Direct Observation of the Wheland Intermediate in Electrophilic Aromatic Substitution. Reversible Formation of Nitrosoarenium Cations

The Wheland intermediate in electrophilic aromatic nitrosation, viz. the nitrosoarenium σ-complex, is directly observed by transient absorption spectroscopy. Femtosecond time-resolved laser experiments based on charge-transfer photoexcitation of electron donor/acceptor (EDA) complexes of nitrosonium cation with various arenes reveal the ultrafast formation of nitrosobenzenium to occur in less than 10 ps via the radical/radical coupling of arene cation radicals and nitric oxide. The lifetimes of the σ-complexes in dichloromethane solution are strongly temperature dependent varying from nanoseconds ( T = 298 K) to microseconds ( T = 195 K). Steady-state photolysis of arene/NO + complexes in n -BuCl glasses at T = 77 K leads to nitrosoarenium σ-complexes which persist for several hours. Based on a reaction scheme that includes an ultrafast equilibrium between the [ArH + •,NO • ] radical pair and the nitrosoarenium σ-complex, energy diagrams are constructed which establish the highly endergonic reaction profile of electrophilic aromatic nitrosations with the arene/nitrosonium π-complex as the thermodynamic sink.