| Meerwein arylation | |
|---|---|
| Named after | Hans Meerwein |
| Reaction type | Coupling reaction |
TheMeerwein arylation is anorganic reaction involving the addition of anaryldiazonium salt (ArN2X) to an electron-pooralkene usually supported by a metal salt.[1] The reaction product is an alkylated arene compound. The reaction is named afterHans Meerwein, one of its inventors who first published it in 1939.
Anelectron-withdrawing group (EWG) on the alkene makes it electron deficient and although thereaction mechanism is unclear,[2] involvement of anaryl radical is presumed after loss of nitrogen in the diazonium salt followed by afree radical addition. In the primary reaction product the intermediate alkyl radical is then captured by the diazonium counterion X which is usually ahalogen or atetrafluoroborate. In a subsequent step anelimination reaction liberates HX (for instancehydrochloric acid) and an aryl vinyl compound is formed. The reaction mechanism from the arene's view ranks as aradical-nucleophilic aromatic substitution.
In a general scope a Meerwein arylation is any reaction between anaryl radical and an alkene.[3] The initial intermediate is an aryl enthenyl radical which can react with manytrapping reagents such ashydrogen orhalogens or with those based onnitrogen orsulfur.
A reported reaction of alkeneacrylic acid with an aryl diazonium salt andcopper(I) bromide andhydrobromic acid yields the α-bromocarboxylic acid.[4] When the alkene isbutadiene the initial reaction product with catalystcopper(II) chloride is a4-chloro-2-butene and after an elimination the aryl substituted butadiene.[5] In a so-called reductive arylation with 3-buten-2-one,titanium trichloridereduces the newly formed double bond.[6]
In a novel kilogram-scale metal-free Meerwein arylation the diazonium salt is formed from 2-nitroaniline, the alkene isopropenyl acetate is an adduct ofpropyne andacetic acid and the reaction product 2-nitrophenylacetone:[7]
