Synthesis and thermal reactions of 1,2-dihydro-1,2,4-benzotriazines

Abstract

The 1,2-dimethyl-1,2,4-benzotriazines (7), (8), and (9) were prepared by acid-catalysed cyclisation of the 2-aminophenylhydrazine derivatives derived from (12), (13), and (14) respectively. Compounds (7) and (9) undergo a thermal elimination to fully aromatic benzotriazines. However, the 2-acyl derivative (18) rearranges to the thermodynamically more stable 4-acyl compound (21). In the presence of traces of water, a ring contraction to benzimidazoles is a competing reaction. For (18) the nitrogen fragment is retained in the benzamide (22). A mechanism for the ring contraction has been suggested in which initial hydration of the imine bond gives 1,2,3,4-tetrahydrobenzotriazines which ring-open, then re-cyclise to benzimidazoles. The benzimidazo[2,1-a]phthalazine (23) was shown not to be an intermediate in the water-mediated ring contraction of (18).