Arenium ion

{{Short description|Forms during electrophilic substitution on benzene ring}}File:Benzenium-3D-balls.png of the benzenium ion]]

An arenium ion in organic chemistry is a cyclohexadienyl cation that appears as a reactive intermediate in electrophilic aromatic substitution.{{March6th|page = 658}}

For historic reasons this complex is also called a Wheland intermediate, after American chemist George Willard Wheland (1907–1976).{{cite book|last=Smith|first=Michael B.|url=https://books.google.com/books?id=1LXMBQAAQBAJ&q=george+willard+wheland+chemist+1907-&pg=PA1041|title=Organic Chemistry: An Acid—Base Approach|date=18 October 2010|publisher=CRC Press|isbn=9781439894620|via=Google Books}} They are also called sigma complexes.More commonly, sigma complex usually refers to a type of organometallic complex. The smallest arenium ion is the benzenium ion ({{chem|C|6|H|7|+}}), which is protonated benzene.

:File:AreniumIon.png

Two hydrogen atoms bonded to one carbon lie in a plane perpendicular to the benzene ring.{{cite book|title=A Guidebook to Mechanism in Organic Chemistry|authorlink=Peter Sykes (chemist)|first=Peter|last=Sykes|pages=130–133}}{{ISBN missing}} 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:

:File:AreniumIonResonance.png

A complexed electrophile can contribute to the stability of arenium ions.

Salts of benzenium ion can be isolated when benzene is protonated by the carborane superacid H(CB₁₁H(CH₃)₅Br₆).{{cite journal|title=Isolating Benzenium Ion Salts|first1=C. A.|last1=Reed|first2=K.|last2=Kim|first3=E. S.|last3=Stoyanov|first4=D.|last4=Stasko|first5=F. S.|last5=Tham|first6=L. J.|last6=Mueller|first7=P. D. W.|last7=Boyd|journal=J. Am. Chem. Soc.|date=2003|volume=125|issue=7|pages=1796–804|doi=10.1021/ja027336o|pmid=12580605|s2cid=8938028 |url=http://www.escholarship.org/uc/item/5c0780qn}} 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:{{cite journal|title=Synthesis and Reactivity of the Methylene Arenium Form of a Benzyl Cation, Stabilized by Complexation|first1=E.|last1=Poverenov|first2=G.|last2=Leitus|first3=D.|last3=Milstein|journal=J. Am. Chem. Soc.|date=2006|volume=128|issue=51|pages=16450–1|type=Communication|doi=10.1021/ja067298z|pmid=17177364|url=https://figshare.com/articles/journal_contribution/3038599|url-access=subscription}}

:File:Methyleneareniumion.png

In this reaction sequence the R–Pd(II)–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 (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.