Phenylnitrene Radical Cation and Its Isomers from Tetrazoles, Nitrile Imines, Indazole, and Benzimidazole

Phenylnitrene radical cations m/ z 91, C₆H₅N, 8a^•+ are observed in the mass spectra of 1-, 2-, and 5-phenyltetrazoles, even though no C-N bond is present in 5-phenyltetrazole. Calculations at the B3LYP/6-311G(d,p) level of theory indicate that initial formation of the C-phenylimidoylnitrene 13^•+ and/or benzonitrile imine radical cation 19^•+ from 1 H- and 2 H-5-phenyltetrazoles 11 and 12 is followed by isomerizations of 13^•+ to the phenylcyanamide ion 15^•+ over a low barrier. A cyclization of imidoylnitrene ion 13^•+ onto the benzene ring offers alternate, very facile routes to the phenylnitrene ion 8a^•+ and the phenylcarbodiimide ion 14^•+ via the azabicyclooctadienimine 16^•+. Eliminations of HNC or HCN from 14^•+ and 15^•+ again yield the phenylnitrene radical cation 8a^•+. A direct 1,3-H shift isomerizing phenylcarbodiimide ion 14^•+ to the phenylcyanamide ion 15^•+ requires a very high activation energy of 114 kcal/mol, and this reaction needs not be involved. The benzonitrile imine -3-phenyl-1 H-diazirine-phenylimidoylnitrene-phenylcarbodiimide/phenylcyanamide rearrangement has parallels in thermal and photochemical processes, but the facile cyclization of imidoylnitrene 13^•+ to azabicyclooctadienimine 16^•+ is facilitated by the positive charge making the nitrene more electrophilic. Furthermore, the benzonitrile imine radical cation 19^•+ can cyclize to indazole 24^•+, and a series of intramolecular rearrangements via hydrogen shifts, ring-openings and ring closures allow the interconversion of numerous ions of composition C₇H₆N₂^•+, including 19^•+, 24^•+, the benzimidazole ion 38^•+ and o-aminobenzonitrile ion 40^•+, all of which can eliminate either HCN or HNC to yield the C₆H₅N^•+ ions of phenylnitrene, 8a^•+, and/or iminocyclohexadienylidene, 34^•+. Moreover, benzonitrile imine 19^•+ can behave like a benzylic carbenium ion, undergoing a novel ring expansion to cycloheptatetraenyldiazene 45^•+. The N-phenylnitrile imine ion 2d^•+ derived from 2-phenyltetrazole 1d cleaves efficiently to the phenylnitrene ion 8a^•+ but may also cyclize to the indazole ion 24^•+. The N-phenylimidoylnitrene 59^•+ derived from 1-phenyltetrazole 5d undergoes facile isomerization to the phenylcyanamide ion 15^•+ and hence phenylnitrene radical cation 8a^•+.