Constructing Correlation Diagrams

A.  4n p system:

  1. Which bonds form and break.
  2. Energy levels.

     

  3. Draw MOs and assign to energy levels. Assign symmetries to each of the orbitals with the symmetry element maintained in the transformation as the orbitals of the starting material are transformed into the orbitals of the product. For conrotatory ring closure, use the C2 rotational axis. For disrotatory, use the mirror plane.  (The Principle of the Conservation of Orbital Symmetry: the starting molecular orbital undergoing transition to the product molecular orbital must be symmetrical.  When ring closures are performed in a disrotatory fashion, a mirror plane allows for this symmetry.  When ring closures are performed in a conrotatory fashion, the axis of rotation allows for this symmetry.)

Thermal controtatory:

electrocyclic conrotatory ground state allowed
The electrons in the starting material wind up in the lower energy orbitals in the product. The ring closure of butadiene is conrotatory in the ground state.

Thermal disrotatory:

The electrons in the HOMO in the ground state for disrotatory ring closure wind up in the antibonding pi molecular orbital in the product. This means that the activation energy would be too high for the reaction to proceed in a disrotatory fashion. The reaction is thus symmetry forbidden in the ground state. Draw the correlation diagram that would show this (BM p34).

 

Photochemical ring closure:

A look at the correlation diagrams for the photochemical ring closure gives the opposite conclusions. Photochemical disrotatory ring closure is allowed and photochemical conrotatory ring closure is forbidden. Draw the correlation diagrams that would show these results (BM 44). Remember to promote one electron into the LUMO for the photochemical reaction.
 

B.  4n + 2 p system:

Follow the same rules to construct the correlation diagram as for the 4n system. The correlation diagram for the allowed ground state disrotatory ring closure is given below. Use correlation diagrams to decide which of the other three possibilities are allowed and which are forbidden.