Arenium ion
An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution. For historic reasons this complex is also called a Wheland intermediate, after American chemist George Willard Wheland, or a sigma complex or σ-complex. The smallest arenium ion is the benzenium ion, which is protonated benzene.
Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring. The arenium ion is no longer an aromatic species; however it is relatively stable due to delocalization: the positive charge is delocalized over 3 carbon atoms by the pi system, as depicted on the following resonance structures:
A complexed electrophile can contribute to the stability of arenium ions.
A benzenium ion can be isolated as a stable compound when benzene is protonated by the carborane superacid H. The benzenium salt is crystalline with thermal stability up to 150 °C. Bond lengths deduced from X-ray crystallography are consistent with a cyclohexadienyl cation structure.
In one study a methylene arenium ion is stabilized by metal complexation:
In this reaction sequence the R–Pd–Br starting complex 1 stabilized by TMEDA is converted through dppe to metal complex 2. Electrophilic attack of methyl triflate forms methylene arenium ion 3 with positive charge located in aromatic para position and with the methylene group 6° out of the plane of the ring. Reaction first with water and then with triethylamine hydrolyzes the ether group.
