Amines arenediazonium salts

Reaction with arenediazonium salts Adding a phe nol to a solution of a diazonium salt formed from a primary aromatic amine leads to formation of an azo compound The reaction is carried out at a pH such that a significant portion of the phenol is pres ent as its phenoxide ion The diazonium ion acts as an electrophile toward the strongly activated ring of the phenoxide ion... 

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. 

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. 

Carboxamides and esters of arenecarboxylic acids are obtainable directly by reacting arenediazosulfones (Ar — N2 —S02 —Ar ) with CO and amines or alcohols, respectively, in the presence of Pd catalysts (Kamigata et al., 1989). Aromatic aldehydes are formed if the reaction is carried out in the presence of triethylsilane (Kikukawa et al., 1984). In an analogous way, arenediazonium salts can be transformed into ketones (ArCO —R R = CH3, C2H5, or C6H5) in the presence of stan-nanes, R4Sn (Kikukawa et al., 1982). 

Doyle et al. (1977 c) and Oae et al. (1980) reported modified Meerwein arylations with significant improvements in the yield by the use of aryl amines and alkyl nitrites in place of arenediazonium salts. However, good yields are only achieved if alkenes activated by electron-withdrawing groups are present. 

In 1882 Griess discovered that in aqueous acidic solution 4-diazobenzenesulfonate and 4-methylaniline react quantitatively to yield 4-toluenediazonium ion and 4-ami-nobenzenesulfonic acid. This phenomenon is called diazo migration or diazo exchange. We now know that it is a consequence of the tautomerism of the initially formed l-(4 -methylphenyl)-3-(4 -sulfophenyl)-triazene, as discussed above. Griess also found another reaction that could not be explained at his time, but which is based on the tautomerism of intermediate triazenes occasionally, the reaction of an arenediazonium salt with a primary aromatic amine in weakly acidic solution yields a mixture of two isomeric aminoazo compounds (Scheme 13-21). 

Through the use of arenediazonium salts, the straightforward transformation of amines into cross-coupling products can be realized. Whenever the diazonium salts do not tolerate bases and strong nucleophiles (e.g., phosphines), base- and phosphine-free protocols have to be used. Heterocyclic carbene ligands serve well in cross-coupling of Aryl- and vinylboronic acids, or alkylboronates with arenediazonium salts.369,370 Several convenient phosphine-free protocols have been developed for the same purpose.371-373... 

A common laboratory procedure converts an aromatic amine to a phenol by way of the arenediazonium salt, ArNH2 —> ArN2 —> ArOH (Section 23-10B). 

Mechanism 19-6 Diazotization of an Amine 910 19-17 Reactions of Arenediazonium Salts 911 Summary Reactions of Amines 915 19-18 Synthesis of Amines by Reductive Amination 918 19-19 Synthesis of Amines by Acylation-Reduction 920... 

The most useful reaction of amines with nitrous acid is the reaction of arylamines to form arenediazonium salts. We consider next how these diazonium salts may be used as synthetic intermediates. 

Various aromatic amines, phenols, and compounds containing active methylene groups can be titrated with arenediazonium salts, from which 4-bromo-l-naphthale-nediazonium chloride seems to be the most widely applicable titrant. Compounds that react slowly with arenediazonium salts can be determined by back-titration when the excess of arenediazonium salt is back-titrated with either sodium tetraphenylborate or 2,4-diaminotoluene. Indirect determination is useful for secondary amines, which react with arenediazonium ions to form triazenes. The determination of diazonium salts of ampholytic character is based on the reaction of these salts with l-phenyl-3-methyl-5-pyrazolone, the excess of which is titrated with 4-bromo-l-naphthalenediazonium chloride solution. 

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. 

Reaction with silyl enol ethers. Derivatives from ketones and esters behave to- ards arenediazonium salts according to their relative nucleophilicities. a-Arylation, 9 ketones by a free radical pathway and nonradical a-amination of esters are. bserved. 

Reaction with arenediazonium salts Adding a phenol to a solution of a diazonium salt fornned fronn a primary aromatic amine leads to formation of an azo compound. 

Arenediazonium salts ArN2Bp4 are an excellent substrate available from aromatic amines for carrying out oxidative addition under mild conditions. The desired cross-coupling with aryl(trifluoro)borates (ArBp3)K is achieved at room temperature with 5 mol% of Pd(OAc)2 or with the paradacycle shown in Eq. 26 (Eq. 46). Biaryl coupling of arene... 

The synthesis of azo-compounds via condensation of perfluorinated w-nitrosoalkanecarboxylic acids or esters with amines was mentioned earlier [item (vi), p. 231]. An alternative method to the AtNHj-RfNO condensation for the preparation of azo-compounds of the type AtNiNRf (Rf = perfluoro-alkyl or -cycloalkyl) has now been discovered, viz. trapping of peifluoro-carbanions (from perfluoro-olefins + caesium or potassium fluoride) with arenediazonium salts (see Scheme 36). ... 

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. 

Aromatic amines can be converted to phenols by first forming the arenediazonium salt in aqueous sulfuric acid and then heating the solution. In this manner, 2-bromo-4-methylaniline is converted to 2-bromo-4-methylphenol. 

The most important reaction of amines with nitrous acid, by far, is the reaction of primary arylamines. We shall see why in Section 20.7. Primary arylamines react with nitrous add to give arenediazonium salts. Even though arenediazonium salts are unstable, they are still far more stable than aliphatic diazonium salts they do not decompose at an appreciable rate in solution when the temperature of the reaction mixture is kept below 5°C ... 

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