benzenes: Model Compounds for the Electrophilic Substitution and Oxidation of Aromatic Compounds

Anionic B complexes have been known and extensively studied as intermediates of nucleophilic aromatic substitution since the beginning of the century.l In a paper that still makes fascinating reading, Meisenheimer describes the formation and isolation of anionic B complexes by reaction of picryl ethers with potassium alkoxides. Structural proof is derived from the fact'that identical u complexes are obtained from methyl 2,4,6trinitrophenyl ether with potassium ethoxide, and from ethyl 2,4,64rinitrophenyl ether with potassium methoxide.2 Cationic u complexes (Wheland intermediates), on the other hand, had been characterized or isolated only in a few cases: despite the ubiquitous implication of electrophilic aromatic substitution in synthesis. Such cationic u complexes were expected to be stabilized best by three amino substituents, in meta positions relative to each other! in analogy to the stabilization of anionic u complexes by three meta-oriented nitro groups (Scheme I).