Thermal Ring-Opening of Pyrazolo[3,4-d][1,2,3]triazin-4-ones: An Experimental and Theoretical Study

Several 3-pyrazolylcarbonyl-pyrazolo[3,4-d][1,2,3]triazin-4-ones have been prepared from 5-amino-1H-pyrazole-4-carbonitriles through a simple sequence. In the first step, diazotization of the corresponding aminopyrazoles afforded pyrazolo[3,4-d][1,2,3]triazin-4-ones. Next, thermal rearrangement of these compounds through nitrogen elimination gave the final products. The proposed mechanism for the ring-opening of the pyrazolotriazinones to give the pyrazolylcarbonyl-pyrazolotriazinones involves the generation of an iminoketene intermediate, which reacts with a second molecule of pyrazolotriazinone. The complete mechanism of product formation involving the iminoketene intermediate, and all other reasonable pathways, have been explored in detail through DFT calculations. Furthermore, additional experiments to corroborate the presence of the iminoketene intermediate were carried out.