 4.2. Selectivity; Frontier MO Theory The strength
of the interaction between a Lewis acid and a Lewis base
can be expressed as the sum of the interactions between
all the orbitals of the acid with all the orbitals of the
base. However, the interaction energy between any pair of
orbitals depends on the reciprocal of the energy
difference between them. Furthermore, interactions
between occupied and virtual orbitals are generally much
larger than those between two occupied orbitals (although
these can also be significant) and interactions between
two virtual orbitals have no energetic consequences at
all because there are no electrons involved. Therefore,
the most important interactions energetically will be
between the occupied and virtual orbitals with the
smallest energy gaps. These are naturally the HOMO of the
base and the LUMO of the acid. It is thus often a very
useful approximation to use only the HOMOs and LUMOs (the
frontier orbitals because they occur at the border
between occupied and virtual orbitals) when considering
reactivity. This approximation does not always work. It
is, for instance, unreliable for extended p-systems with many high-lying occupied
orbitals and low-lying virtual orbitals. It is, however,
very useful for small, non-delocalized systems. 4.2.1. Nucleophilic Oxirane Ring-Opening Let us now
consider a special case of the SN2 reaction,
the nucleophilic ring-opening of substituted oxiranes.
For 2,2-dimethyloxirane this reaction can occur in two
ways (Figure 4.3):
We saw above that nucleophilic substitution involves the s*CX-orbital of the substrate; in this case a CO-antibonding orbital that is the LUMO of 2,2-dimethyloxirane. Visual inspection of this orbital shows that it has a larger contribution at the unsubstituted carbon atom than at the fully substituted one. This can also be seen from the atomic orbital coefficients in the MO. We can therefore conclude that, because the contribution at the unsubstituted carbon is larger, overlap with the HOMO of an attacking nucleophile N will be larger and the interaction energy will be more favourable. In this way it is possible to rationalise the experimental observation that under basic conditions 2,2-dimethyloxirane opens the ring to give the lower of the two products shown above. This sort of approach is often very useful in rationalising regioselectivity and sometimes stereoselectivity. Lewis Acid/Lewis Base Interactions |
