Q6(a). Photochemical ring expansion of cyclobutanone (Norrish type I)

Introduction. Irradiation of cyclobutanone causes Norrish type I (α) cleavage. The strained four-membered ring opens to an acyl–alkyl 1,4-biradical, which closes on oxygen to give a ring-expanded oxacarbene (a cyclic alkoxycarbene). An alcohol traps the carbene to give the 2-alkoxytetrahydrofuran (a cyclic acetal).

Scheme: Excited ketone → 1,4-acyl–alkyl biradical → tetrahydrofuran-2-ylidene → acetal.

1. Excitation. Light promotes an oxygen lone-pair electron into π* (n→π*). The excited (mainly singlet) ketone has a half-filled n orbital on oxygen.
2. α-Cleavage (Norrish I). The C1–C2 bond next to the carbonyl breaks homolytically, giving an acyl radical and a primary alkyl radical linked by a chain: a 1,4-biradical. In cyclobutanone this step is unusually fast because it releases the ring strain of the four-membered ring.
3. Ring expansion. The alkyl radical end bonds to the oxygen of the acyl radical instead of to its carbon. This forms a five-membered ring containing O and leaves a divalent carbon at C-2: tetrahydrofuran-2-ylidene, a singlet oxacarbene, stabilised by the lone pair of the ring oxygen.
4. Trapping by alcohol. The nucleophilic carbene takes a proton from ROH to give a cyclic oxocarbenium ion, and the alkoxide adds to it at once (net insertion into the O–H bond). The product is the 2-alkoxytetrahydrofuran, a cyclic acetal.
5. Competing paths. The biradical can also lose CO (giving cyclopropane and propene) or split into ethene and ketene; in alcohol solvent the ring-expanded acetal is a major product.

Conclusion. hν (n→π*) → Norrish type I α-cleavage to a 1,4-acyl–alkyl biradical → ring closure on oxygen to the oxacarbene (ring expansion) → O–H insertion of ROH: 2-alkoxytetrahydrofuran.

Application. Photochemical ring expansion of cyclobutanones to tetrahydrofuran acetals has been used to make furanose (sugar-like) rings and nucleoside analogues.