Q6(d). Photodecarbonylation of norcamphor and bicyclo[3.2.0]heptan-3-one

Introduction. Photodecarbonylation of a cyclic ketone is a Norrish type I process: α-cleavage of the excited C=O gives an acyl–alkyl biradical, which loses CO. The remaining 1,4-biradical either closes (a ring smaller by one carbon) or fragments its central C–C bond to two C=C.

Scheme: From norcamphor: the two radical centres (C-1 and C-3) are 1,4-related through C-7–C-4.

Scheme: From bicyclo[3.2.0]heptan-3-one: radical centres on C-2 and C-4, linked through the C-1–C-5 bond.

1. A (norcamphor, C₇H₁₀O): C1–C2 cleavage and loss of CO leave radicals at C-1 and C-3 (C₆H₁₀). Bonding C-1 to C-3 gives I = bicyclo[2.1.1]hexane; cleavage of the C-4–C-7 bond, which links the two radical-bearing arms, gives II = hexa-1,5-diene.
2. B (bicyclo[3.2.0]heptan-3-one): loss of CO leaves radicals at C-2 and C-4, attached to the fused carbons C-1 and C-5. Closure (C-2–C-4) gives III = bicyclo[2.2.0]hexane; cleavage of C-1–C-5 (also releasing cyclobutane strain) gives IV = hexa-1,5-diene.
3. All four products are C₆H₁₀ isomers; the ring-closed products are strained bicyclic alkanes that are hard to make by thermal methods.

Conclusion. I = bicyclo[2.1.1]hexane, II = hexa-1,5-diene; III = bicyclo[2.2.0]hexane, IV = hexa-1,5-diene.

Application. Photodecarbonylation was the first route to bicyclo[2.1.1]hexane and is still used to make strained small rings (cyclobutanes, bicyclobutanes) from easily made ketones.