1. 10.14Halogen Addition to Dienes gives mixtures of 1,2 and 1,4-addition products Dr. Wolf's CHM 201 & 202

2. CH2 H2C CHCH BrCH2CHCH BrCH2CH CH2 CHCH2Br Br Example Br2 + (37%) (63%) Dr. Wolf's CHM 201 & 202

3. 10.15 The Diels-Alder Reaction Synthetic method for preparing compounds containing a cyclohexene ring Dr. Wolf's CHM 201 & 202

4. In general... + conjugated diene alkene (dienophile) cyclohexene Dr. Wolf's CHM 201 & 202

5. via transition state Dr. Wolf's CHM 201 & 202

6. Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

7. Mechanistic features concerted mechanism cycloaddition pericyclic reaction a concerted reaction that proceeds through a cyclic transition state Dr. Wolf's CHM 201 & 202

8. Recall the general reaction... + The equation as written is somewhat misleading because ethylene is a relatively unreactive dienophile. alkene (dienophile) conjugated diene cyclohexene Dr. Wolf's CHM 201 & 202

9. EWG C O C C C N What makes a reactive dienophile? The most reactive dienophiles have an electron-withdrawing group (EWG) directly attached to the double bond. Typical EWGs Dr. Wolf's CHM 201 & 202

10. O CH2 H2C CHCH CH benzene 100°C O CH Example + H2C CH (100%) Dr. Wolf's CHM 201 & 202

11. O CH2 H2C CHCH CH benzene 100°C O O CH CH Example + H2C CH via: (100%) Dr. Wolf's CHM 201 & 202

12. Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

13. O CH2 H2C CHC O CH3 O O H3C O O Example + benzene 100°C (100%) Dr. Wolf's CHM 201 & 202

14. O CH2 H2C CHC O CH3 O via: O O H3C H3C O O O O Example + benzene 100°C (100%) Dr. Wolf's CHM 201 & 202

15. O O CH2 CCOCH2CH3 CH3CH2OCC H2C CHCH benzene 100°C O COCH2CH3 COCH2CH3 O Acetylenic Dienophile + (98%) Dr. Wolf's CHM 201 & 202

16. Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

17. 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. Dr. Wolf's CHM 201 & 202

18. + CH2 H2C C CHCH C H C6H5 COH H O O Example C6H5 COH H H only product Dr. Wolf's CHM 201 & 202

19. O COH H + CH2 H2C C CHCH C H C6H5 C6H5 H COH H O Example only product Dr. Wolf's CHM 201 & 202

20. Cyclic dienes yield bridged bicyclicDiels-Alder adducts. Dr. Wolf's CHM 201 & 202

21. Diels-Alder Reaction Dr. Wolf's CHM 201 & 202 Dr. Wolf's CHM 201 & 202

22. Diels-Alder Reaction Dr. Wolf's CHM 201 & 202

23. O COCH3 H C C H CH3OC O H O COCH3 H COCH3 O + Dr. Wolf's CHM 201 & 202

24. O O H COCH3 COCH3 H H H COCH3 COCH3 O O is thesame as Dr. Wolf's CHM 201 & 202

25. 10.16 The p Molecular OrbitalsofEthylene and 1,3-Butadiene Dr. Wolf's CHM 201 & 202

26. 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. Dr. Wolf's CHM 201 & 202

27. 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. Dr. Wolf's CHM 201 & 202

28. 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 Dr. Wolf's CHM 201 & 202

29. The p MOs of Ethylene Antibonding p orbital of ethylene;no electrons in this orbital LUMOHOMO Bonding p orbital of ethylene;two electrons in this orbital Dr. Wolf's CHM 201 & 202

30. The p MOs of 1,3-Butadiene • Fourp orbitals contribute to the p system of 1,3-butadiene; therefore, there are fourp molecular orbitals. • Two of these orbitals are bonding; two are antibonding. Dr. Wolf's CHM 201 & 202

31. The Two Bonding p MOs of 1,3-Butadiene HOMO 4 p electrons; 2 ineach orbital Lowest energy orbital Dr. Wolf's CHM 201 & 202

32. The Two Antibonding p MOs of 1,3-Butadiene Highest energy orbital LUMO Both antibondingorbitals are vacant Dr. Wolf's CHM 201 & 202

33. 10.17A p Molecular Orbital Analysisof theDiels-Alder Reaction Dr. Wolf's CHM 201 & 202

34. 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. Dr. Wolf's CHM 201 & 202

35. 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) Dr. Wolf's CHM 201 & 202

36. MO Analysis of Diels-Alder Reaction HOMO of 1,3-butadiene LUMO of ethylene (dienophile) Dr. Wolf's CHM 201 & 202

37. H2C CH2 H2C CH2 A "forbidden" reaction • The dimerization of ethylene to give cyclobutane does not occur under conditions of typical Diels-Alder reactions. Why not? + Dr. Wolf's CHM 201 & 202

38. H2C CH2 + H2C CH2 A "forbidden" reaction HOMO of one ethylenemolecule HOMO-LUMOmismatch of twoethylene moleculesprecludes single-stepformation of two news bonds LUMO of other ethylenemolecule Dr. Wolf's CHM 201 & 202

39. End of Chapter 10 Dr. Wolf's CHM 201 & 202