Arenium ion

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

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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). ["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] 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: ["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.]

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References