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.