Photon assisted cycloaddition
· We have seen that the thermal cycloaddition of two ethenes is “forbidden” because the energy of the reactants goes up significantly as the reactants (two ethenes) approach each other.
·
We reach this conclusion
through the use of the correlation diagram, which correlates or joins up
orbitals of the reactants and products which have the same symmetry.
· From the fact that the HOMO goes up in energy and the energy of the reactants increases as the reaction proceeds the conclusion is reached that the reaction has a substantial barrier to reactions and is therefore “forbidden”.
How can we
change this situation?
·
We can rearrange the situation so that the reaction does not
increase as the reaction proceeds.
·
It is possible to do this by exciting the electrons in the
reactants using UV light.
The correlation diagram then looks like:
