Cheletropic Reaction - Theoretical Analysis

Theoretical Analysis

In the pericyclic transition state, a small molecule donates two electrons to the ring.The reaction process can be shown using two different geometries. The small molecule can approach in a linear or non-linear fashion. In the linear approach, the electrons in the orbital of the small molecule are pointed directly at the π system. In the non-linear approach, the orbital approaches at a skew angle. The π-systems ability to rotate as the small molecule approaches is crucial in forming new bonds. The direction of rotation will be different depending on how many π-electrons are in the system. Shown below is a diagram of a two-electron fragment approaching a four-electron π-system using frontier molecular orbitals. The rotation will be disrotatory if the small molecule approaches linearly and conrotatory if the molecule approaches non-linearly. Disrotatory and conrotatory are sophisticated terms expressing how the bonds in the π-system are rotating. Disrotatory means opposite directions while conrotatory means the same direction. This is also depicted in the diagram below.

Using Huckel's Rule, one can tell if the π-system is aromatic or anti-aromatic. If aromatic, linear approaches use disrotatory motion while non-linear approaches use conrotatory motion. The opposite goes with an anti-aromatic system. Linear approaches will have conrotatory motion while non-linear approaches will have disrotatory motion.