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, CF 3, OMe, NMe 2 ) phenyl azides bearing a dimethylpyrazolyl group in position 2 allowing intramolecular trapping of singlet nitrene have been photolyzed at both 295 and 90 K in ethanol. For three significant models (H, CF 3, NMe 2 ), the reaction has been further studied in the presence of diethylamine (DEA) and of oxygen. With all substituents but NMe 2, singlet nitrene (trapped intramolecularly to give pyrazolobenzotriazoles) and didehydroazepine (trapped with DEA to give 5 H -azepines and then rearranging to 3 H- azepines) are in equilibrium. With the NMe 2 derivative, the nonelectrophilic singlet is not trapped, while DEA adds to the benzoazirine, the precursor of the didehydroazepine. Thus, electronic effects do not hinder the equilibrium between singlet nitrene and its cyclic isomers, while determining which of the above intermediates decays to a stable end product. The electron-donating group NMe 2 has a second important effect, causing a drastic enhancement of the triplet nitrene energy and reduction of the S−T gap, so that triplet nitrene is also in equilibrium with the singlet and the benzoazirine. As for triplet nitrenes, these have been characterized in matrix at 90 K, and the competition between dimerization (to give azo compounds, as typical of such stabilized species) and hydrogen abstraction from the solvent (involving a sizable barrier) has been studied. The energetic p -dimethylamino triplet undergoes hydrogen abstraction exclusively. 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 excitation of the triplet in matrix leads to intramolecular hydrogen abstraction.