1. The Diels-Alder Reaction
Synthetic method for preparing compounds containing a cyclohexene ring 1

2. In general... +
conjugated diene
alkene (dienophile)
cyclohexene 2

3. via
transition state 3

5. a concerted reaction that proceeds through a cyclic transition state
Mechanistic features
concerted mechanism
[4+2] cycloaddition
pericyclic reaction
a concerted reaction that proceeds through a cyclic transition state 4

6. Recall the general reaction...+
conjugated diene
alkene (dienophile)
cyclohexene
The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile. 2

7. diene
dienophile
transition
state
new s bond

8. What makes a reactive dienophile?
The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond.
Typical EWGs C
EWG C O C N 5

11. Example CH O
H2C
CHCH
CH2 +
H2C CH
benzene
100°C CH O
(100%) 6

12. Example CH O H2C CHCH CH2 + H2C CH benzene 100°C CH O via: CH O (100%)6

13. Example O
H2C
CHC
CH2
CH3 +
benzene
100°C
H3C O
(100%) 6

14. Example O H2C CHC CH2 CH3 + benzene 100°C via: H3C O O H3C O O (100%)6

15. Acetylenic Dienophile
CCOCH2CH3
CH3CH2OCC
H2C
CHCH
CH2 +
benzene
100°C
COCH2CH3 O
(98%) 6

16. Diels-Alder Reaction is Stereospecific*
syn addition to alkene
cis-trans relationship of substituents on alkene retained in cyclohexene product
*A stereospecific reaction is one in which stereoisomeric starting materials give stereoisomeric products; characterized by terms like syn addition, anti elimination, inversion of configuration, etc. 9

17. Stereospecific, concerted, syn addition:

18. Predict the reaction products:
1. Consider the alignment of the reactants

19. 2. Consider the charge distribution in each of the
reactants

20. Example O C6H5 COH C + H2C CHCH CH2 H H
Only the s-cis conformation of the diene can participate in a Diels–Alder reaction H
C6H5
COH O
Only product.
But, is it enantiomeric? 10

21. Example C
C6H5
COH H O +
H2C
CHCH
CH2 H
C6H5
COH O
only product 10

22. Cyclic dienes yield bridged bicyclic Diels-Alder adducts.12

24. Since only cis dienes can participate in Diels–Alder reactions: 5- & 6- membered rings are ideal

25. C
COCH3 H O
CH3OC + H
COCH3 O 13

26. H
COCH3 O H
COCH3 O
is the same as 14

27. The p Molecular Orbitals of Ethylene and 1,3-Butadiene

28. Reactants’ HOMO and LUMO

29. Orbitals and Chemical Reactions
A deeper understanding of chemical reactivity can be gained by focusing on the frontier orbitals of the reactants.
Electrons flow from the highest occupied molecular orbital (HOMO) of one reactant to the lowest unoccupied molecular orbital (LUMO) of the other. 6

30. Orbitals and Chemical Reactions
We can illustrate HOMO-LUMO interactions by way of the Diels-Alder reaction between ethylene and 1,3-butadiene.
We need only consider only the p electrons of ethylene and 1,3-butadiene. We can ignore the framework of s bonds in each molecule. 6

31. red and blue colors distinguish sign of wave function
The p MOs of Ethylene
red and blue colors distinguish sign of wave function
bonding p MO is antisymmetric with respect to plane of molecule
Bonding p orbital of ethylene; two electrons in this orbital 6

32. Antibonding p orbital of ethylene; no electrons in this orbital
The p MOs of Ethylene
Antibonding p orbital of ethylene; no electrons in this orbital
LUMO HOMO
Bonding p orbital of ethylene; two electrons in this orbital 6

33. Two of these orbitals are bonding; two are antibonding.
The p MOs of 1,3-Butadiene
Four p orbitals contribute to the p system of 1,3-butadiene; therefore, there are four p molecular orbitals.
Two of these orbitals are bonding; two are antibonding. 6

34. The Two Bonding p MOs of 1,3-Butadiene
HOMO
4 p electrons; 2 in each orbital
Lowest energy orbital 6

35. The Two Antibonding p MOs of 1,3-Butadiene
Highest energy orbital
LUMO
Both antibonding orbitals are vacant 6

37. Two Possible Configurations of Bridged Bicyclic Compounds

38. Secondary orbital overlap favors the endo product
formation

39. A p Molecular Orbital Analysis of the Diels-Alder Reaction1

40. MO Analysis of Diels-Alder Reaction
Inasmuch as electron-withdrawing groups increase the reactivity of a dienophile, we assume electrons flow from the HOMO of the diene to the LUMO of the dienophile. 6

41. MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
HOMO of 1,3-butadiene and LUMO of ethylene are in phase with one another
allows s bond formation between the alkene and the diene
LUMO of ethylene (dienophile) 6

42. MO Analysis of Diels-Alder Reaction
HOMO of 1,3-butadiene
LUMO of ethylene (dienophile) 6

43. A "forbidden" reaction H2C CH2 + H2C CH2
The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? 6

44. HOMO of one ethylene molecule
A "forbidden" reaction
H2C
CH2 +
H2C
CH2
HOMO of one ethylene molecule
HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new s bonds
LUMO of other ethylene molecule 6