Sandmeyer reaction

The Sandmeyer reaction is a chemical reaction used to synthesize aryl halides from aryl diazonium salts using copper salts as reagents or catalysts.^[1][2][3][4] It is an example of a radical-nucleophilic aromatic substitution. The Sandmeyer reaction provides a method through which one can perform unique transformations on benzene, such as halogenation, cyanation, trifluoromethylation, and hydroxylation.

The reaction was discovered in 1884 by Swiss chemist Traugott Sandmeyer, when he attempted to synthesize phenylacetylene from benzenediazonium chloride and copper(I) acetylide. Instead, the main product he isolated was chlorobenzene.^[5] In modern times, the Sandmeyer reaction refers to any method for substitution of an aromatic amino group via preparation of its diazonium salt followed by its displacement with a nucleophile in the presence of catalytic copper(I) salts.

The most commonly employed Sandmeyer reactions are the chlorination, bromination, cyanation, and hydroxylation reactions using CuCl, CuBr, CuCN, and Cu₂O, respectively. More recently, trifluoromethylation of diazonium salts has been developed and is referred to as a 'Sandmeyer-type' reaction. Diazonium salts also react with boronates, iodide, thiols, water, hypophosphorous acid and others,^[6] and fluorination can be carried out using tetrafluoroborate anions (Balz–Schiemann reaction). However, since these processes do not require a metal catalyst, they are not usually referred to as Sandmeyer reactions. In numerous variants that have been developed, other transition metal salts, including copper(II), iron(III) and cobalt(III) have also been employed.^[7] Due to its wide synthetic applicability, the Sandmeyer reaction, along with other transformations of diazonium compounds, is complementary to electrophilic aromatic substitution.