Q5(c). Photochemistry of norbornenone: 1,3-acyl shift and ketene loss

Introduction. Bicyclo[2.2.1]hept-5-en-2-one is a β,γ-unsaturated ketone: the C=O and the C=C are not conjugated but are held close together. Its typical photoreaction is a Norrish type I (α-cleavage) of the bond between C=O and the allylic bridgehead carbon, followed by a 1,3-acyl shift. The cyclobutanone formed then loses ketene on further irradiation.

Scheme: 1,3-Acyl shift, then [2+2] cycloreversion of the cyclobutanone.

1. α-Cleavage. The excited ketone (nπ*, singlet) breaks the C1–C2 bond (C-1 is the bridgehead next to the C=C). This gives an acyl radical and an allyl radical (C-1, C-6, C-5) held together in one molecule.
2. Recombination at the other end. The acyl radical bonds to C-5, the far end of the allyl radical. The net change is a [1,3]-sigmatropic acyl shift, which is photochemically allowed (suprafacial). A new C=C forms between C-1 and C-6.
3. The product X = bicyclo[3.2.0]hept-2-en-6-one: a cyclobutanone fused to a cyclopentene (C₇H₈O, an isomer of the starting ketone).
4. Second photoreaction. The strained cyclobutanone undergoes α-cleavage again and the 1,4-biradical fragments (photochemical [2+2] cycloreversion) into CH₂=C=O and Y = cyclopenta-1,3-diene (C₅H₆).

C₇H₈O →C₇H₈O →C₅H₆ + C₂H₂O

Conclusion. X = bicyclo[3.2.0]hept-2-en-6-one, by a Norrish type I α-cleavage/1,3-acyl shift of the β,γ-unsaturated ketone; Y = cyclopentadiene, by photochemical cycloelimination (Norrish type I fragmentation) of the cyclobutanone with loss of ketene.

Application. 1,3-Acyl shifts of β,γ-enones and photo-decarbonylation or ketene loss from cyclobutanones are used to make strained four-membered rings and in prostaglandin intermediates (bicyclo[3.2.0]heptenones).