Syntheses and 15 N NMR Spectra of Iminodiaziridines – Ring‐Expansions of 1‐Aryl‐3‐iminodiaziridines to 1 H ‐ and 3a H ‐Benzimidazoles, 2 H ‐Indazoles, and 5 H ‐Dibenzo[ d , f ][1,3]diazepines

Iminodiaziridines are synthesized by (i) 1,3‐dehydrochlorination with potassium tert ‐butoxide of N ‐chloroguanidines, generated in situ from N , N′ , N″ ‐substituted guanidines with tert ‐butyl hypochlorite, and (ii) base‐mediated 1,3‐elimination of sulfuric acid from N , N′ , N″ ‐substituted hydroxyguanidine O ‐sulfonic acids. At elevated temperatures, (alkylimino)diaziridines undergo valence isomerization by 1,3‐shift, [2+1] cycloelimination to afford isocyanides and diazenes, and ring‐opening elimination to yield alkylideneguanidines. N′ ‐Aryl‐ N ‐hydroxyguanidine O ‐sulfonic acids furnish ( N ‐arylimino)diaziridines, but no 1‐aryl‐3‐iminodiaziridines, instead giving rearranged isomers. Precursors containing perdeuterated tert ‐butyl groups give rearranged products that show complete scrambling. This indicates that 1‐aryl‐3‐iminodiaziridines are intermediates that undergo very rapid degenerate valence isomerization. Provided that the ortho ‐aryl positions are substituted, high yields of (arylimino)diaziridines are obtained, along with 2‐imino‐2,3‐dihydro‐3a H ‐benzimidazoles. Otherwise, 2‐amino‐1 H ‐benzimidazoles and strongly fluorescent 3‐amino‐2 H ‐indazoles, originating from rearrangements of the elusive 1‐aryl‐3‐iminodiaziridines, predominate. N′ , N″ ‐Diaryl‐ N ‐hydroxyguanidine O ‐sulfonic acids give only rearranged products: a 2‐amino‐1 H ‐benzimidazole and a 6‐amino‐5 H ‐dibenzo[ d , f ][1.3]diazepine if aryl = phenyl, or a 2‐imino‐2,3‐dihydro‐3a H ‐benzimidazole if aryl = mesityl. 3a H ‐Benzimidazoles slowly dimerize through Diels–Alder reactions. 15 N NMR signals were assigned to the syn and anti ring nitrogen atoms of iminodiaziridines with the help of a combination of homonuclear NOE and HN‐HMBC or HN‐gHMBC experiments. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)