Photodimerization of Solid Pleiadiene; Isomerization and Cycloreversion of the Dimer

Article abstract of DOI:10.1002/hlca.19810640845

Irradiation (λ>400 nm) of solid pleiadiene (1) yields a single, head-to-head <_?4_s+_?4_s>-photodimer (2) the structure of which was determined by X-ray analysis.The formation of 2 is entirely suppressed at 77 K, since properly oriented pairs of molecules arise only from thermal disorder in crystals of 1.Upon pyrolysis (80 deg), the strained photodimer 2 rearranges to the <_?2_s+_?4_s> dimer 3 by a 'forbidden' suprafacial <1,3>-C-atom migration.Both 2 and 3 are reconverted to 1 by UV. irradiation in solution, but the latter, 'forbidden' photoreaction is suppressed at 77 K.Discrepancies of the experimental observations with the predictive schemes of Kaupp or Michl are discussed.