The Frontier Orbital approach

The Frontier orbital approach is getting more complicated, and this is the only example considered in detail. The HOMO of one molecule and the LUMO of the other must overlap in phase. In this example of a Diels-Alder cycloaddition reaction, 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 or butenal:

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.