Ring-Fused 1,4-Dihydro[1,2,4]triazin-4-yls through Photocyclization

Halogen lamp irradiation of benzo­[e]­[1,2,4]­triazines 2­[X] in CH2Cl2 solutions leads to planar ring-fused 1,4-dihydro­[1,2,4]­triazin-4-yl radicals 1 through a novel, potentially general, cyclization mechanism. The scope and efficiency of the method were established for unsubstituted and ortho-substituted (X = NH2, Br, NO2) phenoxy, naphthyloxy, and quinolinoxy derivatives 2­[X]. The regioselectivity of 2­[X] photocyclization was rationalized with DFT computational methods. Radicals 1 were characterized by spectroscopic (UV–vis, EPR), electrochemical, and XRD methods.