Organic Chemistry 2 · Sigmatropic shifts, cycloadditions and electrocyclic reactions
Ene reaction of pent-1-ene with maleic anhydride
Conditions: Pent-1-ene (excess), maleic anhydride, sealed tube, 200 °C
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Concerted ene reaction
The terminal alkene carbon bonds to one alkene carbon of maleic anhydride, the alkene π bond moves to the allylic position, and the allylic C–H bond transfers its hydrogen to the other anhydride alkene carbon. All six electrons move in one cyclic transition state.
The curved arrows
- The ene π pair forms the new C–C bond from the terminal CH2 to the anhydride alkene.
- The allylic C–H pair becomes the new internal C=C.
- The enophile π pair forms a C–H bond to the transferred hydrogen.
What does the ene component need?
The allylic hydrogen plays the role of the second diene π bond in a Diels–Alder: the ene supplies the C=C π pair and the allylic C–H σ pair.
2-(Pent-2-en-1-yl)succinic anhydride
The anhydride ring is now saturated and carries a pent-2-enyl chain; the alkene has moved one position along the former pentene chain, mainly with E geometry.
What you'll practice
Draw the concerted ene reaction in which pent-1-ene transfers an allylic hydrogen to maleic anhydride while forming a C–C bond, giving 2-(pent-2-en-1-yl)succinic anhydride.
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