Arenium ion
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An arenium ion 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]:
[edit] See also
[edit] 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.
