Metal-Free Oxidative Annulation toward an Ultrathermostable Nitrogen-Rich Fused Azaarene

The development of nitrogen-rich fused azaarenes with exceptional thermal stability remains a critical challenge because the incorporation of high-energy N─N and N═N linkages typically compromises molecular stability. Herein, we report a metal-free oxidative annulation strategy for the construction of an ultrathermostable pyrazole-fused azaarene (TYX-40) through concurrent N═N and C─C bond formation from a simple aminopyrazole precursor. Using hydrogen peroxide as a green oxidant under mild conditions, this transformation enables direct self-coupling of 3,5-diamino-1H-pyrazole to furnish a highly conjugated [5,6,5]-fused nitrogen-rich framework (TYX-40) without transition-metal catalysis. Mechanistic investigations, including radical-trapping experiments and density functional theory calculations, support a radical-mediated sequential annulation pathway involving oxidative azo formation followed by intramolecular C─C coupling. Benefiting from its highly planar and symmetric fused-ring architecture, TYX-40 exhibits exceptional thermal stability with a decomposition temperature of 414°C, among the highest values reported for nitrogen-rich fused heterocycles. Computational analyses reveal that enhanced aromatic delocalization and uniform electronic distribution play critical roles in stabilizing the fused azaarene framework.