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
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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
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3. Comparison of p-bond among 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
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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
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5. 2 Prep of Conjugated Dienes
A sterically hindered base can be used to form dienes while avoiding the competing substitution reaction ?
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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
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7. Stability of Conjugated Dienes
Hydrogenation of conjugated diene releases less heat than 2 times of the single alkene, so conjugation stabilizes the diene.
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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
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9. Stability among Isomers of Conjugated Dienes
About 98% of the molecules are in the s-trans form
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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
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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
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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
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13. Electrophilic addition of diene depends on temperature
The ratio of 1,2 vs. 1,4 addition is often temperature dependant
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14. Product stability: 1,4-adduct vs. 1,2-adduct
Gauche effect in 1,2- vs. 1,4-adduct
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15. Temperature affects selectivity
high temps favors 1,4-addition: Thermodynamically favored
Low temperature favors 1,2-addition: kinetically favored
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16. practice: Thermodynamic Control vs. Kinetic Control
Predict the MAJOR product for the following reactions
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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)
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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
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19. Intro to Pericyclic Reactions
Electrocyclic reactions
Sigmatropic rearrangements
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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.
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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
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22. Practice on Diels-Alder Reactions
Write Reaction mechanism for the following reaction
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23. Energy diagram for Diels-Alder Reactions
products generally have lower free energy due to the position of equilibrium.
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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
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25. Diene vs. Dienophile
Reactants are generally classified as either the diene or dienophile
If a dienophile is not substituted with an electron withdrawing group, it will not be kinetically favored
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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
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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:
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28. Stereochemistry of Diels-Alder Rxns
Diels-Alder reactions are stereospecific depending on whether the (E) or (Z) dienophile is used
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29. Alkyne as Dienophile
A CΞC triple bond can also act as a dienophile, forming ring and a C=C double bond
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30. Practice on Diels-Alder Reactions
Predict products for the following reaction
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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:
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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
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33. Practice on Diels-Alder Reactions
Draw four potential bicyclic Diels-Alder products for the reaction below
Two of the potential stereoisomers are impossible.
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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
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35. Endo position is preferred Diels-Alder Reactions
The electron withdrawing groups attached to dieneophiles tend to occupy the ENDO position
Major Product
Minor Product
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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.
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37. UV-Vis Spectroscopy princple
UV-Vis spectroscopy gives structural information about molecules
A beam of light ( 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
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38. Additional Practice Problems
Explain the relationship between heat of hydrogenation and stability of a pi system.
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39. Additional Practice Problems
Explain why the instability of electrons in a pi MO directly relates to the number of nodes the orbital posses.
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40. Additional Practice Problems
Give a complete mechanism and predict the major product for the addition reaction below.
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41. Additional Practice Problems
Predict the products and give necessary conditions for the compound below to undergo a retro Diels-Alder
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42. Additional Practice Problems
Predict the products for the electrocyclic ring-opening below with proper stereochemical configuration.
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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
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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
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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.
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46. Conjugated Triene: HOMO vs. LUMO
MOs that affects chemical rxns:
Highest Occupied MO (HOMO)
Lowest Unoccupied MO (LUMO)
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47. Frontier MOs
Reactions that molecules undergo can often be explained by studying their frontier orbitals (Fukui, 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.
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48. 17.8 MO of Diene vs. Dienophile
Note the molecular orbital descriptions below
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49. HOMO-LUMO interaction
In the Diels-Alder, the HOMO of one compound must
interact with the
LUMO of the other
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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
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51. Phase matching between HOMO and LUMO
The phases of the MOs align to allow for orbital overlap
If there is conservation of orbital symmetry, the process is symmetry-allowed
Note the carbons that change their hybridization from sp2 to sp3
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52. MO Prediction of [2+2] cycloaddition
Similar to a Diels-Alder ([4+2] cycloaddition),the reaction below is a [2+2] cycloaddition
Draw a reasonable concerted mechanism
Unless the reaction is symmetry-allowed, the process will not occur, so let’s analyze the MOs
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53. HOMO-LUMO in [2+2] Cycloadditions
The LUMO of one reactant must overlap with the HOMO of the other
WHY can’t both of the HOMOs interact together?
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54. No favorable HOMO-LUMO for [2+2]
The phases of the HOMO and LUMO can not line up to give effective overlap, so the reaction is symmetry-forbidden
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55. Excitation of HOMO e- helps [2+2]
[2+2] cycloadditions are only possible when light is used to excite an electron
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56. 17.9 Electrocyclic reactions
Determine how the number of σ and π bonds change for the representative electrocyclic reactions below
Explain why the equilibrium favor either products or reactants in the examples above
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57. 17.9 Electrocyclic reactions
When substituents are present on the terminal carbons, stereoisomers are possible
Note that the use of light versus heat gives different products. See next few slides for explanation
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58. 17.9 Electrocyclic reactions
Often the energy present at room temperature is sufficient to promote thermal electrocyclic reactions
Note that with the use heat, the configuration of the reactant determines the product formed
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59. 17.9 Electrocyclic reactions
The symmetry of the HOMO determines the outcome
The terminal carbons rotate as they become sp3 hybridized and overlap lobes that are in phase
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60. 17.9 Electrocyclic reactions
The terminal carbons rotate as they become sp3 hybridized and overlap lobes that are in phase
Disrotatory rotation (one rotates clockwise and the other counterclockwise) gives the cis product
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61. 17.9 Electrocyclic reactions
Use MO theory to explain the products for the reactions below
Will disrotatory or conrotatory rotation be necessary?
Practice with conceptual checkpoint 17.21
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62. 17.9 Electrocyclic reactions
Predict the major product for the reaction below. Pay close attention to stereochemistry
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63. 17.9 Electrocyclic reactions
Under photochemical conditions, light energy excites an electron from the HOMO to the LUMO
What was the LUMO becomes the new HOMO
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64. 17.9 Electrocyclic reactions
Will the new excited HOMO react disrotatory or conrotatory?
Draw the expected product
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65. 17.9 Electrocyclic reactions
Make the same MO analysis to predict the symmetry-allowed product for the reaction below. Pay close attention to stereochemistry
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66. 17.9 Electrocyclic reactions
The disrotatory nature of the photochemical [2+2] electrocyclic ring-closing is difficult to observe directly, because the thermodynamically favors ring opening
The ring-opening reaction gives products that result from disrotatory rotation. Predict products below
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67. 17.9 Electrocyclic reactions
The Woodward-Hoffmann rules for thermal and photochemical electrocyclic reactions are found in table 17.2
Practice with SkillBuilder 17.4
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68. 17.10 Sigmatropic Rearrangements
Sigmatropic Rearrangements – a pericyclic reaction in which one sigma bond is replaced with another
Note that the pi bonds move their location
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69. 17.10 Sigmatropic Rearrangements
The notation for sigmatropic rearrangements is different from reactions we have seen so far
Count the number of atoms on each side of the sigma bonds that are breaking and forming
This is a [3,3] sigmatropic rearrangement
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70. 17.10 Sigmatropic Rearrangements
The reaction below is a [1,5] sigmatropic rearrangement
Practice with conceptual checkpoint 17.25
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71. 17.10 Sigmatropic Rearrangements
The Cope rearrangement is a [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are CARBONS
In general, what factors affect the spontaneity of the reaction (product favored vs. reactant favored)?
Practice with conceptual checkpoint 17.26
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72. 17.10 Sigmatropic Rearrangements
The Claisen rearrangement is a [3,3] sigmatropic reaction in which one of the 6 atoms in the cyclic transition state is an OXYGEN
In general, what factors affect the spontaneity of the reaction?
Practice with conceptual checkpoints and 17.28
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73. 17.10 Sigmatropic Rearrangements
Two pericyclic reactions occur in the biosynthesis of Vitamin D
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