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.
Third Year Organic Chemistry Course CHM3A2 Applied Frontier Molecular Orbitals Part 3ii: Photochemical Electrocyclic Reactions
Tetraenes: Electrocyclic Reaction Outcomes. 2. trans-Me cis-Me 8e
Exercise 1: Electrocyclics Use frontier molecular orbitals to rationalise the following reaction Scheme.
Answer 2: Electrocyclics Use frontier molecular orbitals to rationalise the following reaction Scheme. H H Answer 1: Electrocyclics
_______________________________________________________ Number of -Electrons ThermalPhotochemical 4n CONrotatory DISrotatory 4n + 2 DISrotatory CONrotatory _______________________________________________________ The following simple rule for “allowed” electrocyclic reactions holds -
Third Year Organic Chemistry Course CHM3A2 Applied Frontier Molecular Orbitals Part 3iii: Photochemical Cycloadditions
Photochemical Cycloadditions (4n Electrons TS) Photochemical [ p 2s + p 2s] ‡
The Paterno-Buchi Reaction (4n Electrons TS) [ 2s + 2s] ‡ Stability 3° radical > 1° radical Alternative Radical Mechanism FMO coeffecients predict this as major product Difficult problem to prove concertedness of photochemical cycloadditions
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: 4n Cycloadditions Rationalise the following reaction scheme utilising frontier molecular orbitals.
Answer 4: 4n Cycloadditions Rationalise the following reaction scheme utilising frontier molecular orbitals. SOMO of Ene y 2 LUMO of Ene y 2 SOMO of Ene y 2 LUMO of Ene y 2 Enantiomers Enantiotopic faces
Number of -ElectronsThermal Photochemical ___________________________________________________________________ 4n sa ss aa 4n + 2 ss sa aa ___________________________________________________________________ NB.s suprafaciala antarafacial The following simple rule for “allowed” cycloadditions hold -