1. 30. Orbitals and Organic Chemistry: Pericyclic Reactions
Based on McMurry’s Organic Chemistry, 6th edition
©2003 Ronald Kluger
Department of Chemistry
University of Toronto

2. Pericyclic Reactions – What Are?
Involves several simultaneous bond-making breaking process with a cyclic transition state involving delocalized electrons
The combination of steps is called a concerted process where intermediates are skipped
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

3. 30.1 Molecular Orbitals of Conjugated  Systems
A conjugated diene or polyene has alternating double and single bonds
Bonding MOs are lower in energy than the isolated p atomic orbitals and have the fewest nodes
Antibonding MOs are higher in energy
See Figure 30.1 for a diagram
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

4. Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003
1,3,5-Hexatriene
Three double bonds and six  MOs
Only bonding orbitals, 1, 2, and 3, are filled in the ground state
On irradiation with ultraviolet light an electron is promoted from 3 to the lowest-energy unfilled orbital (4*)
This is the first (lowest energy) excited state
See the diagram in Figure 30.2
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

5. 30.2 Molecular Orbitals and Pericyclic Reactions
If the symmetries of both reactant and product orbitals match the reaction is said to be symmetry allowed under the Woodward-Hoffmann Rules (these relate the electronic configuration of reactants to the type of pericyclic reaction and its stereochemical imperatives)
If the symmetries of reactant and product orbitals do not correlate, the reaction is symmetry-disallowed and there no low energy concerted paths
Fukui’s approach: we need to consider only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the frontier orbitals
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

6. 30.3 Electrocyclic Reactions
These are pericyclic processes that involves the cyclization of a conjugated polyene
One  bond is broken, the other  bonds change position, a new σ bond is formed, and a cyclic compound results
Gives specific stereoisomeric outcomes related to the stereochemistry and orbitals of the reactants
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

7. Example: Electrocyclic Interconversions With Octatriene
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

8. Example: Electrocyclic Interconversions with Dimethylcyclobutene
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

9. The Signs on the Outermost Lobes Must Match to Interact
The lobes of like sign can be either on the same side or on opposite sides of the molecule.
For a bond to form, the outermost  lobes must rotate so that favorable bonding interaction is achieved
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

10. Disrotatory Orbital Rotation
If two lobes of like sign are on the same side of the molecule, the two orbitals must rotate in opposite directions—one clockwise, and one counterclockwise
Woodward called this a disrotatory (dis-roh-tate’-or-ee) opening or closure
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

11. Conrotatory Orbital Rotation
If lobes of like sign are on opposite sides of the molecule: both orbitals must rotate in the same direction, clockwise or counterclockwise
Woodward called this motion conrotatory (con-roh-tate’-or-ee)
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

12. 30.4 Stereochemistry of Thermal Electrocyclic Reactions
Determined by the symmetry of the polyene HOMO
The ground-state electronic configuration is used to identify the HOMO
(Photochemical reactions go through the excited-state electronic configuration )
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

13. Ring Closure of Conjugated Trienes
Involves lobes of like sign on the same side of the molecule and disrotatory ring closure
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

14. Contrast: Electrocyclic Opening to Diene
Conjugated dienes and conjugated trienes react with opposite stereochemistry
Different symmetries of the diene and triene HOMOs
Dienes open and close by a conrotatory path
Trienes open and close by a disrotatory path
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

15. 30.5 Photochemical Electrocyclic Reactions
Irradiation of a polyene excites one electron from HOMO to LUMO
This causes the old LUMO to become the new HOMO, with changed symmetry
This changes the reaction stereochemistry (symmetries of thermal and photochemical electrocylic reactions are always opposite)
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

16. Rules for Electrocyclic Reactions
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

17. 30.6 Cycloaddition Reactions
Two unsaturated molecules add to one another, yielding a cyclic product
The Diels–Alder cycloaddition reaction is a pericyclic process that takes place between a diene (four  electrons) and a dienophile (two  electrons) to yield a cyclohexene product Stereospecific with respect to substituents
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

18. Rules for Cylcoadditions - Suprafacial Cycloadditions
The terminal  lobes of the two reactants must have the correct symmetry for bonding to occur
Suprafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

19. Rules for Cylcoadditions - Antarafacial Cycloadditions
These take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on opposite faces of the other reactant (not possible unless a large ring is formed)
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

20. 30.7 Stereochemistry of Cycloadditions
HOMO of one reactant combines with LUMO of other
Possible in thermal [4 +2] cycloaddition
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

21. Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003
[2+2] Cylcoadditions
Only the excited-state HOMO of one alkene and the LUMO can combine by a suprafacial pathway in the combination of two alkenes
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

22. Formation of Four-Membered Rings
Photochemical [2 + 2] cycloaddition reaction occurs smoothly
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

23. 30.8 Sigmatropic Rearrangements
A s -bonded substituent atom or group migrates across a p electron system from one position to another
A s bond is broken in the reactant, the p bonds move, and a new s bond is formed in the product
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

24. Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003
Sigmatropic Notation
Numbers in brackets refer to the two groups connected by the s bond and designate the positions in those groups to which migration occurs
In a [1,5] sigmatropic rearrangement of a diene migration occurs to position 1 of the H group (the only possibility) and to position 5 of the pentadienyl group
In a [3,3] Claisen rearrangement migration occurs to position 3 of the allyl group and also to position 3 of the vinylic ether
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

25. Sigmatropic Stereospecificity: Suprafacial and Antarafacial
Migration of a group across the same face of the  system is a suprafacial rearrangement
Migration of a group from one face of the  system to the other face is called an antarafacial rearrangement
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

26. Stereochemical Rules of Sigmatropic Rearrangements
Electron Pairs
Thermal Reaction
Photochemical Reaction
Even Number
Antarafacial
Suprafacial
Odd Number H H
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

27. 30.9 Some Examples of Sigmatropic Rearrangements
A [1,5] sigmatropic rearrangement involves three electron pairs (two  bonds and one s bond)
Orbital-symmetry rules predict a suprafacial reaction
5-methylcyclopentadiene rapidly rearranges at room temperature
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

28. Another Example of a Sigmatropic Rearrangement
Heating 5,5,5-trideuterio-(1,3Z)-pentadiene causes scrambling of deuterium between positions 1 and 5
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

29. Orbital Picture of a Suprafacial [1,5] H Shift
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

30. Cope and Claisen Rearrangements are Sigmatropic
Cope rearrangement of 1,5-hexadiene
Claisen rearrangement of an allyl aryl ether
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

31. Suprafacial [3,3] Cope and Claisen Rearrangements
Both involve reorganization of an odd number of electron pairs (two  bonds and one s bond)
Both react by suprafacial pathways
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003

32. 30.10 A Summary of Rules for Pericyclic Reactions
Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003