Cycloaddition and Cycloreversion Reactions

Examples:

Classifying cycloaddition and cycloreversion reactions:

a.  number of electrons moving

b.  HOMO-LUMO arguments: 4q + 2 systems

1.  Diels-Alder - thermally allowed

2.  Diels-Alder - photochemically forbidden

promote electron from HOMO of diene into LUMO of diene (lower energy electron pair). LUMO of diene now HOMO

c.  Correlation diagram

1.  Thermal Diels-Alder

 

2.  Photochemical Diels-Alder

 

d.  HOMO-LUMO arguments: 4q systems

1.  thermally forbidden

2.  photochemically allowed

promote electron from HOMO of half the alkenes into LUMO. LUMO of half the alkenes now HOMO.