Sm. Hubig et Jk. Kochi, Direct observation of the wheland intermediate in electrophilic aromatic substitution. Reversible formation of nitrosoarenium cations, J AM CHEM S, 122(34), 2000, pp. 8279-8288
The Wheland intermediate in electrophilic aromatic nitrosation, viz. the ni
trosoarenium sigma-complex, is directly observed by transient absorption sp
ectroscopy. Femtosecond time-resolved laser experiments based on charge-tra
nsfer photoexcitation of electron donor/acceptor (EDA) complexes of nitroso
nium cation with various arenes reveal the ultrafast formation of nitrosobe
nzenium to occur in less than 10 ps via the radical/radical coupling of are
ne cation radicals and nitric oxide. The lifetimes of the sigma-complexes i
n dichloromethane solution are strongly temperature dependent-varying from
nanoseconds (T = 298 K) to microseconds (T = 195 K). Steady-state photolysi
s of arene/NO+ complexes in n-BuCl glasses at T= 77 K leads to nitrosoareni
um sigma-complexes which persist for several hours. Based on a reaction sch
eme 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 aro
matic nitrosations with the arene/nitrosonium pi-complex as the thermodynam
ic sink.