Photochemistry of ketones in solution. 53. Stereospecific triplet-state photorearrangements of chiral 2-cyclohexenones: type A lumiketone rearrangement and phenyl migrations

Visible Light Mediated Generation of trans-Arylcyclohexenes and Their Utilization in the Synthesis of Cyclic Bridged Ethers

“…While irradiation with UV light led to multiple products, it has also been shown that if excited in acidic methanol, the trans -phenylcyclohexene underwent methanolysis of the olefin , (Scheme B). Additionally, Schuster proposed a similar intermediate for the rearrangement of cyclohexenones. Further studies from the McClelland group with UV flash photolysis have demonstrated the presence of a transient cationic intermediate, which in some cases could be intercepted with a solvent .…”

Photochemistry of ketones in solution. 57. Synthesis and photochemistry of a constrained 2-cyclohexenone

“…3 We have previously argued that the very low quantum efficiency ( < 0.01) of the so-called "lumiketone rearrangement" of 4,4-dialkyl enones, 1 -*• 2,4 cannot be adequately explained on the basis of a mechanism involving C-C bond scission to form diradical or zwitterionic intermediates which revert to starting material in competition with progress to product. 5 We have suggested32•5 that the planar enone triplet, formed in unit efficiency from the excited singlet state(s), relaxes rapidly by twisting around the C=C bond to give a twisted species which serves as an ideal "funnel" for crossing to the ground state (So) potential surface from T1, since the difference in energy of So and T¡ is minimized at this geometry.5•6 A partitioning on the So surface between return to starting material and conversion into rearrangement products is postulated, the latter being the minor decay pathway.…”

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

“…Nevertheless, the question of the mechanism of this rearrangement remains unanswered, both concerted and stepwise mechanisms having been proposed but not proven. The lifetime of an eventual propane-l,3-diyl-biradical intermediate A has to be short, as a ) the reaction proceeds with retention of configuration at the dialkyl-disubstituted C(4) [7], and b ) no products derived from cyclopropyl-…”