1. Part 3i: Photochemical Pericyclic Reactions
Third Year Organic Chemistry Course
CHM3A2
Applied Frontier Molecular Orbitals
Part 3i:
Photochemical Pericyclic Reactions

2. KEY POINT: The Symmetry of the FMOs Reverses
OUTCOME: (I) Reactions which were not allowed thermally are allowed photochemically
(ii) Stereochemical outcomes of reactions can be reversed

3. – Summary Sheet Part 1ii –
Electronically Excited States
CHM3A2
– Applied Frontier Molecular Orbitals –
For polyenes in their ground states, the highest occupied molecular orbital (HOMO) is symmetric with respect to the mirror plane for 2, 6,  electron systems and antisymmetric for 4, 8,  electron systems. The lowest unoccupied molecular orbital (LUMO) has the symmetry opposite to that of the HOMO. In the first excited state, the LUMO of the ground state becomes singly occupied because of the promotion of an electron and it will become the new ‘highest occupied’ orbital. In the first excited state, therefore, the symmetry of the highest occupied orbital is opposite to that of the ground state, and the HOMO is termed the singly occupied molecular orbital (SOMO).
This promotion of an electron reverses the symmetries of the frontier molecular orbitals, relative to the ground state, and thus reactions that were
(i) not possible under thermal control become possible, as overlap between species is possible in the transition state, and/or
(ii) the stereochemical outcome of reactions are reversed.

4. Part 3ii: Photochemical Electrocyclic Reactions
Third Year Organic Chemistry Course
CHM3A2
Applied Frontier Molecular Orbitals
Part 3ii:
Photochemical Electrocyclic Reactions

5. Trienes
Electrocyclic Reaction Outcomes. 1.
cis-Me
trans-Me

6. Electrocyclic Reaction Outcomes. 2.
Tetraenes:
Electrocyclic Reaction Outcomes. 2.
trans-Me 8e 8e
cis-Me

7. Exercise 1: Electrocyclics
Use frontier molecular orbitals to rationalise the following reaction Scheme.

8. Answer 2: Electrocyclics Answer 1: Electrocyclics
Use frontier molecular orbitals to rationalise the following reaction Scheme. H

9. The following simple rule for “allowed” electrocyclic reactions holds -
_______________________________________________________Number of -Electrons Thermal Photochemical
4n CONrotatory DISrotatory
4n DISrotatory CONrotatory
_______________________________________________________

10. Part 3iii: Photochemical Cycloadditions
Third Year Organic Chemistry Course
CHM3A2
Applied Frontier Molecular Orbitals
Part 3iii:
Photochemical Cycloadditions

11. Photochemical Cycloadditions (4np Electrons TS)
Photochemical [p2s + p2s]‡

12. The Paterno-Buchi Reaction (4np Electrons TS)
FMO coeffecients
predict this as
major product
[p2s + p2s]‡
Alternative Radical Mechanism
Stability 3° radical > 1° radical
Difficult problem to prove concertedness of photochemical cycloadditions

13. Major
Product
[p2s + p2s]‡
Most Conjugated

14. Singlet and Triplet Photochemical Reactions
Pericyclic Mechanism Difficult to prove unequivocally
D.O Cowan, R.L.E. Drisko, J. Am. Chem. Soc., 1970, 92, 6286

15. Exercise 4: 4np Cycloadditions
Rationalise the following reaction scheme utilising frontier molecular orbitals.

16. Answer 4: 4np Cycloadditions
Rationalise the following reaction scheme utilising frontier molecular orbitals.
Enantiotopic faces
Enantiomers
SOMO of Ene y2
LUMO of Ene y2
SOMO of Ene y2
LUMO of Ene y2

17. The following simple rule for “allowed” cycloadditions hold -
Number of -Electrons Thermal Photochemical
___________________________________________________________________
4n sa ss aa
4n ss sa
NB. s  suprafacial a  antarafacial