Synthesis of 2‐Aryl‐2H‐1,2,3‐triazoles via P(OMe)3‐Promoted Intramolecular Transannulation of 4‐Diazenylisoxazol‐5(4H)‐ones

Trimethyl phosphite promotes the synthesis of 2,4,5‐trisubstituted 2H‐1,2,3‐triazoles. The protocol developed affords a two‐step method towards triazoles from available reagents. 3,4‐Disubstituted isoxazolones are initially coupled with diazonium tetrafluoroborates. The subsequent reaction is triggered by a single electron transfer from the phosphite to the diazenylisoxazolone under heating or blue LED irradiation at room temperature. The radical anion cascade proceeds via a ring opening/decarboxylation/cyclization sequence. The synthesized triazoles show promising fluorescence in the region from 340 nm to 450 nm with a quantum yield up to 81.6%. The emission energy depends slightly on the substituent nature at the triazole carbon atoms. If an electron‐donor group is introduced into the N2 substituent of the triazole ring, it results in a bathochromic shift of the luminescence energy.