Unanticipated formation of a novel octaazacyclodecane ring upon oxidation of a 1,1-bis-urazole

Tetrahydrotetrazoles are a little-explored class of five-membered heterocycles with four contiguous singly-bonded N atoms. Recent work in our labs has demonstrated that urazole radicals are amenable to N-N bond formation via radical combination to form such a chain of four N atoms. Previously described 1,1-bis-urazole compounds appeared to be convenient precursors to the target tetrazoles via their oxidation to intermediate urazole diradicals, which upon N-N bond formation would complete the tetrazole framework. While oxidation proceeded smoothly, the novel 10-membered octaaza heterocycle 7,7,18,18-tetraacetyl-4,10,15,21-tetraphenyl-1,2,4,6,8,10,12,13,15,17,19,21-dodecaazapentacyclo[17.3.0.0^2,6.0^8,12.0^13,17]docosan-3,5,9,11,14,16,20,22-octone, C₄₂H₃₂N₁₂O₁₂, was obtained (36% yield) instead of the expected tetrazole product, as confirmed by X-ray crystallography. Calculations at the (U)B3LYP/6-311G(d,p) level of theory suggest that the desired tetrazoles have weak N-N bonds connecting the two urazole units.