The Diels–Alder reaction joins a diene (four π-electrons) and a dienophile (two π-electrons) to form a cyclohexene. It makes two C–C σ-bonds and up to four stereocentres in one step, with no intermediate.
CH₂=CHCO₂Meheat
What the partners need
- The diene must be conjugated and able to reach the s-cis conformation. Only then are its two ends close enough to bond to the dienophile at the same time. Dienes locked s-trans do not react. Dienes locked s-cis, such as cyclopentadiene, are especially fast.
- Electronics. The usual pairing is an electron-rich diene with an electron-poor dienophile, one bearing C=O, CN or NO₂. In orbital terms, the diene HOMO overlaps with the dienophile LUMO, and a smaller energy gap gives a faster reaction.
Stereochemistry is conserved
The reaction is concerted and suprafacial on both partners, so geometry carries straight through into the product:
- substituents that are cis on the dienophile end up cis on the ring
- substituents that are trans stay trans
- the "outside" groups of an (E,E)-diene end up cis to each other
Regiochemistry: the "ortho/para" rule
With unsymmetrical partners, two orientations are possible. As a rule of thumb, the products that form are the ones that put substituents in "ortho" or "para" positions:
- a 1-substituted diene gives mainly the 1,2 ("ortho") product
- a 2-substituted diene gives mainly the 1,4 ("para") product
To justify this, put a partial negative charge on the diene terminus that the donor makes most nucleophilic. Then join it to the dienophile carbon β to the withdrawing group.