Q6(b). o-Xylylene from a tetrabromide; photochemical ring closure

Introduction. Iodide removes two bromine atoms from the two CHBr₂ groups by a 1,4-elimination, generating the reactive diene α,α'-dibromo-o-xylylene (an o-quinodimethane). Light then closes it by a 4π electrocyclic reaction to a benzocyclobutene; closure restores the aromatic ring.

Scheme: Debromination gives an o-quinodimethane; photochemical 4π closure is disrotatory.

1. Debromination. I⁻ attacks a bromine atom of one CHBr₂ group (halophilic attack, giving IBr, which reacts with more iodide to I₂ and Br⁻). The carbanion formed pushes its electrons through the ring and expels Br⁻ from the other CHBr₂ group: a 1,4-elimination.
2. Intermediate. α,α'-Dibromo-o-xylylene (5,6-bis(bromomethylidene)cyclohexa-1,3-diene). The ring has lost its aromaticity, so the diene is very reactive. The bulky Br atoms prefer the outward (E,E) positions.
3. Photochemical closure. The terminal =CHBr carbons form a σ bond: a 4π electrocyclic ring closure. Under irradiation the HOMO is ψ₃, whose end lobes are in phase on the same face, so the closure is disrotatory.
4. Stereochemistry. Disrotation of the (E,E) diene turns both outward Br atoms the same way: the product is cis-1,2-dibromobenzocyclobutene. Closure is strongly favoured because it regenerates the benzene ring.
5. Contrast. Without light (warming with NaI in ethanol), the same o-xylylene closes conrotatorily and gives trans-1,2-dibromobenzocyclobutene; the two routes show the thermal/photochemical switch of the Woodward–Hoffmann rules.

Conclusion. I⁻ effects 1,4-debromination to (E,E)-α,α'-dibromo-o-xylylene; hν closes it by a disrotatory 4π electrocyclisation to cis-1,2-dibromobenzocyclobutene (the thermal, conrotatory route gives the trans isomer).

Application. Benzocyclobutenes made this way reopen on heating to o-xylylenes, which are trapped in Diels–Alder reactions to build steroid and anthracycline ring systems.