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.