Replacement reactions, of arenediazonium

In another paper Zakhardin s group investigated the reaction of arenediazonium salts with carboranes in which one of the CH protons was replaced by copper(i). The... 

Most arenediazonium salts are unstable at temperatures above 5—10 °C, and many explode when dry. Fortunately, however, most of the replacement reactions of diazonium salts do not require their isolation. We simply add another reagent (CuCI, CuBr, Kl, etc.) to the mixture, gently warm the solution, and the replacement (accompanied by the evolution of nitrogen) takes place ... 

The discovery of the Balz-Schiemann reaction in 1927 replaced the earlier Wallach procedure (1886) based on fluorodediazoniation of arenediazonium piper-idides (aryltriazenes) in aqueous hydrogen fluoride [40 41]... 

The diazonio group can also be replaced by —OH to yield a phenol and by —H to yield an arene. A phenol is prepared by reaction of the arenediazonium salt with copper(I) oxide in an aqueous solution of copper(ll) nitrate, a reaction that is especially useful because few other general methods exist for introducing an -OH group onto an aromatic ring. 

Mechanistically, these diazonio replacement reactions occur through radical rather than polar pathways. In the presence of a copper(I) compound, for instance, it s thought that the arenediazonium ion is first converted to an aryl radical plus copper(II), followed by subsequent reaction to give product plus regenerated copper(l) catalyst. 

The diazonio group of arenediazonium salts can be replaced by alkenes and alkynes or, seen from the other reaction partner, alkenes and alkynes can be arylated with arenediazonium salts. The reactions are catalyzed by copper salts and, as found more recently, also by salts of palladium and other metals. 

The most useful reaction of arylamines is conversion by diazotization with nitrous acid into arenediazonium salts, ArNg X . The diazonio group can then be replaced by many other substituents in the Sandmeyer reaction to give a wide variety of. substituted aromatic compounds. Aryl chlorides, bromides, iodides, and nitriles can be prepared from arenediazonium salts, as can arenes and phenols. In addition to their reactivity toward substitution reactions, diazonium salts undergo coupling with phenols and arylamines to give brightly colored azo dyes. 

Nucleophiles such as C N, CF, and Br will replace the diazonium group if the appropriate cuprous salt is added to the solution containing the arenediazonium salt. The reaction of an arenediazonium salt with a cuprous salt is known as a Sandmeyer reaction. 

Primary aromatic amines react with nitrous acid to form arenediazonium salts, which, unlike their aliphatic counterparts, are stable at 0°C and can be kept in solution for short periods without decomposition. When an arenediazonium salt is treated with an appropriate reagent, nitrogen is lost and replaced with another atom or functional group. What makes reactions of primary aromatic amines with nitrous acid so valuable is the fact that the — NH2 group can be replaced with the groups shown. 

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 reactions are not fully understood the reactions appear to be radical in nature, not ionic. 

The name Sandmeyer reaction - is used for the replacement of the diazonium group in an arenediazonium compound by halide or pseudohalide, taking place in the presence of a metal salt. However this is not a strict definition, since the replacement of the diazonium group by iodide, which is possible without a metal catalyst, is also called a Sandmeyer reaction. 

The classical method for replacing a diazonio group by fluorine is the Balz-Schie-mann reaction (Balz and Schiemann, 1927) in which the arenediazonium tetrafluoro-borate is heated without a solvent. Although it gives fairly good yields in the majority of cases, it can lead to difficulties. These have been discussed by Wulfman (1978,... 

It was mentioned earlier (Sec. 8.6) that for iodo-de-diazoniation no catalyst is necessary because the redox potential of the iodide ion (E° = 1.3 V) is sufficient for an electron transfer to the arenediazonium ion. The reaction was actually observed by Griess (1864 c). Four iodo-de-diazoniation procedures are described in Organic Syntheses. For the syntheses of iodobenzene and 4-iodophenol (Lucas and Kennedy, 1943, and Daines and Eberly, 1943, respectively) KI is used in equimolar quantity and in 1.2 molar excess. However, for 2-bromoiodobenzene and for 1,3,4-triiodo-5-nitrobenzene (replacement of a diazonio group in the 4-position by iodine), up to... 

The subject of this chapter is the replacement of protons in primary and secondary aromatic and aliphatic amines by arenediazonium ions. In these reaction (Scheme 13-1) derivatives of the unknown substance triazene, HN = N-NH2, are formed. ... 

This diazotization reaction is compatible with the presence of a wide variety of substituents on the benzene ring. Arenediazonium salts are extremely important in synthetic chemistry, because the diazonio group (N=N) can be replaced by a nucleophile in a radical substitution reaction, e.g. preparation of phenol, chlorobenzene and bromobenzene. Under proper conditions, arenediazonium salts react with certain aromatic compounds to yield products of the general formula Ar-N=N-Ar, called azo compounds. In this coupling reaction, the nitrogen of the diazonium group is retained in the product. 

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. 

Replacement of the Diazonium Group by Fluoride and Iodide When an arenediazonium salt is treated with fluoroboric acid (HBF4), the diazonium fluoroborate precipitates out of solution. If this precipitated salt is filtered and then heated, it decomposes to give the aryl fluoride. Although this reaction requires the isolation and heating of a potentially explosive diazonium salt, it may be carried out safely if it is done carefully with the proper equipment. There are few other methods for making aryl fluorides. 

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