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Arenium ion



 

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]

See also

References

  1. ^ 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
  2. ^ 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
  3. ^ 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
  4. ^ 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
  5. ^ 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.
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Arenium_ion". A list of authors is available in Wikipedia.
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