Synthesis of 1,2,4-Triazinones Exploiting the Reactivity of Diazadiene and N-Isocyanate Intermediates

1,2,4-Triazinones are useful compounds, but their synthesis can be challenging. Herein, we report a strategy to build 1,2,4-triazinones using α-bromohydrazones to access diazadienes and exploiting their ability to undergo facile substitution with nitrogen nucleophiles. The N-isocyanate intermediate formed in situ can then undergo cyclization to give the desired triazinones. This provides access to products with various substituents at the 4-position, and with suitable hydrazone precursors (R² = Ph), the cascade reaction yields 1,2,4-triazin-3(2H)-ones at room temperature.