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Third Year Organic Chemistry Course CHM3A2 Applied Frontier Molecular Orbitals Part 3i: Photochemical Pericyclic Reactions Electronic Excited State Electronic Ground State LUMO HOMO Therm al Reactions 4 3 nodes 3 2 nodes 2 1 node 1 0 nodes LUMO Photochemical Reactions SOMO The Effect of Light on Frontier Molecular Orbitals: SOMO HOMO – Highest Occupied Molecular Orbital LUMO – Lowest Unoccupied Molecular Orbital SOMO – Singly Occupied Molecular Orbital Electronic Excited State Electronic Ground State 4 LUMO 3 nodes Light 3 HOMO 2 nodes SOMO LUMO 2 1 node 1 0 nodes HOMO 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 CHM3A2 – Applied Frontier Molecular Orbitals – – Summary Sheet Part 1ii – Electronically Excited States 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. Third Year Organic Chemistry Course CHM3A2 Applied Frontier Molecular Orbitals Part 3ii: Photochemical Electrocyclic Reactions Electronic Excited State Electronic Ground State LUMO HOMO Therm al Reactions 4 3 nodes 3 2 nodes 2 1 node 1 0 nodes LUMO Photochemical Reactions SOMO Trienes Electrocyclic Reaction Outcomes. 1. 3 Triene DISrotatory Thermal 6e Me Me Me Me cis-Me Me 4 Triene Me 6e CONrotatory Photochemical Me Me Me Me trans-Me Tetraenes: Electrocyclic Reaction Outcomes. 2. 4 Tetraene CONrotatory Thermal 68e e Me Me Me Me trans-Me Me Me 5 Tetraene 6e 8e Photochemical DISrotatory Me Me Me Me cis-Me Exercise 1: Electrocyclics Use frontier molecular orbitals to rationalise the following reaction Scheme. H h H H H racemic mixuture Answer 2: 1: Electrocyclics Use frontier molecular orbitals to rationalise the following reaction Scheme. H h H H H racemic mixuture h H H H SOMO5 H HOMO4 H H H H The following simple rule for “allowed” electrocyclic reactions holds - _______________________________________________________ Number of -Electrons Thermal Photochemical 4n CONrotatory DISrotatory 4n + 2 DISrotatory CONrotatory _______________________________________________________ Third Year Organic Chemistry Course CHM3A2 Applied Frontier Molecular Orbitals Part 3iii: Photochemical Cycloadditions Electronic Excited State Electronic Ground State LUMO HOMO Therm al Reactions 4 3 nodes 3 2 nodes 2 1 node 1 0 nodes LUMO Photochemical Reactions SOMO Photochemical Cycloadditions (4n Electrons TS) SOMO 2 h LUM O 2 Photochemical [p2s + p2s]‡ SOMO 2 LUMO 2 The Paterno-Buchi Reaction (4n Electrons TS) FMO coeffecients predict this as major product n-* is electronically excited oxetanes O Ph h Ph [2s + 2s]‡ O O Ph Ph Ph Ph 90% 10% Alternative Radical Mechanism Stability 3° radical > 1° radical O O Ph Ph Ph Ph Difficult problem to prove concertedness of photochemical cycloadditions O n-* is electronically excited h O Ph Ph Ph Ph Major Product O Ph Ph Most Conjugated O [2s + 2s]‡ O Ph Ph Ph Ph Singlet and Triplet Photochemical Reactions h LUMO 2 singlet state h SOMO 2 triplet state Pericyclic Mechanism Difficult to prove unequivocally D.O Cowan, R.L.E. Drisko, J. Am. Chem. Soc., 1970, 92, 6286 Exercise 4: 4nCycloadditions Rationalise the following reaction scheme utilising frontier molecular orbitals. h O H Me H H O H Me H H O Answer 4: 4n Cycloadditions Rationalise the following reaction scheme utilising frontier molecular orbitals. Enantiotopic faces Enantiomers h O H LUMO of Ene y2 H Me H H O Me H H O Me SOMO of Ene y2 O H H SOMO of Ene y2 H H H LUMO of Ene y2 O H Me The following simple rule for “allowed” cycloadditions hold Number of -Electrons Thermal Photochemical ___________________________________________________________________ 4n sa aa ss 4n + 2 ss aa sa ___________________________________________________________________ NB. s suprafacial a antarafacial