A Photo-Smiles Rearrangement: Mechanistic Investigation of the Formation of Blatter Radical Helicenes

Photocyclization of 8-aryloxy-3-phenylbenzo[e][1,2,4]triazines
leads to helicene radicals. Structural analysis of radicals leuco forms by two-dimensional correlation nuclear magnetic
resonance methods demonstrated that the photocyclization involves
a Smiles rearrangement and exclusive formation of a single rearranged
product for all substrates. Density functional theory investigations
indicate that the mechanism requires the T1 state with
(n, π*) character localized on the benzo[e][1,2,4]triazine
(BT) fragment and at least one occupied π molecular orbital
(MO) localized on the aryloxy fragment with an energy that is higher
than that of the n MO. This electronic structure is favorable for
aryl-to-BT single-electron transfer and formation of a zwitterion,
which undergoes an intramolecular polar cyclization followed by ring
opening of the resulting spirooxazole. The proposed mechanism represents
a new variation of photo-Smiles rearrangement and appears to be general
for the photochemical formation of planar Blatter radicals.