Diels-Alder [4+2] cycloaddition of butadiene and maleic anhydride

Question

Butadiene (buta-1,3-diene) reacts with maleic anhydride via a Diels-Alder [4+2] cycloaddition. Draw the mechanism, showing the required s-cis conformation of the diene, the concerted bond-forming step, and the resulting bicyclic product. Explain the endo vs exo selectivity of this reaction and why the endo product is typically favoured kinetically.

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Diels-Alder [4+2] cycloaddition of butadiene and maleic anhydride — question structures

Expert answer

Butadiene must first rotate about its central C2-C3 single bond into the s-cis conformation, since only this arrangement allows both terminal carbons of the diene to align simultaneously with the two carbons of the dienophile's double bond. This is a concerted, single-step [4+2] cycloaddition: all bonds break and form in one transition state — the diene's electron-rich π system reacts with maleic anhydride's electron-poor, activated alkene (activated by the two adjacent carbonyl groups), forming two new sigma bonds and a new six-membered ring in one step. The product is a bicyclic anhydride, formed preferentially as the endo isomer. This is identifiable because the ring-junction hydrogens end up on the same side as (cis to) the anhydride bridge — a defining structural feature of the endo product, distinguishing it from the exo isomer where these groups sit on opposite faces. The endo pathway is favoured kinetically due to secondary orbital interactions: as the transition state forms, the carbonyl carbons of the anhydride group have their own π* orbitals, which align favourably with and are stabilised by additional overlap with the diene's π system (beyond just the primary bond-forming interactions). This secondary orbital contribution from the carbonyl carbons lowers the activation energy specifically for the endo approach, making it the kinetically preferred product even though the exo product is often thermodynamically more stable once formed. This kinetic vs thermodynamic distinction is a classic point of confusion and a common exam focus. Transition state: concerted, six-membered TS — endo approach shown, with secondary orbital overlap between the carbonyl π* orbitals and the diene π system.