Electron-Deficient Strong Bases. Generation of the 4-Biphenylyl- and 2-Fluorenylnitrenium Ions by Nitrene Protonation in Water

摘要

Aryl azide irradiation results in a singlet arylnitrene, an unstable species that rapidly reacts by several routes, the principal ones being ring expansion to a didehydroazepine and intersystem crossing to a triplet. A nitrene is also the conjugate base of a nitrenium ion, and this raises the possibility of trapping this species by protonation as a route for effecting photochemical entry into these cations. The paucity of photochemical reactions that produce nitrenium ions is a principal reason that these ions have seen limited study using flash photolysis methods. Knowledge of the reactivity of nitrenium ions is important since these electrophiles are proposed to be the DNA-binding intermediates responsible for the carcinogenicity of aromatic amines such as 4-aminobiphenyl and 2-aminofluorene. We have recently reported lifetimes of the N-acetyl analogs of the nitrenium ions derived from these two amines, the ions being generated in aqueous solution by photoheterolysis of precursors with various leaving groups. However, the high solvolytic reactivity of the derivatives lacking the N-acetyl group precludes their use, and we therefore turned to the azides as possible precursors. To our surprise, these produce excellent yields of nitrenium ions without the need for added acids, that is, with water as proton donor. These experiments place an upper limit on the aqueous acidity constants for N-H deprotonation of the nitrenium ions, showing that these are very weak acids. In analogy with nucleophilic carbenes, the 4-biphenyl- and 2-fluorenylnitrenes are remarkably strong bases despite their formal electron-deficient character.