C. Carra,†§ T. Bally,†* and A. Albini‡*

†Department of Chemistry, University of Fribourg, CH-1700 Fribourg,
‡ Switzerland, and Department of Chemistry, University of Pavia, I-27100 Pavia, Italy.

Abstract: The chemistry of 2-(1-pyrazolyl)- (2a) and 2-[1-(3,5-dimethylpyrazolyl]phenylnitrene (2b)
has been studied in EtOH solution at room temperature, in EtOH glasses at 90 K and in Ar
matrices at 12 K. These nitrenes were chosen as suitable models for clarifying the mechanism
of intramolecular reactions because attack at the pyrazole ring could occur according to different
modes and the asymmetry of the substituent gave rise to different conformations.
Detailed DFT and CASSCF/CASPT2 studies on the conformation and decay paths of both
spin states of the nitrenes have been carried out. Ring expansion to dehydroazepines (via
benzoazirines) is calculated to be competitive in both nitrenes, but in the dimethyl derivative,
2b, attack onto the N lone pair (which us made more nucleophilic by the methyl groups) is
favored. The higher barriers (by 4-8 kcal/mol) in singlet 2a make that 60-70% of this nitrene
decay by intersystem crossing to the triplet. Thus, the seemingly straightforward formation of
benzo-fused heterocycles through intramolecular attack of the pyrazoline N lone pair by the
singlet phenylnitrene can only win against ring expansion and intermolecular reactions under
favorable circumstances. The comparatively persistent triplet nitrenes are characterized in
matrices, and the yields of photocyclization products (mainly pyrazolo[1,5-a] benzimidazole
(7) from 2a and 5,6-dihydropyrazolo[1,5-a]quinoxaline (8) from 2b) are shown to depend on
the preferred conformation of the starting azide and nitrene.