Ene reaction (Alder ene) template

Propene and maleic anhydride give allylsuccinic anhydride: the C=C of the ene bonds to the enophile, the double bond moves along and the allylic hydrogen is delivered to the enophile, all in one cyclic step.

Mechanism step by step

An alkene with an allylic H adds to an enophile: new C–C bond, shifted C=C and H transfer

1. Concerted ene step – Six electrons (π, σ C–H and π of the enophile) move round a six-membered cycle

Key points

• Thermally allowed [σ2s + π2s + π2s] (six electrons, like a Diels–Alder in which one σ C–H bond replaces a π bond).
• Breaking the C–H σ bond gives a higher barrier than a Diels–Alder: simple ene reactions need 150–250 °C, or a Lewis acid (carbonyl-ene with Me₂AlCl).
• Electron-poor enophiles (maleic anhydride, acetylenedicarboxylates, glyoxylates, singlet O₂ in the Schenck ene reaction) react best.
• Used industrially: alkenylsuccinic anhydrides (detergents, lubricant additives) come from alkenes and maleic anhydride.

Drawn with 41 atoms, 5 arrows, 3 shapes – every one editable in MolDraw.

Sources

• Fleming, Molecular Orbitals and Organic Chemical Reactions, Wiley 2010 Textbook
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook