Reactivity of Singlet and Triplet Arylnitrenes: Temperature-Dependent Photodecomposition of 1-(2-Azidophenyl)-3,5-dimethylpyrazole

At >200 K photolysis of 1-(2-azidophenyl)-3,5-dimethylpyrazole ( 5 ) gives 1,3-dimethylpyrazolobenzotriazole ( 6, by electrophilic cyclization of singlet nitrene 1 4 ) or, in the presence of diethylamine, aminoazepine 8 (by addition of the nucleophile and rearrangement). At lower temperatures, the yield of these products decreases and the azo derivative 9 (from the dimerization of triplet nitrene 3 4 ) as well as products from intramolecular radical cyclization (again via 3 4 ) is obtained, to become the only products at <100 K. Differential thermodynamic parameters for the reactions of 1 4 and 3 4 are determined by analysis of the temperature dependence of products distribution (ΔΔ H ⧧ = −10 kJ mol -1 and ΔΔ S ⧧ = 34 J mol -1 K -1 in ethanol). Addition of the amine may require previous ring enlargement to give the dehydroazepine 12; there is no indication that this is an intermediate of any stability, however, and if formed, this is in equilibrium with 1 4 . Triplet nitrene 3 4 is characterized in matrix by UV spectroscopy, and its photoreactions (to give mainly intramolecular hydrogen abstraction) are separately studied.