Photoinduced Electron-Transfer Degenerate Cope Rearrangement of 2,5-Diaryl-1,5-hexadienes: A Cation-Radical Cyclization−Diradical Cleavage Mechanism

Under the 9,10-dicyanoanthracene (DCA)-sensitized photoinduced electron-transfer (PET) conditions, 2,5-diaryl-3,3,4,4-tetradeuterio-1,5-hexadiene ( d 4 - 1 ) undergoes a degenerate Cope rearrangement to give a photostationary mixture of d 4 - 1 and its 1,1,6,6-tetradeuterio analogue d 4 - 1 ‘ in 52:48. This reaction involves 1,4-diaryl-2,2,3,3-tetradeuteriocyclohexane-1,4-diyl cation radical ( d 4 - 2 • + ), which can be captured by molecular oxygen as 1,4-diaryl-2,3-dioxabicyclo[2.2.2]octane ( 3 ). In contrast, nonphotoinduced electron-transfer (non-PET) reaction of d 4 - 1 with cerium(IV) ammonium nitrate similarly forms d 4 - 2 • +, but the degenerate Cope does not take place. This observed striking contrast between PET and non-PET was ascribed to the formation of 1,4-diaryl-2,2,3,3-tetradeuteriocyclohexane-1,4-diyl ( d 4 - 2 ) in the PET process by back-electron transfer from DCA • - to d 4 - 2 • + . The results of similar PET and non-PET reactions of 1,4-diphenyl-2,3-diazabicyclo[2.2.2]oct-2-ene ( 4c ) and its 5,5,6,6-tetradeuterio analogue ( d 4 - 4c ) support this conclusion. Photoacoustic calorimetry of the PET reaction of 2,5-diphenyl-1,5-hexadiene ( 1c ) suggests that 1,4-diphenylcyclohexane-1,4-diyl ( 2c ) lies ca. 18.8 kcal/mol higher in energy than 1c and ca. 25.6 kcal/mol below the ion-radical pair [ 2c • + /DCA • - ]. Deazetation of d 4 - 4c demonstrates that d 4 - 2c undergoes cleavage to d 4 - 1c and d 4 - 1c ‘ with bond-breaking kinetic isotope effects, k ( d 4 - 2c ) H 4 / k ( d 4 - 2c ) D 4 = 1.11, completing a cation-radical cyclization−diradical cleavage mechanism.