A One-Pot Cyclization Reaction of Tetrazole with Methylene to Construct a Fully Modified Pyrazolo[3,4- d ]pyridazine-Fused Ring

Generally, a fused-ring skeleton exhibits high stability due to its extensive conjugated system, which results in reduced reactivity at modification sites, making complete modification difficult and thereby affecting the overall performance of energetic materials. This work achieved complete modification of the fused-ring skeleton by first introducing energetic functional groups, followed by one-pot fused-ring-closing reactions. A series of pyrazolo[3,4-d]pyridazine derivatives with high energy and low sensitivity were synthesized through the cyclization of tetrazoles with methylene under alkaline conditions. A possible reaction mechanism, including ring-opening reaction, electron transfer, cyclization, substitution, and salt formation, was proposed. All target pyrazolo[3,4-d]pyridazine fused-ring compounds (4–6) possess higher energy (D > 8088 m s–1) comparable to that of traditional high-energy explosive TNT. Furthermore, the target compounds exhibit good thermal stabilities (Tdec > 219 °C) and low sensitivities (IS > 40 J). The excellent comprehensive performance suggests that pyrazolo[3,4-d]pyridazine-fused-ring compounds have the potential to become new-generation high-energy, low-sensitivity explosives.