Diels–Alder cycloaddition: cyclopentadiene + maleic anhydride → endo-norbornene-2,3-dicarboxylic anhydride

Cycloaddition [4+2]

Diene + Dienophile →[4+2] cyclic adduct

Textbook [4+2] cycloaddition. Electron-rich diene (cyclopentadiene, freshly cracked from its dimer) reacts with electron-poor dienophile (maleic anhydride) in a concerted, suprafacial, endo-selective addition. The endo product crystallises from cold toluene. Frontier-MO analysis predicts the rate (HOMO_diene/LUMO_dienophile gap) and the ortho/endo regio+stereo selectivity — see Houk, JACS 95, 4092 (1973).

Conditions: Run at 20–25 °C in toluene (~1 M substrate) for 1–4 h, or neat above maleic anhydride's mp (~52 °C) for 30 min. No catalyst required — the EDG/EWG-matched pair is fast at room temperature. Cyclopentadiene must be freshly cracked from dicyclopentadiene (170 °C distillation, used within hours). Workup: cool to 0 °C, filter the white crystalline endo product, wash with cold toluene, dry under vacuum.

Expected yield: 80–95 %
Expected duration: 1–4 h at 20–25 °C in toluene; 30 min neat above 52 °C

Reagents

Reagents involved in Diels–Alder cycloaddition: cyclopentadiene + maleic anhydride → endo-norbornene-2,3-dicarboxylic anhydride, grouped by role.
Reagent	Role	Equiv	MW	ρ
Cyclopentadiene 542-92-7 · C5H6 Cyclopentadiene — freshly cracked from dicyclopentadiene (170 °C). Dimerises on standing; use within hours.	reactant	1.0	66.10	0.800
Maleic Anhydride 108-31-6 · C4H2O3 Maleic anhydride — recrystallised from chloroform if discoloured.	reactant	1.0	98.06	1.480
108-88-3 SDS 108-88-3 · C7H8 Toluene — non-coordinating aromatic solvent; doubles as the recrystallisation medium for the product.	solvent	—	92.14	0.860
826-62-0 826-62-0 · C9H8O3 endo-Bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylic anhydride — kinetic product. White crystalline solid, mp 165 °C; the exo isomer (mp 142 °C) is the thermodynamic product but requires heating beyond the room-temperature conditions used here.	product	1.0	164.16	1.420

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