Effenberger, Franz2009-10-192016-03-312009-10-192016-03-311989312051166http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-43577http://elib.uni-stuttgart.de/handle/11682/1232http://dx.doi.org/10.18419/opus-1215Anionic σ complexes have been known and extensively studied as intermediates of nucleophilic aromatic substitution since the beginning of the century. In a paper that still makes fascinating reading, Meisenheimer describes the formation and isolation of anionic σ complexes by reaction of picryl ethers with potassium alkoxides. Structural proof is derived from the fact that identical σ complexes are obtained from methyl 2,4,6-trinitrophenyl ether with potassium ethoxide, and from ethyl 2,4,6-trinitrophenyl ether with potassium methoxide. Cationic σ 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.eninfo:eu-repo/semantics/openAccessElektrophile Substitution , Oxidation , Aromaten540article2009-10-20