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) gi ves 1,3-dimethylpyrazolobenzotriazole (6, by electrophilic cyclization of singlet nitrene (1)4) or, in the presence of diethylamine, aminoaz epine 8 (by addition of the nucleophile and rearrangement). At lower t emperatures, the yield of these products decreases and the azo derivat ive 9 (from the dimerization of triplet nitrene (3)4) as well as produ cts from intramolecular radical cyclization (again via (3)4) is obtain ed, to become the only products at < 100 K. Differential thermodynamic parameters for the reactions of (1)4 and 34 are determined by analysi s 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 enl argement to give the dehydroazepine 12; there is no indication that th is is an intermediate of any stability, however, and if formed, this i s in equilibrium with (1)4. Triplet nitrene (3)4 is characterized in m atrix by UV spectroscopy, and its photoreactions (to give mainly intra molecular hydrogen abstraction) are separately studied.