Heteroarylcarbene–Arylnitrene Radical Cation Isomerizations

Abstract

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 N 2 generate the pyridinium diazomethyl radical 22 •+ or pyridinium-2-carbyne 23 •+ . These two ions can isomerize to 2-pyridyldiazomethane 24 •+ and 2pyridylcarbene 26 •+ , the latter rearranging to the phenylnitrene radical cations 9a •+ . 13 C-labeling of the tetrazole rings confirmed that 2-(5-tetrazolyl)pyridine 12a generates 2-pyridylcarbene/phenylnitrene radical cations retaining the 13 C label, but 4-(5tetrazolyl)pyridine 12c generates 4-pyridylnitrene 18c •+ , which has lost the 13 C 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, 4pyridylnitrene radical cation 18c •+ or its isomers (m/z 92) is obtained from 1,2,3-triazolo[1,5-a]pyrazine 36. Several other αheteroaryltetrazoles behave in the same way as 2-(5-tetrazolyl)pyridine, yielding heteroarylcarbene/arylnitrene radical cations in the mass spectrometer, and this was confirmed by 13 C-labeling in the case of 1-(5-tetrazolyl)isoquinoline 42-13 C. In general, 5aryltetrazoles generate arylnitrene radical cations under electron impact, but α-heteroaryltetrazoles generate α-heteroarylcarbene radical cations.