Recent Advances in Mechanistic Studies of the Aromatic C-H Bond Substitution and Related Chemistry - A Mini-Review

The electrophilic C-H bond substitution of arenes (ArH) by the linear nitronium (ONO+) ion or nitrating agents (Y-NO2), giving nitroarenes (ArNO2), has been of interest in organic chemistry for many years. The reactions were found to take place via a nitroarenium ion [Ar(H)NO2]+ (σ-complex). Research in the recent twenty years has revealed that two reactive intermediates, a charge-transfer complex (π-complex) [ArH, NO2]+ or [ArH, YNO2]+ and an ion-radical pair [ArH+. , NO2•], are produced consecutively in the aromatic nitration prior to formation of the σ-complex [Ar(H)NO2]+. The full mechanism for this classical type of reactions has been established. Electrophilic aromatic substitution reactions of various structurally related electrophiles, such as dithionitronium NS2+ and zwitterionic Cl2SO+--AlCl3 adduct, have been discovered and studied most recently. The related nucleophilic aromatic substitution has been further studied recently. Effective biological applications of several types of functionalized aromatic compounds have been developed in the recent decade.