Meerwein arylation

{{Short description|Organic reaction}}

{{redirect|Meerwein reaction|other reactions named after Hans Meerwein}}

{{Use dmy dates|date=May 2023}}

{{Reactionbox

| Name = Meerwein arylation

| Type = Coupling reaction

| NamedAfter = Hans Meerwein

}}

The Meerwein arylation is an organic reaction involving the addition of an aryl diazonium salt (ArN₂X) to an electron-poor alkene usually supported by a metal salt.{{cite journal|first1 = Hans|last1 = Meerwein|authorlink1 = Hans Meerwein|first2 = Eberhard|last2 = Buchner|first3 = Konrad|last3 = van Emster|title = Über die Einwirkung aromatischer Diazoverbindungen auf α,β-ungesättigte Carbonylverbindungen|language = German|journal = J. Prakt. Chem.|year = 1939|volume = 152|issue = 7–10|pages = 237–266|doi = 10.1002/prac.19391520705}} The reaction product is an alkylated arene compound. The reaction is named after Hans Meerwein, one of its inventors who first published it in 1939.

Image:Meerwein arylation.png

An electron-withdrawing group (EWG) on the alkene makes it electron deficient and although the reaction mechanism is unclear,{{cite book|title = Strategic Applications of Named Reactions in Organic Synthesis: Background and Detailed Mechanisms|first1 = Laszlo|last1 = Kürti|first2 = Barbara|last2 = Czakó|year = 2005|publisher = Elsevier Academic Press|isbn = 9780124297852}} involvement of an aryl radical is presumed after loss of nitrogen in the diazonium salt followed by a free radical addition. In the primary reaction product the intermediate alkyl radical is then captured by the diazonium counterion X which is usually a halogen or a tetrafluoroborate. In a subsequent step an elimination reaction liberates HX (for instance hydrochloric acid) and an aryl vinyl compound is formed. The reaction mechanism from the arene's view ranks as a radical-nucleophilic aromatic substitution.

In a general scope a Meerwein arylation is any reaction between an aryl radical and an alkene.{{cite journal|title = Intermolecular Olefin Functionalisation Involving Aryl Radicals Generated from Arenediazonium Salts|first = Markus R.|last = Heinrich|doi = 10.1002/chem.200801306|journal = Chem. Eur. J.|year = 2009|volume = 15|issue = 4|pages = 820–833|pmid = 19086045}} The initial intermediate is an aryl enthenyl radical which can react with many trapping reagents such as hydrogen or halogens or with those based on nitrogen or sulfur.