Sandmeyer reaction, mechanism

The following mechanism of the Sandmeyer reaction has been proposed as a result of a kinetic study, and incidentally accounts for the formation of the azu compounds as by-products. The catalyst is the CuCl ion produced in the dissolution of cuprous chloride in the chloride solution ... 

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

Nitro compounds can be formed in good yields by treatment of diazonium salts with sodium nitrite in the presence of cuprous ion. The reaction occurs only in neutral or alkaline solution. This is not usually called the Sandmeyer reaction, although, like 14-25, it was discovered by Sandmeyer. The BFJ ion is often used as the negative ion to avoid competition from the chloride ion. The mechanism is probably like that... 

A Sandmeyer reaction leading to the 3-chloro derivatives was observed upon treatment of 3-diazotriazoles with aqueous hydrochloric acid [1898LA33 26JCS1729 78ZN(B)216]. 3-Diazotriazole was reduced to the parent triazole by treatment at 0°C with primary and secondary alcohols [86DIS(B) (46) 3052]. The mechanism is not clear, but the process may be envisaged as involving hydride transfer from the intermediate 244 obtained by nucleophilic addition of alcohols to the diazo compound (Scheme 70). 

The mechanism by which a nucleophile displaces the diazonium group depends on the nucleophile. While some displacements involve phenyl cations, others involve radicals. Nucleophiles, e.g. CN , Cl and Br , replace the diazonium group if the appropriate cuprous salt is added to the solution containing the arene diazonium salt. The reaction of an arene diazonium salt with cuprous salt is known as a Sandmeyer reaction. 

The diazonium group can be replaced by a number of groups.222 Some of these are nucleophilic substitutions, with SnI mechanisms (p. 644), but others are free-radical reactions and are treated in Chapter 14. The solvent in all these reactions is usually water. With other solvents it has beeen shown that the SnI mechanism is favored by solvents of low nucleo-philicity, while those of high nucleophilicity favor free-radical mechanisms.222 (For formation of diazonium ions, see 2-49.) The N2 group can be replaced by Cl, Br. and CN, by a nucleophilic mechanism (see OS IV, 182). but the Sandmeyer reaction is much more useful (4-25 and 4-28). As mentioned on p. 651 it must be kept in mind that the N2 group can activate the removal of another group on the ring. 

This reaction, also called the Sandmeyer reaction, is similar to 4-25 in scope and mechanism. It is usually conducted in neutral solution to avoid liberation of HCN. 

The mechanism of the Sandmeyer reaction involves the transfer of electron from Cu" " to diazonium salt. Elimination of nitrogen generates aryl free radical, which reacts with CuXCl to give aryl halide via either path a or path b (Scheme 2.33). 

In the Sandmeyer reaction, the cold diazonium salt solution is run into a solution of the copper(I) halide dissolved in the halogen acid. The complex, which usually separates, is decomposed to the aryl halide by heating the reaction mixture. The mechanism (Scheme 8.16) involves generation of an aryl radical by electron transfer from Cu(I), which then reacts with the halide ion. 

The mechanism of the Sandmeyer reaction is not completely understood. For a long time it was believed to proceed via aryl cations, but later W.A. Waters and then later J.K. Kochi proposed a radical mechanism which was catalytic for... 

Waters, W. A. Decomposition reactions of the aromatic diazo compounds. X. Mechanism of the Sandmeyer reaction. J. Chem. Soc., Abstracts 1942, 266-270. 

The preparation of unsymmetrical biaryls,6 the Sandmeyer reaction,6 the replacement of an aromatic diazonium group by hydrogen,7 and the reaction of diazonium salts with ,0-unsaturated carbonyl compounds 8 appear to proceed by free radical mechanisms and will not be discussed. Diazonium coupling reactions, however, have the characteristics of ionic processes. 

The reactions involving copper salts are known as the Sandmeyer reactions. These reactions proceed via radical, rather than ionic, mechanisms. 

For reviews of the mechanism of the Sandmeyer reaction see Cowdrey and Davies1197 and Hodgson.1198... 

Copper is commonly used as a catalyst for a variety of organic reactions applying organometallic intermediates (114,115, 148-150). These processes include the Ullmann reaction, the Sandmeyer reaction, the Meerwein reaction, the Click reaction, a variety of atom transfer processes including pol5maerizations, etc. Many of these processes involve radicals and redox processes initiated by the copper species. All these processes are usually carried out in aprotic solvents and are therefore beyond the scope of this review. However, the mechanism of some of them was studied in aqueous solutions with the hope to perform them in this medium and in order of determining their detailed mechanisms. 

The most important finding for the synthetic applications of the Sandmeyer reaction is the clear experimental evidence of Galli that both oxidation states of copper ion are necessary for high yields. This claim is understandable on the basis of the reaction mechanism cupric ions are a ligand transfer reagent (see reviews ). The fact that the presence of Cu" ions was not realized much earlier is understandable, because cuprous salts are rarely completely free of cupric impurities. In aqueous systems they form cupric ions by equilibration as well as by air oxidation. The following comparative experiments of Gain in the chloro-de-diazoniation of benzenediazonium sulfate in water at room temperature are instructive. Yields of chlorobenzene are with 0.25 M CuCl 45% with 0.25 M CuCl -F 0.25 M Cu(N03)2 63% with 0.25 M Cu(N03)2 <0.1%. 

This reaction was initially reported by Gattermann in 1890. It is the preparation of aromatic halides or aromatic cyanides by decomposition of corresponding diazo salts in the presence of copper powder, in which copper powder is freshly obtained from a copper (II) sulfate aqueous solution. Therefore, it is generally known as the Gattermann reaction. Occasionally, it is also referred to as the Gattermann method. It should be pointed out that this reaction is almost equivalent to the Sandmeyer Reaction It is believed that a radical mechanism might be involved in this reaction, by which electron transfer occurs at the surface of metallic copper. However, this reaction is not applicable for the preparation of aromatic fluorides because HF exists as H2F2 and ionized as H+ and 10 2 . ... 

Arenediazonium salts react with cuprous chloride, cuprous bromide, and cuprous cyanide to give products in which the diazonium group has been replaced by —Cl, —Br, and — CN, respectively. These reactions are known generally as Sandmeyer reactions. Several specific examples follow. The mechanisms of these replacement reacaiions are not fully understood the reactions appear to be radical in nature, not ionic. 

There are numerous multistep processes that generate electrophiles. As examples of these types of reactions, we will consider the diazotization of anilines and the formation of chloroiminium ions in the Vilsmeier-Haack reaction. Aryl diazonium ions are useful in the modification of arenes by the Sandmeyer reaction and as electrophilic intermediates in diazonium coupling reactions for the synthesis of dyes and pigments. Several types of synthetic methods have been developed for this chemistry, and the mechanism varies depending on the methodology [13]. Under some conditions, the nitrosonium ion (18) initiates the process (Scheme 1.5). NjOj and NOCl have also been proposed as intermediates in diazotization—both are considered as nitrosonium ion carriers. The aniline reacts... 

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