Photodecomposition of some para-substituted 2-pyrazolylphenyl azides. Substituents affect the phenylnitrene S-T gap more than the barrier to ring expansion

A series of para-substituted (H, Me, Cl, F, CF3, OMe, NMe2) phenyl azides b earing a dimethylpyrazolyl group in position 2 allowing intramolecular trap ping of singlet nitrene have been photolyzed at both 295 and 90 K in ethano l. For three significant models (H, CF3, NMe2), the reaction has been furth er studied in the presence of diethylamine (DEA) and of oxygen. With all su bstituents but NMe2, singlet nitrene (trapped intramolecularly to give pyra zolobenzotriazoles) and didehydroazepine (trapped with DEA to give 5H-azepi nes and then rearranging to 3H-azepines) are in equilibrium. With the NMe2 derivative, the nonelectrophilic singlet is not trapped, while DEA adds to the benzoazirine, the precursor of the didehydroazepine. Thus, electronic e ffects do not hinder the equilibrium between singlet nitrene and its cyclic isomers, while determining which of the above intermediates decays to a st able end product. The electron-donating group NMe2 has a second important e ffect, causing a drastic enhancement of the triplet nitrene energy and redu ction of the S-T gap, so that triplet nitrene is also in equilibrium with t he singlet and the benzoazirine. As for triplet nitrenes,these have been ch aracterized in matrix at 90 K, and the competition between dimerization (to give;azo compounds, as typical of such stabilized species) and hydrogen ab straction from the solvent (involving a sizable barrier) has been studied. The energetic p-dimethylamino triplet undergoes hydrogen abstraction exclus ively. When present, oxygen adds efficiently to all of the nitrenes, giving a nitroso oxide, Likewise characterized in the matrix, which then converts to the nitroso and nitro,derivatives in good yields. Photochemical excitat ion of the triplet in matrix leads to intramolecular hydrogen abstraction.