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 sigma-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 sigma-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 sigma-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 sigma-complex, energy diagrams are constructed which establish the highly endergonic reaction profile of electrophilic aromatic nitrosations with the arene/nitrosonium pi-complex as the thermodynamic sink.