91. Photorearrangement of 4,4-Dimethylcyclohex-2-enones with Alkyl or Fluoro Substituents at C(5) and C(6): in Search of the Mechanism

Article abstract of DOI:10.1002/hlca.19900730415

The photorearrangement of cyclohex-2-enones 4a-h to bicyclo<3.1.0>hexan-2-ones 5 and cyclopent-2-enones 6 (λ = 350 nm, MeCN) was investigated.Both the quantum yield (Φ_-4 = 0.004 - 0.024) and the product ratio (5/6 = 65:35 - 31:69) vary only over a rather small range, indicating the rearrangement to be relatively insensitive to substituents on C(5) or C(6).Compounds 4b, 4c, and 4g with just one alkyl group at either C(6) or C(5) rearrange selectively to the diastereoisomer 5 with alkyl group and three-membered ring in trans-configuration, while 6-fluorocyclohex-2-enones 4d and 4f afford mixtures of diastereoisomeric bicyclohexanones.Mechanistic conclusions regarding an intermediate trimethylene biradical are presented.