Q1(e). Diels–Alder of o-xylylene; photochemistry of butadiene

Introduction. Both parts are pericyclic reactions. (i) is a thermal [4πₛ+2πₛ] Diels–Alder reaction of a reactive diene, o-xylylene (o-quinodimethane), with maleic anhydride. (ii) is the photochemistry of buta-1,3-diene, which gives two isomeric C₄H₆ products.

(i) o-Xylylene and maleic anhydride

o-Xylylene contains two diene units. The exocyclic diene (the two =CH₂ groups, held s-cis) gives the major product: the new ring is a cyclohexene fused to a ring that becomes a benzene ring, so the adduct regains aromaticity (about 150 kJ mol⁻¹ of extra stabilisation). The endocyclic ring diene can also react; the bicyclo[2.2.2]octene adduct keeps two exocyclic CH₂= groups and gains no aromatic ring, so it is the minor product.

Scheme: Major: cis-1,2,3,4-tetrahydronaphthalene-2,3-dicarboxylic anhydride. Minor: adduct of the ring diene (two =CH₂ kept).

Both adducts have the anhydride ring cis (syn addition; the dienophile keeps its geometry). For the major adduct the endo and exo approaches give the same compound, because the diene termini carry only H; for the minor adduct the endo approach is favoured (secondary orbital overlap).

(ii) Butadiene to cyclobutene and bicyclo[1.1.0]butane

The conversion is photochemical: irradiate buta-1,3-diene in solution (direct UV, about 230–254 nm). Both products form from the excited singlet.

• Cyclobutene: a 4π electrocyclic ring closure of s-cis butadiene. In the excited state the HOMO is ψ₃, whose terminal lobes on C-1 and C-4 have the same phase on the same face only after a disrotatory turn; so the closure is disrotatory.
• Bicyclo[1.1.0]butane: an intramolecular [π2ₛ+π2ₛ] cycloaddition of the two C=C bonds of a twisted s-cis conformer, forming the C1–C3 and C2–C4 bonds; [2+2] cycloadditions are allowed photochemically.

Figure: Left: disrotatory closure of s-cis butadiene. Right: crossed internal [2+2] (dashed = bonds forming).

Conclusion. (i) The major product is the aromatic cis-1,2,3,4-tetrahydronaphthalene-2,3-dicarboxylic anhydride (exocyclic diene); the minor one comes from the endocyclic diene. (ii) Photolysis of butadiene gives cyclobutene by disrotatory 4π closure and bicyclo[1.1.0]butane by an internal [π2ₛ+π2ₛ] cycloaddition.

Application. o-Xylylenes generated in situ (from benzocyclobutenes) are used to build the tetralin rings of steroids and anthracyclines; photochemical ring closure is the key step in the body's synthesis of vitamin D.