Arenediazonium salts replacement

Reduction of the Diazonium Group to Hydrogen Deamination of Anilines Hypophos-phorus acid (H3P02) reacts with arenediazonium salts, replacing the diazonium group with a hydrogen. In effect, this is a reduction of the arenediazonium ion. 

Arenediazonium salts are extremely useful because the diazonio group (N2) can be replaced by a nucleophile in a substitution reaction. 

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. 

Arylamines are converted by diazotization with nitrous acid into arenediazonium salts, ArN2+ 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. 

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 diazonio group in an arenediazonium salt can be replaced by one of several transition metal ions in subgroups lb (Cu), Illb (Tl), IVb (Ge, Sn, Pb), or Vb (P, As, Sb, Bi) or by certain compounds of the transition elements. There is only one report of a substitution by a main group metal, magnesium, but the primary product has not been clearly identified (Nesmeyanov and Makarova, 1959). 

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... 

The replacement of the diazonio group by metals and related transition elements was investigated intensively until the mid-20th century, particularly by Nesmeyanov and coworkers (reviews173). Most intensively studied were mercury-de-diazoniations. Since about 1970 there has been very little activity in the whole field of aryl-element chemistry as far as arenediazonium salts are involved. This decrease is probably due to the lack of interest for technological purposes, and to the environmental problem, which the synthesis and the use of the compounds cause. 

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 Hydroxide Hydrolysis Hydrolysis takes place when the acidic solution of an arenediazonium salt is warmed. The hydroxyl group of water replaces N2, forming a phenol. This is a useful laboratory synthesis of phenols because (unlike nucleophilic aromatic substitution) it does not require strong electron-withdrawing substituents or powerful bases and nucleophiles. 

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. 

Replacement of the —N=N group in an arenediazonium salt by a cuprous salt usually cuprous chloride, bromide, or cyanide, (p. 912)... 

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. 

We have seen that primary arylamines react with nitrous acid to form stable arene-diazonium salts (Section 16.12). Arenediazonium salts are useful to synthetic chemists because the diazonium group can be replaced by a wide variety of nucleophiles. This reaction allows a wider variety of substituted benzenes to be prepared than can be prepared solely from electrophilic aromatic substitution reactions. 

Trimethylsilyl)phenyldiazene (10.23) is an interesting alternative to arenediazonium salts, although the N(J3) — Si bond is clearly covalent. Abel and Burton (1979 see also earlier references there) used it to replace a carbonyl group in monobromo(pentacarbonyl)manganese (10-7). The trimethylsilyl part of 10.23 (as... 

Arenediazonium salts are highly useful intermediates in the synthesis of aromatic compounds, because the diazonium group can be replaced by any one of a number of other atoms or groups, including —F, —Cl, —Br, —I, —CN, —OH, and —H. 

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 ... 

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. 

Arenediazonium salts react with potassium iodide to give products in which the diazo-nium group has been replaced by — I. An example is the synthesis of/>-iodonitrobenzene ... 

Arenediazonium salts react with hypophosphorous acid (H3PO2) to yield products in which the diazonium group has been replaced by —H. 

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. 

Sandmeyer Reaction (Section 23.8E) Treatment of an arenediazonium salt with CuCl, CuBr, or CuCN results in replacement of the diazonium group by —Cl, — Br, or —CN, respectively. 

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