Q8(c)(ii). Photodecarbonylation; trapping the o-xylylene

Introduction. Indan-2-ones lose carbon monoxide on irradiation (photodecarbonylation, Norrish type I). Both bonds to the C=O are benzylic and break easily, so the product is a reactive o-quinodimethane (o-xylylene). This diene is trapped by maleic anhydride in a Diels–Alder reaction that also restores the benzene ring.

Scheme: CO is lost; the diene is trapped and the ring becomes aromatic again.

1. α-Cleavage. The excited ketone breaks one C(O)–CHPh bond to give a benzylic radical and an acyl radical (Norrish type I). The acyl radical loses CO, leaving a 1,4-biradical (two benzylic radical centres held ortho on the ring).
2. X. The biradical closes its π system to the quinonoid diene α,α'-diphenyl-o-xylylene (5,6-dibenzylidenecyclohexa-1,3-diene), mainly the (E,E) isomer with both Ph pointing outwards. It is not aromatic and is very reactive.
3. Y. The s-cis exocyclic diene adds to maleic anhydride (suprafacial [4+2], endo). Formation of the new six-membered ring restores the aromatic sextet, a large driving force. Y = 1,4-diphenyl-1,2,3,4-tetrahydronaphthalene-2,3-dicarboxylic anhydride.
4. Stereochemistry of Y. The (E,E) diene puts both Ph on one face; endo addition puts the anhydride on the same face: all four substituents cis.

Conclusion. X = α,α'-diphenyl-o-xylylene, formed by photochemical loss of CO (Norrish type I); Y = its Diels–Alder adduct with maleic anhydride, the all-cis 1,4-diphenyltetralin-2,3-dicarboxylic anhydride.

Application. Photochemical or thermal generation of o-xylylenes and trapping by dienophiles is a standard route to tetralins and to the steroid and anthracycline ring systems.