Meerwein arylation

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.^[1] 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.

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An electron-withdrawing group (EWG) on the alkene makes it electron deficient and although the reaction mechanism is unclear ^[2], 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 tetrafluorborate. 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.

Scope

A reported reaction of alkene acrylic acid with an aryl diazonium salt and copper(I) bromide and hydrobromic acid yields the alpha bromo-carboxylic acid ^[3]. When the alkene is butadiene the initial reaction product with catalyst copper(II) chloride is a 4-chloro-2-butene and after an elimination the aryl substituted butadiene ^[4]. In a so-called reductive arylation with 3-buten-2-one, titanium trichloride reduces the newly formed double bond.^[5]

In a novel kilogram-scale metal-free Meerwein arylation the diazonium salt is formed from 2-nitroaniline, the alkene isopropenyl acetate is an adduct of actone and acetic acid and the reaction product 2-nitrophenylacetone ^[6]:

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