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

Abstract

Phenylnitrene radical cations m/z 91, C 6 H 5 N, 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 1H-and 2H-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-1H-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 C 7 H 6 N 2•+ , including 19 •+ , 24 •+ , the benzimidazole ion 38 •+ and o-aminobenzonitrile ion 40 •+ , all of which can eliminate either HCN or HNC to yield the C 6 H 5 N •+ 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 •+ .

Formation of (Aza)fulvenallene, Cyanocyclopentadiene, and (Aza)fluorenes in the Thermal Rearrangements of Indazoles, Azaindazoles, and Homoquinolinic Anhydride

“…The triplet states of nitrilium betaines are of much higher energies 21 and will not, therefore, be considered here. Due to the high degree of double-bond character, the cleavage of the N−N bond 22 does not occur under thermal conditions. A bent, carbenic, or dipolar form 40b is not an energy minimum but a point on the IRC curve for the cyclization to the 3aH-indazole 41 (Scheme 4 and Figure S2, Supporting Information).…”

Substituent Effects on EI-MS Fragmentation Patterns of 5-Allyloxy-1-aryl-tetrazoles and 4-Allyl-1-aryl-tetrazole-5-ones; Correlation with UV-Induced Fragmentation Channels

“…In mass spectrometry, cyclic aromatic species carrying a single charged group attached to them, such as toluene and other benzyl analogues, frequently undergo a ring expansion process leading to more charge-stabilising seven-membered ring systems [33]. Previous EI-MS works on phenyl azide and analogues [34], including recent computational studies on tetrazole derivatives, at DFT and CASPT2 (7,9) levels of theory [35,36], aimed at determining if the same could be observed for aryl nitrenes, corroborated this proposal for phenyl nitrene (m/z 91) and other substituted analogues. Hence, it appears reasonable to consider that the same process takes place for the product at m/z 91 and its substituted analogues, the phenyl nitrene radical cation being subjected to a ring expansion process to produce a more stable chemical entity, a 1-aza-1,2,4,6-cycloheptatetraene (ACHT) derivative.…”