Diels-Alder cycloaddition: the mechanism, step by step
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.
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
Six electrons move round at once
The two double bonds of the diene and the one of the alkene all shift together, going round in a circle. Two new bonds appear at the ends and a new double bond appears in the middle.
A suprafacial-suprafacial [4+2] cycloaddition. Three pairs of π electrons move round a six-membered cycle in one step, with no intermediate: two σ bonds are made and the remaining π bond ends up between the middle two carbons.
What this drawing assumes
- Drawn as a single concerted step, which is what the evidence for the ordinary cases supports: both new bonds form together rather than one at a time.
- The diene has to be able to reach the shape where both ends point the same way. A diene locked the other way about does not react, and nothing here checks that.
Other pathways this class runs by
- A strongly polarised pair can go stepwise through a zwitterion, and Lewis acid catalysis pushes it that way.
- Which face meets which - the endo and exo products - is a separate question this drawing does not answer.
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.
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.
Organometallic addition to a carbonyl
The carbon attached to the metal is nucleophilic. It adds to the carbonyl, and the alkoxide that results is protonated when the reaction is worked up.