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 34 are determined by analysis of the temperature dependence of products distribution (Delta Delta H-not equal = -10 kJ mol(-1) and Delta Delta S-not equal = 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.