Q6(b). Photolysis of cyclobutanone

Introduction. Light of 325 nm excites the weak n→π* band of the C=O group. Cyclobutanone is strained, so the excited ketone undergoes fast Norrish type I (α) cleavage of a ring C–C bond next to C=O.

Scheme: The intermediate: a 1,4-acyl–alkyl biradical.

Scheme: Three fates of the biradical: decarbonylation, cleavage, and ring expansion to an oxacarbene trapped by methanol.

1. Intermediate: α-cleavage of the C1–C2 bond gives the acyl–alkyl 1,4-biradical °CH₂CH₂CH₂C°=O. Ring strain makes this step much faster than in cyclopentanone or cyclohexanone.
2. Decarbonylation: the acyl radical loses CO; the trimethylene biradical °CH₂CH₂CH₂° closes to cyclopropane or undergoes H-shift to propene.
3. Cycloelimination: the biradical splits at C2–C3 into ethene and ketene (CH₂=C=O).
4. Ring expansion: the acyl oxygen bonds to the alkyl radical end, giving a five-membered oxacarbene (2-oxacyclopentylidene). In methanol it is trapped as the acetal 2-methoxytetrahydrofuran (2-methoxyoxolane).

Conclusion. Intermediate: acyl–alkyl 1,4-biradical (Norrish type I). Products: CO + cyclopropane/propene, ethene + ketene, and (in methanol) 2-methoxytetrahydrofuran via the ring-expanded oxacarbene.

Application. Photochemical ring expansion of cyclobutanones to oxacarbenes is used to make tetrahydrofuranyl acetals and C-nucleosides; decarbonylation of cyclic ketones is a route to strained small rings.