6.02 | Conservation of Symmetry in Cyclo-Addition Reactions

Monday August 12, 2024 | I this class we continue to discuss pericyclic reactions with a particular emphasis on cyclo-addition reactions. In this class we learned that the possibility of a cyclo-addition reaction with 4n+2 or 4n electrons depend on the symmetry of the frontier orbitals. We demonstrated that for 4n+2 (electron) cyclo-addition reactions, there is conservation of symmetry (mirror plane) where the ground-state of the reactants lead to the ground state electronic structure of the products. This was demonstrated by means of an orbital correlation diagram. Using the same methodology we also discovered that for 4n (electron) cyclo-addition reactions, there is no conservation of orbital symmetry (two mirror planes) and that the ground-state of the reactants lead to the excited state electronic structure of the products. As a result the 4n (electron) cyclo-addition reactions are thermally forbidden. However, we also realized that during excited state chemistry, through irradiation of the olefin, the 4n (electron) cyclo-addition is allowed. We came to the conclusion that 4n+2 (electron) cyclo-addition reactions are thermally allowed but photochemically forbidden, while 4n cyclo-addition reactions are thermally forbidden but photochemically allowed. Overall the rules discovered in this lecture are the so-called Woodward-Hoffman Rules

The link to the video for this lecture can be found below: