Conjugated Dienes: Stability, Bonding Theory & Reactions

1. Pericyclic Reaction • Conjugated diene: stability, bonding theory • Reaction of conjugated diene: Diels-Alder rxn • Electrophilic addition: regiochemistry • Diels-Alder rxn: regio/stereochemistry, MO interpretation UV-Vis spectroscopy overview

2. Dienes • There are three categories for dienes • Cumulated – pi bonds are adjacent • Conjugated – pi bonds are separated by exactly ONE single bond • Isolated – pi bonds are separated by any distance greater than ONE single bond

3. Comparison of p-bondamong Dienes • There are three categories for dienes • Cumulated – pi bonds are perpendicular • Conjugated – pi bond overlap extends over the entire system • Isolated – pi bonds are separated by too great a distance to experience extra overlap

4. Identify the Conjugated p-bonds • This chapter focuses on conjugated systems • Heteroatoms may be involved in a conjugated system • Where are the conjugated p-bonds? • Practice with conceptual checkpoint 17.1

5. 2 Prep of Conjugated Dienes • A sterically hindered base can be used to form dienes while avoiding the competing substitution reaction ?

6. Bonding in Conjugated Dienes • Single bonds that are part of a conjugated pi system are shorter than typical single bonds • The hybridization (more s character) of a carbon as well as overlapping of p orbitals shortens bond length

7. Stability of Conjugated Dienes • Hydrogenation of conjugated diene releases less heat than 2 times of the single alkene, so conjugation stabilizes the diene.

8. Isomers of Conjugated Dienes • Due to the free rotation of single bonds, there are two most stable rotational conformations for 1,3-butadiene: s-cis and s-trans • The s-cis and s-trans both allow for full pi system overlap

9. Stability among Isomers of Conjugated Dienes • About 98% of the molecules are in the s-trans form

10. 4 Electrophilic Addition of HX • Recall the Markovnikov addition of H-X to a C=C double bond from section 9.3 (due to stability of carbocation!) • With a conjugated diene as the substrate, two products are observed

11. Resonance of allylic carbocation • The resonance stabilized carbocation can be attacked by the halide at either site that is sharing the (+) charge • 1,2-addition vs. 1,4-addition

12. Electrophilic addition of X2 • The addition of bromine to a diene also gives both 1,2 and 1,4 addition • Predict the MAJOR products for the reaction below. Pay close attention to stereochemistry

13. Electrophilic addition of diene depends on temperature • The ratio of 1,2 vs. 1,4 addition is often temperature dependant

14. Product stability: 1,4-adduct vs. 1,2-adduct • Gauche effect in 1,2- vs. 1,4-adduct

15. Temperature affects selectivity • high temps favors 1,4-addition: Thermodynamically favored • Low temperature favors 1,2-addition: kinetically favored

16. practice: Thermodynamic Control vs. Kinetic Control • Predict the MAJOR product for the following reactions ° °

17. Application of 1,4-addition: Polymerization of Isoprene • Many polymerization reactions rely on 1,4 addition, such as polymerization of isoprene (industrial procedure for synthetic rubber)

18. 6 Pericyclic Reactions Pericyclic reactions occur without ionic (SN or Elimination, electrophilic addition, etc.) or free radical intermediates • There are three main types of pericyclic reactions • Cycloaddition reactions

19. Intro to Pericyclic Reactions • Electrocyclic reactions • Sigmatropic rearrangements

20. 7 Diels-Alder Reactions • Diels-Alder reactions can be very useful • They allow a synthetic chemist to quickly build molecular complexity • [4+2] cycloaddition: There are FOUR and TWO p electrons from each of the two reactants.

21. Arrow pushing in Diels-Alder Reactions • Like all pericyclic reactions, the mechanism is concerted • The arrows could be drawn in a clockwise or counterclockwise direction

22. Practice on Diels-Alder Reactions • Write Reaction mechanism for the following reaction

23. Energy diagram for Diels-Alder Reactions • products generally have lower free energy due to the position of equilibrium.

24. Temperature Effects on Diels-Alder Reactions • Most Diels-Alder reactions (DS < 0) are thermodynamically favored at low and moderate temperatures • At temperatures above 200 C, the retroDiels-Alder can predominate

25. Diene vs. Dienophile • If a dienophile is not substituted with an electron withdrawing group, it will not be kinetically favored • Reactants are generally classified as either the diene or dienophile

26. Electron Withdrawing Group • Electron withdrawing group (EWG): an atom or functional group that removes electron density from a conjugated π system via resonance (such as –CHO, -COOH, -COOR, -CN) or inductive electron withdrawal (such as –CF3, ), thus making the π system more electrophilic. • Examples: functional groups in red

27. EWG enhances Dienophile • When an electron withdrawing group is attached to the dienophile, the reaction is generally spontaneous • The electron withdrawing groups are in red in the following dienophiles:

28. Stereochemistry of Diels-Alder Rxns • Diels-Alder reactions are stereospecific depending on whether the (E) or (Z) dienophile is used

29. Alkyne as Dienophile • A CΞC triple bond can also act as a dienophile, forming ring and a C=C double bond

30. Practice on Diels-Alder Reactions • Predict products for the following reaction

31. S-cis of Diene for Diels-Alder Rxn • Recall that many dienes can exist in an s-cis or an s-trans rotational conformation • Diels-Alder reactions can ONLY proceed when the diene adopts the s-cis conformation • Dienes that can only exist in an s-trans conformation can not undergo Diels-Alder reactions:

32. Reactivity of Diene Practice: Dienes that are locked into the s-cis conformation undergo Diels-Alder reactions readily. Cyclopentadiene is so reactive, that at room temperature, two molecule will react together. Show the reaction and products

33. Practice on Diels-Alder Reactions • Draw four potential bicyclic Diels-Alder products for the reaction below • Two of the potential stereoisomers are impossible.

34. Stereochemistry of Bicyclic ring in Diels-Alder Rxns • When bicyclic systems form, the terms ENDO and EXO are used to describe functional group positioning

35. Endo position is preferred Diels-Alder Reactions • The electron withdrawing groups attached to dieneophiles tend to occupy the ENDO position Minor Product Major Product

36. Rationale for the Endo preferrence • The Diels-Alder transition state that produces the ENDO product benefits from favorable pi system interactions • EWG as electron attracting party; Diene as electron offering party.

37. UV-Vis Spectroscopy princple • UV-Vis spectroscopy gives structural information about molecules • A beam of light (200-800 nm) is split in two • Half of the beam travels through a cuvette with the analyte in solution • The other half of the beam travels through a cuvette with just the solvent (used as a negative control) • The intensities of the light that pass through the cuvettes are compared to determine how much light is absorbed by the analyte

38. Additional Practice Problems • Explain the relationship between heat of hydrogenation and stability of a pi system.

39. Additional Practice Problems • Explain why the instability of electrons in a pi MO directly relates to the number of nodes the orbital posses.

40. Additional Practice Problems • Give a complete mechanism and predict the major product for the addition reaction below.

41. Additional Practice Problems • Predict the products and give necessary conditions for the compound below to undergo a retro Diels-Alder

42. Additional Practice Problems • Predict the products for the electrocyclic ring-opening below with proper stereochemical configuration.

43. 17.3 Recap of Molecular Orbital Theory • A MO forms when atomic orbitals overlap • A MO extends over the entire molecule The p bond in ethylene: • p2p (bonding orbital) • p*2p (antibonding orbital) • The node: electron density = 0

44. MO of conjugated diene The number of MOs must be equal to the number of AOs combined Note how the shorthand drawing matches the actual MOs

45. C2-C3 bond has partial p bond • The 4 p electrons in butadiene will occupy the lowest energy MOs. MO also explains why central C-C single bond is shorter and stronger than a typical C-C single bond.

46. Conjugated Triene: HOMO vs. LUMO MOs that affects chemical rxns: • Highest Occupied MO (HOMO) • Lowest Unoccupied MO (LUMO)

47. Frontier MOs • Reactions that molecules undergo can often be explained by studying their frontier orbitals (Fukui, 1981 Nobel Prize) • Light can be used to excite an electron from the HOMO to the LUMO. • The wavelength of such photon corresponds to the engergy gap between HOME and LUMO.

48. 17.8 MO of Diene vs. Dienophile • Note the molecular orbital descriptions below

49. HOMO-LUMO interaction • In the Diels-Alder, the HOMO of one compound must interact with the LUMO of the other

50. EWG’s promote HOMO  LUMO • An electron withdrawing group lowers the dieneophile’s LUMO thus helps LUMO accept electrons from the diene’s HOMO