Diels-Alder with an aromatic diene: the mechanism, step by step
The same six electrons moving round at once, except that the diene is an aromatic ring and gives up its aromaticity to react.
This class has one step and no intermediate. Everything happens at once, and that is not a simplification — it is what the class means. Bonds break and form in the same motion, through a single transition state that is never isolated.
Below is the sequence as it is taught. On the platform it is drawn on whatever structures you supply, with the curly arrows resolved to the actual atoms involved rather than to a general case with R groups on it.
The steps
The aromatic ring adds across the alkene
The ring gives up being aromatic and behaves as a diene. Its electrons and the alkene's move round together, making two new bonds at the ends and leaving a bridge.
A [4+2] cycloaddition in which the 4π component is an aromatic ring. The aromatic stabilisation lost is what makes this reversible, and it is why benzene does not do it.
What this drawing assumes
- Concerted, like the ordinary case.
- The ring pays its aromatic stabilisation to react, which is why only the five-membered heterocycles do this and why the reaction runs backwards on heating.
Other pathways this class runs by
- The adduct can revert to the starting materials, so the product seen depends on the temperature as much as on the rate.
Run it on your own structures
Enter the reactants and the product you expect. If the transformation is one the engine can perform and this pathway reaches that product from those structures, it is drawn on them — with the arrows on the right atoms and the intermediates you would actually pass through. If it does not reach the product, you are told that instead of being shown a drawing that does not apply.
Other mechanisms
Nucleophilic acyl substitution
The nucleophile adds to the carbonyl, the carbon holds four groups for a moment, and the leaving group is pushed out as the double bond comes back.
Bimolecular nucleophilic substitution
One step. The nucleophile comes in on the opposite side from the leaving group, and the carbon turns inside out as the exchange happens.
Imine formation
The amine adds to the carbonyl, the resulting alcohol-amine loses water, and a carbon–nitrogen double bond is left.
Diels-Alder cycloaddition
One step. Six electrons move round a ring at once, two new single bonds form at the ends and the double bond ends up in the middle.
Bimolecular elimination
One step. The base takes a hydrogen from one carbon while the leaving group departs from the next, and a double bond forms between them.
Unimolecular nucleophilic substitution
The leaving group goes first, on its own, leaving a flat carbon with a positive charge. Whatever is around then attacks it from either side.