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

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

– 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, 10 ..  electron systems and antisymmetric for 4, 8, 12 ...  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.

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

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

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

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

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

The following simple rule for “allowed” electrocyclic reactions holds - _______________________________________________________Number of -Electrons Thermal Photochemical 4n CONrotatory DISrotatory 4n + 2 DISrotatory CONrotatory _______________________________________________________

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

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

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

Major Product [p2s + p2s]‡ Most Conjugated

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

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

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

The following simple rule for “allowed” cycloadditions hold - Number of -Electrons Thermal Photochemical ___________________________________________________________________ 4n sa ss aa 4n + 2 ss sa NB. s  suprafacial a  antarafacial