Arenediazonium salts benzenes

Many different nucleophiles—halide, hydride, cyanide, and hydroxide among others—react with arenediazonium salts, yielding many different kinds of substituted benzenes. The overall sequence of (1) nitration, (2) reduction, (3) diazotization, and (4) nucleophilic substitution is perhaps the single most versatile method of aromatic substitution. 

Hydro-de-diazoniation seems to be an unnecessary reaction from the synthetic standpoint, as arenediazonium salts are obtained from the respective amines, reagents that are normally synthesized from the hydrocarbon. Some aromatic compounds, however, cannot be synthesized by straightforward electrophilic aromatic substitution examples of these are the 1,3,5-trichloro- and -tribromobenzenes (see below). These simple benzene derivatives are synthesized from aniline via halogenation, diazotization and hydro-de-diazoniation. Furthermore hydro-de-diazoniation is useful for the introduction of a hydrogen isotope in specific positions. 

Reaction of diarylditellurides with arenediazonium salts (typical procedure). p-Methoxy phenyl p-tolyl telluride. With heating and stirring in an atmosphere of nitrogen, sodium tetrahydroborate was added in small portions to a solution of 9.4 g of bis(p-methoxyphenyl) ditelluride in a mixture of 50 mL of ethanol and 15 mL of benzene until the solution was decolourized completely (1.5 g was required). Then 8.24 g of p-toluenediazonium fluo-roborate was added rapidly and the mixture was stirred for 1 h and poured into dilute HCl. The oil formed was extracted with ether, and the extract was washed with water and dried over CaCl2. Ether was evaporated, and the residue was dissolved in benzene and chromatographed on alumina (eluent hexane). After the evaporation of hexane, 4.8 g (36%) of the telluride was isolated, m.p. 64-64.5°C (hexane). 

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. 

Gomberg-Bachmann biphenyl synthesis. Reaction of stable arenediazonium tet-rafluoroborates or hexafluorophosphates in an aromatic solvent with potassium acetate (2 equiv.) and a phase-transfer catalyst results in biar Is in high yield. Crown ethers, Aliquat 336, and tetrabutylammonium hydrogen sulfate arc all effective catalysts. The reaction is useful for synthesis of unsymmetrical biaryls. The ortho-isomer predominates in reactions with a monosubstituted benzene. The most selective method is to couple a substituted arenediazonium salt with a symmetrical arene. 

Synthesis of Substituted Benzenes Using Arenediazonium Salts... 

So far, we have learned how to place a limited number of different substituents on a benzene ring—those substituents listed in Section 16.2 and those that can be obtained from these substituents by chemical conversion. However, the kinds of substituents that can be placed on benzene rings can be greatly expanded by the use of arenediazonium salts. 

Aniline and substituted-anilines react with nitrous acid to form arenediazonium salts a diazonium group can be displaced by a nucleophile. Arenediazonium ions can be used as electrophiles with highly activated benzene rings to form azo compounds that can exist in cis and trans forms. Secondary amines react with nitrous acid to form nitrosamines. 

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. 

Arenediazonium salt A derivative of the resonance-stabilized benzenediazonium cation, C6H5N2 +. Aromatic compound (1) Benzene or a derivative—represented by a structure with three double bonds in... 

THE SYNTHESIS OF SUBSTITUTED BENZENES USING ARENEDIAZONIUM SALTS... 

Arenediazonium salts can be converted into other substituted benzenes... 

When arenediazonium salts are decomposed in the presence of nucleophiles other than water, the corresponding substituted benzenes are formed. For example, diazotization of arenamines (anilines) in the presence of hydrogen iodide results in the corresponding iodoarenes. 

In Summary Arenediazonium salts, which are more stable than alkanediazonium salts because of resonance, are starting materials not only for phenols, but also for haloarenes, arenecarbonitriles, and reduced aromatics by displacement of nitrogen gas. The intermediates in some of these reactions may be aryl cations, highly reactive because of the absence of any electronically stabilizing features, but other, more complicated mechanisms may be followed. The ability to transform arenediazonium salts in this way gives considerable scope to the regioselective construction of substituted benzenes. 

Aprotic nitrosation of 1,3-diaryl-triazenes RC6H4NHN2C6H4R (R = H, 4-Me, 4-OMe, 4-Cl, 4-NO2, 2-Me, 2-Cl, or 3-Cl) with Me(CH2)40NO yields the A-nitroso-compound, which decomposes to arenediazonium salts and radicals and yields biphenyls, which are also formed together with (183), by electrophilic substitution of sulphonyl-triazenes (184) with benzene and AICI3. 

---