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Arenium ion

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An arenium ion in organic chemistry is the cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution.^[1] For historic reasons this complex is also called a Wheland intermediate^[2] or a sigma complex or σ-complex.

In the ion the positive charge is delocalized over 5 carbon atoms as a result of resonance:

The smallest arenium ion is protonated benzene C₆H₇⁺. The benzenium ion can be isolated as a stable compound when benzene is protonated by the carborane superacid H(CB₁₁H(CH₃)₅Br₆).^[3] 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.

More methods exist for the generation of stable arenium ions. In one study a methylene arenium ion is stabilized by metal complexation:^[4]^[5]

References

^ Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent (classical) carbenium ions from three-center bound penta- of tetracoordinated (nonclassical) carbonium ions. Role of carbocations in electrophilic reactions George A. Olah J. Am. Chem. Soc.; 1972; 94(3) pp 808 - 820; doi:10.1021/ja00758a020
^ A Quantum Mechanical Investigation of the Orientation of Substituents in Aromatic Molecules G. W. Wheland J. Am. Chem. Soc.; 1942; 64(4) pp 900 - 908; doi:10.1021/ja01256a047
^ Isolating Benzenium Ion Salts Christopher A. Reed, Kee-Chan Kim, Evgenii S. Stoyanov, Daniel Stasko, Fook S. Tham, Leonard J. Mueller, and Peter D. W. Boyd J. Am. Chem. Soc.; 2003; 125(7) pp 1796 - 1804; doi:10.1021/ja027336o
^ Synthesis and Reactivity of the Methylene Arenium Form of a Benzyl Cation, Stabilized by Complexation Elena Poverenov, Gregory Leitus, and David Milstein J. Am. Chem. Soc.; 2006; 128(51) pp 16450 - 16451; (Communication) doi:10.1021/ja067298z
^ In this reaction sequence the R-Pd(II)-Br starting complex 1 stabilized by TMEDA is oxidized by dtpp to metal complex 2. Electrophilic attack of methyl triflate forms methylene arenium ion 3 with (based on X-ray crystallography) 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.