition reactions
Thermal [2+2]-cycloaddition:
Examples:

28. Photohemical [2+2]-cycloaddition:
Examples:

29. Problems on Cyclo-addition reactions

30. Problems on Cyclo-addition reactions contd….

31. Problems on Cyclo-addition reactions contd….

32. Problems on Cyclo-addition reactions contd….

33. Synthetic application of Cyclo-addition reactions
Propose two possible retrosynthesis paths for the synthesis of the following target
molecule using Diels-Alder reaction. Which one will lead to better synthesis of the
target molecule and why?

34. Tasks on Cyclo-addition reactions
Q. 1 Write down the structure chiral allene which forms the Diels-Alder adduct,
given below with cyclopentadiene.
Q. 2 Optically active A was found to racemizes on heating at 100 oC. Give a rational
explanation
Q. 3 Predict the stereochemistry of the products in the following reactions and show
FOI.

35. Tasks on Cyclo-addition reactions
Q. 4 Predict the products in the following reactions and show FOI.
Q. 5 What is the structure of the product formed when A is heated with maleic
anhydride? Explain why only one product is formed even though A has four double
bonds?

36. Tasks on Cyclo-addition reactions
Q. 6 Identify the kind of pericyclic reaction, and explain how the products are formed
Q. 7 Identify the missing structures in he following schemes of reactions
Q. 8 What diene and dienophile would react to give the following D.A. product?