Photorearrangement of 4,4‐Dimethylcyclohex‐2‐enones with Alkyl or Fluoro Substituents at C(5) and C(6): In Search of the Mechanism

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.