Competitive cyclisations of singlet and triplet nitrenes. Part 8. The 1-(2-nitrenophenyl)pyrazoles and related systems

Abstract

The title nitrenes, derived by nitro-group deoxygenation with triethyl phosphite or by thermolysis or photolysis of the corresponding azide, have been studied. The effect of substituents (Cl, Br, OMe, NMe₂, Me, CF₃, and NO₂) both meta and para to the nitrene has been examined as a determinant of the preferred mode of cyclisation to either a pyrazolobenzotriazole or a pyrazoloquinoxaline. Similarly, the role of solvents, sensitisers, and quenchers has been studied. Routes to the isomeric 1- and 2-(2-nitrophenyl)-4,5,6,7-tetrahydroindazoles have been defined and the literature corrected by studying the nitrene-mediated cyclisation of these products. The chemistry of the analogous 1-(2-carbenophenyl)- and the 1-(2-nitrenosulphonylphenyl)-3,5-dimethylpyrazoles has also been examined, the former giving 2-(3,5-dimethylpyrazol-1-yl)-benzaldazine and -benzyl alcohol while the latter gave products of intramolecular nitrene attack (a pyrazolobenzothiatriazine) and intermolecular reaction. Rationalisations for all the reaction pathways have been advanced.