Cuprous chloride reaction with aryl diazonium salts

Replacement of diazonium groups by halide is a valuable alternative to direct halogenation for preparation of aryl halides. Aryl bromides and chlorides are usually prepared by reaction of aryl diazonium salts with the appropriate Cu(I) salt, a process which is known as the Sandmeyer reaction. Under the classic conditions, the diazonium salt is added to a hot acidic solution of the cuprous halide.92 The Sandmeyer reaction is formulated as proceeding by an oxidative addition reaction of the diazonium ion with Cu(I) and halide transfer from the Cu(III) intermediate. 

For the preparation of chlorides or bromides, the diazonium salt is decomposed with a solution of cuprous chloride or bromide in the corresponding halogen acid (Sandmeyer reaction). It is possible to prepare the aryl bromide from the diazonium chloride or sulfate. A variation Involves the use of copper powder and a mineral acid for the decomposition step (Gattermann reaction). Both procedures are illustrated by the syntheses of the isomeric bromotoluenes and chlorotoluenes. The usual conditions of the Sandmeyer reaction fail in the preparation of the chloro- and bromo-phenanthrenes. However, these compounds can be successfully obtained by the interaction of the diazonium compound with mercuric and potassium halides (Schwechten procedure). Another procedure for formation of aryl bromides involves treatment of the amine hydrobromide with nitrogen trioxide in the presence of excess 40% hydro-bromic acid. The Intermediate diazonium perbromide is then decomposed by heat. ... 

Examples of the three mechansims are, respectively (a) hydrolysis of aryl diazonium salts to phenols (b) reaction of aryl diazonium ions with Ns to give the aryl azides " and (c) the Sandmeyer reaction, involving cuprous chloride or bromide for synthesis of aryl halides. Specific synthetically important substitution processes are considered in the succeeding sections. 

Treatment of diazonium salts with cuprous chloride or bromide leads to aryl chlorides or bromides, respectively. In either case the reaction is called the Sandmeyer reaction The reaction can also be carried out with copper and HBr or HCl, in which case it is called the Gatterman reaction (not to be confused with 11-16). The Sandmeyer reaction is not useful for the preparation of fluorides or iodides, but for bromides and chlorides it is of wide scope and is probably the best way of introducing bromine or chlorine into an aromatic ring. The yields are usually high. 

Examples of the three mechanistic types are, respectively (a) hydrolysis of diazonium salts to phenols89 (b) reaction with azide ion to form aryl azides90 and (c) reaction with cuprous halides to form aryl chlorides or bromides.91 In the paragraphs that follow, these and other synthetically useful reactions of diazonium intermediates are considered. The reactions are organized on the basis of the group that is introduced, rather than on the mechanism involved. It will be seen that the reactions that are discussed fall into one of the three general mechanistic types. 

Replacement of the Diazonium Group by Chloride, Bromide, and Cyanide The Sandmeyer Reaction Copper salts (cuprous salts) have a special affinity for diazonium salts. Cuprous chloride, cuprous bromide, and cuprous cyanide react with arenediazonium salts to give aryl chlorides, aryl bromides, and aryl cyanides. The use of cuprous salts to replace arenediazonium groups is called the Sandmeyer reaction. The Sandmeyer reaction (using cuprous cyanide) is also an excellent method for attaching another carbon substituent to an aromatic ring. 

Treatment of diazonium salts with cuprous, Cu(I), salts generates aryl halides. When 398 reacts with CuCl (cuprous chloride) or CuBr (cuprous bromide), the products are chlorobenzene or bromobenzene via what is probably a radical reaction.29l jhis conversion is known as the Sandmeyer reaction. 2 The use of copper powder rather than cuprous salts for this transformation is often called the Gattermann reaction. 93,292b,c Aryl iodides are also produced from diazonium salts by reaction with potassium iodide (KI) but the actual reactive species may be l3-.294,295 Treatment of aniline derivative 403 with sodium nitrite and HCl followed by treatment with KI, for example, gave a 89% yield of 404.Aryl nitriles are generated under Sandmeyer conditions using cuprous cyanide (CuCN), as in the conversion of 405 to benzonitrile derivative 407 via diazonium chloride, 406. 

Conversion of diazonium compounds to aryl chlorides, bromides, or cyanides is usually accomplished using cuprous salts, and is known as the Sandmeyer reaction. Since a CN group is easily converted to a CO2H group (eq. 10.13), this provides another route to aromatic carboxylic acids. The reaction with KI gives aryl iodides, usually not easily accessible by direct electrophilic iodination. Similarly, direct aromatic fluorination is difficult, but aromatic fluorides can be prepared from diazonium compounds and tetrafluoroboric acid, HBF4. 

Another common transformation of diazonium salts is their conversion to aryl halides by reaction with cuprous salts (CuX), in what is known as the Sandmeyer reaction, named after Traugott Sandmeyer (Switzerland 1854-1922). This means that the Ar-NH2 ArX conversion is possible, where Ar = an aryl group. When 132 is treated with cuprous bromide (CuBr), the product is bromobenzene (18). The reaction works with many other cuprous salts as well, including cuprous chloride (CuCl). A variation of this reaction treats the diazonirun salt with cuprous cyanide (CuCN) to give a nitrile. In this manner, 4-methylanihne (24) is treated with HCI and NaN02 and then with CuCN to give 4-methyl-l-cyanobenzene (134). 

Meenvein reaction [1, 166, before references]. The arylation of olefinic com pounds by diazonium halides with copper salt catalysis was discovered by Meerweit (1939).8 Cupric chloride has been usually employed. Cleland,9 however, prefers cuprous bromide (MCB reagent grade) and recommends that the salt (light green) be washed with acetone until the washings are colorless and then with benzene and then with hexane. The resulting solid is dried at 120° and is only faintly colored. He... 

W. A. Waters (Oxford University) Investigations of the extent to which complexes such as (CuCl)+ and undissociated CuCL affect the chain length in the polymerization associated with the Sandmeyer reaction are in progress at Oxford. It is well known that ions that complex well with cupric, e.g., (CN) , can be introduced into aryl nuclei by the Sandmeyer procedure in preference to chloride even when diazonium chlorides have initially been taken. The system, however, is complicated by the fact that the complexing of cuprous and cupric salts alters the redox potential, and this affects the facility of both stages 1 and 3 of the reaction sequence. The effects of introducing polar substituents into the aryl nuclei (Table I) indicates the importance of such effects. 

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