N-Phenyl-1,2,4-triazoline-3,5-dione (PTAD) as a Dienophilic Dinitrogen Equivalent: A Simple Synthesis of 3-Amino-1,2,4-benzotriazines from Arylcarbodiimides

N-Arylcarbodiimides react with PTAD to provide [1,2,4]triazolo[1,2-a][1,2,4]benzotriazines by a [4+2] cycloaddition reaction. When asymmetrically substituted diarylcarbodiimides are used, the cycloaddition proceeds with total chemoselectivity because only the more electron-rich aryl nucleus is involved. This observation has been rationalized by a computational study using DFT methods that shows that, in the reaction of HTAD with arylcarbodiimides, the magnitude of the energy barriers depend on the electronic features of the substituents at the aryl nucleus; the calculations also indicate that the reaction proceeds through asynchronous states with polar characteristics. The treatment of the final cycloadducts with potassium hydroxide affords 3-aryl(alkyl)amino-1,2,4-benzotriazines.