Nitrene–Nitrene Rearrangement under Thermal, Photochemical, and Electron‐Impact Conditions: The 2‐Azidopyridines/Tetrazolo[1,5‐ a ]pyridines

Heteroarylcarbene-Arylnitrene Radical Cation Isomerizations

5-Phenyltetrazole 1e is an important source of phenylnitrene or the phenylnitrene radical cation ( m/ z 91) under thermal, photochemical, and electron impact conditions. Similarly, 3- or 4-(5-tetrazolyl)pyridines 12b,c yield pyridylnitrene radical cations 9a^•+ ( m/ z 92) upon electron impact. In contrast, 2-(5-tetrazolyl)pyridine 12a^•+ generates 2-pyridyldiazomethane 24^•+ and 2-pyridylcarbene 26^•+ radical cations ( m/ z 119 and 91) upon electron impact. The 2-pyridylcarbene radical cation undergoes a carbene-nitrene rearrangement to yield the phenylnitrene radical cation. Calculations at the B3LYP/6-311G(d,p) level have revealed facile H-transfer from the tetrazole to the pyridine ring in 2-(5-tetrazolyl)pyridine, 12a^•+ → 21^•+, taking place in the radical cations. Subsequent losses of N₂ generate the pyridinium diazomethyl radical 22^•+ or pyridinium-2-carbyne 23^•+. These two ions can isomerize to 2-pyridyldiazomethane 24^•+ and 2-pyridylcarbene 26^•+, the latter rearranging to the phenylnitrene radical cations 9a^•+. ¹³C-labeling of the tetrazole rings confirmed that 2-(5-tetrazolyl)pyridine 12a generates 2-pyridylcarbene/phenylnitrene radical cations retaining the ¹³C label, but 4-(5-tetrazolyl)pyridine 12c generates 4-pyridylnitrene 18c^•+, which has lost the ¹³C label. 2-Pyridylcarbene/phenylnitrene radical cations ( m/ z 91) also constitute the base peak in the mass spectrum of 1,2,3-triazolo[1,5- a]pyridine 34. Similarly, 4-pyridylnitrene radical cation 18c^•+ or its isomers ( m/ z 92) is obtained from 1,2,3-triazolo[1,5- a]pyrazine 36. Several other α-heteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations in the mass spectrometer, and this was confirmed by ¹³C-labeling in the case of 1-(5-tetrazolyl)isoquinoline 42-¹³C. In general, 5-aryltetrazoles generate arylnitrene radical cations under electron impact, but α-heteroaryltetrazoles generate α-heteroarylcarbene radical cations.

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^•+.

Phenylnitrene Radical Cation Rearrangements

The electronic structure and the rearrangements of the phenylnitrene radical cation C₆H₅N^.+ 2^.+ have been investigated at DFT and CASPT2(7,9) levels of theory. The ²B₂ state has the lowest energy of five identified electronic states, and it can undergo ring expansion to the 1-azacycloheptetetraene radical cation 4^.+ with an activation energy of ca. 28 kcal/mol. Ring opening and recyclization provide a route to 5-cyanocyclopentadiene radical cation 8^.+, which may undergo facile 1,5-hydrogen shifts. The 2-, 3-, and 4-pyridylcarbene radical cations 31^.+, 35^.+ , and 39^.+ interconvert with the phenylnitrene radical cation via azacycloheptatetraenes with activation barriers <35 kcal/mol. The carbene-carbene and carbene-nitrene rearrangements, ring expansions, ring contractions, ring openings (e.g., to cyanopentadienylidene 28^.+), and cyclizations taking place in all these radical cations are completely analogous to the thermal and photochemical rearrangements.