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

Phenylnitrene radical cations m/ z 91, CHN, 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 CHN, including 19, 24, the benzimidazole ion 38 and o-aminobenzonitrile ion 40, all of which can eliminate either HCN or HNC to yield the CHN 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.