1. 10.12 The Diels-Alder Reaction Synthetic method for preparing compounds containing a cyclohexene ring

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

3. transition state via

4. concerted mechanism cycloaddition pericyclic reaction a concerted reaction that proceeds through a cyclic transition state Mechanistic features

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

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

7. + benzene 100°C (100%) H2CH2CH2CH2C CHCH CH 2 H2CH2CH2CH2C CH CHO CHOExample

8. + benzene 100°C (100%) H2CH2CH2CH2C CHCH H2CH2CH2CH2C CH CHO CHOExample CHOvia:

9. + benzene 100°C (100%)OO O Example H2CH2CH2CH2C CHC CH 3 H3CH3CH3CH3COO O

10. + benzene 100°C (100%)OO O Example H2CH2CH2CH2C CHC CH 2 CH 3 H3CH3CH3CH3COO O via: H3CH3CH3CH3C O O O

11. + benzene 100°C (98%) H2CH2CH2CH2C CHCH CH 2 Acetylenic Dienophile O CCOCH 2 CH 3 CH 3 CH 2 OCC O COCH 2 CH 3 OO

12. syn addition to alkene cis-trans relationship of substituents on alkene retained in cyclohexene product Diels-Alder Reaction is Stereospecific* *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.

13. only product + H2CH2CH2CH2C CHCH CH 2 Example C C C6H5C6H5C6H5C6H5 COH HH OH C6H5C6H5C6H5C6H5 H COH O

14. only product + H2CH2CH2CH2C CHCH CH 2 Example C C C6H5C6H5C6H5C6H5 COH H HO H C6H5C6H5C6H5C6H5H COH O

15. Cyclic dienes yield bridged bicyclic Diels-Alder adducts.

16. + C C COCH 3 H HO CH 3 OC O H H COCH 3 O O

17. is the same as H H COCH 3 O O H H O O

18. 10.13 The  Molecular Orbitals of Ethylene and 1,3-Butadiene

19. 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.

20. 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  electrons of ethylene and 1,3-butadiene. We can ignore the framework of  bonds in each molecule. Orbitals and Chemical Reactions

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

22. The  MOs of Ethylene Bonding  orbital of ethylene; two electrons in this orbital Antibonding  orbital of ethylene; no electrons in this orbital LUMO HOMO

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

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

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

26. 10.14 A  Molecular Orbital Analysis of the Diels-Alder Reaction

27. 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.

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

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

30. A "forbidden" reaction The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? H2CH2CH2CH2C CH 2 H2CH2CH2CH2C +

31. A "forbidden" reaction H2CH2CH2CH2C CH 2 H2CH2CH2CH2C + HOMO of one ethylene molecule LUMO of other ethylene molecule HOMO-LUMO mismatch of two ethylene molecules precludes single-step formation of two new  bonds