The frontier orbital description of cycloadditions

When an ionic cyclization reaction occurs, such as the lactonization at the head of this chapter, one important new bond is formed. It is enough to combine one full orbital with one empty orbital to make the new bond. But in a cycloaddition two new bonds are formed at the same time. We have to arrange for two fi lled p orbitals and two empty p orbitals to be available at the right place and with the right symmetry. See what happens if we draw the orbitals for the reaction above. We could try the HOMO (π) of the alkene and the LUMO (π*) of the double bond in the anhydride (as in the margin). This combination is bonding at one end, but anti-bonding at the other so that no cycloaddition reaction occurs. It obviously doesn’t help to use the other HOMO/LUMO pair, that is the HOMO of the aldehyde and the LUMO of the alkene, as they will have the same mismatched symmetry. Now see what happens when we replace the alkene with a diene. We shall again use the LUMO of the electron-poor anhydride. Now the symmetry is right because there is a node in the middle of the HOMO of the diene (the HOMO is ψ2 of the diene) just as there is in the LUMO of the dienophile. If we had tried the opposite arrangement, the LUMO of the diene (ψ3) and the HOMO of the dienophile, the symmetry would again be right. The LUMO of the diene has two nodes and gives the same symmetry as the HOMO of the dienophile, which has no nodes. So either com bination is excellent. In fact most Diels–Alder reactions use electron-deficient dienophiles and electron-rich dienes so we prefer the fi rst arrangement. The electron-deficient dienophile has a low-energy LUMO and the electron-rich diene has a high-energy HOMO so this combi nation gives a better overlap in the transition state. The energy levels will look like this, and the interaction shown in orange is better than the interaction shown in brown because the orbitals are closer in energy.

This is why we usually use dienophiles with conjugating groups for good Diels–Alder reactions. Dienes react rapidly with electrophiles because their HOMOs are relatively high in energy, but simple alkenes are not suitable electrophiles because they have relatively high energy LUMOs. The most effective modifi cation we can make is to lower the energy of the alkene’s LUMO by conjugating the double bond with an electron-withdrawing group such as carbonyl or nitro. These are the most common type of Diels–Alder reactions—between electron-rich dienes and electron-deficient dienophiles.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct