The Frontier orbital approach is getting more complicated,
and this is the last example we will consider in detail. The
HOMO of one molecule and the LUMO of the other must overlap in
phase. In this example, there are therefore two combinations
that must be considered; the HOMO of the "diene" with the LUMO
of the "ene", and the LUMO of "diene" with the HOMO of "ene".
For this example, both give the same result, but this need not
always be the case.
An extension of this frontier orbital approach is to state
that the two LARGEST orbital nodes will overlap with one
another, and the two SMALLEST will also match up. The size of
the orbitals must be obtained from detailed quantum mechanical
calculations. Such considerations can explain much pericyclic
regiochemistry, as for example in the dimerisation of
acrolein;
One final extension of this approach, to consider not just the
primary HOMO/LUMO overlap but other overlaps as well (so called
secondary orbital overlaps) is required to explain why the
Diels Alder reaction shows an endo rather than an exo
preference. This however is beyond the scope of the present
lectures.
