Photochemical Transformations. 38. Novel Transformations of Diarobicyclooctadienes to Phenanthrenes and Dihydrophenanthrenes.

Article abstract of DOI:10.1021/jo00214a019

Irradiation of 2,3:6,7-diarobicyclo<3.2.1>octadienes having nucleofugal groups at C-8 in an unprecedented rearrangement, produces 9-functionalized or 9,10-difunctionalized phenanthrenes (e.g., 8-11).The reactions involve triplet excited states, as demonstrated by acetone sensitization when the C-8 substituent was Cl or Br and by quenching studies when the C-8 was HgOAc.When C-8 is unsubstituted, dihydrophenanthrenes, rather than phenanthrenes, are formed.The products of the latter reaction are presumably formed from a carbene intermediate resulting via a frustrateddi-?-methane rearrangement.On the other hand, the formation of the phenanthrene products is consistent with a pathway involving intramolecular electron transfer from the excited aromatic ring to the carbon-nucleofuge bond to form a zwitterionic biradical, loss of nucleofuge, and a convoluted rearrangement of the resulting biradical cation.