AbstractIminodiaziridines are synthesized by (i) 1,3‐dehydrochlorination with potassiumtert‐butoxide ofN‐chloroguanidines, generated in situ fromN,N′,N″‐substituted guanidines withtert‐butyl hypochlorite, and (ii) base‐mediated 1,3‐elimination of sulfuric acid fromN,N′,N″‐substituted hydroxyguanidineO‐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‐hydroxyguanidineO‐sulfonic acids furnish (N‐arylimino)diaziridines, but no 1‐aryl‐3‐iminodiaziridines, instead giving rearranged isomers. Precursors containing perdeuteratedtert‐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 theortho‐aryl positions are substituted, high yields of (arylimino)diaziridines are obtained, along with 2‐imino‐2,3‐dihydro‐3aH‐benzimidazoles. Otherwise, 2‐amino‐1H‐benzimidazoles and strongly fluorescent 3‐amino‐2H‐indazoles, originating from rearrangements of the elusive 1‐aryl‐3‐iminodiaziridines, predominate.N′,N″‐Diaryl‐N‐hydroxyguanidineO‐sulfonic acids give only rearranged products: a 2‐amino‐1H‐benzimidazole and a 6‐amino‐5H‐dibenzo[d,f][1.3]diazepine if aryl = phenyl, or a 2‐imino‐2,3‐dihydro‐3aH‐benzimidazole if aryl = mesityl. 3aH‐Benzimidazoles slowly dimerize through Diels–Alder reactions.15N NMR signals were assigned to thesynandantiring 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)