Copper chloride with aromatic rings

For the in situ preparation of the required arenediazonium salt from an aryl amine by application of the diazotization reaction, an acid HX is used, that corresponds to the halo substituent X to be introduced onto the aromatic ring. Otherwise—e.g. when using HCl/CuBr—a mixture of aryl chloride and aryl bromide will be obtained. The copper-(l) salt 2 (chloride or bromide) is usually prepared by dissolving the appropriate sodium halide in an aqueous solution of copper-(ll) sulfate and then adding sodium hydrogensulfite to reduce copper-(ll) to copper-(1). Copper-(l) cyanide CuCN can be obtained by treatment of copper-(l) chloride with sodium cyanide. 

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. 

Chlorobiphenyts. These compounds can be synthesized by direct chlorination of biphenyl in [he presence of iron or other calalysts. Other means of preparation include reaction of diazotized aminobiphenyl with copper chloride. Treatment of chlorobiphenyls at elevated temperatures (300—400°C] with strong caustic yields hydroxybiphenyls. Various reactions, normal to aromatic systems, will occur—usually on the unsubstlluted ring. 

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. 

A diazonium salt reacts with copper(I) chloride or copper(I) bromide to form an aryl chloride or aryl bromide, respectively. This is called the Sandmeyer reaction. It provides an alternative to direct chlorination and bromination of an aromatic ring using CI2 or Br2 and a Lewis acid catalyst. 

The air oxidation of substituted catechols to the corresponding 0-benzoquinones is catalyzed by a variety of Cu(II)-amine systems. It appears that the mechanism involves the formation of a dicopper(II) catecholate intermediate electron transfer then occurs from the aromatic ring to give the o-benzoquinone and two Cu(I) centers. The latter then react with dioxygen and the catechol to regenerate the dicopper(II) catecho-late. A study of the effect of chloride ions on the kinetics of the copper-catalyzed oxidation of ascorbic acid by dioxygen does not rule out the involvement of Cu(I) intermediates but a mechanism involving Cu(III) is preferred. Kinetic studies on Cu(II)-catalyzed oxidation of enamines [e.g., equation (26)] and 3-phenylpropanal have been reported. 

Addition of carbenes to aromatic systems leads to ring-expanded products. Methylene itself, formed by photolysis of diazomethane, adds to benzene to form cycloheptatriene in 32% yield a small amount of toluene is also formed by an insertion reaction. The cycloheptatriene is formed by a Cope rearrangement of the intermediate cyclopropane (a norcaradiene). More satisfactory is the reaction of benzene with diazomethane in the presence of copper salts, such as copper(I) chloride, which gives cycloheptatriene in 85% yield (4.87). The reaction is general for aromatic systems, substituted benzenes giving mixtures of the corresponding substituted cycloheptatrienes. 

---