Orbital Symmetry Considerations for Photochemical Reactions of Alkenes and Dienes

Orbital Symmetry Considerations for Photochemical Reactions of Alkenes and Dienes 

How does the situation change when a photochemical reaction involving one ground state alkene and one excited state alkene is considered If we assume a 

Consideration of the HOMO-LUMO interactions also indicates that the [2Tr+2Tr] addition is allowed photochemically. The HOMO in this case is the excited alkene tt orbital. The LUMO is the tt of the ground state alkene, and a bonding interaction is present between both pairs of carbons where new bonds must be formed. Similarly, the concept of aromatic transition states shows that the reaction has an antiaromatic 4tt combination of basis set orbitals, which predicts an allowed photochemical reaction. Thus, orbital symmetry considerations indicate that photochemical [2tt- -2tt] cycloaddition of alkenes is feasible. 

Aromatic Mobius basis set orbial array tor [2+ 2] photocycloaddition 

Although orbital symmetry provides a starting point for analyses of photochemical reactions of conjugated dienes and polyenes, experimental studies have identified a number of additional facets of the problem, some of which have to do with the fundamental assumptions of the orbital symmetry analysis. One of the underlying 

---