Symmetry controlled reactions sigmatropic rearrangements

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

Like electrocyclic reactions and cycloadditions, sigmatropic rearrangements are controlled by orbital symmetries. There are two possible modes of reaction migration of a group across the same face of the tt system is suprafacial, and migration of a group from one face of the tt system to the other face is antara-facial (Figure 30.11). 

Therefore, sigmatropic rearrangements are controlled by the same orbital symmetry considerations as cycloaddition reactions. Table 25.3 hsts the rules for thermal and photochemical sigmatropic rearrangements. 

Sigmatropic rearrangements are also controlled by orbital symmetry considerations— the HOMO and LUMO determine the outcomes of thermal and photochemical reactions, respectively. 

---