Presentation on theme: "Part 3i: Photochemical Pericyclic Reactions"— Presentation transcript:
1Part 3i: Photochemical Pericyclic Reactions Third Year Organic Chemistry CourseCHM3A2Applied Frontier Molecular OrbitalsPart 3i:Photochemical Pericyclic Reactions
2KEY 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 StatesCHM3A2– 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.
4Part 3ii: Photochemical Electrocyclic Reactions Third Year Organic Chemistry CourseCHM3A2Applied Frontier Molecular OrbitalsPart 3ii:Photochemical Electrocyclic Reactions
14Singlet and Triplet Photochemical Reactions Pericyclic Mechanism Difficult to prove unequivocallyD.O Cowan, R.L.E. Drisko, J. Am. Chem. Soc., 1970, 92, 6286
15Exercise 4: 4np Cycloadditions Rationalise the following reaction scheme utilising frontier molecular orbitals.
16Answer 4: 4np Cycloadditions Rationalise the following reaction scheme utilising frontier molecular orbitals.Enantiotopic facesEnantiomersSOMO of Eney2LUMO of Eney2SOMO of Eney2LUMO of Eney2
17The following simple rule for “allowed” cycloadditions hold - Number of -Electrons Thermal Photochemical___________________________________________________________________4n sa ssaa4n ss saNB. s suprafacial a antarafacial