Electron-Transfer Mechanism for Aromatic Nitration via the Photoactivation of EDA Complexes. Direct Relationship to Electrophilic Aromatic Substitution

Charge-transfer nitration of various aromatic hydrocarbons (ArH) is readily achieved by the deliberate photoactivation of their electron donor-acceptor (EDA) complexes with N-nitropyridinium (PyNO₂⁺). Time-resolved spectroscopy unambiguously identifies (ArH·⁺, NO₂, Py) as the reactive triad resulting directly from the charge- transfer activation of the [ArH,PyNO₂⁺] complex; and the subsequent homolytic annihilation of the aromatic cation radical with NO₂ leads to the Wheland intermediate pertinent to aromatic nitration. Charge-transfer nitration of such aromatic donors as toluene, anisole, mesitylene, and tert-butylbenzene as well as the polymethylbenzenes and substituted anisoles forms the basis for detailed comparisons with the products from electrophilic aromatic nitration, especially with regard to the unique isomer distributions, nuclear versus side-chain nitrations, and ipso adducts in nonconventional nitrations. Mechanistic implications are discussed in terms of the electron-transfer activation of electrophilic aromatic nitration.